Variable HV Power Supply ZVS Driver  - Page 1
Forum section: electronics / beginners

author: Mailed
author_rank: Newbie
timestamp: August 30, 2021, 03:28:35 pm
content: Hello,i have a Question regarding a ZVS Driver for a flyback transformer. I want to build a variable HV power supply with a Cockcroft–Walton multiplier after the flyback transformer. My thought was: If I can regulate the voltage going into the ZVS driver, that should vary the voltage at my output, correct? The problem is that the ZVS driver needs at least 12V to run, so I can’t go any lower than that. Do you have any suggestions how I could build a supply, that can vary the voltage from 0-X V?That you very much for your Help!Kind regards!

author: Wolfram
author_rank: Frequent Contributor
timestamp: August 30, 2021, 08:23:03 pm
content: Yes, the primary voltage with the classic self-oscillating MOSFET CFPR circuit is proportional with the supply current.Most high voltage transformer have have a resonant frequency, where the voltage gain differs from the transformer ratio, and depends on the load current. Make sure you size the primary inductance + capacitance to operate below this frequency if you want good regulation. Cockroft-Walton multipliers also have significant voltage drop with loading, so it would make sense to use feedback from the output to control the primary voltage, if you want a stable output voltage despite load current variations.You can power the gate pullup resistors from a separate +12 V supply, this will allow you to lower the transformer primary voltage further, without the circuit stopping oscillating.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2021, 10:47:44 pm
content: Yes, it's gate bias that's critical to maintain, indeed, dropping it unexpectedly will most likely destroy the transistors due to flyback of the supply inductor!I used the same mechanic here, varying the supply voltage with a linear supply (a switching supply would also do well, I just didn't need the efficiency).Behavior is kind of weird at low voltages and light loads, as it oscillates in weird modes, or squeggs, or quenches entirely. It also can't go down forever, as the gate bias itself is a power source, and if it's sufficient to cause oscillation, it'll generate some output (in this case, something like 200V). Such is the consequence of such a simple circuit.Mind that you'll similarly need a nice high voltage resistor, to make the sense divider.Also, make sure you're using the right kind of flyback transformer, without internal rectifiers. You can't run a C-W from the DC kind, at least not in any reasonable way.Tim

author: Mailed
author_rank: Newbie
timestamp: August 31, 2021, 02:02:39 pm
content: I am still having troubles understanding how I can separate the gate drive from the power input voltage. Do you have a circuit diagram for this? Or could you explain me how to do it on this diagram:https://content.instructables.com/ORIG/FSW/2MIC/GNBEVVKF/FSW2MICGNBEVVKF.jpg?auto=webpAnd what would be the best way to implement a voltage feedback, to keep the output stable?Sorry if these are pretty basic questions. I’m still trying to get into this topic.Thank you a lot!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2021, 02:59:38 pm
content: Uh, think you got the wrong clipboard contents there...On my circuit anyway, it's where the +12V goes to 1k's to the gates, and not to the inductor to the transformer and drains.Tim

author: Mailed
author_rank: Newbie
timestamp: August 31, 2021, 04:38:36 pm
content: Hi Tim,something went wrong there with the Link, sorry. I changed it. So I would supply the gates with +12V and supply the rest of the circuit (where the 100uH 2A inductor in your circuit is) with the variable voltage for the primary?I don't fully understand what the upper part of your circuit is. I can see that there is a voltage feedback from the HV output.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 31, 2021, 07:25:30 pm
content: The mosfet will die if it sees more than 20V Vgs. Use a suitable TVS like a SMBJ14A from gate to ground a zener is far to slow. You might want to beef up the wattage of the 1K resistors so the TVSs dont melt. What does the voltage across the IRF644 look like, is it exceeding 250V, is it swinging negative and exceeding the repitetive avalanche energy rating?You can repalce the inductor with a current source so that the oscillator still sees a high source impedance at resonance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2021, 09:10:51 pm
content: TVS at gates doesn't really matter, nothing to shock them around and they're only ever supplied by the 12V. Recovery shouldn't matter, the rate of rise is low. Or I suppose a clamp diode to +12 would do the same thing, then a bypass cap +12 to GND. But that's as much work as adding the zeners. Anyway 1N4742s will do.Oh, also there should be series gate resistance (or a ferrite bead on the gate pin) so they don't oscillate. Putting a capacitance (zener junction is a modest capacitance) across a MOSFET gate is a sure way to make it oscillate at frequencies you can't measure.To be perfectly clear on wiring, see attachment:Also deleted the JPEG, because internet rot.Tim