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Registered Member #1879
Joined: Mon Dec 22 2008, 04:11AM
Location:
Posts: 40
I have a scratch built DRSSTC that is based on the Minibrute.
It used to run just fine but after taking it apart and changing the transformer and a few other things I have all kinds of problems getting it to run.
I tuned both the primary and the secondary to about 235kHz
I can get a wimpy little spark if I drive the CT input with a signal generator at 240kHz but when I hook it up with the CT on the primary it drives the bridge at 470kHz and it rings out after the drive turn of at over 1.7MHz.
I checked the caps in the primary with a capacitance meter and they seem ok. The output of the half bridge seem ok to and both gate drives are reasonable but on the slow side, I have about 500ns rise time but I don't think that is causing it.
The trace is the gate signal when I drive the CT input at about 70kHz
Any ideas? I also posted some traces earlier on the Minibrute thread.
Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
Flip the phasing of your feedback transformer, your phasing is 180* out I think. If your phasing is wrong then your driver will switch as soon as any current is built up in the primary circuit. In other words your driver will not switch when the current crosses zero.
Let me elaborate a little on that. When operating normally your DRSSTC is an oscillator with the frequency determined by the resonant frequency of the series resonant circuit of the primary and tank capacitance. With the current transformer providing the necessary feedback with a 360* phase shift. When the current transformer is phased to provide feedback with 180* of phase shift the resonant circuit (Lpri & tank C) is no longer the primary frequency determining portion of the circuit. What determines the frequency is any phase shift between the primary current and output voltage of the CT, rise time of the input circuit, rise time of the gate driver and rise time of the bridge.
Registered Member #1879
Joined: Mon Dec 22 2008, 04:11AM
Location:
Posts: 40
Thanks for the suggestion, I will take a look at that possibility. When trying earlier it does not do anything when I reverse the CT. I finally got myself a reliable function generator so it will be easier now.
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Frquency doubling problem with DRSST
