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Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
Hi, I rebuilt my ZVS flyback driver (built another one) and it worked fine. Recently I can't draw arcs from flybacks anymore because the ZVS flyback driver stops oscillating when I try to draw an arc. Otherwise it is oscillating fine. I've checked that the fets and diodes are good and they are fine. Also the cap bank reads about .5uF which is what it is supposed to be. Anyone know what might be causing it to stop oscillating?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
What is your supply voltage and what are your gate supply resistors? If the resistors are 470ohm and supply is over 35V, there might be "parasitic" oscillations in the gate drive circuit. Try to double the resistors and see if that helps.
Registered Member #1407
Joined: Fri Mar 21 2008, 07:09AM
Location:
Posts: 222
Hi Myke,
If your fets are fine then there can be something wrong with your flyback, like a blown multiplier, this is usually the case with flybacks (well for me at least for me ).
I dont think its that critical but maybe your 0.50 uf cap bank is not ideal for your flyback.
also check that all your connections are good and check again that there are no parts that are causing an inbalance in the circuit.
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374
Hi there,
I've been reading here for some time but I think now is the time to sign up and participate. The resistors you used look like wire-wound types, so the parasitic inductance could be a probleme here. Apart from that you could recalculate the resistors for the higher voltage used.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
Ah, forgot to mention that I made this for a split supply so that the switching part's supply is 12V and the flyback's voltage can be varied from 12-40V. All I've done is 12-36V and they worked fine. Now it'll turn off when I try to draw an arc at 24V which it shouldn't do.
I also thought it was the flyback but i tried it will many flybacks and they all did the same thing. I reheated all the connections and it still does it...
The resistors are 470ohms wirewound but I don't think the inductance of the resistors should affect the circuit that much. I'll measure the inductance of them if I can and if it has significant inductance then I'll get 2W carbon film ones and see if that does anything.
It has worked fine with .5uF cap bank. I chose that value so that it doesn't keep dropping into the audio frequency range. I had before used only 2 caps to get .5uF but they started getting warm at 300W or so power input so I made a bigger bank cap.
EDIT: I can't measure the inductance fo the 470ohm resistors but I'd guess they are around 5-10uH each.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
About 9-10 inches coming right from the bank to the flyback. Mesured to be about .18uH. Is this too much inductance? I'd assume that the leakage would just be swamped by the actual inductance of the pri.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Myke wrote ...
About 9-10 inches coming right from the bank to the flyback. Mesured to be about .18uH. Is this too much inductance? I'd assume that the leakage would just be swamped by the actual inductance of the pri.
Like I said try to double your resistors and see what happens. The inductnace between pri. and driver does not matter.
I got the problem "runs but stops with first spark" too, and this is related to parasitic oscillation in the gate circuit, because of too low resistors or too fast transistors.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
What do you mean by too fast transistors? Well I replaced the 470 ohm resistors with 2 2W 560ohm resistors and now it works again. Thanks.
I wouldn't expect parasitic oscillations at 24V input though... I think they are caused by the stray capacitances and inductances oscillating with the circuit. Is this caused by little stray capacitance and inductance or a lot? Also why to they tend to appear at a higher input voltage? Wouldn't split supplies reduce the chance of parasitic oscillations from happening?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Myke wrote ...
I wouldn't expect parasitic oscillations at 24V input though... I think they are caused by the stray capacitances and inductances oscillating with the circuit. Is this caused by little stray capacitance and inductance or a lot?
No, noo... The stray and parasitic inductances do NOT trigger parasitic oscillations within this circuit, they happen actually in the gate drive circuit, and they oscillate between gate, turn on/off delay time of transistors and through diodes back to the other gate. With higher input voltage, higher base current is present which makes the transistors switch faster. This results in sharper transients and the gate drive is more prone to self-oscillation.
To prove this, I redesigned the gate drive scheme with feedback winding on the transformer fed into two gate driver chips. There were NO oscillations except the wanted one, at any voltage.
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ZVS flyback driver problems
Thanks.