May 93

Snubber Circuits: Theory , Design and Application

Philip C. Todd

Introduction Passive Snubber Types Snubbers are an essential part of power electron- The basic function of a snubber is to absorb ics. Snubbers are small networks of parts in the energy from the reactances in the power circuit. power switching circuits whose function is to The fIrst classification of snubber circuits is wheth- control the effects of circuit reactances. er they absorb energy in controlling a voltage or a Snubbers enhance the performance of the switch- current. A capacitor placed in parallel with other ing circuits and result in higher reliability, higher circuit elements will control the voltage across efficiency, higher switching frequency, smaller size, those elements. An inductor placed in series with lower weight, and lower EMI. The basic intent of other circuit elements will control the current a snubber is to absorb energy from the reactive through those elements. Figure I shows this con- elements in the circuit. The benefits of this may cept. A voltage snubber (Fig. la) has energy stor- include circuit damping, controlling the rate of age capacitors in it and a current snubber (Fig. Ib) change of voltage or current, or clamping voltage has inductors for energy storage. The networks overshoot. In performing these functions a snubber associated with the inductor and capacitor shown in limits the amount of stress which the switch must Figure I determine how energy is passed to the endure and this increases the reliability of the storage element and how the energy is removed switch. When a snubber is properly designed and from it implemented the switch will have lower average All of the other power dissipation, much lower peak power dissipa- classifications of snub- tion, lower peak operating voltage and lower peak berg relate to the ways operating current. This article describes some of the in which the energy is various types of snubbers, where they are used, transferred to and from how they function, how they are designed and what the snubber. If the their limitations are. energy stored in the Snubbers may be either passive or active networks. snubber is dissipated in This article is limited to the main types of passive a resistor the snubber is snubbers. Passive snubber network elements are classed as dissipative limited to resistors, capacitors, inductors and diodes. but if the energy is Active snubbers include transistors or other active moved back to the input switches, often entail a significant amount of extra or ahead to the output circuitry and introduce another level of parasitics the snubber is classed which must be dealt with (usually with a passive as non-dissipative even snubber). However, active snubbers are appropriate though there may be in some applications. A good example of an active some small losses. A snub is what you would like to say to your boss snubber is classed as when he or she decides not to give you a raise. polarized or non-polar- Fig. lb ized depending on

Snubber Circuits 2-1 whether energy moves in or out of the snubber on Table 2 is an index to the snubber circuits de- one edge of the switching waveform or both. The scribed in this article and gives the page number last classification for snubber circuits is rate of rise that the description of that snubber begins on. of voltage or voltage clamping. Current snubbers Duality in Snubber Operation are all rate of rise control types as there is no Snubbers have a duality which is a drawback in passive current limiting device available yet. volt- some applications. A snubber which controls the age snubbers l1Jay clamp a voltage to a fixed or switch voltage at turn off will create a current pulse variable level or may control the dV/dt of the in the switch at turn on. A snubber which controls voltage. the switch current at turn on will create a voltage Snubbers are often used in combinations and a pulse across the switch at turn off. given application may have two or three snubbers Converters with alternating switches, such as a merged into one network to control both the current push-pull converter, with a voltage snubber on one and voltage of the switch. switch to control the voltage at turn off will have a Table I is an applications guide and it gives a current spike in the other switch when it turns on. breakdown of the basic snubber types and their The same is true for snubbers on the output diodes uses. The simple damping snubbers are dissipative of a converter. Some diodes are driven off by the by definition. Rate of rise control and voltage switches and others are driven on so if a snubber is clamp snubbers may be either dissipative or non- not properly designed it will present a low imped- dissipative. Non-dissipative snubbers are more ance to the switches when they are turning on and complex than dissipative snubbers but this complex- result in a large current spike. ity is justified when the power dissipation is too high or the efficiency is too low. Snubber Fundamentals Each snubber in this discussion will be shown in Table I -QUICK GUIDE TO SNUBBER CIRCUIT USAGE an example circuit which is as generic as possible. Figure 2 shows the basic buck, boost and flyback RATEOF R~gQ~JROL SNUBBERS converter circuits drawn so that the switch is always ~ grounded. The figures which follow will generally --reducepower dissipa~on and stress in switchat turn-off --preventovershoot and ringing by not exci~ngresonance show a snubber in relation to a generic grounded --reduceEMI by reducinghigh frequency noise switch so Figure 2 may be used to apply the snub- Current ber to most other topologies. In this approach the --reducepower dissipanon and stress in switchat turn-on switch and the snubber may be thought of as a --reducediode reverse recovery current single unit, a snubbed switch, which may be used VOLTAGECLAMP --reducepeak switch voltage Table II .INDEX TO SNUBBER CIRCUITS --reducepeak switch power dissipauon at turn-off --reduceringing at switchturn-off Dissipa~veSimple RCSnubbers Voltage Snubber 3 DAMPING 3 ~ RCD Voltage Snubber. ... 5 --reduce overshoot and ringing at switch tum-olf Simple RL Current Snubber 9 --reduce switch power dissipation 10 --reduce EMI Non-dissipanveSnubbersTwoThree Terminal3D.2C-1LTerminaI3D-2C-1LTerminal Voltage SnubberVoltageVoltage Snubber.with Snubber. Intermediate Voltage

Current 10 --reduce overshoot and ringing at switch turn-on 11 --reduce power dissipauon in switch 12 --reduce EMI TransformerResetVoltageClampFlybackResonant Reset Recovery Current Current Snubber. Snubbers c c. 13 13 Rate of rise control snubbers and voltage clamp snubbers may be either 15 dissipativeor non-dissipative. Non-dissipauvesnubbers reducethe pow- er dissipauonof the snubber and increase the efficiency of the system. SnubbingDiodes c 15

