High Voltage, Low Noise - Wide Range, Medium Power, High Efficiency - Page 1
Forum section: electronics / projects

author: JoergR
author_rank: Contributor
timestamp: May 20, 2025, 06:54:21 pm
content: Hi all,for quite some time now, I've been intrigued (obsessed) by the idea of building a high voltage "range extender" for my bench power supply (typical 30V output) that would allow me play with some tube circuits. Jim Williams' AN-118 has some nice ideas of how to go about this and Joerg Rehrmann has some circuits , too (https://joretronik.de/Web_NT_Buch/Kap11_2/Kapitel11_2.html) and I really like the idea of self oscillating converters. They should make a great first stage for a tracking regulator on the secondary side and the current fed architecture already gives me a factor of pi on the voltage on the primary side, so I don't need a massive turns ratio and can work with off the shelf transformers and would not need to adapt the design to specific transformers.It's fairly easy to simulate and build these circuits and for a specific input voltage and optimise them for maximum efficiency. For example, at 30V input, 300V output, the capacitive coupling works with about 90% efficiency. See MosfetRoyerCapacitiveRampUp_SimpleTransformer.ascMosfetRoyerCapacitiveRampUp_SimpleTransformer.asc(4.91 kB - downloaded 16 times.)Looking at Capacitive30VInput.png I think it has quite nice waveforms.But I would like this "range extender" to work down to an input voltage of 3V so that I have a 10x range in output voltages I can cover - also at light loads and it should be efficient - because what good is all the effort on a switched power supply if one doesn't strive for maximum efficiency. And this really presents a challenge.As I lower the input voltage down to 3 volts input = 30V output, things get worse. There a two main reasons: i) the voltage V1 that is applied to the gates to bias M1 and M2 and get the oscillation started is too large for the Mofets to turn off once the oscillation has started ii) the voltages at D1 and D2 are too small to fully turn on the Mosfets during oscillation. The capacitive divider on the gates needs to be like this to protect the gates from too high voltage at 30V input.Looking at the waveforms, it's clear that all one really needs to do is rectify the voltage across the capacitor C5 in parallel with the transformer. One half-Wave goes across the M1 and the other across M2. So it seems that a lot of ideas of active rectification should be useful here again.To isolate what an ideal gatedrive would look like, I've replaced the capacitive coupling with a behavioural voltage source of the formlimit(1.5*V(Drain), 0, 15) - so just a little bit of gain and clamped to 15 V.This works nicely over the desired range of inputs from 3V to 30V - with a little bit of trickery to get the oscillation started in LTspice - See MosferRoyerIdealRampUp_SimpleTransformer.ascMosfetRoyerIdealRampUp_SimpleTransformer.asc(3.45 kB - downloaded 11 times.)and the waveformsI would expect this supply to be really quiet given that there is no hard switching and the output ripple can be passively filtered.So now the task is reduced to implementing a gate drive the mimics this behavioural voltage source. Let's see what this needs to do.At 30V input, V_Drain goes up to about 100V. At 100KHz oscillating frequency, the slew rate of V_Drain = 2 * pi * 100kHz * 100V = 600V/us - so that is demanding, but not impossible, especially as we only need a small gain. Plus, at 30V input, one doesn't need the gain of 2, that is really only needed at smaller input voltages.So here is what I've come up with so far: MosfetRoyerDriverRampUp_SimpleTransformer.ascMosfetRoyerDriverRampUp_SimpleTransformer.asc(13.84 kB - downloaded 11 times.)I've used a JFET Diode Clamp and an LT1357 to drive the Mosfet and that works quite nicely.Except now, most of the power is lost in the voltage clamp instead of the Mosfet and there is still considerable shoot through at the end of each switch cycle. The driver does not turn off the Mosfet as fast as it should.So here are my questions:0) Any thoughts about whether this is a crazy idea in the first place? I've played with the classic BJT implementation, but the voltage range I desire directly transfers to the windings providing the base currents, so again, it's not optimal for a wide range of input voltages.1) What can I do to limit the power loss in the clamp but still keep the clamp levels? Just increasing the resistances will make the clamp slow, but maybe there are more advanced parts that are faster?2) I would like to run this at higher frequencies, but the clamp introduces delay and the OpAmp, too. Any ideas on how to compensate this delay esp. for turning the mosfets off? I'm thinking of using a MosfetDriver that is triggered from a comparator, but all my experiments with this could not start the oscillation. Also, one would need to turn on the driver chip for M2 at D1=2V and turn off the driver for M2 at D2=6V to compensate for delays - but this sounds like I need to build a full scope-like triggering with trigger level of 2V at the rising flank and 6V at the falling flank, plus some dead time ...3) In order to start the oscillation, it's really important to have linear feedback with some gain around V_Drain = zero volts and only let a possible high current driver kick in only once a certain amplitude of oscillation is already established. But that means one would have a comparator on the output of the opamp, feed the input of the comparator into a driver chip like ADP3654. The output of this driver needs to go to the gate and the output of opamp also needs to go to the gate - so I have a short circuit. I summing opamp for the linear feedback and the driver would provide high impedance buffering, but that is precisely the problem - I need low impedance to drive the gate.Thanks for any suggestions and ideas!Cheers,Joerg