Application Guide, Snubber Capacitors Designing an RC Snubber Snubbers are any of several simple energy- The selection process is easy in this datasheet— absorbing circuits used to eliminate voltage spikes peak current and rms current capability is provided caused by circuit inductance when a switch— with the capacitance ratings. The peak current either mechanical or semi-conductor—opens. The capability is the dV/dt capability times the nominal object of the snubber is to eliminate the voltage capacitance. The rms current capability is the transient and ringing that occurs when the switch lower of the current which causes the capacitor to opens by providing an alternate path for the cur- heat up 15¡C or the current which causes the rent flowing through the circuit’s intrinsic leakage capacitor to reach its AC voltage. inductance. Snubbers in switchmode power sup- We’ve included dV/dt capability tables to allow you plies provide one or more of these three valuable to compare CDE snubber capacitors to other functions: brands. Snubber Mikes can withstand dV/dts of ❏ Shape the load line of a bipolar switching tran- more than 100,000 V/µs for all ratings and Type sistor to keep it in its safe operating area. DPPs can withstand more than 2000 V/µs. For ❏ Remove energy from a switching transistor and high-voltage snubbers, Types DPFF and DPPS handle more than 3000 V/µs, and Types DPMF dissipate the energy in a resistor to reduce µ junction temperature. and DPPM, more than 1000 V/ s. See the table for values according to case length. ❏ Reduce ringing to limit the peak voltage on a switching transistor or rectifying diode and to Assuming that the source impedance is negligi- reduce EMI by reducing emissions and lower- ble—the worst case assumption— the peak cur- ing their frequency. rent for your RC Snubber is: V = open circuit voltage The most popular snubber circuit is a capacitor V o Pulse, Hi-Frequency Ipk = o & Snubber and a series resistor connected across a switch. Rs = snubber resistance Capacitors Rs Here’s how to design that ubiquitous RC Snubber: Cs = snubber capacitance Component Selection: Choose a resistor that’s And the peak dV/dt is: noninductive. A good choice is a carbon composi- tion resistor. A carbon film resistor is satisfactory dV/dt = V unless it’s trimmed to value with a spiral abrasion pk o pattern. Avoid wirewound because it is inductive. RsCs Choose a capacitor from this datasheet to with- While for a sinewave excitation voltage, rms cur- stand the stratospherically high peak currents in rent in amps is the familiar: snubbers. For capacitance values up to 0.01 µF, look first at Snubber Mike dipped mica capacitors. f = frequency in Hz π 6 µ For higher capacitance values, look at the Type Irms = 2 fCVrms x 10 C = capacitance in F DPP radial-leaded polypropylene, film/foil capaci- V = voltage in Vrms tors. Axial-leaded Type WPP is as good except for For a squarewave you can approximate rms and the higher inductance intrinsic to axial-leaded peak current as: devices. I = CVpp The highest Type DPP rated voltage is 630 Vdc rms V = peak-peak volts and the highest Type WPP voltage is 1000 Vdc. pp .64 tT µ For higher voltages and capacitances, stay with q t = pulse width in s polypropylene film/foil capacitors, choosing the and T = pulse period in µs case size you prefer from Types DPFF and DPPS Ipeak = CVpp selections. For the smallest case size, choose Type DPPM or DPMF, but realize that these types .64t include floating, metallized film as common foils to Other Capacitor Types: Here’s a last word on achieve small size. The use of metallized film capacitor choice to help you venture out on your reduces the peak current capability to from a third own into the uncharted territory of capacitors not to a fifth of the other high-voltage choices. specified for use in snubbers and not in this
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datasheet. Realize that metallized film types and Cs = 1 = 780pF high-K ceramic types have limited peak-current (160)2(50 x 103) and transient withstanding capability, on the order of 50 to 200 V/µs. Polyester has 15 times the loss Optimum Snubber Design: For optimum snubber of polypropylene and is fit only for low rms cur- design using the AC characteristics of your circuit, rents or duty cycles. And, be sure to take voltage first determine the circuit’s intrinsic capacitance and temperature coefficients into account. While a and inductance. Suppose you were designing a Snubber Mike’s or a Type DPP’s capacitance is snubber for the same transistor switch as in the nearly independent of voltage and temperature, by “Quick” example. Then on a scope note the ringing comparison, a high-K ceramic dielectric like Y5V frequency of the voltage transient when the transis- can lose 3 of its capacitance from room tempera- tor turns off. Next, starting with a 100 pF Snubber ture to 50¡C (122¡F) and lose another 3 from zero Mike capacitor, increase the capacitance across volts to 50% rated voltage. the transistor in steps until the ringing frequency is half of the starting frequency. The capacitance you Quick Snubber Design: Where power dissipation have added in parallel with the transistor’s intrinsic is not critical, there is a quick way to design a capacitance has now increased the total capaci- snubber. Plan on using a 2-watt carbon composi- tance by a factor of four as the ringing frequency is tion resistor. Choose the resistor value so that the inversely proportional same current can continue to flow without voltage to the square root of the circuit’s inductance- overshoot after the switch opens and the current capacitance product: is diverted into the snubber. Measure or calculate 1 the voltage across the switch after it opens and fo = the current through it at the instant before the 2π q LC switch opens. For the current to flow through the a resistor without requiring a voltage overshoot, So, the transistor’s intrinsic capacitance, Ci, is Ohm’s Law says the resistance must be: of the added capacitance, and the circuit induc- tance, the the above equation, is: Capacitors & Snubber # R Vo Vo = off voltage
Pulse, Hi-Frequency L = 1 I I = on current i fi = initial ringing frequency C (2πf )2 C = intrinsic capacitance The resistor’s power dissipation is independent of i i i the resistance R because the resistor dissipates (added capacitance) /3 the energy stored in the snubber capacitor, Li = intrinsic inductance 2 2 CsVo , for each voltage transition regardless of the resistance. Choose the capacitance to cause When the transistor switch opens, the snubber the 2-watt resistor to dissipate half of its power rat- capacitor looks like a short to the voltage change, and only the snubber resistor is in the circuit. ing, one watt. For two times fs transitions per sec- ond, the resistor will dissipate one watt when: Choose a resistor value no larger than the charac- teristic impedance of the circuit so that the induc- 2 2 1 = ( CsVo )(2fs) fs = switching frequency tive current to be snubbed can continue unchanged without a voltage transient when the Cs = 1 switch opens: 2 Vo fs As an illustration, suppose that you have designed R = q Li /Ci a switchmode converter and you want to snub one You may need to choose an even smaller resis- of the transistor switches. The switching frequency tance to reduce voltage overshoot. The right is 50 kHz and the open-switch voltage is 160 Vdc resistance can be as little as half the characteristic with a maximum switch current of 5A. The resistor impedance for better sampling of the Intrinsic LC value must be: circuit. R # 160/5 = 32 Ω The power dissipated in the resistor is the energy 2 2 and the capacitance value is: in the capacitance, CsVo , times the switching
1605 East Rodney French Blvd. CORNELL New Bedford, MA 02744 (508) 996-8564, Fax (508) 996-3830 DUBILIER http://www.cornell-dubilier.com 5.030 E-mail: [email protected] Your Source For Capacitor Solutions Pulse, Hi-Frequency & Snubber Capacitors - - 5.031 µH -6 ” Quick ) = 0.2 W ) = 0.2 ” “Optimum “ 3 on 10R (10)(54) 2 )