170V adjustable HV regulator - Page 1
Forum section: electronics / projects

author: LukeW
author_rank: Frequent Contributor
timestamp: July 29, 2020, 03:39:40 pm
content: The attached schematic is a 20-year-old in-house design in a physics research application, which is used for pulsed gas injection using a specialised solenoid valve at a fairly high voltage.It's a bit old and rough but hopefully I can rebuild this apparatus and get it working.The schematic is pretty messy, but there are basically two parts to it - a low-side switch which includes a monostable 555 to provide the variable-duration edge-triggered gas pulse width (say 100us). I don't really care about this part - it is working fine.And then there's the variable-voltage power supply, which is used to supply a fairly high voltage to a capacitor, have this voltage adjustable with a pot, and maintain this voltage amplitude reasonably well during the width of the pulse. This regulator, which ideally should be pot-adjustable up to 170V or so, is not working - I can't control it.Because the design is ancient, I've made a few changes. Also because of the parts I have.Changed the main transformer to one with a 115VAC secondary, which will limit the output voltage to 170V or so, which is fine.Changed Q2 and Q11, both to FJP13009.Replaced C2, C3 and C4 with a single 56uF 400V capacitor.Replaced R43-R46 with a single 47k 5W resistor.Changed Q7 to a BC559.IC1 replaced with a NE5534 opamp.Removed the opamp compensation capacitor C1.Removed R18 from the input-trigger circuit.VR2 changed to a 10k pot since I didn't have 5k.R38 changed to a 0.33 ohm pulse-withstanding 5W resistor.Replaced Q1, Q8 and Q9 with 2N3904.Replaced R32-R35 with a single 2.4 ohm 3W resistor.Three FETs (Q4-Q6) changed to FQP3P50.Q3 changed to TIP32.The three MOSFETs and two high-voltage BJTs are mounted on heatsinks with mica insulators.The triggering works, the monostable adjustable-width circuit works, there is current passing through my dummy load resistor with the expected pulse duration.The +170V rectified supply is working, the 5V and 12V and -6V (or so) rail is working.However, the voltage adjustment stage doesn't seem to be working.The voltage adjustment with the pot, into the noninverting input, does vary when adjusted.Actually, I should increase R5 and make it about 27k, so that the pot adjustment voltage range into the opamp is about 0-3.24V.Any ideas or suggestions how to fix this?

author: magic
author_rank: Super Contributor
timestamp: July 29, 2020, 04:23:23 pm
content: A few thoughts:The 5534 is a fair bit faster opamp, it might be oscillating without compensation, so check it. Luckily, it can be externally compensated if necessary.Mind the common mode input range of the 5534, it doesn't work down to ground.And really, what does "doesn't seem to be working" actually mean? What is it doing instead?What are the voltages at the inputs and outputs of IC1?edit5534 should have 100nF between the supplies close to the chip.Maybe just buy a 301A? I think they still make them.