2-2 UNITRODE CORPORATION in almost any switch for a particular purpose. An inductor in switching series with the switch, a comnt snubber, presents L~Do Co RL regulator Lo the switch with an inductive load at turn-on so that I- topology. v-=- it switches on with zero comnt. This is a modifi- r-;=- Not all cation of the load which would typically be some- snubbers are Q what capacitive at turn-on. The same is true at turn- applicable to off with a voltage snubber. At turn-off the load on ~ a particular the switch will typically look inductive so a capaci- problem. The tor in shunt with the switch, a voltage snubber, will BUCK circuits in change the load line to be capacitive so that the Figure 2 are switch can turn off at zero voltage. r all c lam ped l Dissipative Voltage Snubbers inductive Dissipative snubbers are those which dissipate loads and ~Do Lo the energy they absorb in a resistor. Dissipative these circuits ~I 4-- snubbers may be either voltage or current snubbers do not need Q I v and may be either polarized or non-polarized. Dissi- voltage clamp , -~ pative snubbers may be designed to control the rate snubbers since of rise of voltage or current or be designed to that function clamp the voltage. is inherent in BOOST Simple RC Voltage Snubber: The simple RC the topology. snubber shown in Figure 3A provides damping of The snubbers I the parasitic resonances in the power stage and is which are I -"Co probably the most widely used of all snubber appropriate in RL ~Do circuits. It is used on output inductors and the these topolo- I Lo ~I secondaries of transformers as well as across diodes gies are volt- -4--- and switches. It is applicable both to rate of rise age and cur- I -=-v Q control and to damping. The simple RC snubber is rent rate of L one of few snubbers which is effective in the rise control classic push-pull switch configuration. snubbers. C II .Fl YBACK Figure 3B shows the RC snubber applied to the ontro mg the rate of Fig 2a. b. c generic switch circuit. As discussed above the generic switch circuit is a clamped inductive load rise of voltage and current and clamping the voltage so in the idealized form shown here there are no and controlling resonances reduces the stress on the parasitic resonances to damp. In this case the RC switch. Snubbers can control the voltage and current snubber may be used to reduce the peak power to the point where switching occurs at zero voltage dissipation in the switch. If the values of R and C and zero current and this raises the reliability of the are chosen correctly the switching losses can be power stage significantly. Taken to the extreme, reduced by up to 40% including both the loss in the zero voltage and current switching becomes reso- switch and the loss in the resistor over the complete nant power conversion. This becomes necessary switching cycle[l]. when the circuit parasitics become large relative to The main application of an RC snubber is damp- the power level. High voltage outputs are one ing the resonance of parasitic elements in the power example. In most cases, however, the optimum circuit. In applications where damping is required efficiency level is reached with small snubbers long the value of the resistor must be close to the before a full resonant approach is necessary. impedance of the parasitic resonance which it is The effect of a snubber on the switch may be intended to damp. The snubber capacitance must be viewed as changing, or shaping, the load line on the

Snubber Circuits 2-3 larger that the resonant circuit capacitance if the impedance of the parasitic resonant circuit is but must be small enough so that the power 1~ C1 known. The value of inductance is usually the dissipation of the resistor is kept to a mini- leakage inductance of the transfonDer and can be mum. The power dissipation in the resistor estimated with some effort. The resistor value is set increases with the value of capacitance. R1 equal to the resulting characteristic impedance. Figure 3C shows an application which It is always good practice to estimate the values has an extra unclamped inductance. This needed fIrst since a calculated value which is could be leakage inductance in an isolated 3a grossly in error indicates that either the circuit is flyback converter, a not built as designed or was not designed as built. forward converter or a Once the circuit has been built and is operating, push-pull converter. It the values of the snubber components may be opti- L Lo might also be the mized experimentally. Start with a small value of inductance from a ~ capacitor and place it in the circuit in the snubber +- current snubber. In :C1 position, often this is directly across the switch, and these cases there is an then observe the voltage wavefonD with and with- Q unclamped inductance 1- out the capacitor in the circuit. Increase the value of Rt and a resonating ca- the capacitor until the frequency of the ringing to pacitance, the switch be damped has been halved. At this point the circuit t" '" output capacitance. capacitance is four times the original value so the When the switch turns Fig 3b additional capacitance is three times the original off, the energy stored circuit value. This is a near optimum value for the in the inductance will ring capacitor since it allows damping very near Q = 1. with the capacitance if there L The circuit inductance may be calculated from the Lo is no snubber. For simplicity two resonant frequencies and the two values of , rTTn-e we assume the switch with- 4-- capacitance. The characteristic impedance of the stands the extra voltage. La parasitic resonant circuit may be calculated from the There is typically very little original circuit capacitance and the inductance and loss in a parasitic resonant the value of the snubber resistance is equal to this circuit so many cycles of impedance. These calculated values of resistance ringing normally occur. and capacitance may be added to the circuit to fonD

The RC snubber will .RI the snubber. damp the ringing and if the Example: Assume that the primary of a forward snubber resistance is equal converter transfonDer has an unclamped leakage to the characteristic imped- Fig. 3c inductance of 2 pH and the power mosfet has an ance of the resonant circuit output capacitance of 330 pF. One ampere flowing [(L/C)~] then the resonant circuit will be critically in the switch at turn off will produce a voltage damped and have no overshoot. The capacitor in spike of 78 volts above Vcc if there is no damping series with the resistor must be larger than the in the circuit. The ringing frequency will be 6.2 circuit parasitic capacitance to reduce overshoot and MHz. The characteristic impedance of the LC tank ringing. is 78 ohms. If a l000pF capacitor is added in The value of the capacitor and resistor can often parallel with the switch the ringing frequency will be estimated from the other circuit components. The decrease to 3.1 MHz. If 78 ohms are added in dominant circuit capacitance is the output capaci- series with the new capacitor the ringing will tance of the switching transistor and its value can disappear. be obtained from the data sheet. The snubber capacitance will generally be two to four times this value. The snubber resistor value can be estimated

