Voltage Controlled Power Supply Tester by OPA277

While testing the power supplies or discovering their potential problems, you must apply dynamic and static tests. This circuit, namely current sink, makes this job easier for you. It can draw 0 to 1.5A current from the load depending on the input voltage between 0 to 5V applied to the input. The two main components of the circuit are OPA277 and IRF530. OPA277 is a precision operational amplifier IC which features a max. V_IO= 100 µV and I_IB=4 nA. It makes a comparison between the voltage applied to its non-inverting terminal and the voltage across the resistor R_SENSE.

The second is the STMicroelectronics' IR530 N-Channel Power MOSFET which is capable of delivering max. 14 A with V_DS voltage of 100V and at a case temperature of 25 °C. It needs adequate heatsinking.  Since the MOSFET can dissipate a finite amount of maximum power, when you increase the power supply voltage, you must reduce the load current accordingly.

The voltage divider part including the R1 and R2 resistors allows to apply 0 to 0,495V to the non-inverting input when the input voltage is between 0 to 5V that results an output current range of 0 to 1.5A.

The output voltage can be found by the following relation;

I_O = V⁺/R_SENSE

where V⁺ is the voltage applied to the non-inverting terminal and calculated by;

V⁺ = [R2/(R1+R2)]V_IN

So you can change the voltage divider resistors to draw more current from the load. In this instance, the supply voltage must be lowered not to exceed the maximum power dissipation of the MOSFET.

C3, C4, R3 and R4 components ensure the loop stability, yielding a circuit with a rise time of 1.4 µsec. So you can apply not only static tests by applying DC to the input, but also  dynamic test for instance by applying a pulsed input voltage to simulate fast load and transients.

The minimum supply voltage can be 0.735 V since the low channel resistance of the IRF530 is low. The negative output giving power supplies also can be tested by reversing the power supply terminal connections.

While connecting the power supply, especially during the dynamic tests, you must shorten the turn area as much as possible  because the pulsed load current produces radiated emissions and this may affect the circuit itself and the measuring equipment. Luca Bruno, ITIS Hensemberger Monza, Lissone, Italy - EDN, 20/3/2008