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Old 12th September 2008, 04:51 AM   #1
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Default spike from SG3525

hi all

I am designing a half- bridge SMPS with SG3525, but im facing some problem here.


when my high side mosfet turns on there is a spike at the low side mosfet. this spike increases as the Vac is increases say from 50Vac to 100 Vac.

Attached is the signal from the that goes to the gate of both the mosfet.

IS this normal..
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Old 12th September 2008, 10:29 AM   #2
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I have seen a similar thing due to slight imbalances in the circuit that can then cause the core to be magnetized more in one direction than the other. The flyback during MOSFET off-time drive shows itself as spikes.
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Old 12th September 2008, 12:11 PM   #3
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Wouldn't that spike cause both the mosfets to on at a split time ?
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Old 12th September 2008, 03:42 PM   #4
luka is offline luka  Slovenia
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Hi

Yes it can...
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Old 12th September 2008, 05:00 PM   #5
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Were you overly generous in the primary winding inductance? What load do you have on the secondary.
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Old 12th September 2008, 05:01 PM   #6
Eva is offline Eva  Spain
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This is not just a spike, this is ringing, a sign of bad PCB layout and decoupling. Furthermore, what you see in the oscilloscope is not only the voltage waveform at the measurement point but also the EMI picked up by the probe, particularly the common-mode component.
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Old 13th September 2008, 03:13 PM   #7
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The load that i applied is very low bout 0.08A..

perhaps the ringing effect might cause by the transformer part, because i connect long wires from PCB to transformer...i never include it in the PCB...
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Old 13th September 2008, 03:23 PM   #8
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Quote:
Originally posted by jegandren
The load that i applied is very low bout 0.08A..

perhaps the ringing effect might cause by the transformer part, because i connect long wires from PCB to transformer...i never include it in the PCB...
Eva -- correct me on this if I am wrong -- but I believe that you have about inductance of 10 nanoHenries per centimeter -- so any part of the layout which isn't compact adds significant impedance and energy storage.
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