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Hi, I got the UCC28711EVM board, and tried to measure some signals using oscilloscope on the board. Once I touch the GND in primary side with the ground probe of oscilloscope, the fuse R9 was blew out. This does not seem to happen when I use the volt meter. Would you know why?
Where exactly where you probing at (pins)? Did you have your GND probe on pin 5, and probing pin 3? if this is the case, then it the fuse will blow. Pin 3 is not connected to ground, thus floats with respect to GND. The oscope GND probe is GND and would be common to the AC ground line through the power plug. The voltmeter has it's own isolated GND, so you would not see the fuse get blown.
If you can confirm where exactly your are probing at?
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In reply to David Wang11:
I just have the oscope GNG probe touched to the C7's terminal connected to the ground, ie pin 4 of UCC28711, nowhere else. The same problem happened on UCC28610EVM-474 when I place the oscope GND probe on the TP4.
In reply to James Hu:
If not doding this already try connecting the ground before you power up the EVM.
In reply to Mike O':
Try and take your handheld meter and probe the same point (C7 - pin 4 on the UCC) relative to TP2 (GND on the secondary side). And then probe the C7-pin 4 relative to scope ground. What may be happening is that the schematic is misleading. If you see a large potential for both of the two location above, then hooking up scope GND to C7-pin4 will trip the FUSE since it is connected to a hot (neutral) line.
Verify those measurements.
I measured C7-pin 4 relative to TP2 the meter reading is 0.12VDC/0VAC, measured C7-pin 4 relative to scope ground the meter reading is 54.2VDC/118VAC.
If you are using an isolation transformer, which you should be doing for safety your power supply is floating. It will have a potential voltage from ground due to parasitic capacitance. When you put a scope ground on this it is shorted to ground and you generate enough current to blow the fuse.
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