Snubber Capacitors - Application Guide

1 Application Guide Snubber Capacitors Designing RC Snubber Networks Snubbers are any of several simple energy The selection process is easy in this catalog . absorbing circuits used to eliminate voltage spikes peak current and rms current capability is provided caused by circuit inductance when a switch either with the capacitance ratings. The peak current mechanical or semi-conductor opens. The object capability is the dV/dt capability times the nominal of the Snubber is to eliminate the voltage transient capacitance. The rms current capability is the and ringing that occurs when the switch opens by lower of the current which causes the capacitor providing an alternate path for the current owing to heat up 15 C or the current which causes the through the circuit's intrinsic leakage inductance.

2 Capacitor to reach its AC voltage. Snubbers in switchmode power supplies provide We've included dV/dt capability tables to allow one or more of these three valuable functions: you to compare CDE Snubber Capacitors to other Shape the load line of a bipolar switching transis- brands. Dipped mica Capacitors can withstand tor to keep it in its safe operating area. dV/dts of more than 100,000 V/ s for all ratings Remove energy from a switching transistor and and Type DPPs can withstand more than 2000 V/. dissipate the energy in a resistor to reduce junc- s. For high-voltage snubbers, Types DPFF and tion temperature. DPPS handle more than 3000 V / s, and Types Reduce ringing to limit the peak voltage on a DPMF and DPPM, more than 1000 V/ s.

3 See the switching transistor or rectifying diode and to table for values according to case length. reduce EMI by reducing emissions and lower- Assuming that the source impedance is negligible . ing their frequency. the worst case assumption the peak current for The most popular Snubber circuit is a capacitor and your RC Snubber is: a series resistor connected across a switch. Here's V0 = open circuit voltage how to design that ubiquitous RC Snubber : V0. Ipk = R RS = Snubber resistance Component Selection: Choose a resistor that's S. Cs = Snubber capacitance noninductive. A good choice is a carbon composition resistor. A carbon lm resistor is satisfactory unless And the peak dV/dt is: it's trimmed to value with a spiral abrasion pattern.

4 Avoid wirewound because it is inductive. V. dV/dtpk = R0 C. s s Choose a capacitor to withstand the stratospherically high peak currents in snubbers. For capacitance While for a sinewave excitation voltage, rms values up to F, look first at dipped mica current in amps is the familiar: Capacitors . For higher capacitance values, look at the Type DPP radial-leaded polypropylene, lm/foil f = frequency in Hz 6. Capacitors . Axial-leaded Type WPP is as good Irms = 2pfCVrms x 10 C = capacitance in F. except for the higher inductance intrinsic to axial- V = voltage in Vrms leaded devices. The highest Type DPP rated voltage is 630 Vdc For a squarewave you can approximate rms and and the highest Type WPP voltage is 1000 Vdc. peak current as: For higher voltages and capacitances, stay with CVpp polypropylene lm/foil Capacitors , choosing the case Irms =.

5 64 tT Vpp = peak-peak volts size you prefer from Types DPFF and DPPS. selections. For the smallest case size, choose and t = pulse width in s CVpp V = voltage in Vrms Type DPPM or DPMF, but realize that these types Ipeak = T=Pulse periods in s include oating metallized lm as common foils .64 tT. to achieve small size. The use of metallized lm Other Capacitor Types: Here's a last word on capacitor reduces the peak current capability to from a third choice to help you venture out on your own into the to a fth of the other high-voltage choices. uncharted territory of Capacitors not speci ed for use in snubbers and are not in this section. CDE Cornell Dubilier s 1605 E. Rodney French Blvd. s New Bedford, MA 02744 s Phone: (508)996-8561 s Fax: (508)996-3830 s Application Guide Snubber Capacitors Realize that metallized lm types and high-K ceramic 1 = 780 pF.

6 Types have limited peak-current and transient Cs = 2 3). (160) (50 x 10. withstanding capability, on the order of 50 to 200 V/ s. Polyester has 15 times the loss of polypropylene and Optimum Snubber Design: For optimum Snubber is t only for low rms currents or duty cycles. And, be design using the AC characteristics of your circuit, sure to take voltage and temperature coef cients into rst determine the circuit's intrinsic capacitance account. While a mica's or a Type DPP's capacitance and inductance. Suppose you were designing is nearly independent of voltage and temperature, by a Snubber for the same transistor switch as in comparison, a high-K ceramic dielectric like Y5V can the Quick example. Then on a scope note the lose of its capacitance from room temperature to ringing frequency of the voltage transient when 50 C (122 F) and lose another from zero volts to the transistor turns off.

7 Next, starting with a 100 pF. 50% rated voltage. mica capacitor, increase the capacitance across Quick Snubber Design: Where power dissipation is not the transistor in steps until the ringing frequency critical, there is a quick way to design a Snubber . Plan is half of the starting frequency. The capacitance on using a 2-watt carbon composition resistor. Choose you have added in parallel with the transistor's the resistor value so that the same current can continue intrinsic capacitance has now increased the total to ow without voltage overshoot after the switch opens and the current is diverted into the Snubber . Measure capacitance by a factor of four as the ringing or calculate the voltage across the switch after it opens frequency is inversely proportional to the square root and the current through it at the instant before the of the circuit's inductance capacitance product: switch opens.

8 For the current to ow through the resistor without requiring a voltage overshoot, Ohm's Law says fo = 1. the resistance must be: 2p LC. Vo = off voltage R Vo So, the transistor's intrinsic capacitance, Ci, is of the I I = on current added capacitance, and the circuit inductance, from the above equation, is: The resistor's power dissipation is independent of the resistance R because the resistor dissipates 1. Li = fi = initial ringing frequency the energy stored in the Snubber capacitor, C (2pf ) 2. i i Ci = intrinsic capacitance CsVo2, for each voltage transition regardless of the (added capacitance) /3. resistance. Choose the capacitance to cause the Li = intrinsic inductance 2-watt resistor to dissipate half of its power rating, one watt.

9 For two times fs transitions per second, When the transistor switch opens, the Snubber the resistor will dissipate one watt when: capacitor looks like a short to the voltage change, 2. 1 = ( CsVo )(2fs) fs = switching frequency and only the Snubber resistor is in the circuit. Choose a resistor value no larger than the characteristic Cs = 12 impedance of the circuit so that the inductive current Vo fs to be snubbed can continue unchanged without a As an illustration, suppose that you have designed voltage transient when the switch opens: a switchmode converter and you want to snub one R = Li/Ci of the transistor switches. The switching frequency is 50 kHz and the open-switch voltage is 160 Vdc You may need to choose an even smaller resistance with a maximum switch current of 5A.

10 The resistor to reduce voltage overshoot. The right resistance value must be: can be as little as half the characteristic impedance for better sampling of the Intrinsic LC circuit. R 160/5 = 32 $. The power dissipated in the resistor is the energy and the capacitance value is: in the capacitance, CsVo2, times the switching CDE Cornell Dubilier s 1605 E. Rodney French Blvd. s New Bedford, MA 02744 s Phone: (508)996-8561 s Fax: (508)996-3830 s Application Guide Snubber Capacitors frequency, fs, times the number of voltage transitions And calculate the circuit inductance: per cycle. For example, if your circuit is a half-bridge 1. converter, there are two voltage transitions per cycle Li = -12. = H. (67 x 10 )(2p44 x 106)2. and the power in the resistor is: Cs = Snubber capacitance And the Snubber resistor value: Pr = CsV02 fs Vo = off voltage R= (10-3) = 54 W.