Snubber Circuits

DESIGN OF POWER 

ELECTRONIC CIRCUITS 

Snubber 

Circuits

SNUBBER 

When a power semiconductor 

switch is either on or off, its 

power dissipation is relatively 

small. However, high voltage 

and high current occur 

simultaneously while 

transitioning from one state to 

the other. Special efforts are 

often required to ensure that 

the device will survive this 

most stressful part of its 

operating cycle.

SNUBBER 

We need to control (limit) the voltage 

and current in a switch when it makes 

transition between the states because: 

• The switch has to be in SOA (Safe 

Operation Area) 

• The rate of change of voltage and 

current has to be low enough to 

provide reliable operation 

• The power dissipation during 

switching has to be low as much as 

possible (to increase efficiency and 

switching frequency) 

• Switching of a controllable switch is 

often accompanied by a change in 

state of a diode. Thus, making this 

transition slow enough to keep the 

diode’s forward recovery voltage and 

reverse recovery current from 

excessively stressing the controllable 

switch is often important.

SNUBBER 

To satisfy all these concerns, we often add special components 

to the power circuit to limit the voltage and current in a switch 

during switching transitions. These components are called 

snubber circuits. 

There are two kinds of snubbers; 

• Turn-off snubber (to control the voltage rise) 

• Turn-on snubber (to control the current rise)

TURN-OFF SNUBBER 

Here is an example for turn-off 

snubber: 

Suppose that there is a buck 

converter with V dc = 400V, I dc = 

50A and t f = 0.5 µs.

• 

TURN-OFF SNUBBER


If the transistor is operated 

near its rating, the SOA can 

easily be exceeded, with 

respect to both peak power 

and second-breakdown limits.


So, if either the peak or the 

average power dissipation are 

too high for the transistors, or 

if the SOA is exceeded by 

switching the transistor, a turnoff 

snubber must be used to 

change the waveforms. The 

capacitor C s is a simpler 

snubber. Now when the 

transistor is turned off, its 

voltage and current waveforms 

look like shown in the figure.


Note that because C s supplies a third path through which I dc 

can flow, it is no longer necessary for v Q to rise to equal V dc 

before i Q can begin to fall as the diode picks up the load 

current. Instead, any difference between I dc and i Q flows 

through C s . The charging of this capacitor, rather than the 

switching time of the transistor, governs the rate at which v Q 

rises from zero to V dc .


Because the voltage across C s increases at a rate proportional 

to 1/C s , we can choose a large enough capacitor to keep the rise 

in voltage to only a small fraction, γ v , of V dc by the time i Q 

reaches zero. The consequence of including C s is that the peak 

power and total dissipation in the transistor are much smaller 

than they were below before the inclusion of C s , as illustrated by 

the bottom graph.


In addition to the reducing the 

dissipation in the transistor, 

the snubber capacitor also 

maintains the switching locus 

in the SOA of the transistor. 

The bottom figure shows the 

turn-off locus as modified by 

the presence of C s and turn-on 

locus by a snubber as well.

• 

CHOOSING A VALUE FOR C S

CHOOSING A VALUE FOR C S 

If we choose C s so that γ v =0.2 for 

V dc =400V, I dc =50A, and if we 

assume that t’ f =0.33µs, then 

W diss =0.12mJ. During the time 

0

A MORE PRACTICAL TURN-OFF SNUBBER 

A capacitor by itself is not a sufficient turn -off snubber because, when 

the transistor is turned on again, C s will discharge through Q.The 

resulting transistor current can be very large and lead to failure. To 

avoid this problem, we add resistor and a diode to C s , as shown above. 

The purpose of the resistance R s is to limit the discharge current when 

the transistor turned on, and the purpose of the diode D s is to allow 

the charging current to bypass the resistor during the turn -off 

transition.

A MORE PRACTICAL TURN-OFF SNUBBER 

A tradeoff is necessary in determining the value of R s , It must be 

small enough to ensure that the capacitor is fully discharged during 

the shortest time that Q might be on but large enough to prevent the 

discharge current from exceeding the transistor rating.

TURN-ON SNUBBER 

The figure on the side shows 

the voltage and current 

waveforms during the turn-on 

in the transistor. Just like the 

turn-off snubber, the turn-on 

snubber serves to modify the 

switching waveforms to 

reduce the loss and maintain 

operation within the SOA.


Dissipation during the turn-on 

transition can be calcutated, 

just like the way that is found 

while turn-off dissipation, with 

t f replaced by t r . A typical value 

of i r , for a 400V 50A transistor 

is 0.5 µs. This value for t r 

results in a peak power of 

20kW and an energy loss of 

5mJ during each turn-on 

transition. With a switching 

frequency of 20 kHz, turn-on 

causes 100W of dissipation in 

the transistor.


The switching locus in the i Q -v Q 

plane for this transition is 

identical to the one shown on 

turn-off, except that the path 

is taken in the reverse 

direction. Once again, the 

transistor’s peak power and 

second-breakdown limits are 

exceeded.


The solution to the turn-on 

problem is the dual of the turnoff 

problem solution. A 

snubber, L s , is placed in series 

with the transistor. The 

waveforms are shown on the 

side.

• 

TURN-ON SNUBBER


At the switching frequency of 

20 kHz, the resulting average 

turn-on dissipation in the 

transistor is 1.2. That quantity 

is almost 1/100 of the turn-on 

losses without a snubber. 

The peak power is also 

reduced, and the turn-on 

swtitching locus now remains 

within the SOA.

A MORE PRACTICAL TURN-ON SNUBBER 

As with the turn-off snubber, the turn-on snubber requires additional 

components to allow the transistor to survive. In this case, if nothing 

were added to the circuit, the transistor would recieve an impulse of 

voltage when it turned off, because the inductor current would be 

forced to change in a very short time. To avoid this potentially 

destructive transient, a resistor and a diode can be added to the 

snubber circuit, as shown above.The resistor R s provides an alternative 

path for the inductor current when the transistor turns off. The purpose 

of the D s is to keep the resistor from conducting during the turn-on 

transition

A MORE PRACTICAL TURN-ON SNUBBER 

The resistance R s must be large enough to completely discharge 

L s during the off state. However to minimize the transistor’s 

voltage stress, it should not be made any larger than necessary.

CHOOSING A VALUE FOR L S 

•

CHOOSING A VALUE FOR L S 

If the shortest time that we expect the transistor to be turned 

off is 2.5µs, or 5% of a 20 kHz cycle, we need an L s /R s time 

constant of about 0.5 µs to ensure that L s is completely 

discharged. Meeting this condition requires a resistor of value 

R s ≈ 2.5 Ω.

COMBINED TURN-ON&TURN-OFF 

SNUBBERS 

A combined turn-on&turn-off snubber circuit with the resistor 

placed to discharge C s and the voltage and current waveforms at 

turn off.

COMBINED TURN-ON&TURN-OFF 

SNUBBERS 

A combined turn-on&turn-off snubber circuit with the resistor 

placed to discharge L s and the voltage and current waveforms at 

turn on.

Snubber Circuits

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