MOSFET Current Source Circuit Questions

Thread Starter

odm4286

Joined Sep 20, 2009
241
Hello everyone. I'm looking for some help understanding a few things about using a N-Channel MOSFET as a voltage-controlled current source. I spent some time playing around in PSPICE today with the following circuit. Note: This is not the actual PSPICE version but it is effectively the same circuit.
Circuit.PNG

According to the simulator I get my desired results. 2V on the non-inverting terminal of U1 from the µC DAC gives me 2A through my load. Thanks to the negative feedback I believe, the relationship between µC DAC Vout and Id is mostly linear . I have a few questions I want to ask to make sure I resolve any potential underlying misconceptions.

1. The biggest question I have is whether or not it is OK to drive an inductive load with an opamp. I've read in the past a capacitive load can cause problems but I've never heard anything about an inductive load. Ultimately, I'd like to use this circuit to drive a small DC motor.

2. My Estop switch pulls the MOSFET gate low turning the MOSFET off. This causes the Opamp to go to the positive rail, 24V, in an attempt to balance both input terminals. While I don't see any real issue with this I don't believe it is ideal. One idea I have is to detect a voltage across R4 using the µC. When the firmware enters the interrupt routine it will set the DAC output to 0. I think this will work, but I'd rather handle this in hardware as well and cannot think of a decent way to do it.

3. Is there any issue with varrying Vds while keeping Vgs steady? I'm used to dealing with BJTs and this seems strange to me even though it appears to work.

4. Lastly, I could really use some help understanding the output characteristics graph from the datasheet. I understand the relationship between Vds and Id. What confuses me are voltages next to each curve. Are these designated curves for different Vgs values? If so, why is the Vgs = 10 value tucked away in the corner?
charateristic output.PNG
Source: STW4N150 Datasheet

Hopefully, I was clear in my questions. I'm open to any suggestions/criticisms, thank you all in advance.
 

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ronsimpson

Joined Oct 7, 2019
168
4. Left side of graph is Drain current.
Bottom is Drain to Source voltage.
If you put a 5 ohm resistor from D-S you will get a line starting at 0,0 and going upward 1A,5V and 2A,10V. It looks like this MOSFET when on has a Rdson of about 5 ohms.
"6V", 7V, 8V, 9V is the G-S voltage. At only 6V the MOSFET is on up to about 1.2 amp, where it tears open.
At 7V G-S it looks pretty well on (closed) up to 4A and a little more.
There is no line for 5.5V GS but the transistor probably can only have 0.5A.

1. I drive a inductive load all the time. A little differently.
2. A current source, when unloaded, will go to maximum voltage.
 

Thread Starter

odm4286

Joined Sep 20, 2009
241
Why are you using the op amp to drive the load instead of the MOSFET?
Because I'm trying to keep Vgs constant. It keeps Id closer to linear than adjusting Vgs and keeping Vds constant. From what I can tell at least. Is there anything wrong with doing that?
 

crutschow

Joined Mar 14, 2008
23,508
Because I'm trying to keep Vgs constant. It keeps Id closer to linear than adjusting Vgs and keeping Vds constant. From what I can tell at least. Is there anything wrong with doing that?
i don't really understand your concern.
The usual configuration with the op amp driving the MOSFET gate has a very linear current out versus voltage is due to the large amount of negative feedback.
And the circuit is largely insensitive to changes in the load impedance due to the high resistance of the MOSFET drain (a perfect current source would have an infinite resistance).
The load is also isolated from the op amp output.
I don't see that your circuit improves on that.

LTspice simulation of typical circuit below:
Note that the Inhibit function does not cause the op amp to go to the rail, as the feedback loop stays in control.

1572756600463.png
 
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