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I am in need of a high voltage supply, capable of 15mA at around 200V for a valve/tube preamp. This supply has to be low noise and any switching noise kept out of the audio band.

The incoming voltage is 24V.

After a little time, I decided to try a boost converter circuit, the switching done with a 80kHz PWM signal from an Arduino, trying at low voltages for testing.

My first attempt at controlling the voltage via feedback was to simply switch on the PWM signal if the voltage fell below the threshold, and off if was above. This resulted in unacceptable ripple and audible noise on the output.

The second attempt was to vary the duty cycle. This was much better, but I wondered if I could do better with a more simple and more 'analog' way of implementing feedback.

This is what I came up with:

boost_fb1 Falstad Simulator Link

This seemed to work quite well and noise was kept to a minimum. Best of all, it's super simple!

The only thing I don't like is it only takes 600mv(ish) to start attenuating the signal, meaning a large voltage divider.

I know I could get lots of different ICs to do this for me, but I prefer a challenge and designing my own gives better control and flexibility.

Is this a good approach? If so, why do we not see this more often?

Any improvements?

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    \$\begingroup\$ How much power is dissipated in the switching transistor with this feedback model? I'd expect it to be pretty inefficient. \$\endgroup\$
    – Klas-Kenny
    Jan 31 at 18:02
  • \$\begingroup\$ 5.4mA peak. I kept the collector resistor low, as to have plenty of current to drive the mosfets gate capacitance. I would probably go with a driver IC for the final design. \$\endgroup\$
    – G Frank
    Jan 31 at 18:07
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    \$\begingroup\$ I think Klas-Kenny is worried that when the feedback transistors starts conducting, the mosfet will be switched less efficiently (be operated in its linear region instead of in saturation) \$\endgroup\$ Jan 31 at 18:16
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    \$\begingroup\$ My point is that the feedback works by not allowing enough gate voltage to fully open the MOSFET. What you have created is basically some hybrid switching/linear regulator. How would you connect the feedback if using a proper gate driver? \$\endgroup\$
    – Klas-Kenny
    Jan 31 at 18:16
  • \$\begingroup\$ I would start by removing the bjt on the left and the 2.2k resistor. then adding a resistor (maybe 220ohm) in series with the mosfets gate. efficiency isn't to much of a concern. as long as it's half reasonable. highest priority is keeping the output as ripple free as possible and any noise kept out of the audio band. \$\endgroup\$
    – G Frank
    Jan 31 at 18:22

1 Answer 1

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On the face of it your circuit will run the mosfet hot due to analog burning and VBE of a transistor is not a temp stable reference .But your circuit is simple so lets try to make it workable Remember that valve preamps when running draw pretty constant current due to class A operation and also remember that they ran off 50Hz rectified DC that followed the mains volts .In other words the line regulation and the load regulation are not very important for your job.If the valves were cold and therefore there was no load you need something to limit the V out of your boost convertor .You could set up your regulation to come in at an abnormally high output volts so you are protected but not burning power .Then you could reduce your digital pwm to get the correct output volts for your tube preamp .

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