2-4 UNITRODE CORPORATION Power Dissipation of Simple RC Voltage may be found from the capacitance and the voltage. Snubber: The power dissipation of the resistor = L\Q'2f and L\Q = cy so = 2CVf must be found also. Finding an exact expression for the power dissipation is a mathematically difficult and task but it may be estimated. The assumption is that p = PR, so p = 4C2V2f2R the time constant of the snubber (t=RC) is short compared to the switching period but is long This calculation is especially useful when the compared to the voltage rise time. If the time time constant of the snubber is on the order of the constant is on the same order as the rise time the rise time of the voltage. In that case the power power dissipation estimate will be too high. If the dissipation will be at least equal to the FR loss time constant is too long the equations are not valid calculated above. and the estimated power will again be too high. Example: The inductance is 2pH and the capaci- The capacitor in a snubber stores energy. In a tance is 33OpF as above. The snubber capacitor is simple RC snubber the capacitor charges and lOOOpF and the resistor is 78 ohms. The voltage discharges. By the principle of conservation of across the switch is 400V and the switching fre- charge an amount of energy equal to that stored quency is 100KHz. The power dissipation will be will be dissipated for each charge and discharge 16.0 watts. The resistor selected must be non- cycle. This amount of power dissipation is inde- inductive since it must handle very high frequen- pendent of the value of the resistor. The power cies. In general, even a non-inductive wire wound dissipation may be calculated from the capacitance, resistor will have too much inductance to be effec- tive. The minimum power dissipation will be 0.5 the charging voltage and the switching frequency. The equation evolves as follows: watts. p = 2f(1/2C V2) = f C V2 Polarized Voltage Snubbers The objective of a polarized voltage snubber is Where p is the power dissipation, F is the switch different from that of a non-polarized snubber. The frequency, C is the value of the snubber capacitor polarized voltage snubber does not necessarily and V is the voltage that the capacitor charges to provide damping since is disconnected from the on each switching transition. This is a handy circuit over much of the cycle. Its main functions equation and is quite useful when designing the are rate of rise control or clamping. power circuit. The dominant value of capacitance is The RCD Voltage Snubber: The circuit in usually the switch output capacitance so three times Figure 4A applicable to either rate of rise control or that is the snubber capacitance. The voltage that the clamping. The circuit variation shown in Figure 4B capacitor charges to is generally known so the is applicable only to the clamp operation. maximum power dissipation of the snubber resistor A typical application of a resistor-capacitor-diode can be calculated. This allows the selection of a snubber is to control the rate of rise of voltage on resistor wattage which will not flame at the first the drain or collector of a switching transistor in a application of power. Note that the frequency forward, flyback or boost converter. At turn-off, the chosen is the switch frequency. In some single snubber will carry a major portion of the switch ended converters the output frequency is half of the current (if not all of it) and this transfers the power clock frequency. In a push-pull converter the switch dissipation of the switch into the snubber. The frequency will be equal to the clock frequency. reliability of the switch increases since its peak It is also possible to calculate a minimum power power dissipation is reduced and the controlled rate dissipation which is based on the average current of rise of voltage also lowers the high frequency through the snubber resistor. The actual dissipation EM! which the uncontrolled switching generates. will be greater than this. The equation is derived When the resistor-capacitor-diode snubber is used from averaging the absolute value of the charge and to control the rate of rise of voltage, the RC time discharge currents over the time period. The current constant must be short compared to the switching

Snubber Circuits 2-5 frequency because the capaci- I tor must be charged and dis- Where I is the maximum peak switch current, .1V is the peak voltage the capacitor will charge to, charged on each cycle. The R1 ~ circuit in Figure 4C shows .1t is the rise time of the voltage and C is the value how the snubber would be of the capacitor. The resistor is then chosen to have connected in the generic a time constant which is small compared to the switch circuit. When the switching period. A typical value for the time switch turns off, the current TCI constant would be one tenth of the switch maxi- Fig 4a from the inductor is diverted mum on time. through the snubber diode to The power dissipation of the resistor is deter- mined by the size of the capacitor since the time the snubber capacitor until the ~ capacitor is charged to the rail constant is short compared to the switching period. voltage and the main diode All of the energy stored in the capacitor is dissipat- turns on to carry the inductor ed on each cycle but there is only one transition C1 :RI current. The snubber is active (the capacitor discharge) so the power dissipated by only during the switching the resistor is given by: transition. When the switch -I p = I/].CV2f turns on the snubber capacitor Fig 4b is discharged through the Where p is the power dissipated, C is the capaci- resistor and tance value, V is the peak voltage the capacitor the switch. It 2 charges to, and F is the number of discharge cycles must be al- Lo per second. Again, note that the power dissipated most fully by the resistor is independent of its value as long as ~ discharged on the time constant is short compared to the switching each cycle to ~ .R1 period. If the time constant is longer than this, the control the snubber is functioning in a different mode and rate of rise of different equations apply. voltage on the Q A point to keep in mind about this kind of switch. snubber is that when the switch turns on, the ~C1 The design current which is discharging the snubber is flowing Fig 4c of the snubber through the switch. This current will add to the begins with the spike on the leading edge of the current waveform. Another point is that the discharge is sensitive to choice of the rise Lo time at maximum the pulse width. If the pulse width becomes very inductor current and .~ narrow, the capacitor will not be fully discharged. the supply voltage or ,f~ This usually happens only during an overcurrent the peak voltage the condition. When this happens the peak stress on the D1 switch will go way up but the average power capacitor will charge -i>f- to. The relationship dissipation on the switch is generally reasonable between the voltage because the duty factor is quite low. Q C1 and current in a R1 Example: The switch current is 1.0 amps and capacitor allows the the maximum voltage is 400v. The voltage rise time calculation of the is to be 400ns. The capacitance necessary is necessary capacitor Fig 4d l000pF. The time constant to discharge the capaci- value. tance is chosen to be 500ns, which is compatible with a looKHz switching frequency. The discharge I = C ~V/~t resistor value is 500 ohms. The power dissipated in

2-6 UNITRODE CORPORATION the resistor is 8.0 watts. A wire wound resistor may simple equation to define this relationship. be used in this case since the time constant is WL + WCl = WC2 relatively long and the self inductance of the resistor will not be important. Where WL is the energy stored in the parasitic The RCD Voltage Snubber in Clamp Mode: inductance, W Cl is the initial energy stored on the The purpose of a resistor

