# Electrical – OP amp PI controlled electrical load

I am building an electrical load, capable of "sinking" 0 – 3A on a maximum of 60VDC. I want to control the load with a microcontrollers ADC (0-3V). On the internet you find hundereds of circuit propositions, that build around bipolar transistors or FET. I chose a design with a NMOS FET as actual sink.
The circuits principally work – but a FET is a capacitive load. So the circuit I built (following an article on design ideas (google "Power-MOSFET-is-core-of-regulated-dc-electronic-load") is oscillating.
I need my design to be as failsafe as possible. Merging all information I have I ended with this around a PowerFET and a PI control loop.
I started by a simulation with TINA-TI. But I have some doubts about the results: • Why do I have this huge Peak in the beginning of a transient analysis (Zero initial values)?
• The current sense IC INA168 should provide a linear output tension – why do I have 72mA flowing after a step response of 500mV, 3.4A after 3V and nothing useful after 50mV?
• Is it normal that the OP Amps are staying at -VCC with 0V input? shouldnt the have 0V at the output aswell?
• Does this transient analyses make sense at all? Which other analysis should I perform?

Unfortunately stackexchange doesn't let me post more than 2 links, so I cannot post the other simulation results.

So the first thing is to decide on an accuracy on the current that one can live with, as this sets other parameters. So lets say 0.03A, or 1% of full scale.

Next, let's define the process we want to control as the circuit from Vgate(Vfet in the schematic) through U4-output. Lets call that Vout. Checking the fet datasheet, the gain given is 13 A/V, although perhaps less depending on actual bias point in the application. Sense resistor 0.01ohm. INA168 setup for 75V/V gain. U4 unity buffer. Total process DC gain 13*0.01*75*1 = roughly 10 V/V.
And the error at Vout will be 22.5mV for a 30 mA current error.

The input at Vgate will be somewhere in the 2-10 V range, say nominally 5 V. Vgate-nominal / Vout-errorlimit roughly equal to total open-loop gain for appreciable gain >> 1. So minimum open loop gain required about 5V/.0225V = 222 V/V. The "process" provides a gain of about 10 V/V above, so control loop gain minimum about 22.

The control circuit above (from Vout, aka U4-output) to Vgate has 4.7 V/V gain at frequencies above 3.4kHz, and unlimited gain at DC, well limited only by the internal gain of U7, which could be 1000V/V or more. That may be super accurate in terms of output error, sort of, but can be challenging for stability. If one is using 1% resistors, then there is already 1% error at R2 and one wonders why one would need more than that from the accompanying control circuit.

Right off the top you could remove R12 and change R8 for a 470k - and probably get ok results. The open loop gain in the control circuit would be 47, which meets our minimum of 22. And taking out the very high gain from U7 might settle the circuit down enough to be stable right there.

If you had to put more DC gain in, you could put a resistor in parallel with C1 to limit the DC gain to something reasonable like 100, eg a 1000k in parallel with C1. But you may not need it if the 470k for R8 above got you acceptable results.

In terms of the accuracy you need: this circuit is driven by a micro+DAC. Ok, perhaps a calibration could be stored in the micro to correct a small %age error in the analog circuit. You might be able to live with more than 1% FS for the error in such case because the micro can correct for that.

In terms of the number of opamps, it is possible to provide the function needed with less micros. From the circuit above I would retain the unity buffer U4. A similar unity buffer is perhaps required on the DAC output. A 20k input impedance may be too much for the DAC output. A third opamp can produce Vgate from the 2 buffered signals. But unless you are tight for cost or board area then the circuit above is ok.

In terms of the simulation, you can put a AC frequency sweep voltage source IN SERIES with R9. Then look at the signal out U5-output that results from the series AC voltage source. Check the gain and phase margin through the process and control loop combined to ensure a robust, stable control.