IGBT question?

[wkyong]

What is the meaning for anti-parallel hyperfast diodes for an IGBT?
Does it means they cannot be parallel up?

 

[Styx]

What it means is included in the package is a DIODE in anti-parallel with the main component

IGBT's are primarily used in inverters and as such freewheel capability for circulating current around an inductive load is needed (unless you want to overvolt & avalaunch and thus destroy your IGBT's after their 1st and only switch ).


Yes IGBT's can be paralleled up since they have a positive temperature coef, just try to keep any stray lead lengths equal between all devices in the parallel chain

 

[dknguyen]

When you suddenly switch off the current in an inductive load (like an inductor or motor), the inductance will try to keep current flowing. THis is because the current in an inductor can only change gradually. If it cannot flow for some reason (like you switching the current off), what happens is that the inductor will try and MAKE the current flow by generating a huge voltage to FORCE the current through. This results in things like sparks across mechanical contacts that have been opened, or in the case of semi-conductor switches, they fry and become short-circuits.

This diode gives the inductive current a path to flow when the switch is turned off so the current can die gradually and a voltage spike produced (or one not as large). For unipolar systems where the current onyl flows through the inductive load in one direction, the diode is placed in anti-parallel with the inductive load to provide this path.

But for bidirectional circuits, you obviously can't do this since you would need two diodes which would form a short-circuit that bypasses the inductive load in both directions. So instead the diodes are placed in anti-parallel with the power switches in the circuit and the "flyback current" find a path this way instead. It is not as efficient placing one diode in antiparallel with the inductor because the flyback current must flow through two diodes and the exact path and voltage spikes formed are dependent on what other transistors were on when the current was disconnected (by turning off some other transistor)...but it's all you can do with diodes in this case.

This pictures of current flow might help:
https://www.modularcircuits.com/h-bridge_secrets2.htm

You can also play RC series circuits and a bunch of other combinations of R, L, C, and diodes to do the same thing. Each one behaves a little differently and does different things. For example, an RC circuit is much faster than the diode, is bidirectional (so you could just place it across the inductive load in a bidirectional circuit). But it dampens the voltage, it doesn't "clamp" it to within the power supply voltages like diodes do. These are called snubber circuits.

THe pictures here will help:
https://en.wikipedia.org/wiki/Flyback_diode

Anti-Parallel is a way of connecting two components and means they are connected in parallel, but are "pointing" in opposite directios (so the diode doesn't form a permanent short-circuit across the transistor, for example). Anti-parallel does not mean they are non-parallelable lol. So yeah, you can parallel them.

Not sure if you can swallow all that, but once you get one bit, the rest fall into place eventually (I think so anyways).