Snubber Circuits 2-7 resistor with the O.5pF capacitor. This gives a lOOps The simple flyback regulator shown in Figure 5 time constant. The average voltage across the is from Unitrode Application Note U-96A. C8, 05, resistor is O.5V and the average current is 2.5mA. and Rll form a rate of rise control voltage snubber The capacitor must have low inductance to handle and C9, 04 and R12 form a clamp to limit the the relatively high peak currents at switch turn-off. drain voltage. This is an example of using two Clamping may also be performed by zener snubber circuits to accomplish different objectives diodes. The advantage of a zener is that it clamps and control the voltage across the switch. at an absolute level. The zener must be rated for high peak power dissipation as well as the average Dissipative Current Snubbers power dissipation. The zener is not necessarily a The purpose of a current snubber is to control high speed device when the package inductance is the rate of rise of current in the switch and, con- taken into account so care must be taken with the versely, it is often used to control the rate of circuit layout to insure that the stray inductance is decrease of current in, and therefore turn- off of, kept low to avoid overshoot. If the zener is large the output diodes. The series inductance allows the then it may not be possible to keep the inductance switch to be fully turned on by the time the current low and it may be necessary to add a small high reaches its operating value. This greatly reduces the frequency capacitor in parallel with the zener and to peak power dissipation in the switch, it reduces the put a diode in series with both of them. The capaci- average dissipation in the switch and it increases tor handles the very high frequency currents. A the reliability . zener clamp may also be integrated into an RCD The current snubber for the switch also benefits snubber and the snubber handles the high frequency the turn off of the diode on the output. In PWM currents and the zener clamps the voltage on the converters one of the output diodes is driven off by snubber capacitor and thereby the voltage across the the switch and high reverse currents result. The switch. current snubber on the switch provides a controlled

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2-8 UNITRODE CORPORATION rate of change of current in the since the square loop core functions as a switch and diode and the low dI/dt lowers the does not provide a controlled dI/dt for the diode. dissipation in the diode and reduces However, in situations where the diode is not a the peak reverse current. consideration the square loop cores can be effective. R, The Simple RL Current Snub- Variations of tbe Simple RL Snubber: One ber: The simple RL circuit shown 111l1 way to reduce the power dissipation of the resistor in Figure 6A. This is the dual of '-T" in a simple RL snubber circuit is to add a diode in the simple RC snubber circuit. It is Fig 60 series with the resistor. This makes it a polarized not often used in switching circuits snubber which is often a desirable characteristic. in this fonn because from a practi- This is shown in Figure 6B and a simple applica- cal standpoint the value of resis- tion is shown in Figure 6C. The inductor functions tance tends to be small for good , R1 normally when the switch turns on and current only damping of the circuit. In a switch- li ing circuit this will have high I e LI flows through the resistor when it is needed to dissipate the energy which is stored in the inductor. L ~ D1 power dissipation. This circuit is The amount of power dissipated by the resistor is rather common in input and output equal to lhLIZP where I is the peak current in the filters where the AC component of ~ Fig 6b inductor and it includes the diode reverse recovery the current is relatively small and current as well as the load current. This snubber the resistor provides damp- can be very effective in increasing the overall ing necessary to control efficiency of the circuit and in increasing the L Lo the Q of the filter transfer reliability . functionl2]. Other variations ~ +- The variation shown in Figure 6D is actually a of this snubber with lower , RI IIE L1 combination of the simple RC snubber and the loss are useful for switch- inductor and has the lowest loss of the RL dissipa- ing circuits. L tive snubbers. The capacitor eliminates DC losses in A ferrite bead is actual- the resistor, which are the major contributor to Iy a simple RL circuit with power loss in switching circuit applications, and Q both the inductance and allows the resistor value to be optimized for damp- resistance incorporated into ing. This configuration is especially useful in non- a single device. It can be dissipative energy recovery networks because there very effective in low pow- Fig 6c are usually low energy, high frequency resonances er situations but the beads which must be damped. have only a very limited Example: A switch is turning on into 4OOV such power dissipation capabili- L Lo as in a boost converter. The circuit is Figure 6C ty. In switching circuits the Switch current is 1.OA. An inductor of 4OpH will bead will have high power give the diode lOOns to turn off. The inductor will dissipation and this gener- IIE L1 ~ store 20pJ of energy. The power dissipation of the ally makes it unusable in .RI resistor is 2.0 watts with a lOOKHz switching all but the lowest power frequency. 80 ohms will give a 500ns reset time situations. constant which should be reasonable at lOOKHz. Square loop cores are ~ Q There will also be an 80V spike across the switch sometimes used for current ~ when it turns off which must be taken into account. snubbers so that the resis- A lower value of resistance would lower this peak tor may be eliminated. The voltage. square loop core will allow F " ." th 19 6d the swItch to turn on WI low loss but it will not benefit the output diodes

