current limited pulsed suply - Page 1
Forum section: electronics / projects

author: Jorge.fv
author_rank: Contributor
timestamp: June 02, 2017, 10:42:37 pm
content: Hi I want to design a current limited power supply with pulsed output with the ability to limit the output current a friend asked me about this so why notbecause I want to learn more about power mosfets and at the end I may build a VFD/inverter but I have a lot to learn first.I want it to be able to generate a pulsed output with 50% duty cicle from 50hz to 1kHz but maybe I will stop at 500Hz.the input voltage is about 280VDC and the output current will be limited up to 1A the load is resistive and capacitive but variable from 150 to 250 ohmsI'm using the dspic30f4011 just because I have 4 of those that I want to use.the input voltage is 200V AC that gets rectified and filtered, I have made a first attempt using the Dave's dummy load for the current limiting part and the irf840 as a power transistor on this first try I used an R2R ladder instead of the pwm on the Dave's dummy load circuit and it worked very well, but for the pulses I just turned the analog voltage otuput on and off, I gave it to my friend and he used it for a month with just 50V AC at the input until he did a bad connection and it got burned out.second attempt was with an irfp460 and 200V AC at the input that got rectified to around 280V DC but the mosfet capacitance is to high so it wont turn off, I added as a patch a bjt to the gate to just turn it off and it worked very well. but the circuit is an absolute abobination and I'm a bit ashamed to show it but I want to learn.now that I know that the waveform generated worked for him I want to do it better with a better way to limit the current I was thinking on the way it is done on the stepper motor drivers but that is a inductive load and here I have a resistive and capacitive load.

author: Marco
author_rank: Super Contributor
timestamp: June 02, 2017, 10:59:29 pm
content: 1 kW in the shunt resistors at 1A?

author: Jorge.fv
author_rank: Contributor
timestamp: June 02, 2017, 11:03:17 pm
content: sorry I missed that, those are 10ohm resistors to get 1 ohm at the output, I forgot to change the default value

author: jbb
author_rank: Super Contributor
timestamp: June 03, 2017, 07:03:00 am
content: This is probably a reasonable architecture.Some things to consider:1) you should probably deploy several MOSFETs in parallel. Have a look into Safe Operating Area and think about what happens with a shorted output and maximum on time.2) add a thermal cutout of some kind.3) consider some analog circuitry to disable the output if your software has a glitch and tries to run the output continuously4) add a discharge resistor to your main DC cap. Otherwise it will lie in wait (I.e. remain charged for a long time) and bite you.5) be aware that the output will be live to mains potential, even when it's 'off.'

author: Jorge.fv
author_rank: Contributor
timestamp: June 03, 2017, 09:54:21 am
content: Quote from: jbb on June 03, 2017, 07:03:00 amthanks for your reply1. I didn't think that several MOSFETs would be necessary because I don't plan to go past 1A and in theory the IRFP460 would be enought but I will check into italso one of my issues is the gate capacitance, several MOSFETSs will increase it.2. I was checking the temperature all the time and also I have a thermal relay I don't know what is called but it opens when it gets to over 100 degrees C and it never tripped also tested it, I got some of those from old copy machines but it is on the plan to integrate the thermal cutout on the main micro.3. this I don't know how to do it, maybe a relay4. bleeder resistor already added to the power supply5. yep also is the firs time for me working on this levels of voltage and power.

author: Jorge.fv
author_rank: Contributor
timestamp: June 03, 2017, 02:32:05 pm
content: a quick Update at this time I don't have an oscilloscope so I'm pretty much blind but since it is a pulsed signal my clamp meter is able to get a reading of the current,I was testing the circuit with a power resitor of 100 ohm and 100W and it was nothing at the output, I checked it and the mosfet was burned its legs are in a short and also took the bjt 3904 with it I think the op amp also is burned.this circuit performed so well on the testing with the final load, I got a reasonable size heat sink on the mosfet so it does not get warm after hours of abuse.now I wold like to improve the design so it wont happen again, I guess the problem is at the mosfet gate and I need something to drive it properly maybe change the linear current regulation to PWM with a proper mosfet gate driver for testing purposes and future projects I have ordered the TC4420 but it will take some time to arrive, on the mean time I will try some arrangements.

author: Marco
author_rank: Super Contributor
timestamp: June 03, 2017, 06:21:35 pm
content: I'd throw a STE53NC50 in there and jury rig it to a CPU heatsink, worst case you're dissipating 150 Watt.PS. irfp460 datasheet doesn't even have DC in it's FSOA graph, even if it did it would be well below 1A at 300V. You have to make assumptions about the load taking voltage to do this within the datasheet limits of any single TO-220 MOSFET AFAICS.

author: Jorge.fv
author_rank: Contributor
timestamp: June 04, 2017, 03:29:28 am
content: Quote from: Marco on June 03, 2017, 06:21:35 pmThat is a really nice part a bit expensive I'm reading the datasheet at this moment I have always overlooked at that graph now I have learned something, I think I will use it on he next version when it arrives, I see that there are ebay sellers that have it used but I wont take the risk to get it used.that is why I dared to get into this because I'm learning a lot and making mistakes like crazy on the process, the downside is that I don't have much money to spare so at some times I may stop until I got enough money to continue.for now I will keep experimenting with the irfp460 because it is the only thing I have at this moment maybe getting it in parallel as I got suggested.the heatsink I'm using at the moment si rated for 200W and I have also attached a cooler fan but even on the short circuit scenario witch I have tested it barely gets a bit warm when the cooler is off, and I have also left it on short for a complete day, it worked no problems at all, until it got burned when I was trying connecting and disconnecting a resistive load for testing how it reacts on that situation and it got burned.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: June 04, 2017, 04:21:48 am
content: Just a note. Be aware that when Q2 is turned on that it will snap the inverting input of U1-b down and cause its output to go to the positive rail. This will put 10-11V across R4 and force the op-amp to supply about 100mA through R4 and Q2. This wont be so good for either opamp or '3904. It maybe better to arrange a way to switch the inverting input high through a fet or some such instead then you wont waste so much power.

