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Current sink/DC Load control circuit

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SebastianH:
Hey all,

I'm working on a DC load (150V; 0-10A; wattage still tbd, somewhere between 60W and 200W, should be scaleable relatively easy).

Power Stage
For the Power stage I'd like to go with a circuit that is very similar to what can be found in commercial DC loads: a couple of IRFP250N, each controlled individually by a reasonably fast opamp (e. g. LM7322, NE5532) using balancing resistors.

--> MOSFET_control.png

This circuit by itself works respectably well in this simple simulation
+ Good step response for changes of the setpoint
+ Good step response for changes of the load
- Poor DC performance (obviously), unusably bad at very low currents
- With no load connected U1 saturates, which is a huge problem when (re-)connecting a load while the output is on
- High dV/dt at the input terminals can be a problem (capacitive voltage divider of Cgs and Cgd), especially in rather theoretical scenarios (but still). D1 certainly does help, but the current flow through D1 into the opamp can get quite substantial. Limiting the output voltage to 10V via an appropriate feedback network would help a lot, especially in combination with a zener diode between gate and source. That might also be overkill for practical use cases.

As a side note: I saw some people connecting the negative input of the MOSFET-opamp to GND or +15V respectively via a (sometimes rather large) resistor in similar applications. Connecting it to GND would change the closed loop gain (ok, if you need that). From what I understand, connecting it to +15V would result in a certain offset that the outer current controller would compensate for in normal operation. This would (for example) allow the “upstream” control circuit to keep the MOSFET off, even if the opamp has a positive offset and the output of the "upstream" control is limited to positive voltages. Is there anything more to it?

Control Circuit
I address the DC problem with a fairly simple current control loop with a few precision opamps to control the total current of all MOSFETs.

--> Current_Control.png

As soon as the controller is in a valid operating point, it works nicely (simulation):
+ Good step response for changes of the setpoint
+ Good step response for changes of the load
+ Very decent DC performance (the simulation results at least would be good enough for me)

Two main problems
1. The aforementioned saturation with no load connected.
One idea would be to implement a second controller and limit how far the voltage across the input may drop. And then connect both controllers like so:

--> TwoControllers.png

I’m not sure whether this actually works. The “min voltage control” would have to transition to current control slow enough for the current controller to reduce its output accordingly.
So I’d like to ask for advice regarding:
•   Whether this is a reasonable idea
•   Whether there are other ways to implement this

2.   Generally a massive overshoot for any current setpoint step starting at 0A („off“). The reason is quite obvious: The opamp drives all the gate to the negative rail (without D2) or one forward voltage drop below GND (with D2) to achieve 0A at the output. And right now I have no idea how to design a controller that can recover from the „off“ state reliably and quickly without a ton of overshot, especially for rather small currents – the transconductance curve of the circuit is extremely steep in this region.
Any suggestions are appreciated. Btw: If that is possible I would like to keep the inner control loop relative close to the one shown above.
Furthermore: If someone has access to a professional DC load: I would really be interested in the step response, both the transition from output „off“ to „on“ with the load connected, as well as with the output „on“ and a load being connected.

Thanks,
Sebastian


Btw: I'm too stupid to insert a link the attached images  :palm:

MarkF:
I had some discussions on the subject of saturation in NO-Load situations, but never spent the time to add a whole other level of complexity to control it.  I took a minimal approach since my PCB was designed and built.  What I did was add a resistor (shown in red) such that it added enough current across the sense resistor, when the control was set to have minimal current, where the op-amp would turn off the MOSFET.  Not pretty but works in one very specific use case.   I can therefore turn the desired current to Off/Minimal and the MOSFET OFF when no load connected.  The circuit will saturate if you even slightly turn up the desired current point.

MarkF:
My process is:
  - Turn the current select to 'off'
  - Connect the load
  - Turn the current up to the desired level

No saturation, no over-shoot.


Note:
  My selection of resistors to the MOSFET gate and op-amp feedback loop was 're-use' of values already in the circuit.
Better selections would be 100 Ω to MOSFET gate and 2.2K Ω in the feedback loop.  The high values I used also protect the op-amp if the MOSFET fails.  Because the MOSFET normally fails shorted and puts high voltage onto the op-amp.  The higher resistor values limit the current and may possibly save the op-amp.  ( :-// Haven't killed yet to find out.)

SebastianH:
Thanks for your quick reply  :)

Do you use the "outer" current control loop?

Since I plan to have multiple MOSFETs in parallel I want to use a common current shunt, which -in turn- means that I have to implement the current control loop.
And then the trouble starts  >:(

The resistor could help without the control loop, but: The outer control loop will sense a current smaller than the setpoint and then the opamp will still go into saturation.
Without the outer loop I don't see any overshoot either, but at the same time it seems to be virtually impossible to set the current to precisely 1mA with a 12 to 16 bit DAC, especially with mulitple FETs in parallel. On the other hand, the outer control loop manages that very easily, as soon as it is in a normal operating point.

Using a larger Gate resistor could help a bit, but at the same time would make the current control much more slowly and more prone to MOSFET self turn on. Maybe 100 Ohms would be ok for this particular FET regarding self-turn on, but the transient response would be certainly much worse, because a dV/dt between Drain and Source would mean an increased effect on the Gate voltage.

So I'd much prefer a solution with a rather low Gate resistance/as low as I can get without instability issues.

MarkF:

--- Quote from: SebastianH on June 10, 2021, 11:55:09 pm ---Thanks for your quick reply  :)

Do you use the "outer" current control loop?

Since I plan to have multiple MOSFETs in parallel I want to use a common current shunt, which -in turn- means that I have to implement the current control loop.
And then the trouble starts  >:(

--- End quote ---

I only have the one MOSFET.  My load is limited to 2.5A @ 25V.

I would have multiple sense resistors.
I would have each MOSFET output stage be its own independent group (i.e. op-amp, MOSFET, sense resistor).
Basically, each circuit I posted duplicated however many times needed to have the desired total load.

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