Linear Supply @300V and 200mA design - Page 1
Forum section: electronics / projects

author: vruhl
author_rank: Newbie
timestamp: July 28, 2025, 01:51:10 pm
content: Hello,I'm new here, but have a question already.I'm trying to build a 300V supply for testing a solar inverter. I already tried a few designs with multiple iterations, but none worked quite the way I wanted it to.Currently, I try to implement it by using a MOS-FET as a low-side-switch and a current-shunt after the source connection. To control the FET I thought of an OP-AMP circuit to compare the voltage generated by the current shunt and a reference voltage (which should be controlled later on by a poti, to allow a range of currents).My question is: Does this even work if the circuit is correctly designed, or is this a bad idea? And furthermore, how do I exactly design the circuit? I have problems with the OP-AMP design.Many thanks in advance.Vitas

author: tooki
author_rank: Super Contributor
timestamp: July 28, 2025, 02:01:35 pm
content: Please share what you’ve tried so far that didn’t work.The schematic shown is not complete, and it shows a high-side switch (which is harder to drive than a low-side switch). But using a MOSFET is tricky anyway: nearly all MOSFETs made today are designed to be switching MOSFETs, that is, they are not designed to be used in a linear mode, and it’s easy to burn them up that way.There do exist specialty linear MOSFETs, for example from Littelfuse. But it might be easier to use a BJT.

author: richard.cs
author_rank: Super Contributor
timestamp: July 28, 2025, 02:18:00 pm
content: 10 years or so ago I built a similar supply using pentodes as the pass element. It was for a general purpose bench supply where getting semiconductors to survive user (tired me) nastiness was looking tricky. Today I would probably do it as a switcher unless I desperately needed low noise.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 28, 2025, 05:02:05 pm
content: Quote from: vruhl on July 28, 2025, 01:51:10 pmIt is possible, but you will encounter two challenges to overcome.The first is the safe operating area of the MOSFET. Most MOSFETs on the market are not designed for linear mode.The second challenge is the stability of the feedback loop.Your schematic with positive feedback doesn't make sense. But it is easy to make something that works.

author: mariush
author_rank: Super Contributor
timestamp: July 28, 2025, 05:45:17 pm
content: Wonder if it would be possible to configure a LR8 high voltage linear regulator with an external transistor to give it higher current capability.LR8 is an adjustable linear regulator from Microchip which can do up to 450v input, up to Vin - 12v output, up to 10mA output :LR8 :https://www.digikey.com/short/v04brqdnexample of LM317 regulator modified with a 2n3055 to give it up to 5A of output current :http://www.circuitdiagram.org/lm317-5a-variable-power-supply.htmlI think you'd only need a npn transistor with a Vce of around 100-200v, only needs to be the difference between input voltage and output voltage... if you rectify 230v AC voltage to DC you get at most ~375 and your output would be 300v ... so you just need to drop < 100v on the transistor.

author: Benta
author_rank: Super Contributor
timestamp: July 28, 2025, 06:24:41 pm
content: I'd probably go for a classic discrete bipolar design scaled up to 300 V.But even using pass elements like MJL4281A (ONSemi) you'll be sailing close to the wind on SOA.@mariush:"I think you'd only need a npn transistor with a Vce of around 100-200v, only needs to be the difference between input voltage and output voltage"Yes, provided you don't short-circuit the output or it doesn't have to charge a load (eg, capacitor) first.

author: vruhl
author_rank: Newbie
timestamp: July 28, 2025, 08:08:37 pm
content: Hello, first I tried to implement the first circuit. This didn't work because it didn't scale up correctly up to 300V.I also tried to modify this circuit, but that also did not work correctly.The second picture is the version with the first suggestions to make it better. I also found a similar circuit in a video from CuriousMarc, from which I also tried to implement the circuit.The cap on the Load is to "stabilize" (remove high-frequent ac signals) the voltage at the load. We added the TVS-Diode to protect the transistors in our previous designs (I figured that it couldn't hurt, but I think it can be removed)I also should say, that this is a project in my vocational training so it does not need to be perfect.Thank you for answering so quickly

author: vruhl
author_rank: Newbie
timestamp: July 28, 2025, 08:40:01 pm
content: Quote from: tooki on July 28, 2025, 02:01:35 pmI'm still exploring how this blog works, I answered your follow-up question down below, I don't know if you have received a notification.

author: Smokey
author_rank: Super Contributor
timestamp: July 28, 2025, 08:42:29 pm
content: Just because no one has said this yet...200mA at 300V = 60WAs everyone has said, your pass element will be operating in the linear resistive region.This is what a 60W rated resistor looks like:https://www.digikey.com/en/products/detail/te-connectivity-passive-product/TE60B3R9J/2367245

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 28, 2025, 09:51:59 pm
content: Quote from: vruhl on July 28, 2025, 01:51:10 pmFor that, throw away the opamp, it will only cause you grief.You do not even want high gain feedback.LT Spice has limits on HV High power transistors, so this is scaled to lower than you need, but it gives the idea.By using simply discretes, you limit the feedback which actually helps emulate a solar cell.You want a slight droop with current and then the current fall to not be too sharp either. A solar inverter is looking for that MPPT point.Q3 and Q2 are high voltage parts. The others are generic.Q5,Q6 are a simple current sink, allows some control range over output voltage with less shift in current limit operation.Increasing R4 softens the current limit, and reduces the power in Q3 - even 1mA is 300mW at 300VQ2 needs to be high power, as mentioned above, and you may need multiple packages to spread the high powerFor Q2 you look for the highest power highest HFE part you can get your hands on, with enough voltage headroom.If this all floats, you could flip everything, so the highest power part is NPN, which is slightly more common.Addit: As mentioned in #5 parts like NPN MJL4281A PNP MJL4302A are 350V 15A with good HFE, but their SOAR is less than 100mA at 300V, even for a 1 second oops, so multiple packages would be needed.

author: Benta
author_rank: Super Contributor
timestamp: July 28, 2025, 10:49:51 pm
content: @PCB.Wiz, that's the kind of circuit I was thinking of.But to mitigate the SOA problem during overload or shorting, adding foldback current limiting might be an idea.Not hard to do and would avoid having to use "monster transistors".

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 28, 2025, 11:35:40 pm
content: Quote from: Benta on July 28, 2025, 10:49:51 pmYes, I was just exploring current foldback.I would still use the 'biggest' parts I could find, on some thermal mass heatsinks, the PNP MJL4302A looks ok, tho not a lot of voltage margin.Certainly for short circuits it can help, but it needs to be Zener offset to avoid affecting the MPPT area of the curve.A 150V Zener looks to offset it almost enough, gives appox 30W peak power at 177Vo, hopefully that is well away from the MPPT working area.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 29, 2025, 12:55:18 am
content: Here is the 300V/200mA operating point with zener fold back - there is some control over the current slope via R4, R3Q3 also needs to be a good package, it runs ~1W at limiting point.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: July 31, 2025, 02:00:01 am
content: i did using audio stereo amplifier ( i think 120w fisher ) feed by SG each channel per single backward connected transformer diode bridge, caps, each produce around 200V, change output voltage by SG amplitude and frequency ..