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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have noticed that when there is ringing on the gate, putting a RC combo across the source and drain reduce the ring. I have seen that just using a very low capacitance element works better without the resistor. Is there any reason not to do this if the capacitor can handle the current?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
A capacitor from drain to source is a primitive turn-off snubber. It reduces ringing because capacitance here limits the rate at which the drain-source voltage rises at turn off.
Any energy stored in the drain-source snubber capacitor discharges into the channel when the MOSFET turns on. So it will increase turn-on losses in the switch, unless the converter is designed to operate with zero-voltage switching.
The Class-E amplifier is a good example of a single switch system using a turn-off snubber. The phase-shifted ZVS full-bridge achieves similar with four switches.
For all non-ZVS converters an RCD snubber is more common. Here the snubbing energy collected in the capacitor at turn-off is dissipated in the resistor at turn-on.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Ok, so there is some extra power loss. But is this a problem, especially if it reduces switching spikes to almost nothing? I found adding the resistor limits the effect of the snubber.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I think this is a fairly fool proof way to get rid of ringing. I have a couple of heatsinks with a halfbridge IGBT, some film caps and power resistors, and occasionally a few other parts like 3 phase rectifiers. They use an RC circuit across each D-S, each using a 35 ohm 50 watt resistor and a 10nf MKP film capacitor. The capacitor was rather small.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Same question, still: is it a big problem to just use a capacitor?
Second, to calculate the power lost is this how it is done? Would it be less since the calculations are assuming all power stored in the capacitor is lost.
C = 2200pf = 2200 e-12 V = 330vac W = (1/2) * 330^2 * (2200pf) = 0.00011979J
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snubbing mosfets