author: duak
author_rank: Super Contributor
timestamp: July 29, 2020, 04:59:08 pm
content: I think @magic is right, IC1 is oscillating. According to the Onsemi data sheethttps://www.onsemi.com/pub/Collateral/NE5534-D.PDFpage 1, para 2, the device is "compensated internally for a gain equal to or greater than three". Substituting a 10 MHz GBW op-amp for a 1 MHz unit can be expected to cause a bit of trouble.I think you will have to add at least 22p compensation capacitance (C1). The voltage gain and frequency response of the output stage will also come into play so I'd expect some more fiddling with C1 & C27 and perhaps adding another capacitor between IC1 pins 2 & 6.Cheers,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 29, 2020, 08:39:59 pm
content: Weird, IC2:C-D does nothing? Yikes, C9 is a destructive spark waiting to happen!Haha, British style has been used on the resistors ("2k7") but nothing else (".1" -- the dreaded decimal, instead of "0u1"; "12.5VAC" instead of "12V5"). Except R38, heck 'em I guess?4-way crossings all over, too, and it looks like it even hit them (assuming IC2 has a mistake)!Poor regulator (7812) having to witness ~4A load pulses. Guess that's what the C13 is there for. Poor regulator, having to charge a cap that size...C10+R40 doesn't do much if anything, because D4 shorts out the solenoid during turn-off. Which makes it turn off as slow as possible. Hmm. Do they only want sharp rising-edge gas pulses? (Is it dumping from a small accumulator, instead of a gas regulator?)They've got a tonne of capacitance on the hot side (C11), but don't use it -- the regulated output voltage supplies charging resistors that supply relatively small capacitors that supply the solenoid. (Which, if C2-C4 are well discharged by the solenoid, D4 could at least return to +400V instead, but they chose not to for some reason.)As for the regulator, the compensation is difficult because it's a circuitous loop: IC1 is... hrm, not just followed by Q1, is that done for range limiting as well? Then Q2 inverts it, then Q4-Q6-Q5 (what, they're out of order, haha, but R11-R13 are in order?) inverts itagain-- the problem is each inverting stage adds considerable phase shift near cutoff. And with no static load, the outputs' cutoff varies with operating point, making it just that much harder. Q2 seems to have the dominant pole compensation (R42 + C15), a pretty substantial impedance.With no feedback path local to IC1, yeah, it's very likely to oscillate, in fact guaranteed to at some point, as you increase GBW. It would seem IC1 has to be slower than the output transistors at least.I would redesign the output side to do a couple things:- We already have a switch in here; why not use it for double duty?- We can get rid of the pulse capacitors if we make the switch rated for peak current.- All the low side stuff is easily done from one supply (5V say) and at a fraction of the current; a small offline DC supply will do (a 1W switching module say).- The 240V isolation transformer is probably a good idea to leave in. It's dumb and easy, it's not terrifically expensive, and SMPS with HV outputs tend to be specialty. And if we need the energy storage anyway, the filter cap is fine.The pulsed regulator would then be much like shown, but rearranged for low side operation (driving the equivalent of Q11). VR1's top end would be pulsed, so that the regulator setpoint sits at 0V for most of the time but can be triggered, and then goes to the setpoint.IC1 might be a TLV2371, or maybe something a bit faster. With everything running from 5V, we have free choice of fast, low power, and cheap amps.A differential amplifier would probably be used, to sense the load voltage, which is now high-side referenced. Make that TLV2372 (dual).We don't have to worry about two inverting stages cascaded, only the one (Q11 equivalent). This is easier to solve, and would probably take the same form, i.e. an R+C across drain to gate. The impedance would be larger (like 1k + 100pF for a ballpark guess?), allowing faster performance. Probably an emitter follower, or IC buffer, would still be needed to get the op-amp level up enough to drive the output transistor.Transistor choice would be SuperJunction type. These are fast and affordable, the dominant high voltage type on the market today. They offer wide SOA, despite their high power densities. FQA9N90C-F109 is a good example, having a high power dissipation-per-dollar value.Ca. 10V would be desirable for gate drive, so I might be a bit hasty in declaring 5V the new standard. We could use a 12V supply and CD4000 logic -- it's older than this circuit I guarantee, but still around and still just as useful. If a 5V logic input is required, we could still add a transistor or something to hack it in (a discrete TTL input stage, basically), or a comparator for even better accuracy.Otherwise, 5V would be just usable with a logic-level MOSFET, but these are rarely found in high voltage ratings, and may not have wide SOA ratings either.Finally, the snubber can still be a clamp diode, or if greater turn-off speed is desired, a diode into a TVS. UF5408 and 1.5KE200A for example. Or SMT equivalents.An R+C may still be desirable across the output, but more for compensation than damping -- having a load impedance for the regulator to switch into,Overall, the regulator should be able to turn on in a few 10s of microseconds, which seems comparable to what's shown here. Faster components can run it hotter, but I would be a bit leery of having to tune something for single microseconds or less (I'd at least want to budget some extra project time to do that).And since we're doing a real time setpoint, if the peak-maintained signal (from the discharge caps and resistors) is required, we can simply drive VR1 with a waveform of the same shape. Which can be created in much the same way (RC networks), using the switches that drive it.To wrap up a few things: a digital logic isolator can be used to communicate the trigger signal, if isolation is desirable; at worst, this can be either a self-powered integrated isolator (e.g. ADUMxxxx), or an optoisolator (e.g. SFH6345) with a small DC-DC converter to bias the isolated side. Assuming the open-collector trigger capability is a requirement. Fusing is already shown on the primary side, but additional fusing may be a good idea to the output; this must be DC rated (500VDC is a typical rating), and should have a small I^2t rating so that the filter cap doesn't utterly obliterate the victim (if the power transistor happens to be it, this won't save it by any means; it's more of a damage-control measure, so its plastic case doesn't become a plasma-driven shrapnel grenade). If using an SMPS module for power, take care to filter it, potentially both input and output; they can be rather noisy, even the ones with onboard filtering (the smallest and cheapest modules don't even have that!).Tim

