ZVT etc... Please, help to decide on control logic! / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

ZVT etc... Please, help to decide on control logic!

1 2 next

Move Thread

LAN_403

baleworker

Mon Jul 28 2008, 09:42PM

Registered Member #1596 Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65

Hello, again!

Please help me decide on a right control circuit for my application.

I am using full bridge off the mains, hope for at least 1.5-2kW of power and need a zero voltage or zero current switching control circuit.

This is a constant current, HV power supply, so I do need to have a current feedback to be able to adjust duty cycle or phase to control output current.

Please advice on any circuits and control techniques which might be worth looking at. My goal is a constant output current, no matter what the load is.

Thanks a lot for your input!

GeordieBoy

Mon Jul 28 2008, 10:27PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

If you want constant current out, it is generally better to design the power electronics around this requirement from the outset. So either use a current fed inverter, or a design like the series load resonant (SLR) converter. Whether or not you want ZVS or ZCS usually depends on the choice of switching devices and the topology. For example, the SLR converter achieves zero-current switching into a short-circuit due to the resonant nature of the load, so is therefore well suited to IGBTs.

I'm not sure why you mentioned the Zero-Voltage Transition ZVT converter. This is a technique usually used to achieve zero-voltage turn-on with MOSFETs in compact high-frequency DC-DC converter applications. It does not intrinsically give you a constant current output, but it can be made to incorporate various flavours of current-mode control.

Remember that constant output current can only be maintained up to a certain output voltage which represents the supply's maximum power rating.

Take a look at the series load resonant converter (SLR), and the various current-fed schemes. You might also want to become familiar with something like a flyback or buck converter using peak current-mode control, as this is a good model for understanding how current-mode control can make a voltage fed-converter *behave like* a constant current source.

-Richie,

Dr. Dark Current

Tue Jul 29 2008, 07:46AM

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

Well today in my bed I was thinking about simple series resonant converter which should give you nice constant current output. You have a series LC circuit and connect your transformer primary across the C. The LC is driven at resonance, and in case of open circuit there is a current feedback from the LC tank which limits duty cycle (down to a few %) if the current gets too high.
Would this work for a CC supply?

GeordieBoy

Tue Jul 29 2008, 03:05PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Yes. What you describe is the familar LCLR resonant network often used for induction heating.

The L-match part act's like a quarter wave impedance-transformer and converts the constant-voltage output of the inverter to a constant-current at the output of the matching network.

-Richie,

Dr. Dark Current

Tue Jul 29 2008, 03:34PM

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

GeordieBoy wrote ...

Yes. What you describe is the familar LCLR resonant network often used for induction heating.

The L-match part act's like a quarter wave impedance-transformer and converts the constant-voltage output of the inverter to a constant-current at the output of the matching network.

-Richie,

Hi Richie, I don't know if you got me right, I mean the resonance is between the external inductor and capacitor. Transformer has ideally infinite inductance (no core gap), very low leakage and it should not play any role in the resonant circuit at all and act only as resistive component.
The problem with LCLR is the crazy phaseshifting around the resonant frequency and I can imagine it would be hard to tune with constant-frequency driver (and I think it's useless for driving transformers anyway)

-J.M.

baleworker

Tue Jul 29 2008, 08:29PM

Registered Member #1596 Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65

Forgive my stupidness, but I cant figure out how to adjust an output of an SLR converter.
As far as I understand, your turn on the switch, current goes up and down as a sinwave. The on time is adjusted so by the moment current reaches zero switch opens. Oscillations of current continue. This is is in a full load mode.
But how do I adjust the output? Changing a frequency, width or pulse will always cause a transition at nonzero currentâ€¦.?
I tried to model Steve Wards circuit

and none of potentiometers seem to affect its operation. Looks like there is at least one mistake in it, center tap of R3 is not connected to anywhere.
Please, help me understand how it works and how should I perform switching if I want to run it at a lower duty cycle?

Steve Ward

Tue Jul 29 2008, 09:01PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Connect the wiper of R3 to either end of R3. Its just a trimming resistance.

That circuit is basically a constant pulse width, variable duty cycle, oscillator. The flip flop alternates the pulses for you. It works fairly well, but for HV transformers you have to be careful not to excite a resonance of the HV winding itself. For your low voltage transformer it should work fine, but keep in mind you are just supplying a series of constant amplitude pulses, so you will have to smooth the output somehow.

Dr. Dark Current

Tue Jul 29 2008, 09:23PM

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

Steve Ward wrote ...

For your low voltage transformer it should work fine, but keep in mind you are just supplying a series of constant amplitude pulses, so you will have to smooth the output somehow.

Actually if the transformer core has low magnetizing inductance, the Fres can drop only to half with the open-circuited output, effectively performing destructive series resonance!

baleworker

Tue Jul 29 2008, 11:15PM

Registered Member #1596 Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65

Alright, so in SLR I always have a constant width ON pulse, but the frequency changes?
I realize that it outputs HF pulses as well, as any other smps, correct me if I am wrong. Will inductor after the full bridge on 0V secondary wire help to smooth out the output?

Steve Ward

Wed Jul 30 2008, 12:03AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Will inductor after the full bridge on 0V secondary wire help to smooth out the output?

With no smoothing cap it may be difficult because the inductor has to store much more energy so that it can provide it when the inverter is not. But something about this doesnt seem quite right for the use of an SLR setup, so im not sure if the results will be useful or not.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob