Use of series mosfets  in High Voltage power supply - Page 1
Forum section: electronics / projects

author: marvi3
author_rank: Contributor
timestamp: June 21, 2021, 02:45:35 pm
content: Hello Frienda,I've found an EDN interesting item about use of series mosfets for power supply.https://www.edn.com/series-connected-mosfets-increase-voltage-power-handling/what about its use in HV PSU?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 21, 2021, 02:55:53 pm
content: What about it? Cascoded stacks are well known. For switching purposes (and probably just AC stability too), the divider chain should have capacitors in parallel with the resistors. And maybe zeners (S to G) too.Not so important these days, with Si MOSFETs up to 4kV available, and SiC up to 1700V commercially available (they go up to 10kV+ as far as I know, but are special order, and probably ITAR or some stupid crap like that).Or there's still good old fashioned tubes, if you don't mind the many downsides, and just need something to handle the voltage.Tim

author: Terry Bites
author_rank: Super Contributor
timestamp: June 21, 2021, 03:37:26 pm
content: Running straight off the mains looks like a recipe for disaster! As T3sl4co1l says thre are plenty of HV devcies around so you dont need a cascode. IGBTs are available well into the 5kV region. I'd only cascode if there was a price constraint. Take care compagno.

author: marvi3
author_rank: Contributor
timestamp: June 21, 2021, 06:27:20 pm
content: I know that now we have mosfet or Igbt with 1000-2000v, but the problem is that those devices have High Rds-on and SOA limited, so they dissipate more power and easily they can burn. This the idea was to use a cascode or serial mosfets with lower voltages, high currents and a very low Rds-on... that can handle more dissipation power, and so more current...What do you think about... is my idea fool?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 21, 2021, 07:32:26 pm
content: Well, is this for the purpose of switching, or linear amplifying? Because switching devices don't need to dissipate much power, but your concern seems to be about power and SOA.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: June 21, 2021, 09:18:04 pm
content: With a series connection the R_on add up and there is additional loss from the gate divider.For high voltage (e.g. > 500V) the good old vacuum tube is still not that bad. A big plus is that they tend to fail open, while a semiconductor tend to fail short. The can also be high power. I have used a linear regulated power supply for 10 kV up to 1 A, that used just 1 tube for the power stage - though with water cooling.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 21, 2021, 10:46:49 pm
content: Hmm, I wouldn't say >500V, more like 3kV+ for tubes?They can still fail shorted, particularly if you melt some electrodes together. Upside is, it takes a LOT of heat to do that. You're physically heating up bulk metal, it can take seconds before grids visibly glow (at which point you can still turn it off, if you're quick), and tens of seconds before the plate glows too many shades of red/yellow.It's harder to work with tubes, you might not be putting in as many protective features perhaps (I mean, there's no excuse with a solid state control, but this was very much the case in the olden days), but it's unlikely to explode the first time you short-circuit it.Transistors take perhaps 100s of microseconds to fail under fault conditions, so protective features must be considered in the primary design.As for Rds(on), SuperJunction MOSFETs scale proportionally -- no need to prefer lower voltage types, the only reason you need stacks here is for voltage rating. (Earlier generations had a penalty going as ~Vds^2, so high voltage types truly were much worse, and there might be reason to stack them even for fairly modest voltages.)And needless to say, tubes don't offer nearly as low Rds(on) -- it was the case as recently as a ~decade ago that high perveance e.g. sweep tubes still slightly outdid HV MOSFETs, if you ignore the heater power and screen grid requirements, the higher gm of the FET, and whether or not the body diode is a bug or feature (which basically means for switching applications, you can use the MOSFET for voltage-fed inverters, or the tube for current-fed inverters, without adding any fiddly diodes to either*).*But you need extra circuitry to prevent grid and screen conduction while the plate is reverse biased.But even with such exceptions, MOSFETs are well and truly the exceptional device now.Tim

author: marvi3
author_rank: Contributor
timestamp: June 22, 2021, 07:50:01 am
content: For example,IRFP460 has a Rds-on of .27 Ohm, so Two has .54 Ohm total Rds-on dividing power between two...FQA10N80 has Rds.on 1 Ohm so dissipating four times with respect to IRPF460...Serial or parallel connections, dissipate the some...I prefer the first, more reliable... for 250-400V 500 ma out PSU...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 22, 2021, 02:20:17 pm
content: Oh, it'll be this thread, right?https://www.eevblog.com/forum/projects/hv-power-supply-0-400v-200-ma/So linear it is.Rds(on) ismeaningless, you're dropping voltage in the linear range.Tim

author: David Hess
author_rank: Super Contributor
timestamp: June 22, 2021, 02:52:12 pm
content: Stacking transistors works fine but gets complex quickly. It is common in the highest power linear audio amplifiers. Some oscilloscope vertical CRT amplifiers do it to get enough voltage swing even at 100 MHz.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 22, 2021, 03:38:23 pm
content: There are ways to combine stacked transistors with 2 (or possibly more ) supply rails. This way the heat loss is not just shared, but at times the loss is reduced - nearly half for the worst case, so one can safe on the heat sink.

author: David Hess
author_rank: Super Contributor
timestamp: June 22, 2021, 04:22:14 pm
content: I have usually considered stacking when I either could not get a device with a high enough voltage rating, or to more effectively avoid secondary breakdown where halving the voltage has a much larger effect than halving the current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 22, 2021, 04:26:54 pm
content: Quote from: David Hess on June 22, 2021, 02:52:12 pmHave also seen stacked SCRs, back when they were all the rage and 200V devices had the shortest Iq.Tim