author: Jorge.fv
author_rank: Contributor
timestamp: June 04, 2017, 10:25:29 am
content: Quote from: basinstreetdesign on June 04, 2017, 04:21:48 amI'm looking on a better way to do it, but the pic only turns Q2 on when there is 0V at the output of the R2R ladder, because the mosfet was not turning off maybe because of the gate capacitance it worked to test the waveform output on to the final load.

author: Marco
author_rank: Super Contributor
timestamp: June 04, 2017, 09:43:56 pm
content: Do you have a small PNP BJT? You can just forget about the Q2 setup and make your own buffer to drive the MOSFET gate with a totem pole, NPN on top, PNP on the bottom.

author: jbb
author_rank: Super Contributor
timestamp: June 05, 2017, 05:52:01 am
content: Hi JorgeSorry your last unit blew up. I expect the opamp died along with the MOSFET. FYI, MOSFETs almost always fail short circuit, and blow up their gate drivers. If you have enough power available, the current flow through the shorted MOSFET can literally blow the wires off the silicone, and make the MOSFET go open circuit.I forgot to say last time: your emergency-off switch shouldnottrust the MOSFET, as it's the most likely thing to fail and will likely fail short-circuited. Additionally, a mechanical relay may not cut it, because it's hard to break DC current above about 24V DC.Here are my guesses for why your test circuit blew up:MOSFET got too hot slowly due to insufficient heat sinkingorpoor attachment to the heatsinkMOSFET got too hotfastdue to high peak dissipation, but average dissipation was low enough that the heatsink didn't get too hot. This would be caused by violating the Safe Operating Area.MOSFET experienced big temperature cycles due to being turned on and off. Temperature cycles fatigue the connections to the MOSFET silicon until something breaks, and then it blows up.You'll note a common theme. There are 2 angles of attack to help the MOSFET last longer:Get a MOSFET with a lower thermal resistance Rth to the heatsink. The STE53NC50 would be a good candidate, but is sadly expensive due to the use of an ISOTOP package (the package itself is expensive). TO-247 packages combine cheapness, good gap between the pins and OK thermal resistance. Often going to a higher voltage (e.g. 500 - 1200V) rating will get you a larger silicon chip and thus lower thermal resistance.Parallel MOSFETs to spread the stress. I've attached an indicative diagram (values are likely to be wrong!) for three MOSFETs Q1-Q3. Each MOSFET has it's own small gate resistor R1-R3 to prevent oscillations between the gates. Each MOSFET has it's own current sense resistor R4-R6 to both measure current and help with current sharing. The current sense voltages are averaged by R7-R9. R10 provides most of the gate resistance, and C1 reduces the high frequency gain to (hopefully!) damp any ringing. All component values are subject to change.On MOSFET gate driving:I think you'll need to get access to an oscilloscope somehow. You'll have to tune the control loop at some point and you can't do that if you can't see it.In general power MOSFETs are used as on/off switches. This is achieved by slamming lots of current into/out of the gate (like 500mA and more) using a great big high current driver (either a totem pole or a dedicated driver chip (also a totem pole (yay recursive brackets))). This charges/discharges the stray capacitances quickly.However, you want the MOSFET to operate in the linear region; partly off and partly on. This allows the MOSFET to act as a controlled current sink, and means that you're more interested in handling the gate precisely, rather than trying to throw it around.Most controlled current sink designs use an opamp just like you did. I'd stick with that concept.

author: Jorge.fv
author_rank: Contributor
timestamp: June 05, 2017, 08:21:09 am
content: thanks for your suggestions, I had to sell my scope along with some things I seriously needed the money, but I took this as an opportunity to improve an get a better one than the one I used to have, I already bought a provisional one that still is on its way so I hope I will be able to te get to see the waveform soon.I don't know if I can buffer the opamp but I found an example maybe it will worki got the BD139/140 pair for that purpose

author: Marco
author_rank: Super Contributor
timestamp: June 05, 2017, 12:31:40 pm
content: I was wrong by the way, the IRFP460 should be able to handle the average 0.5A with good cooling. Since MOSFET's are simply thermally limited near DC and it can handle more than 150W.You will need to use a heatpipe CPU cooler though to have hope of getting near that cheaply with a single MOSFET. Or if you only have a non heatpipe cooler lying around, two MOSFETs.

author: Jorge.fv
author_rank: Contributor
timestamp: June 05, 2017, 01:33:15 pm
content: I will go to the two mosfet configuration until I could get something betternow I'm trying to find a better way to drive it at the gate, maybe the totem pole transistor after the opamp will be enough, but now I'm experimenting with pwm instead of the linear configuration.

author: Marco
author_rank: Super Contributor
timestamp: June 05, 2017, 01:45:28 pm
content: You'll still want a beefy cooler regardless ...

author: Jorge.fv
author_rank: Contributor
timestamp: June 05, 2017, 09:48:24 pm
content: I'm getting a better cooler, also adding the bjt buffers to the output of the opamp