Difference between DC Link and Snubber Capacitors

[profdc9]

Hello,

I am a little confused about the terminology with DRSSTC.  The capacitor that goes between the high and low voltage rails I thought is called a DC Link capacitor.  It is intended to hold the voltage at the IGBT terminals constant despite the inductance of the connection from the electrolytic doubler capacitors to the IGBT terminals.   Snubbers I thought were on the part where the lower collector and the upper emitter of the half-bridge meet to the rails.  I thought that was intended to prevent spikes coming back from the load from spiking over the VCE limit of the IGBTs.

I am wondering this because I am using the Kaiser snubber calculator and if I place a value of capacitor between the collector/emitter junction and the rails (around 1 to 2 uF), it's going to basically short out the IGBT and the capacitor is going to blow up.  Do I have this wrong?

Thanks,

Dan

[Mads Barnkob]

DC Link Capacitors and Snubber capacitors are really only differentiated by the amount of capacitance. To some extend there is also the difference in material (electrolytic vs. polypropylene)

The large capacitance that prevents DC bus voltage sagging by being able to supply a lot of peak current will just act as a energy storage, with its large capacitance and high ESR it does not take care of transients.

The small capacitance, high dv/dt, peak current ability of a snubber capacitor will absorb the switching transients from hurting the IGBT, this is why it should be mounted directly on the IGBT terminals, even the internal inductance of a large IGBT brick can give us problems when we push things above the limit.

current sharing in parallel capacitors is done capacitively, so if you have a 1000 uF electrolytic capacitor and a 1 uF snubber capacitor, the snubber capacitor will only supply 1/1000 of the current to the bridge.

Are you doing simulations since you say that 1-2 uF will short out the bridge?

It might not even be necessary with snubber capacitors if you only use MKP capacitors as DC link capacitance.

[Hydron]

I think there's a little confusion here with terminology.

A snubber capacitor can be either:
a) a suitably sized capacitor across the DC bus, e.g. a 1-2uF film cap with bolt-on terminals, used to provide a low impedance to minimise the magnitude of any switching transients at the IGBT terminals
b) a capacitor used together with resistor/diode on the half bridge output (C2E1 on IGBT module), used for reducing the stress on the IGBT or diode when hard-switching a lot of current by providing another path for some of the energy.

In DRSSTC use we normally just use type a), as it's easy and anything further isn't normally needed for a soft-switching. Obviously putting a large film cap from half-bridge output to ground is not a good idea, and this is not what is done in type b), as there are additional components involved as well!

[profdc9]

Thanks for the explanation Mads and Hydron.  I get it now, I was just a bit confused.  Anyways, as a matter of interest, I am laying out this monster.  It is a half-bridge with 4 IGBTs on each side.  Not sure what I'm going to do with it yet, but I thought I would give it a go and see how wide I could get the traces on a 100 mm board.

Dan