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Parallel resonant inverter variant

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Dr. Dark Current

Sun May 04 2008, 06:53AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Do you see any problems with placing the resonant cap like this? There would be one really big advantage, and that is you don't have to care about implementing cross-conduction because the resonant cap "freewheels" any spikes that can occur during deadtime (which still should be kept as small as possible).

Dr. Dark Current

Fri May 09 2008, 07:54AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Anyone? I have plans for a relativly large inverter and this would simplify its design considerably.

Steve Conner

Fri May 09 2008, 09:01AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

How will the capacitor resonate, if the voltage across it can't reverse? It has diodes across it. I'm not familiar with this circuit, I take it the two inductors are coupled? Where was the resonant cap before you moved it?

What's the problem with implementing cross-conduction anyway? The larger the device you're building, the more effort it's worth putting into the control circuit. A big inverter should have lots of protection circuits because it costs more to fix when it blows up.

Dr. Dark Current

Fri May 09 2008, 09:38AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Dr. Spoke wrote ...

How will the capacitor resonate, if the voltage across it can't reverse? It has diodes across it. I'm not familiar with this circuit, I take it the two inductors are coupled? Where was the resonant cap before you moved it?

What's the problem with implementing cross-conduction anyway? The larger the device you're building, the more effort it's worth putting into the control circuit. A big inverter should have lots of protection circuits because it costs more to fix when it blows up.

Sorry for not mentioning, yes the inductors are coupled of course (a "dual winding" choke). It does not (or at least should not :p) take part in the resonant circuit, it just provides constant current supply.

As for the cap not being able to resonate, well that was my question. The inverter switches in the instant when there is no voltage on the cap, so it is not storing any energy, so it should not matter for the cap "which way" it will go... The inductor (transformer) will change polarity and it still "sees" the same capacitance, just the cap charges the other way.

The problem with implementing x-conduction is added complexity (surprise :D), I like to keep things simple. I'd have to use optocoupler driver with some additional things for adjustable x-conduction, this way I can use a GDT without any extras.

The original schematic is this, basically it is the "Mazilli" inverter adapted for half bridge operation:

Steve Conner

Fri May 09 2008, 10:47AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Oh, well no then, you can't move the resonant cap in the way you suggested. It's doing something totally different in the new position.

One thing I remember seeing by Blackplasma/Matt Bingham, was a GDT with a tertiary winding that was rectified to provide a DC bias for the gate. You could do this so that your devices were both normally on, and the gate drive pulses turn them off. It might not be practical if you really are going to use BJTs, though.

As another alternative, you could use a third "reset winding" on your coupled inductor, to dump spikes back into the DC bus through a diode. I guess it should have 1/pi the number of turns of the other two windings. Or 2/pi or 1/(2pi) or something

I like this method because it will also give a safe escape route for the stored energy in the DC bus choke, in the event that the gate drive fails.

Dr. Dark Current

Fri May 09 2008, 01:04PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Dr. Spoke wrote ...

As another alternative, you could use a third "reset winding" on your coupled inductor, to dump spikes back into the DC bus through a diode. I guess it should have 1/pi the number of turns of the other two windings. Or 2/pi or 1/(2pi) or something

I like this method because it will also give a safe escape route for the stored energy in the DC bus choke, in the event that the gate drive fails.

That looks like a cool trick, thanks!

Could you please explain why it won't work? The choke is coupled so the other "end" of the cap will mirror the voltage on where the transformer is "connected" to the cap at any time through one of the transistors. I don't see any problem with the cap voltage not reversing because all the energy in the event of transition is stored in the transformer and it sees the same capacitance all the time.

Well it seems i'm gonna try it anyway

Steve Conner

Fri May 09 2008, 01:16PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It won't work because the resonant capacitor isn't connected across the load any more. What you've done is the same as putting it across the DC bus choke in a Mazzilli oscillator, if I'm picturing it right. So it can't resonate with the load like it did before.

Dr. Dark Current

Fri May 09 2008, 02:18PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I think I'm starting to get it. The first realisation is that ALL of the reactive current will flow through the switches, to avoid this was the original idea of this inverter! So even if it would work like in the schematic, there would be no point in building it.

Peter02

Sat May 17 2008, 11:15AM

Registered Member #1488 Joined: Sat May 17 2008, 10:41AM
Location: Germany
Posts: 18

Hi,
I am new to this forum and like it very much. I didn't want to open a new Thread and found this one best fitting for my topic.

Recently i got the idea to build a highpower ignition system for gasoline Motors, for that i have to build a 12V to 1200V converter which has to deliver about 1KW for charging a capacitor.
Since that is realy much for a 12V car system, i thought of using some kind of resonant converter topology to improve efficiency, reduce RF interference and maybe better reliability.

Because it is for repitative charging of a 4 ÂµF capacitor i read through the CCPS thread and looked at many PDF's on that topic. I concluded that if i were not to run it of 12V but some higher voltage (like mains) a SLR Converter would be pretty good for it.
But since the reactive current can become quite large compared to average current and has to be completely conducted by the semiconductor switches, that might be very problematic.
My question now is if someone here could give me a hint on what topology would be best fitted.
My thought was that a parallel resonant converter might do the job but i did not completely understand if it behaves as a voltage- or a currentsource where a currentsource is what i would need to efficiently load a capacitor.

Best Regards
Peter

Dr. Dark Current

Sat May 17 2008, 11:37AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hi Peter,
search Google for "zvs flyback driver". It is a very simple circuit, a type of a current fed parallel resonant inverter. Provided you have a high-quality resonant capacitor, this circuit can push insane amounts of power with commonly available semiconductors (MOSFETs).

Ok here is the schematic:

You might want to change the number of turns, series inductor value and resonant cap value to suit your design and application.

-J.M.

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