Snubber Circuits 2-9 Non-dissipative Snubbers To under- Non-dissipative snubbers are sometimes called stand the resonant snubbers although the class is more exten- operation of C1 L1 \ D2 sive than just resonant energy recovery. Non-dissip- the snubber , D3 L ative snubbers are the class of snubbers which assume that ~ recycle the energy they collect either to the input or the switch in , 01 C2 the output or they circulate it to prepare for the Figure 7B is L next cycle. They include both current and voltage off, the induc- snubbers and both rate of rise and clamp types of tor is conduct- voltage snubbers. The basic principles of non- ing through Fig 7a dissipative snubbers are the same as for dissipative the main snubbers but in general non-dissipative snubbers do diode and that not provide damping. Damping is by defmition a the two ca- ~~ dissipative function and small resistors are often pacitors are necessary in non-dissipative snubbers to control discharged. :c. LD tertiary resonances. All non-dissipative snubbers are The snubber -mY\- -- polarized snubbers. must be reset when the Non-dissipative Voltage Snubbers switch turns A voltage snubber controls voltage by transfer- on. As the Fig 7b ring energy into a capacitor and in a dissipative switch is snubber this energy is removed from the capacitor turning on, diodes D I and D2 will turn off and the and turned into heat but in a non-dissipative snub- capacitors will apply V cc across the snubber induc- ber a way is found to transfer the energy either tor because they are discharged. Current will flow back into the source or forward into the load or to through the inductor and it will ring with the cycle it back and forth within the snubber. There capacitors until the current reaches zero and the are only a few basic types of non-dissipative or diode in series with the inductor turns off. At this resonant energy recovery voltage snubbers. They point both capacitors are charged to V cc and the are all polarized and they operate on only one edge snubber is ready for the switch turn-off. When the of the switching waveform and are reset on the switch turns off all of the current from the main other edge. In some applications they can become inductor will flow into the two capacitors. The two very complex, especially when combined with diodes in series with the capacitors now conduct so current snubbers. that the capacitors are effectively in parallel. The Two Terminal Voltage Snubber: One of the two capacitors control the rate of change of voltage basic circuits is a three diode, two capacitor, one across the switch. The turn off dissipation of the inductor (3D-2C-IL) circuit which is configured as switch is very small since the capacitors take the a two terminal network. There are two versions of full inductor current. The main diode clamps the 3D-2C-IL networks, the first of which is a two snubber voltage when the capacitors are fully terminal network. This network is shown in Figure discharged and the cycle is ready to begin again. 7 A and the connection to the generic switch circuit The first step in the design process is knowing is shown in Figure 7B. Note that this snubber is what the peak switch current is to be, what the applicable to all three converters, buck, boost and maximum value of V cc is and what the desired buck-boost. This is a rate of rise control snubber switch voltage rise time is. The value of the two and cannot be used as a clamp. The two capacitors capacitors in parallel is found from the equation for generally have equal values and the resonant fre- a capacitor which has been rearranged into the quency of the two capacitors and the inductor is following form: much higher than the switching frequency. 2C=I.t./Vcc

2-10 UNITRODE CORPORATION Where c is the value of one of the capacitors, I stress and power dissipation in the switch. is the peak current in the switch, ~ is the desired Example: The current in the switch is l.OA and maximum rise time and V cc is the maximum the switch voltage is 4OOV. The rise time is to be supply voltage. 400ns. The value of the capacitors is SOOpFeach. The length of time it takes to charge the capaci- A l.Ops recovery time gives an inductor value of tors is half of one complete resonant period of the 4OOpH. The peak current in the inductor will be two capacitors in series with the inductor. This time 3l6mA. Note that with this small a capacitance the period must be less than the smallest expected recovered charge of the diodes becomes significant. switch on time. If the on time becomes less than The diodes must be very fast and have a very low this, the snubber will not be fully reset and the recovered charge. The avemge current through them switch dissipation will go up. Note that this nor- is very small so they do not have to be large. If the mally happens only when the power supply is in recovered charge in the diodes is too large the current limit. Under a short circuit condition the voltage stored on the capacitors will not be suffi- snubber will be completely ineffective and care cient to reset them for the next cycle. must be taken to see that the switch will survive Three Terminal Voltage Snubber: Another during this condition. Once the reset time is known configuration of the three diode, two capacitor, one the value of inductance may be calculated from the inductor non-dissipative voltage snubber has three following equation: terminals rather than two. In operation it is similar L = 2(2 to the two terminal circuit The three terminal snubber is shown in Figure SA. Figure SB is the C1t2 same circuit with the top and bottom terminals Where L is the value of the inductance, t is the reversed The circuit in Figure SC shows the snub- reset time and C is the value of one of the snubber ber connected to the generic converter of Figure 2. capacitors. This snubber may be used with either buck, boost The peak current in the inductor must be found or flyback configurations. The so that the inductor and the diode in series with it L ~ ::1 snubber is a rate of rise control can be sized to handle the current. The peak current type. is found by equating the inductive and capacitive C Referring to Figure SC, the D3 L 1 operation of the circuit begins energy when each is at its peak during the cycle. The equation reduces to the following: with the switch off, the upper 2 l1 , It= ~ capacitor, C 1, discharged and 2 CVcc 1=- the lower capacitor, C2, 2L charged to V cc. The main Where I is the peak current in the inductor, C is diode, DO, is conducting the the value of one of the snubber capacitors, L is the current in the main inductor, Fig 80 value of the snubber inductor and V cc is the maxi- LO. When the switch turns on, mum supply voltage. An important consideration is 1 the diodes in series with the snub- that the half sine current pulse which resets the ber capacitors turn off and V cc is snubber when the switch is on must flow through applied across the snubber inductor, the switch. The switch must be able to handle this Ll, because Cl is discharged and current in addition to the load current. The snubber C2 is charged to Vcc. Current current should not flow through the current sense flows from C2 through Ll and D3 mechanism when the switch turns on. lie to charge Cl. When the lower Some designers make one of the two capacitors capacitor is discharged, the upper 10-20% larger than the other to insure that at least capacitor, Cl, is charged to Vcc, one of the two capacitors will be completely the inductor current is zero, and charged to Vcc. They feel that this gives the lowest diode D3, in series with the induc- Fig 8b

