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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
No... You can't put a capacitor in parallel with the output of a voltage-source inverter. The output is a square wave, and the capacitor presents a short circuit to the harmonics. The capacitor and maybe the IGBTs will suffer huge losses and maybe blow up.
To get resonant operation, you have to use a series capacitor like in a DRSSTC. The parallel capacitor is for current-source inverters like the ZVS.
Before messing with capacitors, I'd try increasing the coupling by moving the primary to the same core leg as the secondary.
Registered Member #4603
Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159
Oh... Thats true Steve! ^^
I want to use this inverter for a QCW VTTC. µC generates the signal, half bridge powers the flyback -> voltage ramp at the output! Should work fine for powerful pulses - i got 10.000µF smoothing xD
But i am not sure about the signal. How does it have to look like? Some kind of PWM with increasing duty cycle?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You need bipolar PWM. Like regular ("unipolar") PWM but every other pulse goes negative instead of positive. This allows modulation of the duty cycle without introducing a DC component to the signal.
The TL494 and SG3525 both support bipolar PWM. To generate it yourself, you normally use a full-bridge driver made of two gate drive ICs. The "off" portions of the waveform are made by switching both gate drivers to the same state, forcing the GDT primary voltage to zero. You may have a problem with this, if you used an intermediate bridge itself driven by a GDT.
Registered Member #4603
Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159
Steve Conner wrote ...
You need bipolar PWM. Like regular ("unipolar") PWM but every other pulse goes negative instead of positive. This allows modulation of the duty cycle without introducing a DC component to the signal.
Thanks a lot Steve! Perfectly explained. Okay, bipolar PWM. Something like this:
Push pull operation. Looks good :)
But i think it would be easier not to use a GDT but drive the gates directly with 2 galvanically isolated gate drivers instead...? How can i instruct the TL494 to ramp up the duty cycle?
EDIT: I think i found a circuit that i could use...?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I don't see how that circuit can work. The TL494 is configured for bipolar PWM, but in this case it's driving a buck converter, which needs unipolar PWM. (The DC component of the PWM is actually the desired output.)
So I don't think it can work as it is, but it would work fine driving your transformer.
Registered Member #4603
Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159
I found a similar version of this circuit here But the part withe the feedback...? It will not work with my transformer. Is there a other possibility to drive the transformer to get a QCW waveform?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You can take feedback from the high voltage DC output, just use a big HV resistor in the divider. As far as the control loop is concerned, it doesn't make a huge difference.
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3kW IGBT Halfbridge Inverter *UPDATE 5kW*
