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Snubber circuit for 3 phase Inverter

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vinu gopalakrishnan

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I am planning to control the speed of 1HP 3 phase induction motor using inverter drive(SPWM technique). My RC snubber design is like 2200pf,1000V capacitor and 33ohm resistor. But i dont knnow which diode to use in parallel with resistor. A power diode or a high ampere general purpose diode?
DC bus is 400V.
IGBT used is 15n120.
What can be the maximum current that can flow through?
shunt snub.jpg
 

FvM

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You won't want to use RCD snubbers in push-pull output stages, they are only useful for singled ended outputs, e.g. flyback. It's primarly creating additional losses for the active switching transistor.

I don't see a purpose in using snubbers for H-bridges at all. You'll want to have a low inductance DC bus, connecting the bridge transistors and freewheeling diodes directly. Overvoltages will be cut by the diodes, EMC behaviour can be tuned by changing the gate resistors.
 

Johansen

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you need a snubber if you're using fast mosfets, i have no experience yet with big IGBTs so all i can say is use an RC snubber across both sides of the half bridge.

if you are using discrete igbt's rather than half bridge, full bridge, or three phase moduals, then you can add inductors to the design to recover the reverse recovery energy and dump that back into the supply or the load, but that is rather complex to say the least.. and the snubber components have to handle the full load current plus the recovery current and or resonant currents needed to reset the inductors, transformers or what not...
 
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vinu gopalakrishnan

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well, i am using film capacitor and 2 watt resistor. But i dont know which diode to use, the power diode or the normal 6A one...pls help.

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well, i am using film capacitor and 2 watt resistor. But i dont know which diode to use, the power diode or the normal 6A one...pls help.
 

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you need a snubber if you're using fast mosfets, i have no experience yet with big IGBTs so all i can say is use an RC snubber across both sides of the half bridge.
In rare cases, I did uses RC snubbers in fast MOSFET bridges, never with IGBT. In my view it's a a kind of confession that you didn't succeed in making the circuit inductance as low as required. In a MOSFET H-bridge, a RC snubber isn't required to protect the switches, but may help to improve EMC.

"Both sides of the half bridge" point is another word to concede that the DC bus isn't as low inductive as it should. Placing the RC snubbers at empirically optimized places of the circuit is a more generalized way. R values are set equal to the characteristic impedance of the resonant circuit, C as small as possible to reduce power dissipation.

well, i am using film capacitor and 2 watt resistor. But i dont know which diode to use, the power diode or the normal 6A one...pls help.
As already said, I don't see a purpose of RCD snubbers in H briges. Do you have specific insights why it should be used in your circuit

if you are using discrete igbt's rather than half bridge, full bridge, or three phase moduals, then you can add inductors to the design to recover the reverse recovery energy and dump that back into the supply or the load, but that is rather complex to say the least.. and the snubber components have to handle the full load current plus the recovery current and or resonant currents needed to reset the inductors, transformers or what not...
In general terms, there are various ways to advance from hard to soft switching. Unfortunately, they also restrict the versatility of the H-bridge in different ways, and add circuit complexity. The series inductor idea sounds interesting to me, do you have a literature reference for it?
 

mtwieg

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In bridges, the proper way to "snub" transients is by placing very low ESL/ESR bypass capacitors on the DC bus, directly at the terminals of the FET/IGBT pair in question. This isn't snubbing in the typical sense, because it doesn't actually dissipate power, but it reduces the effective stray inductance, therefore making "real" snubbing on the output of the bridge unnecessary (in most cases). You will see this applied in small DC-DC controllers, all the way up to large VFDs using huge semiconductor modules.

However, there are some cases where RC snubbers on the output of the bridges can be justified as well. This is because even if you protect the voltage of the switches from overshoot using local bypass caps, there may still exist significant stray inductance between phases, or from each phase to the main DC link capacitors. This form of ESL can cause ringing between different nodes in the power train, which is bad for EMC/EMI reasons, and can also be hazardous to many gate drive circuits which aren't truly isolated. For this reason, I have seen many high power inverter/VFD power trains utilize RC snubbers on the output of each bridge which all use the same return node (as opposed to each snubber going to the ground node of each bridge). The location of this node varies (I've seen it at the DC link capacitor terminal, the ground node of the gate drive board, or some arbitrary point on the DC bus), but I think it always has the purpose of reducing EMI in order to protect the control system.
 
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