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How does this power mosfet work in a constant current load?

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onemilimeter:
Hi,

The electronic load shown in attached image is constructed using a power mosfet Q1 (IRF540). Let's say the opamp MAX480 is powered by a 9V battery, thus the maximum output voltage of the opamp should be no more than 9V. If the load current is 10A, the voltage drop across the R10 resistor is 0.1x10=1.0V. If it's a 12V load, then the Vds=12-(0.1x10)=11V. To conduct current, the Vgs must be larger then Vth of the power mosfet. In this case, the Vds>(Vgs-Vth) and the power mosfet should be operating in "saturation" region. However, the power mosfet is said to act like a voltage controlled variable resistor (in "linear" region) in several articles. Which one is correct?

Thanks.

scrat:
The MOSFET acts like a voltage controlled current source, in the case you described. It can't be in triode region, your guess is correct. However, since the voltage is almost constant (from 11 to 12 V, since resistor is very small) the result is that the V/I ratio is variable with current, acting like a voltage controlled resistor.

jimmc:
I don't think that that is meant to be taken literally, it merely indicates that the current varies with gate voltage in a similar manner to a voltage controlled resistor. The analogy does not extend as to how the current varies with output voltage.

To be honest it doesn't matter what region the MOSFET is working in, the current is stabilised by negative feedback taken from the source resistor.
The output voltage of the Op-Amp (and hence the MOSFET's gate voltage) will rise until the current through the 0.1 ohm resistor produces a voltage equal to that on the wiper of the pot (R8).
The Op-Amp has a gain of over 100,000 so the voltage change across the 0.1ohm resistor required adjust the gate voltage to compensate for the MOSFETs characteristics is negligable.
As long as the MOSFET can conduct sufficient current its characteristics do not matter.

Jim 

onemilimeter:
I thought, by understanding how the circuit works, it will help to select a suitable power MOSFET for my application. I'm not sure if every power MOSFET (with the right voltage and current ratings) can be used to build the similar electronic load. Some articles said "logic level" power MOSFET is better. Is that true?

scrat:
Jim is right, in the sense that it is strictly a current source, controlled to reach a reference.

If your opamp was required to be operated into saturation (as in the case of switching), then you would need a voltage much above 5V to achieve a low resistance with a normal MOS.
In this case you won't need any logic level MOSFET, because you don't need to put it in saturation and (as far as you wrote) you don't mean to operate the opamp at 5V or so.

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