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.