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Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
If you are seeing large spikes at the switching events your 330v bus is probably ringing up to some voltage well above your 500v max reverse voltage for the fets. Also one thing to check as Matt B pointed out is that many fets can't handing switching their full rated voltage at their rated current; but rather have a 'safe operating area' that limits the voltage/current they can safely switch at. Check this in the datasheet of the top secret few you are using.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Richie, the problem with my scope is that it's an ancient tube one, and will display the spikes even when I short the probes together and connect to the circuit, also the magnitude of the spikes is affected by the vert.ampl. switch (the signal can be clean at lesser amplification and get extremely spikey when increasing the amp)
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
> ...and will display the spikes even when I short the probes together and connect to the circuit.
If you short the probe tip to probe ground, and spikes appear when you bring it NEAR TO your system being tested, then this is caused by magnetic pickup. Most likely of the fringing flux from the flyback transformer. The solution is to twist the scope probe's earth-clip wire around the length of the probe to reduce the loop area, therefore reducing it's sensitivity to pickup.
On the other hand...
If you short the probe tip to probe ground, and spikes appear as soon as you CONNECT this to the ground rail of your system being tested, then this is caused by common-mode current being driven down the shield of the scope probe cable. You can reduce this effect by looping the scope probe's coaxial cable through a high-perm ferrite toroid a few times. (The added common-mode inductance discourages fast edges from travelling down the shield because it raises the impedance at high frequencies. It won't distort the signal being measured, it just removes the unwanted common-mode component.)
These are tricks I picked up from working on SMPSUs for the last decade.
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Mysterious MOSFET/IGBT deaths
