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| Example of why people say you sholdn't use MOSFETs in parrallel as dummy load |
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| pqass:
--- Quote from: MrYakimovYA on June 20, 2022, 12:02:18 am ---Hi! If all MOSFETs are intended for linear mode you should use opamp with negative feedback (and current shunt) to limit current through every transistor. See the circuit diagram any Agilent Electronic Load. There are plenty of information there. In switch (not linear) mode, AFAIK, there is no need neither opamp circuits nor equalizing resistors. --- End quote --- That's what I'm doing with my modular load (see attached). Just stack as many modules as you need such that each is limited to at most 50W or 5A. I plan to mount each on an aluminum plate with standoffs and exhaust fans on an end. I'm sure I can squeeze the opamp and passives to a in2 in my next iteration. Cable management is easy as you just parallel the spade connectors and P1 pins except for shunt voltage/current readback (pin 3) which is individually connected to an ADC mux input; all pin 4s are fed from a single DAC output. My shunt is made from 3 twisted strands of 24AWG nichrome wire giving 0R1. The MOSFET is an IRFP150M. The opamp is a shitty 4558 (a dual 741) that I salvaged as I didn't have another SOIC on hand. I gave the module a bipolar supply in case a non rail-to-rail opamp is used. |
| MrYakimovYA:
--- Quote from: pqass on June 20, 2022, 09:46:14 pm ---That's what I'm doing with my modular load (see attached). --- End quote --- Nice design! Could I ask you: what about stability of amplifier? Did you simulate the circuit in any SPICE software like LTSpice, Micro Cap and son on? Isn't 10 microfarad capacitor too big in the feedback loop? The circuit could be stable but the bandwidth could be limited to hundreds hertz. So the step response could be a bit slow :) AFAIK, any electronic load should be as fast as possible in comparison to any DUT (device under test). |
| pqass:
--- Quote from: MrYakimovYA on June 21, 2022, 12:04:32 am --- --- Quote from: pqass on June 20, 2022, 09:46:14 pm ---That's what I'm doing with my modular load (see attached). --- End quote --- Nice design! Could I ask you: what about stability of amplifier? Did you simulate the circuit in any SPICE software like LTSpice, Micro Cap and son on? Isn't 10 microfarad capacitor too big in the feedback loop? The circuit could be stable but the bandwidth could be limited to hundreds hertz. So the step response could be a bit slow :) AFAIK, any electronic load should be as fast as possible in comparison to any DUT (device under test). --- End quote --- My goal was to make a generic (ie. common footprints), repeatable design block; keeping the feedback loop short. Given that there can be many MOSFETs and they each need their own personal space, I thought it best to put the long lines (back to the MCU) on the current set and readback signals and not on long gate leads to a close collection of opamps. I haven't done any software simulation although I'd like to get familiar with LTSpice eventually (I think it works under Wine for Linux). The cap is probably overkill but necessary (due to past experience with oscillations on another load implementation). This load is rock-solid at 5A continuous (as seen on my 'scope). Out of curiosity, I connected this load to my XANTREX XT7-6 lab power supply (set to 5V, 6A) via 3ft/1m 18AWG wires. See attached for various waveforms; YELLOW is from my wavegen into the +Vin of the opamp (P1, pin 4) and GREEN is the shunt voltage/current readback (P1, pin 3) signal. The first screenshot is a 570mV, 2Khz, square wave signal. The second is the same but at 20%; pulsewidth is 100us wide. The third is the same as the second but zoomed in. A continuous 570mV produces 5.00A on the display of my PS. The 50% signal shows 2.50A, and the 100us pulse shows 1.00A. As you can see the GREEN is slow to ramp up. Also, that negative edge spike! It could be my PS. I'll try another to see if it's still there. I can always substitute a better opamp, smaller cap/resistor, another MOSFET. Now I have something I can experiment with. On the whole, I'm happy with this eload so far. P.S. The other day, I substituted a 10R shunt instead. With a the simple analog pot control (see attachment in my last message), the eload was now capable of 0 to 120mA with 1 mA resolution. I used it to test a salvaged DVD blue laser. It was very cool seeing the light output ramp up at 30mA to 45mA (where I stopped). |
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