My IGBT bridge is BAD and needs redoing

[Ignoramus22022]

I am working on a high power inverter bridge to make square wave AC
from DC supplied by my welding machine. I got ot to kind of work.

I tested my bridge with about 85 volts, without anything connected to
the AC circuit. Using a big power supply.

The bridge is of shorting variety, I tried to make sure that it never
is open.

Unfortunately, the current leak was about 5 amps, without the inductor
in series. The snubber capacitor that I had, seriously overheated its
supply wires.

That means that the short time is too much and needs to be reduced. I
will need to replace turn off resistors leading to every gate, with a
lower resistor value.

I

 

[Tim Wescott]

I am working on a high power inverter bridge to make square wave AC
from DC supplied by my welding machine. I got ot to kind of work.

I tested my bridge with about 85 volts, without anything connected to
the AC circuit. Using a big power supply.

The bridge is of shorting variety, I tried to make sure that it never
is open.

Unfortunately, the current leak was about 5 amps, without the inductor
in series. The snubber capacitor that I had, seriously overheated its
supply wires.

That means that the short time is too much and needs to be reduced. I
will need to replace turn off resistors leading to every gate, with a
lower resistor value.

I

Did you ever post schematics of this thing anywhere? Your website would
be good; there's also an alt.binaries.electronics.schematics or some
such thing but not everyone has access to binary newsgroups (including
me, as of this morning, it seems -- I must call my ISP about that!).

I've been watching your various threads, and have thought "shorting
bridge, ho hum, he knows what he's doing..." -- I just remembered you're
a newbie.

If you have a shorting bridge it _must_ be operated with a substantial
inductor on the input side -- other wise you'll get high currents. In
fact, my one truly dramatic experience with exploding FETs came from a
power supply circuit with shoot-through problems. If the snubber cap
that you're installing is on the input side then it's just not
compatible with make-before-break H-bridge action.

That having been said, if you're putting the snubber on the input side
then you may be better off with a more conventional break-before-make
H-bridge. This _will_ require a driver with a controlled dead time, and
probably some work to make sure that your IGBTs don't volunteer to turn
on because of high dV/dt between the emitter and collector (this is what
was exploding those FETs at one point).

 

[Ignoramus22022]

Did you ever post schematics of this thing anywhere? Your website
would be good; there's also an alt.binaries.electronics.schematics
or some such thing but not everyone has access to binary newsgroups
(including me, as of this morning, it seems -- I must call my ISP
about that!).

I will try to do that over the weekend.

I've been watching your various threads, and have thought "shorting
bridge, ho hum, he knows what he's doing..." -- I just remembered you're
a newbie.

Absolutely, I am a complete newbie.

If you have a shorting bridge it _must_ be operated with a substantial
inductor on the input side -- other wise you'll get high currents.

Got it. I will retry my test with an inductor made of 2/3 of a
standard 500 ft home depot spool of 12 gauge wire.

(this is a cool experience to watch that inductor. When it is powered,
the slightly loose coils of wire start moving about)

In fact, my one truly dramatic experience with exploding FETs came
from a power supply circuit with shoot-through problems. If the
snubber cap that you're installing is on the input side then it's
just not compatible with make-before-break H-bridge action.

That having been said, if you're putting the snubber on the input side
then you may be better off with a more conventional break-before-make
H-bridge. This _will_ require a driver with a controlled dead time, and
probably some work to make sure that your IGBTs don't volunteer to turn
on because of high dV/dt between the emitter and collector (this is what
was exploding those FETs at one point).

I would prefer to avoid having any dead time. I think that you are
right indeed and I should first redo my test _with_ the inductor in
series. And then, I should make sure that short time is small enough,
by adjusting the off resistors, all 8 of them.

Regarding having settable dead time, yes, my Semikron SKHI 23 allows
me to do that very easily with a resistor. I can set dead time to,
say, 0.9 uS (one of the values in the datasheet) But, like I said, I
would prefer not to go there.

My rough calculations suggest that I simply have too much of short
time and need to adjust that. Having an inductor may make it less of
an issue. By the way, I have little understanding of "saturated
inductors" and I am not sure if they act like simple inductors. If the
inductor in my welder is saturated and if saturated inductors do not
act like inductors, then I may have surprises.

The datasheet for my Semikron SKHI 23, with schematics for paralleled
bridges, is here:

http://igor.chudov.com/projects/Homemade-TIG-DC-to-AC-Inverter/Schematic/SKHI_23_12.pdf

The picture of my current test setup is here:

http://igor.chudov.com/projects/Homemade-TIG-DC-to-AC-Inverter/01-Prototype-1/


i

 

[Tim Wescott]

I will try to do that over the weekend.




Absolutely, I am a complete newbie.




Got it. I will retry my test with an inductor made of 2/3 of a
standard 500 ft home depot spool of 12 gauge wire.

