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Old 26th September 2013, 11:57 AM   #1
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Default Aleph J problem

My Aleph J dc offset is now at 24V!

I have checked the Q5, Q6, Q7, Q8, Q9 and Q10 and it seems they are working fine. The attached image shows the measurement when it was working fine (my measurement in red). I have now removed all the IRFP 240.

I have 3 pairs of mosfets installed.

Which component is most likely be the fault?
Thanks.
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File Type: png 28. Amp measurement.png (282.0 KB, 334 views)

Last edited by pmchoong; 26th September 2013 at 12:00 PM.
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Old 26th September 2013, 12:06 PM   #2
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I don't think you can make any reasoned measurements with the MOS-FETs removed. Q4 for one might well be turned ON as a result of their being removed.
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Old 26th September 2013, 12:23 PM   #3
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Hi and thanks for helping. The dc offset was 21v with the mosfets still intact. So I removed them and check if they are faulty but it seems that they are working fine.

Should I install them back? Will it get burn?
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Old 26th September 2013, 02:01 PM   #4
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Are lower MOSFETs(well check all) biased properly?
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Old 26th September 2013, 02:35 PM   #5
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Use a light bulb tester to minimize damage after reinstalling the output mosfets. Install one in the upper and one in the lower position to get meaningful measurements. (keep track of them if your current source and output devices have different Vgs) If it works with one, add another, and repeat. You may find one or more of the output devices check OK with a DMM but cannot handle the stress of operation.

As PKI suggests, check the voltage across R7 and between the negative rail and the gates of the lower mosfets. These should be the same, unlike the voltages shown on your schematic. If the 4.32V was measured on the gate pin and you really did have a drop across the gate stopper resistors, something is amiss. It could be an oscillation or a leaky gate. Leaky gate means replace the mosfet(s). Oscillation means you need to check your layout and install C5 if you haven't already.
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Old 26th September 2013, 03:15 PM   #6
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What is the polarity of the offset?

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Old 26th September 2013, 03:31 PM   #7
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if offset is positive (ref to gnd ) , increase R7
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Old 26th September 2013, 08:46 PM   #8
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Default DMM Diode Test Function

Like I have always pointed out, the diode test function is an easy enough test in or out of the circuit for these power mosfets. The IRFP240 N-type has a reversed biased diode across its D-S junctions meaning D has cathode (black probe).

Test this D-S diode with the power OFF. This diode is usually shorted out on bad mosfets and will return a very low mV reading. A good one will show a normal diode junction at 0.4V to 0.6V. Reverse probes for P-type.
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Old 26th September 2013, 08:47 PM   #9
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That much offset tells me that there might be a problem with one of the J74s. Maybe there could be a simple dry joint problem there or one of the J74s is faulty? But I am just a novice that is thinking out loud.
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Old 26th September 2013, 08:53 PM   #10
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If the output fets are still good, I would pull them and group R16 with R18 and R17 with R19. It should improve current sharing.
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