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Application Guide,Snubber Capacitors

Designing an RC Snubber

Snubbers are any of several simple energy-

absorbing circuits used to eliminate voltage spikes

caused by circuit inductance when a switch—

either mechanical or semiconductor—opens. The

object of the snubber is to eliminate the voltage

transient and ringing that occurs when the switch

opens by providing an alternate path for the cur-

rent flowing through the circuit’s intrinsic leakage

inductance. Snubbers in switchmode power sup-

plies provide one or more of these three valuable

functions:

❏

Shape the load line of a bipolar switching tran-

sistor to keep it in its safe operating area.

The selection process is easy in this catalog—peak

current and rms current capability is provided with

the capacitance ratings. The peak current capabili-

ty is the dV/dt capability times the nominal capaci-

tance. The rms current capability is the lower of the

current which causes the capacitor to heat up 15°C

or the current which causes the capacitor to reach

its rated AC voltage.

We’ve included dV/dt capability tables to allow you

to compare CDE snubber capacitors to other

brands. Mica can withstand dV/dts of more than

100,000 V/

❏

Remove energy from a switching transistor and

dissipate the energy in a resistor to reduce

junction temperature.

s, and Types DPMF

and DPPM, more than 1000 V/

s. See the table for

values according to case length.

Assuming that the source impedance is negligi-

ble—the worst case assumption— the peak cur-

rent for your RC Snubber is:

Reduce ringing to limit the peak voltage on a

switching transistor or rectifying diode and to

reduce EMI by reducing emissions and lower-

ing their frequency.

The most popular snubber circuit is a capacitor

and a series resistor connected across a switch.

Here’s how to design that ubiquitous RC Snubber:

Component Selection: Choose a resistor that’s

noninductive. A good choice is a carbon composi-

tion resistor. A carbon film resistor is satisfactory

unless it’s trimmed to value with a spiral abrasion

pattern. Avoid wirewound because it is inductive.

Choose a capacitor from this datasheet to with-

stand the stratospherically high peak currents in

snubbers. For capacitance values up to 0.01

I pk = V o

R s

V o = open circuit voltage

R s = snubber resistance

C s = snubber capacitance

And the peak dV/dt is:

dV/dt pk = V o

R s C s

While for a sinewave excitation voltage, rms cur-

rent in amps is the familiar:

f = frequency in Hz

C = capacitance in

look first at dipped mica capacitors. For higher

capacitance values, look at the Type DPP radial-

leaded polypropylene, film/foil capacitors. Axial-

leaded Type WPP is as good except for the higher

inductance intrinsic to axial-leaded devices.

The highest Type DPP rated voltage is 630 Vdc

and the highest Type WPP voltage is 1000 Vdc.

For higher voltages and capacitances, stay with

polypropylene film/foil capacitors, choosing the

case size you prefer from Types DPFF and DPPS

selections. For the smallest case size, choose

Type DPPM or DPMF, but realize that these types

include floating, metallized film as common foils to

achieve small size. The use of metallized film

reduces the peak current capability to from a third

to a fifth of the other high-voltage choices.

I rms = 2

fCV rms x 10 6

V = voltage in V rms

For a squarewave you can approximate rms and

peak current as:

I rms = CV pp

.64 q tT

and

I peak = CV pp

.64t

Other Capacitor Types: Here’s a last word on

capacitor choice to help you venture out on your

own into the uncharted territory of capacitors not

specified for use in snubbers and not in this section.

V pp = peak-peak volts

t = pulse width in

T = pulse period in

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s for all ratings and Type DPPs can

withstand more than 2000 V/

s. For high-voltage

snubbers, Types DPFF and DPPS handle more

than 3000 V/

❏

Application Guide,Snubber Capacitors

s. Polyester has 15 times the loss of

polypropylene and is fit only for low rms currents

or duty cycles. And, be sure to take voltage and

temperature coefficients into account. While a

mica’s or a Type DPP’s capacitance is nearly inde-

pendent of voltage and temperature, by compari-

son, a high-K ceramic dielectric like Y5V can lose

3 of its capacitance from room temperature to

50°C (122°F) and lose another 3 from zero volts

to 50% rated voltage.

Quick Snubber Design: Where power dissipation

is not critical, there is a quick way to design a

snubber. Plan on using a 2-watt carbon composi-

tion resistor. Choose the resistor value so that the

same current can continue to flow without voltage

overshoot after the switch opens and the current

is diverted into the snubber. Measure or calculate

the voltage across the switch after it opens and

the current through it at the instant before the

switch opens. For the current to flow through the

resistor without requiring a voltage overshoot,

Ohm’s Law says the resistance must be:

R # V o

The resistor’s power dissipation is independent of

the resistance R because the resistor dissipates

the energy stored in the snubber capacitor,

2 C s V o 2 , for each voltage transition regardless of

the resistance. Choose the capacitance to cause

the 2-watt resistor to dissipate half of its power rat-

ing, one watt. For two times f s transitions per sec-

ond, the resistor will dissipate one watt when:

1 = ( 2 C s V o 2 )(2f s ) f s = switching frequency

C s = 1

V o 2 f s

As an illustration, suppose that you have designed

a switchmode converter and you want to snub one

of the transistor switches. The switching frequency

is 50 kHz and the open-switch voltage is 160 Vdc

with a maximum switch current of 5A. The resistor

value must be:

R # 160/5 = 32

C s = 1 = 780pF

(160) 2 (50 x 10 3 )

Optimum Snubber Design: For optimum snubber

design using the AC characteristics of your circuit,

first determine the circuit’s intrinsic capacitance

and inductance. Suppose you were designing a

snubber for the same transistor switch as in the

“Quick” example. Then on a scope note the ringing

frequency of the voltage transient when the transis-

tor turns off. Next, starting with a 100 pF mica

capacitor, increase the capacitance across the

transistor in steps until the ringing frequency is half

of the starting frequency. The capacitance you

have added in parallel with the transistor’s intrinsic

capacitance has now increased the total capaci-

tance by a factor of four as the ringing frequency is

inversely proportional to the square root of the

circuit’s inductance-capacitance product:

f o = 1

q LC

So, the transistor’s intrinsic capacitance, C i , is a

of the added capacitance, and the circuit induc-

tance, from the above equation, is:

L i = 1

C i (2

V o = off voltage

I = on current

f i ) 2

f i = initial ringing frequency

C i = intrinsic capacitance

(added capacitance) /3

L i = intrinsic inductance

When the transistor switch opens, the snubber

capacitor looks like a short to the voltage change,

and only the snubber resistor is in the circuit.

Choose a resistor value no larger than the charac-

teristic impedance of the circuit so that the induc-

tive current to be snubbed can continue

unchanged without a voltage transient when the

switch op ens:

Ω

and the capacitance value is:

R = q L i /C i

You may need to choose an even smaller resis-

tance to reduce voltage overshoot. The right

resistance can be as little as half the characteristic

impedance for better sampling of the Intrinsic LC

circuit.

The power dissipated in the resistor is the energy

in the capacitance, 2 C s V o 2 , times the switching

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Realize that metallized film types and high-K

ceramic types have limited peak-current and tran-

sient withstanding capability, on the order of 50 to

200 V/

Application Guide,Snubber Capacitors

frequency, f s , times the number of voltage transi-

tions per cycle. For example, if your circuit is a

half-bridge converter, there are two voltage transi-

tions per cycle and the power in the resistor is:

P r = C s V o 2 f s

44 x 10 6 ) 2

And the snubber resistor value:

R = q 0.196/67 (10 -3 ) = 54

C s = snubber capacitance

V o = off voltage

f s = switching frequency

Ω

Before you can calculate the resistor power dissi-

pation, you must first choose the snubber capaci-

tance:

Choose a snubber capacitance value which meets

two requirements:

1) It can provide a final energy storage greater

than the energy in the circuit inductance

2 C s V o 2 > 2 L i I 2 I = closed-circuit

C s > LI I 2

V o 2

L i I 2

<C s < on

V o 2

10R

(0.196 x 10 -6 )(5) 2

<C s < 2 x 10 -6

(10)(54)

192 < C s < 3700 pF

Since power dissipation in the resistor is propor-

tional to the capacitance, choose a standard

capacitance value near the low end of the above

range. For a 220 pF capacitor and two transitions

per cycle, the power dissipation in the resistor is:

P r = (220 x 10 -12 )(160) 2 (50 x 10 3 ) = 0.2 W

Comparing the “Quick” design to the “Optimum”

design, you see that for the same converter switch

the required snubber resistor’s power capability

was reduced by a factor of 5, from 1 W to 0.2 W,

and the snubber capacitance was reduced by a

factor of 3.5, from 780 pF to 220 pF. This was

possible because the additional circuit measure-

ments revealed that the source impedance was

actually 54

and,

2) it produces a time constant with the snubber

resistor that is small compared to the shortest

expected on-time for the transistor switch.

RC s < t on /10

C s < t on /10R

Choosing a capacitance near the low end of the

range reduces power dissipated in the resistor,

and choosing a capacitance 8 to 10 times the

intrinsic capacitance, C i , almost suppresses the

voltage overshoot at switch turn off. Try a capaci-

tance at the low end of the range as the initial

value and increase it later if needed.

Now revisit the “Quick” example with the added

data permitting “Optimum” design. You’ve taken

some more measurements on your switchmode

converter: the ringing frequency of the voltage

transient when the transistor switch opens is 44

MHz and an added parallel capacitance of 200 pF

reduces the ringing frequency to 22 MHz. The

switching frequency is 50 kHz with a 10% mini-

mum duty cycle, and the open-switch voltage is

160 Vdc with a maximum switch current of 5A. So

you know the following:

f i = 44 MHz

C i = 200/3 = 67 pF

f s = 50 kHz

t on = 0.1/(50 x 10 3 ) = 2

, and that the circuit

inductance permitted a smaller capacitance to

swallow the circuit’s energy.

Usually the “Quick” method is completely ade-

quate for final design. Start with the “Quick”

approach to prove your circuit breadboard, and go

on to the “Optimum” approach only if power effi-

ciency and size constraints dictate the need for

optimum design.

NOTE: For more on RC snubber design, for RCD snubber

design, and for snubber design using IGBT snubber

modules, get the application note, “Design of Snubbers for

Power Circuits,” at www.cornell-dubilier.com/pdfframe.htm.

V o = 160 Vdc

= A

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And calculate the circuit inductance:

L i = 1 = 0.196

(67 x 10 -12 )(2

(160) 2

rather than 32

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