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 StageFor 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 CircuitI 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 problems1. 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