mosfetsnubber
I met this kind of snubber network in Switchmode Power Supply Handbook (by Keith Billings and Taylor Morey):
They were using it with BJTs, and the reason was to limit dv/dt to avoid secondary breakdown of the BJT. Now if I'm using MOSFET in a Flyback converter, is this snubber needed (I have clamp in the circuit)? As far as I know, MOSFETs do not have secondary breakdown, and with proper clamp circuit they can withstand the voltages.
Here's a better way to drive a high-side P-channel MOSFET:
simulate this circuit – Schematic created using CircuitLab
Note that the BJT Q1 is a current sink; the current value is the control voltage divided by R2. If your control circuit puts out 12V, the current will be 12 mA.
This current will create a voltage drop across R1 as well, and if you happen to make the two resistors equal, this voltage drop will be essentially equal to the control voltage supply (minus the VBE drop of Q1). Or you can make the resistors different in order to get a particular relationship.
You need to be aware of two things: The current level that you set will have some bearing on how fast the MOSFET M1 switches; higher current means faster switching. However, keep in mind that Q1 has a lot of voltage across it. When off, it has the full high voltage supply across it. And even when switched on, it has the high voltage minus 2× the control voltage across it. This means that it will be dissipating power equal to that voltage drop times the current when on.
In these circumstances I'd use a zener diode across Q1 - then there is no doubt that the maximum voltage that is imposed by the relay coil (when it is open circuited) is limited to the zener voltage thus protecting the transistor. If the transistor AMR (absolute maximum rating) was 60 volts, then choose a zener that's rated at two-thirds of 60V i.e. 40 volts (a 39 volt zener will do just fine).
The problem with a snubber is that when the MOSFET open-circuits, 100mA will flow thru the 100 ohm resistor and charge the capacitor up rapidly to some voltage that is hard to determine - if you knew the inductance of the relay coil (and hence the stored energy) this could be calculated but I guess you don't or you would have said.
Best Answer
This snubber circuit is used to limit dv/dt during turn-off of the switch by presenting a higher load capacitance to the inductive load usually driven by the transistor. Without the snubber, the rate would be more or less only limited by the difference of load and switch current and the output capacitance of the switch.
Besides limiting the dv/dt it also 'delays' the voltage rise, thereby limiting the turn-off losses in the switch. Of course the resistor has to dissipate the energy stored in the snubber capacitor but there is actually a point, where the overall switching losses are lower as if you didn't use the snubber at all. Here you can find a nice description of it: Snubber Circuits by William P. Robbins
It is rarely used in nowadays flyback converters because the component and assembly cost are too high in comparison to a better power MOSFET, something that is completely different in a 1200V/200A IGBT module.