Author Topic: UD2.7 output stage keeps dying  (Read 1251 times)

Offline Miki_407

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UD2.7 output stage keeps dying
« on: December 29, 2020, 06:26:08 PM »
        As stated in the subject output stage of my Ud2.7 keeps shorting one of the p-channel transistors. i used a function generator to test if the board works. To answer some questions in advance:

1. I connected my function generator to the header without closing the switch to either a inductor+ resistor or resistor alone so  it doesn't run in parallel to them.
2. I didn't see any over voltage on gates of transistors with my oscilloscope.

        Until now I destroyed few of them and I wouldn't like to lose more. H bridges in question are FDD8424H. First time i had no gdt connected. Second time i had it connected but it still shorted. Last one died so i can get this waive form on the output just before it dies. The space in between the pulses  is my power supply overcurrent protection kicking in. It draws over 4 amps. I hope you can help me. I dont want this project to go to waste. I will try to provide any information that is required.

Offline Paultesla

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Re: UD2.7 output stage keeps dying
« Reply #1 on: December 29, 2020, 08:28:10 PM »
I would check that the board is built correctly with no solder shorts to the supply planes on the top side of the board.

To help prevent them blowing put the board on a 24V supply current limited to 200mA and scope the waveforms on the the devices. If built correctly these should not blow.

When I built mine I found that the resistors and capacitors supplied for the components that are in series with the GDT outputs were larger than the pads and I had to cut away the copper of the 24V power plane on the top side to prevent shorts to these components.

Paul

Offline davekni

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Re: UD2.7 output stage keeps dying
« Reply #2 on: December 29, 2020, 09:10:30 PM »
Also, test with very-low duty cycle.  Short interrupter enable pulses at low repetition rate (ie. 50us enable pulses at 10Hz repetition rate).
David Knierim

Offline Miki_407

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Re: UD2.7 output stage keeps dying
« Reply #3 on: December 29, 2020, 09:14:30 PM »
I will be sure  to test it tomorrow. One question: Can it handle CW mode as I didn't have a interrupter?
« Last Edit: December 29, 2020, 10:02:32 PM by Miki_407 »

Offline AstRii

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Re: UD2.7 output stage keeps dying
« Reply #4 on: December 29, 2020, 09:55:54 PM »
Yes the driver is capable of CW mode up to few hundreds of kilohertz.
I have tried this with a frequency of 260kHz, the driver was drawing 1.8A at 24V from lab bench power supply and have only heated up a very little :)

Offline Miki_407

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Re: UD2.7 output stage keeps dying
« Reply #5 on: December 30, 2020, 10:37:03 PM »
So I tested again and no surprise it died again :( .I don't have much of them left to spare and a pad is starting to go away from the PCB so I think I can only try once more before I have to  solder a new board. In the picture attached a circled transistors always dies. On the  second attachment you can see  a part of my PCB that is dying. Electrolytic capacitors are removed so i can desolder the transistors when they die. If anyone can help please do.
Miki

Offline davekni

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Re: UD2.7 output stage keeps dying
« Reply #6 on: December 31, 2020, 12:25:46 AM »
Did you test it at low current as Paul suggested?  Did it die at low current, or only after providing more power?  If you don't have an interrupter, then low duty cycle isn't an option, so low voltage/current is the best way to test.  Test initially with no GDT connected, as it should then operate at low current.

Are you using single packages with both PFET and NFET, or separate PFETs and NFETs?  If separate, have you checked that the NFET below the frying PFET isn't shorted?
David Knierim

Offline Miki_407

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Re: UD2.7 output stage keeps dying
« Reply #7 on: December 31, 2020, 12:43:01 AM »
I am testing at low current, low duty and gdt connected-still dies. N-channel is still working but the p-channel is shorted. If only one is connected it still dies. I am using a single package one. Last test was at 15v 200ma. I am starting to think I got some faulty ones.

Offline davekni

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Re: UD2.7 output stage keeps dying
« Reply #8 on: December 31, 2020, 01:10:04 AM »
Yes, faulty parts is sounding likely.  Still, 15V at 200mA is 3W.  If the output is shorted to ground or the NFET gate is shorted high, then 3W might be enough to fry the PFET.  If it was running before connecting GDT, then such a short wouldn't be the explanation.
David Knierim

Offline Miki_407

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Re: UD2.7 output stage keeps dying
« Reply #9 on: December 31, 2020, 01:01:09 PM »
What should i do for the test today? What should I scope with my 2 channel scope?

Offline davekni

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Re: UD2.7 output stage keeps dying
« Reply #10 on: December 31, 2020, 06:27:45 PM »
Do you have a signal generator you could use to test your GDT?  Does the GDT use 4 twisted pairs with one half of each pair in parallel for the primary?  That is a recommended design for low leakage inductance.  If so, and if one or more of the four primary windings is reversed, that would make the GDT almost a shorted load, so fry FETs.

Do I understand correctly that it works until the GDT is connected?  If you don't have a signal generator, connect a power resistor, 10 to 100 ohms, in series with the GDT (between GDT and driver board).  Scope on either side of the resistor to see what current the GDT is drawing.  The resistance will also protect the driver from a bad GDT (unless the FETs are themselves bad parts).
David Knierim

Offline Miki_407

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Re: UD2.7 output stage keeps dying
« Reply #11 on: December 31, 2020, 06:50:41 PM »
It always died.
1.No GDT, CW mode -died;
1.GDT, CW mode-died;
1.GDT, low duty cycle, limited current- died.
 And that is why I thought that parts are bad. Maybe i messed something up on my PCB that is why I sent the  circuit and the picture. Voltage on the gate doesn't go above 8v so I don't think it is overvoltage on the gate. I am going to test it again so what should i scope and do i place a GDT or should I use a resistive load or something else.

