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!)
Hi, Have you got a DSO to look at your gate/source waveform? In my experience the quality of the gate drive is critical to reliable operation. Also you need the voltage overshoot on the primary in order to get the output to work. You just either have to work on the layout of the PCB to minimise ringing, or reduce the speed fo the gate drive in order to to make things ring up too much. You can increase the gate resistor value to do this.
Back almost 30 years ago, when I was designing off-line switchers, I used a Tek current probe along with a voltage probe to plot the operating SOA on the 'scope screen. Learned a lot about designing snubbers that way, and made a lot of high efficiency stuff.
...Jim Thompson
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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.
You can also consider a zener diode to clamp the peak voltage to what the FET can handle.
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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.
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
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.
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