Author Topic: 3kv 50khz LLC inverter for 100kv voltage multiplier,secondary ringing problem  (Read 735 times)

Offline Axizep

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Salute to engineering folks!
Iam trying to build a HV multiplier for my x-ray tube using cheap 6kv 10nf ceramic capacitors that i bought and  500ns reverse recovery r5000 diodes.
As i have built many different LLC power supplies before,i know this topology quite good and was confident that it will also be good (zvs) for slow diodes in multiplier i have bought.
I have chosen 46-146 khz frequency range with a beefy ferrite EE85,which allowed me to make primary 6 turns without getting saturation at lowest frequency. some impressive 25v per turn!
a half bridge topology with a split resonant capacitor. also transformer appeared to have very low leakage inductance 1.8-8 uhn. i have tried two different configurations : 1.8uhn is for secondary wound on top of primary,whole bobbin wide,1 layer each winding. 8uhn is for windings separated in space: primary on one side and secondary on other,3 layers secondary. low leakage forced me to apply an external resonant inductor around 90 uhn.
Secondary winding consists of 120 turns and is isolated from everything in a circuit with a 51k resistor load and voltmeter.

Half bridge and controller part of a job went quick and easy,but when i have finished a circuit result appeared to be unexpected.

I have noticed that output voltage rises with frequency,inspite of current doing the opposite. the reason appeared to be that secondary is ringing on its own resonant frequency near 330khz. That ringing also introduces to primary circuit and distorts a current waveform.
When i leave secondary disconnected from everything it rings with a higher frequency (550khz) and also introduces ringing to primary circuit.
When i disassemble upper half of a ferrite core,where the secondary is, ringing disappears and converter works like it should- current waveform is triangle at higher freq and perfect sinusoidal at lower freq,also current drops with frequency as like in other llc converters.

Ringing ,depnding on what is connected to secondary,rises output voltage 2-3 times as much ( i also observe it on 1 turn loop into a transformer to monitor waveform and magnitude with oscilloscope). i have no opportunity to load secondary more than 51k (even that is tested on low voltage output,on full power,idle power load resistor should be around 2M to dissipate 5 W)
secondary is wound with magnet wire 0,25 enameled with extra silk isolation wound in 3 layers with thick fiber glass isolation between layers. when transformer was 1 layer full width,it was ringing as much as 3 layer,half width.

Converter construction:

transformer with separated primary and secondary

simplified schematic

osciloscope


yellow trace is inverter output.purple-primary current form ct. light blue- 1 turn loop on a transformer. dark blue- voltage across secondary (51k loaded)

also i have noticed a funny fact- when i connect one side of a primary to inverter ground,ringing lowers its freq and amplitude rises,when i connect other end of secondary,freq also lowers but ringing amplitude rises. in both cases it does not effect inverter frequency,which in both cases stays the same.
introducing air gap into a transformer core changes ringing amplitude also.

if its what iam thinking-self ringing of a winding,than i dont have a clue how to suppress that,other than a big load resistor,which i cannot install.
i hope that might be some other reason of that effect,which iam not aware of,so iam asking for help of a dear community!

Offline davekni

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Simulation is likely the easiest way to see what's going on.  Have you used LTSpice or one of the other analog simulators?

Before making any useful suggestions, a few questions:

What configuration is used for the scope traces?  Is that with primary and secondary separated, one on each E-Core half?  If separated, is the one-turn feedback winding on the primary half?  Does it have all 300 secondary turns?  Is the E85 core gapped or not?  If gapped, what is the primary inductance (unloaded - not leakage inductance, which you already listed as 8uH)?

Is the half-bridge drive controlled by measured primary current, 1-turn voltage, or some combination?

What AC voltmeter are you using on the secondary?  Many normal digital meters have quite significant input capacitance, which could be confusing your results.  They also often don't register 46+kHz frequencies accurately.  (Did you remove the volt meter for the scope traces?  I presume one side of the secondary was grounded for the scope traces.)
David Knierim

Offline Anders Mikkelsen

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It seems like you have some resonance up around 400 kHz, I suspect this is the reflected secondary capacitance together with the primary series inductor. My preferred approach to high voltage transformer drive is to pull the secondary resonance down below your intended operating frequency, and control the drive frequency toward and away from resonance to control the output voltage. If you want to operate at 50 kHz, you would need to dramatically increase your series inductance, but this would also greatly reduce the power that can be delivered. The trick is to put a capacitor in parallel with your primary. 100 nF would be a good starting point, if my guess is right. You can also increase the reflected secondary capacitance by winding more turns, this will also give you the bonus of a higher voltage into your multiplier. I would aim for around 400 turns on the secondary, then enough inductance in series with the primary to bring the resonant frequency down to 50 kHz. If this cannot deliver enough power, then reduce your series inductor and put a capacitor in parallel with the primary.

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