Snubber Circuits 2-11 tor, is off. The Vcc age, while in a boost converter this will be the switch turns off difference between the input and the output voltag- some time later and i l es. When the switch turns on the capacitor will the current through force the voltage on the snubber inductor negative the main inductor, D3L CI Lo which will ring with the capacitor until the current LO, flows into the ~ through the snubber reaches zero or until the diode snubber. This current 1+-- Dl conducts to clamp the voltage. The capacitor L'111- ., D2 discharges the upper voltage will have reversed sign but will not be capacitor through D I ra larger than V2 in Figure 9B. When the switch turns and charges the off, the current from the main inductor will flow lower capacitor ;C2 '~ into the capacitor, through diode Dl and back to through D2. This Fig Bc V2, and this controls the rate of rise of voltage controls the rate of across the switch. When the voltage is high enough rise of voltage across the switch. The two capaci- to turn on, the main diode and capacitor will be tors are effectively operating in parallel even though charged to the initial state (VI-V2). one is charging and the other is discharging. If VI-V2 in Figure 9B is less than V2, the The design of this snubber is exactly the same as charge on the snubber capacitor at the end of the for the two terminal snubber discussed previously. reset period will not be equal to V2. When this Voltage Snubber With Intermediate Voltage: happens, the switch will not turn off at zero voltage The voltage snubber shown in Figure 9A requires but instead will turn off at some intermediate an intermediate voltage which makes this snubber voltage which is determined by the point at which especially useful in forward and flyback converters. D 1 is conducting. This is still a dramatic improve- ment over not having a snubber at all but it must be This is a three terminal network and it may be used as either a rate of rise considered in switch selection and heat sinking. control snubber or as a clamp mode L The design procedure for this snubber is similar snubber. Figure 9B shows how the to the procedure given above for the two terminal non-dissipative snubber. The capacitor value, C, is snubber connects to the generic con- --i 1- verters of Figure 2 and in this applica- C1 found from: I = C dV/dt tion it is operating as a rate of rise II ~ L 1 control snubber. In clamp mode of E Where I is the maximum switch current and operation the circuit is the same but the dV /dt is the maximum rate of change of voltage values of the components are different. desired across the switch. The inductor value, L, is L This mode is normally used in associa- found from the length of time available for the reset tion with a current snubber for resonant and the capacitor value. The equation is: Fig 9a energy recovery so discussion of this 4(2 mode of operation will be v, L=- C1t2 discussed later. .-DolO The operation of the ~ Where t is the length of time available for snubber begins with the -V2 resetting the capacitor and it is shorter than the switch off and the capacitor I ~ ~ Dl smallest pulse width under normal operation. The I charged to some voltage peak current in the inductor is given by: ~I which is the difference be- CI [2 = CV2 tween the diode anode volt- LI L age (VI) and the right in- ductor terminal (V2). In a Q Where I is the peak inductor current and V is the buck or flyback converter initial voltage on the capacitor and is equal to (V 1- this will be the output volt- Fig 9b V2) in Figure 9B.

2-12 UNITRODE CORPORATION Non-dissipative Current Snubbers turns ratio and either the source or load voltage. The basic principle of a current snubber is that The major problem with this type of snubber is energy is collected in an inductor as part of its the leakage inductance between the primary and function of controlling the rate of rise of current in secondary of the inductor. This leakage inductance the switch. Non-dissipative current snubbers are can cause a large voltage spike across the switch. very similar to their dissipative counterparts. The This type of snubber has been used effectively but basic functionality is the same, an inductor is it is generally used with high power converters placed in series with the switch to control the rate where the rise and fall times are slow or where rate of rise of current. In a non-dissipative current of rise voltage snubbers are also used. Simple RC snubber the energy stored in the inductor on each damping networks are usually needed across both cycle is transferred back to the input or ahead to the primary and secondary of the inductor. the output instead of being dissipated. There are Resonant Recovery Current Snubbers: Figure several methods for removing the energy from the IIA shows the generic converter of Figure 2 with inductor on each cycle and some of the more a current snubber where the energy is being re- common methods are discussed in this section. turned to one of the converter voltages. The energy Flyback Reset Current Snubber: One of the recovery in this snubber is being handled in the most obvious ways to get the energy out of the clamp mode and the energy is recovered when the inductor is to put an extra switch turns off. The voltage across the switch is winding on it T~s allows the ! }11:NI~ clamped to the highest voltage in the converter energy to be directed any- .~ ~ whether it is an input or an output voltage. Obvi- where and provides a con- ously. a single diode could have been used in place trolled overvoltage condition Fig lOa of the RLD network so there must be some justifi- on the switch set by the turns cation for using the network. The object of the ratio and the recovery volt- network is to reduce the current in the snubber age. Figure lOA shows the inductor to zero as rapidly as is practical. If a L basic snubber and Figure single diode were used the switch would be lOB and Figure lOC show +-- clamped but there would be no voltage across the 1:N snubber inductor so it would continue conducting two ways of hooking the ~11i;-!*- snubber to the generic con- .:!II!: until the switch turned on again and it would verter of Figure 2. The con- therefore be ineffective as a current snubber. figuration chosen depends 1--'" The purpose of the snubber in Figure llA is to on whether it is a buck, fly- 1-,.Q provide voltage to reset the snubber inductor to zero back (Figure lOB) or boost , cuITent on each cycle. The clamping of the switch (Figure lOC) converter. In voltage at turn off is a secondary benefit. The Fig lOb all cases the energy is trans- snubber ca- ferred to the load although pacitor needs other connections could trans- to be small so D2 L. that it has a fer the energy back to the L~Do Lo input --JTTn--. significant l211~ The design of this type of voltage 1:N change due to snubber is rather simple. The ~II~ I~ ,.. the energy Dl value of the primary induc- ,--1 tance is the same as it would from the be for a dissipative snubber snubberinduc- Q C1 and the voltage which is tor. The volt- added to the switch during age change on J reset is determined by the the capacitor '1 Fig IOc Fig lla

Snubber Circuits 2-13 is what allows the inductor to reset to zero current. inductor is larger Again, the simple relationship between the energy than the switching Dl l stored in the inductor and the energy in the capaci- frequency the volt- Lo

