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  #1  
Old 05-09-2008, 09:38 PM
ShaunManners ShaunManners is offline
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Default 3 phase inverter bridge

Hi All,

I am still trying to get my 3 phase inverter working, I have successfuly managed to get my PIC MCU working great, and have used it to generate 3 signals which I then fed into 3 L6384's (half bridge drivers). This has all been tested and works a treat...

I have put together a bridge using 6 P55NF06 MOSFETs... When I connected them to 24v (nothing else connected) they quite literally blew up and caught fire... see attached photo.

The high side drains are all connected to +24v via the heatsink, and the source is bolted to the drain of the low side. The low side source is soldered to a wide track and bolted to ground.

All gate pins go to the pins on the left of the board.

I do not have any protection diodes in - partly because I wasn't planning on putting a load on it yet.. and also, I'm not sure what diodes to use, any suggestions? bearing in mind I'm planning on using an alternator as a 3phase induction motor.

I also have a circuit breaker on the positive side... which I reset when I want to turn it on. This is one of my theories... could resetting a breaker produce a large enough spike to damage the MOSFETs?

Any suggestions, observations, ideas or help would be welcome!

Cheers
Shaun
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  #2  
Old 05-10-2008, 12:20 AM
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SgtWookie SgtWookie is offline
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My first knee-jerk reaction is that your upper side and lower side were ON at the same time, resulting in a "shoot through" condition.

This can happen when the turn-off time is less than the turn-on time, or vice-versa, which is usually the case.

"Shoot-through" means that the H-bridge drivers are trying to short the supply to ground. This is a very unhappy situation, as smoke and fire may be emitted from the weakest side of the driver, and causing the circuit to not function as desired. However, it can be quite entertaining here in the U.S. around Independence Day, the 4th of July.

I suggest that you re-visit your code, and ensure that an appropriately long delay is inserted between the "off" commands and the "on" commands so that you will no longer encounter the "shoot-through" condition.

Unless you like smoke and flames.
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  #3  
Old 05-10-2008, 04:18 AM
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S_lannan S_lannan is offline
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wow.. nice job!

Some what reminscent of my own prototyping

Anyhow you can get the old scope out to check this too.

Perhaps you might of made an error when connecting the floating source of the high side. I did it once, luckily enough i had current limiting but both the fets were red hot, but lived.
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Old 05-10-2008, 10:39 AM
ShaunManners ShaunManners is offline
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Quote:
Originally Posted by SgtWookie View Post
My first knee-jerk reaction is that your upper side and lower side were ON at the same time, resulting in a "shoot through" condition.
<snip>

However, it can be quite entertaining here in the U.S. around Independence Day, the 4th of July.

I suggest that you re-visit your code, and ensure that an appropriately long delay is inserted between the "off" commands and the "on" commands so that you will no longer encounter the "shoot-through" condition.

Unless you like smoke and flames.
I don't particularly like flames it costs me 6 each time they blow! and that photo was of my 3rd attempt!
However I do agree its quite good fireworks

Your intial reaction is the same as mine, I went back to the code to make sure it was leaving a large enough deadtime... and it was giving me about 1uS... and considering the MOSFETs switch in the order of 8-16nS! it should be fine... Now I am using the drivers, they have a deadtime thingy that you control using an external resistor, which I have chosen to give me roughly 1uS...

The problem with this theory is that I didn't have any of the MCU/driver circuit connected when I turned the 24v on for the bridge... so all of the MOSFET gates were floating... which to me means they should all have been off and no short circuit or shoot through conditions.. yet they still go bang.

This is why I was wondering if it was a spike created by resetting the breaker that damaged the gate (I read something like that on a site) and then shortly after it makes it blow up... see here: http://www.4qd.co.uk/serv/MOSFETfail.html#dvdt

Otherwise I'm stumped... I appreciate your input

Cheers
Shaun
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Old 05-10-2008, 10:42 AM
ShaunManners ShaunManners is offline
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Quote:
Originally Posted by S_lannan View Post
wow.. nice job!

