Electronics > Projects, Designs, and Technical Stuff

400V/10A-100W mini dynamic electronic load

(1/5) > >>

ElectronSurf:
I bought an old AMD CPU heat-sink years ago in the hope of designing a small pure analog electronic load around it for draining batteries etc.
I've designed several electronic loads in the past (up to 400W) but each had their own disadvantages and problems, this time I'm going to do it right hopefully with the help of you guys.

I have several old K2372 MOSFETs laying around, it's adequate for this job and I'm not going to pay $ for IXYS linear MOSFETs!

The main problem was differential current reading, I used to use single ended current reading but the wire length adds resistance to the shunt value and cause error in the readings.

Then I tried differential current reading and fed the output of that op amp to the error amplifier, it was accurate but a bit slow. after some research I found an article showing how to drive a MOSFET with a single differential amplifier, I'm going to use that design as a base.

The other problem was when there's no input voltage connected, op amp output clips to positive rail and making the MOSFET fully ON, causing a huge current surge when input voltage applied.

To fix that I used another op amp to sense input voltage and if it's not connected, pull the voltage reference to negative supply so the error amplifier (now differential amplifier) clips to negative rail thus making the MOSFET fully OFF.

The question is; can I replace the extra op amp with some discrete circuitry? if so how?

There are a few more problems but we'll discuss them as design progress.

Link to the simulator if you wanted to play with it.

Kleinstein:
One can replace the OP-amp for the voltage detection with a discrete circuit, but it would not be very simple if the threshold is supposed to be small. An OP-amp is the proper solution, but it would still need something like a diode.

A suitable idea would be to add some kind of automatic cross over between 2 modes, a bit like with a lab supply:
If the voltage is high enough, the load would operate in constant current mode.
If the voltage is small the load would work in a kind of constant resistance mode - so clamp the set voltage (usually before a divider) of the current regulator to a value proportional to the voltage.
The simulated resistance has to be larger than the shunt and MOSFET R_on combined.  A little offset can turn of the load all the way if the voltage is very low (e.g. < 10 or 100 mV).
So the current circuit is missing some kind of diode for clamping. As shown it does some linear combination, so a mix in between constant current and constant resistance, but not switch over.

I would consider 400 V and 100 W a bit optimistic for the MOSFET. The FET type is pretty old and may thus handle power dissipation reasonable well.  As there is a thermal transient curve in the datasheet, I would be a bit sceptical about the SOA curve. It is not sure that the cuves are real or just calculated from the transient termal impedance.
As the gate capacitance is already relatively large, it would be more separate channels with a seprate OP-amp per FET if more power is needed.

ElectronSurf:

--- Quote from: Kleinstein on March 10, 2023, 10:31:38 pm ---One can replace the OP-amp for the voltage detection with a discrete circuit, but it would not be very simple if the threshold is supposed to be small. An OP-amp is the proper solution, but it would still need something like a diode.

A suitable idea would be to add some kind of automatic cross over between 2 modes, a bit like with a lab supply:
If the voltage is high enough, the load would operate in constant current mode.
If the voltage is small the load would work in a kind of constant resistance mode - so clamp the set voltage (usually before a divider) of the current regulator to a value proportional to the voltage.
The simulated resistance has to be larger than the shunt and MOSFET R_on combined.  A little offset can turn of the load all the way if the voltage is very low (e.g. < 10 or 100 mV).
So the current circuit is missing some kind of diode for clamping. As shown it does some linear combination, so a mix in between constant current and constant resistance, but not switch over.

I would consider 400 V and 100 W a bit optimistic for the MOSFET. The FET type is pretty old and may thus handle power dissipation reasonable well.  As there is a thermal transient curve in the datasheet, I would be a bit sceptical about the SOA curve. It is not sure that the cuves are real or just calculated from the transient termal impedance.
As the gate capacitance is already relatively large, it would be more separate channels with a seprate OP-amp per FET if more power is needed.

--- End quote ---

If it's going to over complicate the circuit, as you said it's better to keep the op amp operate there. Thank you.

ElectronSurf:
Just finished the schematic, used TL431 as voltage reference even though I had better options at hand like LM4040. I think it's good enough for this little project.

The range switch, switches from ~22A to ~2.2A. it then goes to a multi-turn potentiometer and buffered to set the current.

U1D "sense" the input voltage and if there's nothing connected it will pull the MOSFET gate to negative voltage.
U1C is a simple relaxation oscillator with 50% duty cycle and frequency can be set from ~120Hz to ~12Hz, I had this idea of adding variable duty cycle but then ditched it.
U1B is overheat protection, the limit is going to be 70℃.

The shunt have very low value (10mΩ) and only precision op amp that I have in hand is OP07, I'm not sure if the low slew rate can cause a problem in dynamic behavior.

Kleinstein:
The capacitor C6 is likely a bit on the small side. I would expect more like 1-10 nF. It may be a good idea to run a simulation to get the right size.
There is no need for R6, the voltage is small enough already.

The voltage set by R8 and R9 is too small - the TL074 can have more offset. One would need a better amplifier or just set the voltage limit higher.
The buffer U1A is not really needed and with it's offset and drive probably doing more harm than good.

The LEDs usually have a quite limited maxium reverse voltage. So one should somehow limit the maximum output voltage of the OP-amps, e.g. with an extra feedback path via diode and series resistor for the set voltage. Or maybe just with a lower supply voltage. U1D is the only part that may want a high voltage so that not that much of a divider is needed.

The shunt for the current is rather small and the MOSFET would not really work well at very high current.

p.s. The MOSFET seems to be an old type, which is good in the sense of a good chance to have an OK SOA. However the R_on is relatively large (0.2 ohm typ) by modern standards. So there is little sense to use it with more than some 5 A, maybe 10 A at the very most. The gate capacitance is already quite large. Not sure how well the OP07 can handle this. Additional FETs would need it's own driver / regulator.

For stablility one would likely need an RC snubber at the output, at least as an option.

Navigation

[0] Message Index

[#] Next page