tor is used to calculate the size of the capacitor. age on the capacitor LaII~ ~ can change with the +-- WL = Wc or l/].LP = l/].CV2 load current. Care Ct This equation may be rearranged to solve for the must be taken to capacitance: insure that the volt- Dl age does not exceed LI c = Ll2 the switch rating. V2 Example: The Where c is the capacitance in the snubber, L is circuit is shown in Q the inductance of the snubber, I is the current in the Figure llA. The switch at turn-off and V is the change of voltage on switch current is Fig 11b the capacitor. Note that the smaller the capacitor is, l.OA and the series the greater the voltage will be and the faster the inductor is 4OpH as was given in the example for inductor will reset. The reset time is approximately dissipative current snubbers. The reset time of the one quarter resonant cycle and is given by: inductor is chosen to be l.Ops for a lOOKHz switching frequency. The capacitor will be O.OlpF. The change of voltage across the capacitor will be t=~ 2 63V. The inductor in the recovery network may be either large or small as discussed above. Where t is the reset time, L is the snubber The snubber circuit shown in Figure llB is inductor and C is the snubber capacitance. intended to solve a somewhat different problem The inductor which is used to discharge the from that of Figure llA. Both snubber circuits capacitor may be large or small. If the value is control the current in the switch when it turns on small, that is, the resonant frequency is smaller than but the energy recovery circuit shown in Figure the switching frequency, the diode in series is llB recovers the diode tom off energy. The snub- necessary to limit the resonance to a single half ber inductor stores the reverse recovery charge from cycle. The L and C will resonate to discharge the the diode and it could drive the diode into an capacitor and at the end of the discharge cycle the overvoltage condition. The snubber in Figure llB capacitor will be as far below the nominal as it was recycles this energy. above at the beginning. A small series resistor or a The energy recovery part of the snubber in large parallel resistor may be necessary to eliminate Figure llB is the same as the voltage snubber in ringing when the diode turns off. If the value of the Figure 9B except that it is operating in the clamp inductor is large, that is, the resonant frequency is mode. The operation of the snubber in Figure llB much greater than the switching frequency, the begins with the switch off and the main diode diode is not necessary since the inductor will be in carrying the inductor current. When the switch turns continuous conduction. The voltage waveform on on the current in snubber inductor Ll will ramp up the capacitor will be similar but the discharge will and the current through the main diode will ramp be a straight line. This is the most efficient config- down. The diode current will eventually reach zero uration of the snubber. The only problem to watch and it will begin to turn off. The voltage across the out for is that the resonant frequency of the dis- diode will not change until it is completely turned charge inductor and capacitor must be high enough off. The diode reverse recovery current as it turns so that under transient conditions the peak voltage off must flow into the snubber inductor since the on the switch will not exceed its specifications. If main inductor current will not change significantly the resonant frequency of the capacitor and reset in that short a time. Once the diode is fully turned

2-14 UNITRODE CORPORATION off the snubber inductor will drive the voltage to verter to 50%. The ground since its current is higher than the current in circuit operation be- the main inductor. The energy which was needed to gins with the switch turn off the diode is now stored in the snubber on and the capacitor inductor. The snubber network in Figure IIB is charged to V cc by the designed to recycle the extra energy stored in the reset winding. When v.. inductor. After the diode turns off and the voltage the switch turns off, has dropped to zero, the snubber capacitor begins both the transfonner charging through diode D 1. The excess energy in leakage inductance the snubber inductor will transfer into the capacitor and the magnetizing and the diode will hold the energy on the capacitor. inductance will drive When the switch turns off the main diode will the voltage on the conduct again and the snubber discharge inductor switch above Vcc. Fig 12 L2 will reverse the charge on the snubber capacitor When the voltage so that it is ready for the next cycle. Since the across the switch energy recovery network is operating in clamp reaches twice Vcc the diode will turn on and the mode the capacitor is relatively large and the current in the transfonner leakage inductance will voltage across it will be small. be clamped by the capacitor and diode. The reset Example: The same example as above. The winding will then conduct the magnetizing current circuit is Figure IIB. The diode reverse recovery through the diode to reset the transfonner core. current is assumed to be O.5A peak and the snubber When the switch turns on, the capacitor, which is inductor is 4OpH. The energy will be transferred still holding the energy from the leakage induc- into the capacitor in I.Ops so the capacitor is tance, will discharge into the reset winding and is O.OlpF. The extra voltage across the diode will be voltage will again be equal to V cc. A small resistor 32V peak. The reset inductor may be small with a may be needed in series with the reset winding of reset time of 2Jls. Note that the reset requires a full the transfonner to damp the resonances which may half cycle so the equation is t=1t.JLC. The reset occur between the capacitor and the windings. The inductor is 4OpH but it only carries O.5A peak. capacitor must be large enough to absorb the energy from the leakage inductance with only a small Transformer Reset Non-dissipative Volt- voltage change and the diode must handle the peak age Clamp CUlTentfrom the spike and must be rated for at least There is one circuit which deserves special twice Vcc. mention and which is not particularly easy to classify since it requires a transformer for operation. Snubbing Diodes It is a voltage snubber and it operates in clamp Some diodes are driven off by the switch and mode. It is particularly applicable to forward others are driven on. Those driven off are usually converters and push-pull converters, both of which turned on naturally by the energy storage elements are notoriously difficult to snub. The forward in the output circuit and those driven on are usually converter version of the circuit is shown in Figure turned off by the energy storage elements. Some 12. care must be taken to insure that adding a snubber Winding Nl on the transfonI1er in Figure 12 is to the diodes will not add to the stress on the the primary power winding. Winding N2 is the switch. For example, adding a simple RC snubber reset winding and it controls the voltage on the to the output of a forward converter will add to the capacitor and provides the core reset that the current spike in the switch at turn-on. And yet, the forward converter needs. Note that the two wind- snubber may be needed to control ringing which ings have the same number of turns and that this results from the reverse recovery CUlTent of the limits the maximum duty factor of the forward con- shunt diode and the transfonner leakage inductance.

