RC snubber on flyback primary

[Jamie Morken]

Hi,

I am testing a flyback and trying to size the primary side RC snubber,
without the snubber my fet blows up, I started with a 100ohm 1206
package resistor and a 47nF 1kV 1812 package capacitor, and now am using
a 100ohm resistor and 10nF 1kV 1206 package capacitor, this still smokes
a 1206 package 1/4 watt resistor. The flyback is low power, 10watts
maximum output, and I would like to dissipate as little power in the
snubber as possible for efficiency. Switching to a smaller snubber cap
reduces the heat in the resistor, and also I noticed in an ltspice sim
that a smaller value R also decreases the heat in the resistor, so which
method is more efficient to snub voltages, dropping the capacitance
value or the resistance value for an RC snubber?

cheers,
Jamie

 

[Jamie Morken]

Hi,

I am testing a flyback and trying to size the primary side RC snubber,
without the snubber my fet blows up, I started with a 100ohm 1206
package resistor and a 47nF 1kV 1812 package capacitor, and now am using
a 100ohm resistor and 10nF 1kV 1206 package capacitor, this still smokes
a 1206 package 1/4 watt resistor. The flyback is low power, 10watts
maximum output, and I would like to dissipate as little power in the
snubber as possible for efficiency. Switching to a smaller snubber cap
reduces the heat in the resistor, and also I noticed in an ltspice sim
that a smaller value R also decreases the heat in the resistor, so which
method is more efficient to snub voltages, dropping the capacitance
value or the resistance value for an RC snubber?

Just a bit of an update, I removed the RC snubbers on the flybacks I am
testing, and they survived, so I think my original mosfet failures were
from an unrelated cause, I then put on a 100ohm/68pF RC snubber and
there is a 100mW extra draw on the bench supply, with a 2watt output
load. I'm not sure if this small of an RC snubber is very useful, but
it seems to be a good fit (ie. low power!)

cheers,
Jamie

 

[[email protected]]

Lots of app notes around IIRC on snubber design too.

Graham

You may need a voltage clamp across your primary as well as a snubber.
Try a diode in series with R//C. Diode Anode to Drain of fet and R//C
to +supply. This will clamp the drain spike and stop your fet blowing.
The r-c cct across your primary will help kill high frequency
ringing , but does not help much with the leading edge spike. Which
can be big!!
I have attached a simple flyback type cct which demo's the "clamping"
cct. After 300us the "clamp " is enabled.
Cheers
Rob.

Version 4
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TEXT 296 232 Left 0 ;Set C2 to 1pF effectivly take the branch of the
circuit away.

 

[Nico Coesel]

Hi,

I am testing a flyback and trying to size the primary side RC snubber,
without the snubber my fet blows up, I started with a 100ohm 1206
package resistor and a 47nF 1kV 1812 package capacitor, and now am using
a 100ohm resistor and 10nF 1kV 1206 package capacitor, this still smokes
a 1206 package 1/4 watt resistor. The flyback is low power, 10watts
maximum output, and I would like to dissipate as little power in the
snubber as possible for efficiency. Switching to a smaller snubber cap

If this is your goal, you should attack the real problem. The snubber
is necessary to dissipate the energy which is stored in the leakage
induction in the transformer. If you can find (or have made) a
transformer with a lower leakage induction, you'll dissipate less heat
in the snubber or clamp circuit.

reduces the heat in the resistor, and also I noticed in an ltspice sim
that a smaller value R also decreases the heat in the resistor, so which
method is more efficient to snub voltages, dropping the capacitance
value or the resistance value for an RC snubber?

You can also consider a zener diode to clamp the peak voltage to what
the FET can handle.

 

[Vladimir Vassilevsky]

If this is your goal, you should attack the real problem. The snubber
is necessary to dissipate the energy which is stored in the leakage
induction in the transformer. If you can find (or have made) a
transformer with a lower leakage induction, you'll dissipate less heat
in the snubber or clamp circuit.

If the goal is the dissipation of the leakage inductance energy, then
the solution could be just slowing down the turn off process of the FET.

You can also consider a zener diode to clamp the peak voltage to what
the FET can handle.

Depending on the application, there could be the significant amount of
power in the leakage inductance. Also, the zener should be able to
handle peak current. For that matter, I prefer non-dissipative clamping
which returns leakage power back to the supply rail.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[neon]

why the 100ohms 68 pf is small enough to make the 100ohms pratcicaly non exsisting. a scope is the only way to realy find out the right snubber. or guess.

 

[Eeyore]

If this is your goal, you should attack the real problem. The snubber
is necessary to dissipate the energy which is stored in the leakage
induction in the transformer. If you can find (or have made) a
transformer with a lower leakage induction, you'll dissipate less heat
in the snubber or clamp circuit.

And knowing how to do that is an art/science in itself. Especially of it's
running off the mains and you have to take safety margins into account.

You can also consider a zener diode to clamp the peak voltage to what
the FET can handle.

Crikey. You said almost exactly the same as me. But then again those are the
facts.

Graham

 

[legg]

Just a bit of an update, I removed the RC snubbers on the flybacks I am
testing, and they survived, so I think my original mosfet failures were
from an unrelated cause, I then put on a 100ohm/68pF RC snubber and
there is a 100mW extra draw on the bench supply, with a 2watt output
load. I'm not sure if this small of an RC snubber is very useful, but
it seems to be a good fit (ie. low power!)

Depending on what you are trying to snub, differing configurations of
RC or RCD in current or voltage are applicable. It takes considerable
abuse to pop fets from switching loss alone - they are not as
SOA-sensitive as bipolars were (and still are).

Power loss in the simplest RC snubber, with no ringing, is
approximated as:

C x V^2 x f / 2

for each specific voltage change in the waveform; all power within the
waveform being cumulative when correlated in the same waveform.

If all transformer leakage inductance energy was expected to be
absorbed in a voltage clamp, the energy would be:

Lleak x Ipk^2 x f / 2.

Not all of the energy need necessarily be dissipative - as some is
redirected to produce the dynamic waveform's edges. If a relatively
slow Trr rectifier is used in the clamp, some of the energy may also
be returned to the circuit during the clamp diode's reverse recovery,
without being burned off in the clamp ( a 'damping' effect ).

As suggested by Jim T, scoping the current and voltage waveforms is
educational in identifying and reducing switch stress - and to
determine effectiveness of any measures taken to alter the stress
profile. Also keep an eyeball on input power consumption - this can be
a useful relative indicator.

Worse case is likely at the heaviest peak switch current turn-off
condition, at some worst-case temperature and input voltage condition,
though the addition of snubbing parts or a stray-capacitive-laden or
rectifier-diode-lossy turn-on may contribute to turn-on losses in the
profile.

RL