One of my hobbies is designing and building tube-based audio equipment. I'm at the point where my Eico 1030 tube-based high voltage power supply isn't really cutting it anymore for a few reasons:
1) At this point it needs a fairly hefty investment in parts and labor (and that means my time), including about $50 worth of tubes and way more time than I want to put into a piece of equipment that doesn't really fit my needs.
2) The 150 mA maximum current is a limitation when prototyping output stages of larger amplifiers. I'd really like to have closer to 300 mA continuous.
3) It's big in an awkward way (tall), and bench real estate is at a premium.
This has led me to the decision to build a new power supply that is better suited to my requirements. In a move that is likely to be controversial, I have decided to go with a solid-state based design using MOSFETs as the series pass element. Unlike many a threads on this, I have no illusions that a 500V 10A MOSFET can handle 10A while dropping 500V across it (I have yet to see a TO-247 device that can dissipate 5 kW). I will likely need multiple devices in parallel in order to get enough current with a comfortable safety margin.
In the interest of simplicity, I have decided to use an LR8 as the voltage reference. This should get me a maximum output of around 450 volts. A few key design specs here:
-It doesn't need to be completely idiot proof. It's a high-voltage power supply, and that in it of itself is anything but idiot proof.
-It should go from 80 to 450 volts (more is better, but the LR8 is limited to 450).
-It should be able to handle 300 mA continuous.
-It should be very quiet (electrically, that is. There will be a fan).
-It should be current limited. If a device under test were to short, It should current limit it so that it doesn't immediately blow the pass elements. In this situation it would dump the full 500V output from the rectification stage into the shorted load- not a good situation, creating lots of smoke and sparks. The filter capacitors will not be very small, so blowing the MOSFETs to pieces would not be hard without some form of protection.
The power transformer in question is a Tektronix 120-037. Information can be found on the web for those who are interested, however for me to get about 500V rectified output from it at sufficient current is easy, and I already have the filter capacitors and rectifiers on hand. This will likely be a cheap project, as aside from the FETs and the LR8, I should have most of what I need in my "junk box".
Looking at the DC SOA, I think that a pair of these FQA8N90C-F109 should be sufficient for the series pass element with a reasonable safety margin.
https://www.digikey.com/product-detail/en/on-semiconductor/FQA8N90C-F109/FQA8N90C-F109-ND/1954558I know that at a lower voltage output (say 100V), this will be dissipating a lot of heat. I'm planning to mount the FETs to the heatsink with a clamping bar. I also plan to take the time to ensure the heatsink is actually flat (I will put it on the mill and skim-cut it with a fly cutter if need be). In order to maximize heat transfer, there will not be any mica spacers. This means extra care will have to be taken in chassis design since the heatsink will be hot (both electrically and physically).
That pretty much leaves my current limiting circuit, which I'm not quite sure how I should go about. It needn't be adjustable, I was planning to leave it fixed at around 500 mA. I have a distinct feeling that an LM317 is not going to be able to take the voltage drop necessary. Most of the designs I've seen for high-voltage current limiting circuits use a BJT, but I'm concerned that I will run into issues with secondary breakdown. If I could use the same MOSFETs as for the pass element, that would be great, but I haven't seen a whole lot of designs that do this.
I would like to keep this reasonably simple. I do not plan to design a PCB for this. Perf-board and point-to-point is the plan.
I'm sure I'm crazy, but this is so far what I've come up with. I have thought very hard about using tubes for the pass element- there are certainly advantages (usually they don't fail short like a FET) but at the desired output it would simply take up too much space. I'd love to hear suggestions on how I should proceed. Hopefully I'll have time to draw up a schematic with my idea this weekend.
Sorry for the long post.