Snubber Circuits 2-15 Figure 13 shows a forward converter with two tance to cause high amplitude ringing at 50 MHz. simple RC snubbers and the transformer leakage Good layout practices are extremely important. A inductance is shown explicitly. There are two ground plane is a necessity and tracks which carry resonances in this output which must be damped. high frequency currents must be kept wide relative The fIrst is the resonant circuit formed by the to the board thickness to keep the inductance down. transformer leakage inductance and diode D l' s The diodes used in snubbers generally do not capacitance in parnllel with the output inductor have to be large diodes. They must handle relative- capacitance and the stray circuit capacitance. This Iy large peak currents but only low average cur- circuit is excited by the recovery current in D 1 as rents. They need low recovered charge, especially it turns off. This occurs when the switch turns on in non-dissipative snubbers as was mentioned in the so it is important to minimize the value of C 1. In examples above. Small diodes are often the best general, this resonant frequency will be the lowest choice and may need small heat sinks. one in the output section because the stray capaci- Inductors have parallel capacitance which must tance is largest. Snubbing this resonance with Cl be minimized. The bandwidth of the inductor must and Rl will result in minimum loss since the be as high as possible for good snubber operation. voltage swing is smallest at this point. The inductor itself may go into parallel resonance ..~--r;:+-T l.I D2 -~ and this cannot be damped electronically since it is Yo a field resonance. It must be eliminated by chang- .C2 ~ ing the winding configuration. Progressive winding R2 L.1 Dl RI -CO Ill" and bank winding techniques will minimize the Fig 13 winding capacitance. Layer winding will maximize the capacitance and random winding will have a The other resonance in the forward converter random distribution of values. output is due to the transformer leakage inductance Capacitors have series inductance which must be and the capacitance of D2. The anode of D2 will minimized. Capacitors are often paralleled to reduce ring due to the reverse recovery cun-ent. This the circuit inductance and this is effective up to a resonance will be higher in frequency than the other point. The series inductance of a large capacitor is one because the capacitance is smaller. This re- quite capable of resonating with a small capacitor quires a smaller capacitor for C2 so the power placed in parallel with it and the resonant circuit dissipation will be minimized in R2. This circuit is will have high Q. This is especially true for the excited when the switch turns off. output capacitor in a boost converter, the input It is possible to use a current snubber in series capacitor in a buck converter and all capacitors in with Dl to control the turn-off of the diode. The a flyback converter. snubber energy may be recovered using both of the The resistors used in RC damping networks must recovery networks shown in Figure 11 or dissipa- be low inductance types. Non-inductive wire wound tive networks may be used but two networks are resistors generally have too much inductance and required. will cause ringing and overshoot at high frequencies Component Problems instead of providing damping. If there is no other alternative it is sometimes possible to parallel a The characteristics of the components used in wire wound resistor with a series RC network to snubbers are very important and this is especially damp the inductance of the wire wound resistor. true in clamping snubbers. The rate of change of current in a snubber is very large and very small General Approach to Snubbing parasitics can make it almost completely ineffective. Switching Regulators The author recalls a clamp snubber in a boost The first step in snubbing a switching regulator converter which had one inch of track on the PWB takes place when the design is still on paper. over a ground plane. That provided enough induc- Formulate an overall snubbing strategy and calcu-

2-16 UNITRODE CORPORATION late the values for each snubber. Where is the remember that more than one snubber may be greatest need in the circuit. How important is required for any particular location. efficiency? How important is cost? Conclusion In the laboratory there is a general approach to A properly snubbed circuit enhances system reli- snubbing and trouble shooting which is effective. ability, is more efficient, and is quieter than an We should point out here that it is sudden death to unsnubbed circuit A properly snubbed circuit a project to snub your technician and we strongly performs well over time, temperature and produc- recommend that you not even try .Statements of tion tolerances. It is well worth the time to under- appreciation are usually much more effective at stand and use snubber circuits. preventing spiking and flaming. The switches are the most vulnerable part of the References system and must be treated gently until the circuit [ 1] W. McMurray, "Optimum Snubbers for Power is well behaved. The power stage can be observed Semiconductors," IEEE Transactionson Industry with only a small fraction of the rated voltage and Applications, September/October 1972, pp 593- current so that measurements can be made without 600. the possibility of switch damage. In this mode the [2] T. K. Phelps and W. S. Tate, "Optimizing effect of each snubber can be ascertained and its Passive Input Filter Design," Proceedings of effectiveness determined. Snubbing generally Powercon 6, May 1979, paper G1. proceeds from the input to the output of the circuit [3] I. C. Fluke Sr., "The Flyback Characteristics of not only because the switches are the most vulnera- Ferrite Power Transformers In Forward Convert- ble part of the circuit but also because the problems ers," Proceedingsof the Power Electronics Show in a switching power supply are staggered in time. and Conference, October 1986, pp128-133. When a switch is on there is no need for a voltage snubber to control the switch voltage. The [4] E. C. Whitcomb, "Designing Non-dissipative switch is connecting the output network of the Current Snubbers for Switched Mode Convert- ers," Proceedings of Powercon 6. May 1979, supply directly to a voltage source. When the paper B1. switch turns off, however, there may be some reactances which are not controlled and which will [5] x. He, S. I. Finney, B. W. Williams and T. C. ring. In a forward converter, for example, when the Green, "An Improved Passive Lossless Tum-on switch turns off the leakage inductance on the and Turn-off Snubber," IEEE Applied Power primary will drive a spike across the switch. The Electronics Conference Proceedings. March leakage inductance on the secondary will continue 1993, pp385-392. to supply current through the diode. The leakage [6] E. T. Calkin and B. H. Hamilton, "Circuit Tech- inductance on the secondary will prevent the diode niques for Improving the Switching Loci of from turning off for a few tens of nanosecondsafter Transistor Switches in Switching Regulators," the switch voltage has reached its clamp level. This IEEE Transactions On Industry Applications. time displacement is observable. This allows the Iuly/August 1976, pp364-369. individual parasitic elements to be separated and [7] W. R. Skanadore, "Load Line Shaping Consid- each one can be dealt an appropriate snubber. It erations for the High SpeedSwitching Transistor usually requires an oscilloscope with matched and In Switching Regulators and Other Highly properly adjusted probes. Inductive Environments," Proceedings of Power- Once a point has been identified as needing a con 4. May 1977, paper H-4. snubber the objective of the snubber must be [8] P. 0. Lauritzen and H. A. Smith, "A Nondissi- considered. Is the objective to provide damping of pative Snubber Effective Over a Wide Range of a resonant circuit, to clamp an overshoot, to control Operating Conditions," IEEE Power Electronics a rise or fall time or to increase efficiency. This Specialist Conference Proceedings. Iune 1983, determines the type of snubber to add. Always pp345-354.

Snubber Circuits 2-17