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SLR inverter question

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LAN_403

baleworker

Tue Dec 09 2008, 06:37AM

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

Here are 2 waveforms of my SLR primary current with an open circuit secondary. first one at 12.4kV Output, second one at 17.4kV
I kept increasing the voltage by reducing a number of primary turns.
Inductance is primarily external.

Is 1st one looks ok?
Is 2nd one indicates saturated core? Why is it not symmetrical anymore?

GeordieBoy

Tue Dec 09 2008, 04:31PM

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

To Baleworker: These waveforms look all wrong! It looks like you are leaving the switches on for too long, so the resonant network is ringing up over several cycles. It also looks like you are turning off the switches when there is still current flowing through them, as evident by the excessive ringing in the first picture.

Switches should be turned on for a minimum time of a 1/2 cycle at the resonant frequency of the load network, and a maximum amount of time of 1 complete cycle. Or to put it another way they must be turned on to support the entire positive half-cycle of current, and may then be turned off when the negative half-cycle is flowing through the free-wheel diodes. They should not remain on for a full cycle.

To Dr kV: I would _aim_ to design the transformer for the required leakage inductance value. This may be an itterative procedure to arrive at a design where Lp, Ls and the primary-side leakage inductance are all within spec. You can always decrease leakage inductance by moving windings closer together if the measured value comes in too high, or supliment an inadequate value with magnetic shunts or an external resonant inductor. (Air-core, gapped ferrite, or type-2 iron powder should all work depending on how much suplimental inductance you need.)

Remember that the leakage inductance is only one part of the series resonant network. The resonant capacitor is the other. Together they both determine the resonant frequency and the surge impedance of the load network. In mass-manufactured transformers there is always some variation in the leakage inductance, so it is often common to tweak either the operating frequency or the capacitance to get the switching just right for that transformer.

I would not recommend gapping the core in an SLR inverter as it will decrease Lp and Ls and is not required.

Hope this helps,

-Richie,

Dr. Dark Current

Tue Dec 09 2008, 04:47PM

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

GeordieBoy wrote ...

To Baleworker: These waveforms look all wrong! It looks like you are leaving the switches on for too long, so the resonant network is ringing up over several cycles. It also looks like you are turning off the switches when there is still current flowing through them, as evident by the excessive ringing in the first picture.

I thought his waveforms were with open-circuit secondary...
Thanks for the help.

GeordieBoy

Tue Dec 09 2008, 04:50PM

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

> I thought his waveforms were with open-circuit secondary...
> Thanks for the help.

With secondary winding open-circuit, the primary current should be triangular in shape if I remember correctly.

-Richie,

Dr. Dark Current

Tue Dec 09 2008, 05:06PM

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

GeordieBoy wrote ...

With secondary winding open-circuit, the primary current should be triangular in shape if I remember correctly.

-Richie,

I think the resonances which can be seen are between the inductance and secondary self-capacitance. If I remember correctly I have observed something similar. Right now I'm designing/building a new inverter so I can post my waveforms when/if I get it going.

baleworker

Tue Dec 09 2008, 07:15PM

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

This is an open circuit, under load my waveforms look exactly as you described. Unfortunately I understand very little of what is happening with an open secondary and how SLR topology is able to provide a constant current.

Why the resonance frequency increased? I thought it supposed to stay constant = 1/(2Pi*Sqrt(LC))?

Can anyone explain a theory of SLR current regulation?

GeordieBoy

Tue Dec 09 2008, 07:45PM

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

The SLR inverter supplies constant average current into a short-circuit - That is what it is designed to do. It can maintain an _approximately_ constant current up to a certain load (output voltage) It is not able to drive this constant current through an open circuit for obvious reasons!!! Hint: It would need infinite voltage to do so.

When you open-circuit the HV secondary of the SLR inverter, the series resonant action breaks down because the leakage inductance is out of the circuit. With the secondary open circuit it does not operate how it is intended to operate. All you need to do is design the supply to maintain constant current over the required load range, (for example up to 10kV if you want to charge some capacitors up to 10kV.)

-Richie,

baleworker

Tue Dec 09 2008, 09:06PM

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

Thanks a lot!

I understand it cant keep the current with an open secondary, so I am wondering if there are any equations to estimate a current under certain load? Up to what Vsecondary it can keep the current realtively constant and how constant?

Also what is the maximum open circuit voltage? According to my probe, I always get slightly more than Vprimary_peak*turns_ratio.

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