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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
When I normally draw arcs from my IGBT half bridge flyback driver, the input current from mains is near-zero when the HV electrodes are shorted together, and as I draw an arc, the input current rises and reaches maximum just before the arc extinguishes. However when I look at the primary current waveform with a current transformer (connected between the halfbridge and transformers), I see that the current is highest when the HV electrodes are shorted! It's a nearly perfect triangle wave. As I draw an arc, the amplitude of the current seems to slightly decrease until it's a weird "chopped" low amplitude sine wave (probably resulting from the transformers resonating) after the arc extinguishes.
How is this possible?! How can be the primary current highest when current draw from mains is the lowest? I "measured" the input power with both DMM and a lightbulb, both show minimum current draw with the output shorted.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
There can't really be something like ''bad power factor'' for DC, right? But shorting the transformer will cause very poor PF on HF-AC side, as most of primary current is reactive.
Energy circulates from and back to bulk filter cap with each cycle. Lots of magnetizing energy is put in each cycle into now low primary inductance only to be returned to bulk cap again in next cycle. So having primary current like order of magnitude bigger than DC input current is nothing odd in that case. DC current will simply be proportional to total ohmic dissipation in the output.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks for the explantation. And I was wondering why the fuse between my filter cap and the bridge was blowing when I shorted the output and there was nearly none input power flowing
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Half bridge, weird behavior
