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Registered Member #2662
Joined: Fri Jan 29 2010, 10:14AM
Location:
Posts: 36
Hi All,
I'm part way through building a half-bridge PSU and have come across a strange problem... The waveforms on the MOSFET gate look reasonable on a 10's of microsecond timescale, but if I zoom in on the rising and falling edges there's actually a considerable amount of ringing. Looking at the output voltage of the bridge (between the two mosfets, no load attached) it seems as though the rising and falling edges of the usual 'square' wave have a few little steps in them and are not as sharp as I would expect; the fet's are also getting warm, not something I would expect with no load at 5kHz. I guess this is due to the strange ringing on the gate waveform, it is not something I have come across before, normally the ringing occurs after the rising/falling edges, not on the actual edges.
The gates are driven via small transformer, I've used the same transformer design numerous times previously without issue, so I can't see that being at fault. The layout of the circuit is pretty good, no long tracks/wires etc; one thing to mention is that there are no gate resistors... whilst troubleshooting I added 10ohms, I was sure this would fix the problem but it made no difference at all (very odd!).
A few images from the scope:
Has anyone got any ideas about what's going on/how to fix it?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I dont know if I can contribute much yet. But please post the pdf's for your MOSFETS, I need to see some of those parameters first before saying anything intelligent.
You do measure the gate waveform with both the ground and the signal pins of the oscilloscope connected directly to the mosfet pins? right? If you have the ground clip connected some place far away you won't see the true signal.
Registered Member #2662
Joined: Fri Jan 29 2010, 10:14AM
Location:
Posts: 36
Daedronus, Yes, the probe is directly across the mosfet gate and source. Ground lead is ~3 inches, not ideal, but shouldn't cause what I am seeing. After a little more probing it seems as though this oscillation is even on the gate drive IC, i'm starting to think it may be the layout/ground around the IC that's causing the problem. It's strange because there's an awful lot of passivation.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
iJim wrote ...
Daedronus, Yes, the probe is directly across the mosfet gate and source. Ground lead is ~3 inches, not ideal, but shouldn't cause what I am seeing. After a little more probing it seems as though this oscillation is even on the gate drive IC, i'm starting to think it may be the layout/ground around the IC that's causing the problem. It's strange because there's an awful lot of passivation.
iJim.
Then desolder the transistor, use only the Gate drive IC and your exact circuit layout to the oscilloscope.
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Gate drive problem
