Help need understanding circuit

[talvor]

Hi,

I am helping my father trouble shoot a board that keeps blowing up.  The board came with an ultrasonic welding machine he purchased from a one-hung-low company.

I have reverse engineered the board and created a schematic from it, which I have attached.  From what I can see it looks like there are 2 sets of 6 power transistors in parallel.  One sets emitter is connected to the other sets collector.

What I would to like know is the following;
1. What is the purpose of chaining the power transistors in parallel?
2. Why connect one sets emitter to the other sets collector?
3. Can any one provide an indication of what the circuit might be used for. Just incase my assumptions are incorrect.

PS. I am very new to electronics, so please be gentle 

Thanks
Phillip

[Rufus]

1. What is the purpose of chaining the power transistors in parallel?
2. Why connect one sets emitter to the other sets collector?
3. Can any one provide an indication of what the circuit might be used for. Just incase my assumptions are incorrect.

1. To share current and power dissipation between them.
2. Search for half bridge circuits (or full or H bridge if it has two circuits like this).
3. It is the power part of a half bridge drive circuit.

The transducer drive is likely high (as in lethal) voltage so be careful.

[WarSim]

First assumption is it is an inverter.  Not knowing how it is driven this is an assumption. 
The transistors are in parallel to increase power capabilities. 

What components keep "blowing up". To diagnose the problem I hope you are not so new you can use an oscilloscope. 
The most likely issue is shoot through.  And the most likely cause is the controller board with a misaligned dead time cct.  If the designer cheeper out and is depending on the transistor characteristics alone for dead time then the cct is doomed to fail sporadically. 

If it is a doomed cct the only good fix is design a new controler cct.  A bit if electronics learning may be required. 

If not possible the second best fix is change all transistors at once with 12 transistors of the same batch.  Repeat as nessary. 



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[talvor]

WarSim, Rufus,

Thanks for your input.  I did a quick google on half-bridge circuit, looks like I have a bit of reading to do.

Be assured that I do not have the transducer or power supply that this board is connected to, so no issues with high voltage here.

It seems that all 12 transistors are shot on this board.  Using my multimeter I am measuring a short between all the pins on each of the transistors.  My understanding of transistors is that there should be a high resistance between the pins.

I will replace the transistors and see what happens.

Thanks
Phillip

[talvor]

The most likely issue is shoot through. 

WarSim, what do you mean by shoot through.  This is not a term I have come across yet.

Thanks
Phillip

[Kremmen]

Shoot through is the term given for the situation where both legs of a half bridge conduct at the same time. The load is connected at the midpoint of a half bridge and by design the load should be switched either to the positive _or_ the negative rail, i.e. the upper or lower switch should conduct.
If both switches conduct they will create a short circuit and the current will "shoot through" the bridge. The designer of a bridge control circuit must carefully avoid the situation by ensuring that there is sufficient gap between closing the switches.

[WarSim]

Kremmen thanks for answering. 
I am still packing my lab for a move and my responses are sporadic. 


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