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Registered Member #793
Joined: Sun May 20 2007, 06:50PM
Location: Minneapolis, MN
Posts: 35
How do you all make measurements on the gate-source of MOSFETS/IGBTS in the lower section of the bridge to look at the gate signal? (I know , I don't want to ground the high-side) My 'lab' is the garage and is full of GFI outlets. Over course anytime I connect the ground of the single ended probe to the bottom of the H-bridge the GFI trips because I am providing a path to ground for the neutral. The supply to the bridge is a variac to a full-wave rectifier/cap. I don't have a differnetial probe and tried the two probe A minus B deal but didn't have very good results. No battery powered scope for me to use either. Use a battery for the supply to the H-bridge? Some other clever isolation scheme? Maybe there is something obvious I am missing.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
You arent simply grounding neutral if you are probing *after* the bridge rectifier. You are literally shorting out half cycle DC.
Anyway, what you need is an isolation transformer. I happened to find a nice 120V to 240v/480v 500va transformer that has been plenty for all of my work (it can go to 1kW++ for short periods of time). Of course, for low power testing a smaller transformer at a lower voltage would work. Id suggest perhaps winding your own transformer on an old MOT, since they are easily found and relatively easy to take apart (just cut off 2 welds along the laminations). You typically dont want to run full voltage while probing on a nice scope anyway, so even if you only get 60VAC that should be enough for most cases.
Registered Member #793
Joined: Sun May 20 2007, 06:50PM
Location: Minneapolis, MN
Posts: 35
Duh. Should have thought that one through a bit more. The bottom of the H-bridge just looks so ground friendly sitting at the bottom of the schematic and so far away from the rectifier (at least the way I was drawing it).
I thought about a transformer for isolation but thought the transformer impeadances would screw with the signal especially at the gate. Are the losses so small at higher frequencies they just don't have a big effect on the signal? Do you put any load on the scope side of the isolation transformer or just run it open with the probe connected? That sounds like a great solution though; I should be able to dig up a decent transformer somewhere. Thanks for the help Steve.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I think you are a little unclear about what we are doing. You clamp you scope directly onto across the gate/source, but you float the whole coil on an isolation transformer.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Yes, that's exactly what you need to do.
A very simple way to make a relatively powerful isolation transformer is to connect the high voltage output of one MOT to the high voltage output of a second MOT, and then take the voltage out at the primary of the second MOT.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Yes, i should have been more explicit. The isolation transformer is for supplying raw AC to the power input of the power electronics. Still, be respectful of the voltage present!
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Scope measurements on an SSTC