(this is a cool experience to watch that inductor. When it is powered,
the slightly loose coils of wire start moving about)




I would prefer to avoid having any dead time. I think that you are
right indeed and I should first redo my test _with_ the inductor in
series. And then, I should make sure that short time is small enough,
by adjusting the off resistors, all 8 of them.

Regarding having settable dead time, yes, my Semikron SKHI 23 allows
me to do that very easily with a resistor. I can set dead time to,
say, 0.9 uS (one of the values in the datasheet) But, like I said, I
would prefer not to go there.

My rough calculations suggest that I simply have too much of short
time and need to adjust that. Having an inductor may make it less of
an issue. By the way, I have little understanding of "saturated
inductors" and I am not sure if they act like simple inductors. If the
inductor in my welder is saturated and if saturated inductors do not
act like inductors, then I may have surprises.

The datasheet for my Semikron SKHI 23, with schematics for paralleled
bridges, is here:

http://igor.chudov.com/projects/Homemade-TIG-DC-to-AC-Inverter/Schematic/SKHI_23_12.pdf

The picture of my current test setup is here:

http://igor.chudov.com/projects/Homemade-TIG-DC-to-AC-Inverter/01-Prototype-1/


i

Actually IIRC saturating inductors were another source of little bits of
FET cases scattered in the lab (I should mention this wasn't my project,
which just means that I learned a bit less from the other guy's mistakes
with _much_ less embarrassment).

When an inductor saturates it's apparent inductance goes way down, so if
it's got drive behind it and it's looking into a short the current goes
_way_ up. You can get some pretty dramatic views on a scope screen
before the current set of final drive transistors scatter bits of
themselves over the bench.

 

[Ignoramus22022]

Actually IIRC saturating inductors were another source of little bits of
FET cases scattered in the lab (I should mention this wasn't my project,
which just means that I learned a bit less from the other guy's mistakes
with _much_ less embarrassment).

Yep, hence my question.

When an inductor saturates it's apparent inductance goes way down, so if
it's got drive behind it and it's looking into a short the current goes
_way_ up. You can get some pretty dramatic views on a scope screen
before the current set of final drive transistors scatter bits of
themselves over the bench.

I think that I will try to reduce short time first and will see what
happens, on the scope. I am able to see the signs of the short time
on the scope signal.

If that does not help, I may switch to dead time. I will experiment a
little bit, I have a relatively simple testing setup that actually
works.

It is an interesting project and I am still playing with low amperages
(under 20A, vs 400A max capacity) and (relative to capacities of my
bridge) peak transient voltages.

i

 

[Don Foreman]

Did you ever post schematics of this thing anywhere? Your website would
be good; there's also an alt.binaries.electronics.schematics or some
such thing but not everyone has access to binary newsgroups (including
me, as of this morning, it seems -- I must call my ISP about that!).

I've been watching your various threads, and have thought "shorting
bridge, ho hum, he knows what he's doing..." -- I just remembered you're
a newbie.

If you have a shorting bridge it _must_ be operated with a substantial
inductor on the input side -- other wise you'll get high currents. In
fact, my one truly dramatic experience with exploding FETs came from a
power supply circuit with shoot-through problems. If the snubber cap
that you're installing is on the input side then it's just not
compatible with make-before-break H-bridge action.

That having been said, if you're putting the snubber on the input side
then you may be better off with a more conventional break-before-make
H-bridge. This _will_ require a driver with a controlled dead time, and
probably some work to make sure that your IGBTs don't volunteer to turn
on because of high dV/dt between the emitter and collector (this is what
was exploding those FETs at one point).

His intent is to drive this with a current source, specifically a
TIG/stick welder. These machines typically have 80 volt OCV, short
circuit current varies with machine setting from 1 amp to maybe 200
amps in Ig's case. Operating voltage (arc established) is typically
in the region of 24 volts. Saturation is not an issue (!)

With no load, energy stored in the driving-point inductance during
short has nowhere to go when the short is removed except for the
load-side snubber.

 

[Ignoramus22022]

His intent is to drive this with a current source, specifically a
TIG/stick welder. These machines typically have 80 volt OCV, short

All correct, my OCV is 85 volt.

circuit current varies with machine setting from 1 amp to maybe 200
amps in Ig's case.
Correct.

Operating voltage (arc established) is typically in the region of 24
volts. Saturation is not an issue (!)

So, would it be correct to conclude that the source for my bridge will
be highly inductive?

With no load, energy stored in the driving-point inductance during
short has nowhere to go when the short is removed except for the
load-side snubber.

That's 100% correct.

So, Don, do you think that my bridge should be shorting, or not
shorting? Is there some hope that by using this bridge after the
welder's inductor, and by reducing short time as much as practicable,
I would be able to run this bridge?

Or do you think that I should use a non shorting bridge with interlock
(dead time)?

i