author: LukeW
author_rank: Frequent Contributor
timestamp: July 30, 2020, 03:27:51 am
content: - Removed C9 because it's a bit risky.- Put C1 back in (33pF).- Measured IC1 pin 7, about 12V.- Measured IC1 pin 4, about -3.5V, not very smooth, with about 1.5v peak-peak ripple.- Measured IC1 pin 3, about 0-6v with pot adjustment linearly, as expected.- Changed IC1 back to a LM301A.Measuring the voltage monitor output (after the 470k series resistor) with a scope probe gives a fairly constant voltage across the pot adjustment range, from 170 to 176V.It does not adjust linearly, but the regulator stage (and the opamp) is abruptly stepping up, about halfway along the pot range.- The DC HV rail is measured, 175V.- Inverting input to the opamp doesn't vary much, about 3.6 - 3.44V across the pot adjustment range.- Output from the opamp is about -2V (quite ripply, because the negative supply pin is ripply) and then once the adjustment pot is wound up about halfway the opamp abruptly switches high and we're getting a 11.5V output.

author: duak
author_rank: Super Contributor
timestamp: July 30, 2020, 06:57:42 pm
content: Whe the op-amp is in the linear operating range, its '-' input should track the '+' input with a few millivolts. When the ouput (pin 6) goes to maximum, the op-amp has lost control and is no longer in its linear or closed loop region. This circuit should be able to regulate to within a few volts of the raw high voltage DC level. As you surmized, If you want to make VR1 less sensitive, R5 will have to be increased.It took me a bit to figure out the driver and pass transistor circuit. It's kind of unusual and @Tim has pointed out a number of its characteristics. The pass section strikes me as sort of a current mirror as it has a high output impedance and the overall voltage regulation is effected by negative feedback to the op-amp. I'll bet the voltage on the op-amp output and the drains of Q4-Q6 go to maximum for a few microseconds when the solenoid is de-energized. Could this be what you are seeing?An NE5534 draws at least twice as much supply current as an LM301A. If you see 1.5 Vp-p now, it was probably worse before. Increasing the value of C5 will reduce the amount of ripple on the -3.5V supply. I'd consider 470 uF or more.

author: magic
author_rank: Super Contributor
timestamp: July 30, 2020, 08:46:21 pm
content: The purpose of R2 absolutely eludes me. It just sinks some 0.5mA from the chip's output, what for?The network around Q2 looks not like dominant pole compensation, but some pole-zero thing. Maybe to limit HF gain of that stage and reduce loop gain? Maybe to push its inevitable parasitic Miller pole higher?I think a simple cascode over the opamp would be more straightforward and less parts, but if it ain't broke

author: LukeW
author_rank: Frequent Contributor
timestamp: August 03, 2020, 11:38:24 am
content: I hopefully don't want to go to too much effort totally redesigning the thing, I'd like to get it working with as much of the existing hardware as possible.The opamp negative feedback loop obviously isn't locking at all, as duak mentioned.Maybe the OA is stuffed - I don't have any other LM301A ICs at the moment, I only scrounged up one, but I could go back to the 5534?Is the change to Q2 (FJP13009), Q4-Q6 (FQP3P50) and Q1 (2N304) likely to make any significant difference to the phase characteristics of the feedback loop?Tried increasing the compensation capacitor C1 to 39pF. I also added another 39pF capacitor as a direct negative feedback path from the output.Neither of these things have made any difference in terms of getting the opamp feedback loop (and hence the overall voltage regulator stage) working.I don't really have any more ideas.

author: magic
author_rank: Super Contributor
timestamp: August 03, 2020, 02:05:14 pm
content: Quote from: LukeW on July 30, 2020, 03:27:51 amI'm not sure if everything is right here.R8 and R9 are effectively about 4.3kΩ and R7 is 220kΩ. So you should see 3.3~3.4V at pin 2.If the opamp is saturated to the negative rail, it means that feedback voltage is higher than pot preset and the opamp is trying to turn off Q2 and hence turn off Q4-Q6.If the output is still at 170V, something isn't quite right, and not with IC1 but with the output stage.