Some what reminscent of my own prototyping

Anyhow you can get the old scope out to check this too.

Perhaps you might of made an error when connecting the floating source of the high side. I did it once, luckily enough i had current limiting but both the fets were red hot, but lived.
Nice job blowing it up, or just in general?

What could I check with the scope?

The floating source goes to the drain of the low side, that is corerct right? what other error could I have made?

Cheers
Shaun
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Old 05-10-2008, 02:26 PM
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Floating gates? Oops...
I suggest that you add pull-down resistors on the gates, so that if you happen to power the thing up again without the drivers connected that the gates will all be pulled to 0v. Floating is floating - you don't really know what voltage might be there. But from the looks of things, I'd say it wasn't zero!
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  #7  
Old 05-10-2008, 02:49 PM
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nice job in general. I like blowing up stuff too!

the high / low side driver circuit won't work unless you connect it up properly.
I'm assuming you might have left out a connection or two.

on the driving ic
pin 4 should connect to the ground of the bridge (eg low side source).
pin 5 should connect to the low side gate.
pin 6 should connect to the high side source, (which is the same point as the low side drain.)
pin 7 should connect to the high side gate.
pin 8 should connect to the 'bootstrap cap' which connects to pin6.

and guess what?
with this ic it seems you can't connect VCC to a low impedance supply!!!
so you have to take a resistor from your high voltage supply (assuming you have one ) and feed it to the VCC pin. It's basically a zener regulator without the resistor. you could feed it of the 24v supply (with a resistor of cause).

I would say...

Use current limiting when testing the bridge = no more blown fets
Possibly try a different gate driver eg.ir21362.
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Old 05-10-2008, 03:13 PM
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oh you left the gates floating.....
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  #9  
Old 05-10-2008, 07:03 PM
ShaunManners ShaunManners is offline
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Quote:
Originally Posted by SgtWookie View Post
Floating gates? Oops...
I suggest that you add pull-down resistors on the gates, so that if you happen to power the thing up again without the drivers connected that the gates will all be pulled to 0v. Floating is floating - you don't really know what voltage might be there. But from the looks of things, I'd say it wasn't zero!
ok then... I'm still not sure why it would blow the first time when I did have it connected though... hmm.. mind you that was before I implemented the drivers... it could be that I have made two errors that ended the same way!

That aside, the pull-down resistors... for the hgh side, do I connect it between the gate and the high side source, or to ground? and what value would you reccomend?

Cheers
Shaun
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  #10  
Old 05-10-2008, 07:08 PM
ShaunManners ShaunManners is offline
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Quote:
Originally Posted by S_lannan View Post
pin 4 should connect to the ground of the bridge (eg low side source).
pin 5 should connect to the low side gate.
pin 6 should connect to the high side source, (which is the same point as the low side drain.)
pin 7 should connect to the high side gate.
pin 8 should connect to the 'bootstrap cap' which connects to pin6.

and guess what?
with this ic it seems you can't connect VCC to a low impedance supply!!!
so you have to take a resistor from your high voltage supply (assuming you have one ) and feed it to the VCC pin. It's basically a zener regulator without the resistor. you could feed it of the 24v supply (with a resistor of cause).

I would say...

Use current limiting when testing the bridge = no more blown fets
Possibly try a different gate driver eg.ir21362.
Thanks for saying its a good job... when it blows up all the time I wonder!!

I have checked all those pins, they are exactly as you say... and I used a 100ohm resistor on Vcc... as I am using 3 of them and they take 25mA each... on a 24v supply... please tell me if this is wrong!

When you say current limiting... presumably you mean put a resistor in line with the supply to the high side drain? what sort of value? and for the moment, with no load on the bridge, is 1/4 watt ok?

Thanks both of you for your help... and sorry if I am making stupid mistakes!

Cheers
Shaun
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