UCC28700: Ucc28700 Waveforms on Oscilloscope

Hagen

You need to do one of the following:

- Isolate your AC mains supply through an isolation transformer

- Use an electronic AC or DC source with an isolated output

- Use isolated or differential scope probes.

- Power your scope through an isolation transformer


The problem you are having is a common problem when probing AC power supplies. Even seasoned experts can get caught out by this sometimes.

The issue is that for the non-isolated AC mains supply, the Neutral and Earth sit at the same potential, they are usually bonded together somewhere back along the AC distribution network, at a sub-station. But the "ground" clip of your standard scope probe is connected to the chassis of the scope, which is also connected to Earth through the AC power cord of the scope.

So when you connect the ground of the scope probe to the DC- of the bulk cap, or any point after the diode bridge, you end up shorting out one side of the bridge rectifier - one end tied to Earth (through the scope ground), the other end tied to Neutral (which is at Earth potential). This causes a huge current to flow, typically blowing the input fuse, diode bridge and probably any surge/inrush resistor if used.

By using an isolation transformer to power the AC supply to either the scope or UUT, you can avoid this short, and can then probe the circuit using regular scope probes.

If you don't have an AC mains freq isolation transformer, or an isolated AC or DC source, you will need to use differential probes - but be sure to NOT connect the Earth/ground clip of the diff probe, just leave it floating.


I hope this helps to answer your question - if so, please click the "verify answer" button.


Thanks,
Bernard