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Registered Member #61469
Joined: Thu Feb 09 2017, 12:26PM
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Posts: 4
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 ,depending 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 fiberglass isolation between layers. when transformer was 1 layer full width,it was ringing as much as 3 layer,half-width.
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 affect inverter frequency, which in both cases stays the same. introducing air gap into a transformer core changes ringing amplitude also.
if its what I am thinking-self ringing of a winding,than i don't 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 I am not aware of,so I am asking for help of a dear community!
traces on scope: 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)
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3kv 50khz LLC inverter for 100kv voltage multiplier,secondary ringing problem