Offline davekni

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Re: UD2.7 output stage keeps dying
« Reply #12 on: December 31, 2020, 07:13:53 PM »
When you said that the gave voltage doesn't go above 8V, is that the PFET gate?  It should be at 24V (or whatever your supply is set to) and drop down ~9v from there (to 15V) and back to 24V.  Is it possible that C26 is shorted?  That would explain the frying.

If you haven't already soldered in the replacement part, I'd scope all the signals around that area of the schematic without Q2 in place.  Verify that the Q1 output is working normally (0 to 24V square wave presuming you are feeding the UD2.7 feedback input from a signal generator).  Verify that Q2 NFET gate is a 9V square wave from 0-9V.  Verify that Q2 PFET gate is a square wave from 15 to 24V (or 16 to 25V counting a diode drop).  All this with no GDT connected.
David Knierim

Offline Miki_407

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Re: UD2.7 output stage keeps dying
« Reply #13 on: December 31, 2020, 07:41:35 PM »
          I should explain better. When i stated that the voltage on the pfet gate isn't bigger than 8 volts i was measuring between 24v and gate not gnd and gate. My oscilloscope was insolated from the ground. I should have told that voltage on gate doesn't go below -8 volts.
          If Q2 isn't connected pfet in Q1 dies so I cant do any of the testing you suggested.
          C26 is not shorted.
          Thanks for taking your time on new year to help me.
Happy new year-Miki
« Last Edit: December 31, 2020, 07:44:58 PM by Miki_407 »

Offline davekni

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Re: UD2.7 output stage keeps dying
« Reply #14 on: December 31, 2020, 08:13:48 PM »
Are the 24V bypass capacitors missing?  I'm grasping at straws here.  Can't think of much else.  If 24V caps were missing, it might be possible for 24V to spike high enough after driver output switching to fry FETs.  Seems unlikely, though.

Or, another unlikely cause would be very-slow transitions on the FET gate signal from the driver chip.  Slow transitions would increase the time when both PFET and NFET are conducting simultaneously.  Or, if your 9V supply were high, but you've already verified 8V waveforms.

Perhaps some Chinese company has counterfeited FDD8424H parts now.  I've had trouble with counterfeit Alpha Omega Semiconductor FETs of several types.

Good luck.  I'm about at the end of ideas.  Will let you know if anything more comes to mind.  Keep us posted on your progress.  If there are counterfeit parts out there, other forum members are bound to run into them too.
David Knierim

Offline Miki_407

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Re: UD2.7 output stage keeps dying
« Reply #15 on: December 31, 2020, 08:35:17 PM »
           I will try to put the gate of pfet on gnd and source on 9 volt. I will pull the nfet low so it doesn't conduct. If it dies with no load i am sure it is some kind of chines fake, and i will report to the lcsc where i got my components. Here is a link if anyone finds it useful https://lcsc.com/product-detail/MOSFET_ON-Semiconductor-FDD8424H_C112174.html 
          Would i be able to get a refund "All LCSC purchases come with a 30-day money-back return policy, plus a 90-day LCSC warranty against any manufacturing defects.
This warranty shall not apply to any item where defects have been caused by improper customer assembly, failure by customer to follow instructions, product modification, negligent or improper operation"?
« Last Edit: December 31, 2020, 08:38:49 PM by Miki_407 »

Offline davekni

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Re: UD2.7 output stage keeps dying
« Reply #16 on: December 31, 2020, 09:31:37 PM »
Good luck on getting any refund :(

Looks like On Semi changed the part marking about 3 years ago to reflect On rather than Fairchild.  But that doesn't prove any counterfeit, as they easily could have been manufactured more than 3 years ago.

Yes, a bench test is a good idea.  Be careful not to ESD zap the gates while handling.
David Knierim

Offline Miki_407

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Re: UD2.7 output stage keeps dying
« Reply #17 on: December 31, 2020, 09:56:30 PM »
I got them not to die ;D . I didn't use a gdt but a resistive load 51ohm, shorted a capacitor C32, added electrolytic capacitors, lowered current limit to 100ma and used a low duty cycle interrupter that is it. Soon i will update with waveform of the currant flowing on the resistor. Voltage drops to 7.5v but i am still getting a wave form on the gates and a output from the transistors. After recording all the waveforms i will post the pictures and increase the current limit.
Edit: It seems my interrupter is not running at low duty cycle 1% but at 99%
« Last Edit: December 31, 2020, 10:01:27 PM by Miki_407 »

Offline davekni

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Re: UD2.7 output stage keeps dying
« Reply #18 on: December 31, 2020, 10:15:07 PM »
I wouldn't increase current limit until you figure out where 100mA is going.  With C32 shorted there should be no activity, so very little current flowing anywhere.  Comparitor uses a bit quiescient current, and the regulators and driver chips.  Doubt that adds up to 100mA.  I didn't check all the data sheets to verify, though.
David Knierim

Offline Miki_407

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Re: UD2.7 output stage keeps dying
« Reply #19 on: December 31, 2020, 10:49:49 PM »
I have turned one of the capacitors around so that was a problem. Happily i noticed before we had any fireworks ;) . After fixing that i am getting some 30ma current draw. So maybe that capacitor C32 was the problem.

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Re: UD2.7 output stage keeps dying
« Reply #19 on: December 31, 2020, 10:49:49 PM »

 


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