When measuring the efficiency and load regulation of linear and switching voltage regulators, it is helpful to have access to an isolated, regulated current source to drive load under test (see the figure). The load under test connects to the drain of N1 and one of the four taps shown, depending on the current range desired.
Isolation enables this current regulator to serve either as a current sink for load configuration (a) or a current source for load configuration (b). When the circuit is acting as a source, the load must also include a floating power supply at least 1.75 V greater than the load voltage to ensure adequate headroom for the regulator.
The current regulator consists of three subcircuits: a voltage-to-current converter centered on U1, another voltage-to-current converter centered on U3, and optocoupler U2. The subcircuit centered on U3 requires an isolated 9-V supply, which can come from a floating supply or a 9-V battery.
The input to op-amp U1, VISET, ranges from 0 to 1 V and establishes the current value that the regulator will provide. U1 drives the optocoupler’s LED so the input photodiode current IP1 reaches VISET/R1.
Because U2 is a linear optocoupler, consisting of an LED, an input photodiode, and an output photodiode, driving current IF through the LED produces a photon flux that stimulates a proportional current in the input (IP1) and output (IP2) photodiodes. Ideally, the LED flux produces equal currents in the photodiodes, achieving unity transfer gain: (IP1/IP2) = 1.
In practice, the two currents aren’t quite equal and the transfer gain can vary ±6%. The resistor RADJUST allows compensation for this gain error. When the transfer gain has been properly adjusted, current IP2 on the other side of the isolation barrier will reproduce voltage VISET at the non-inverting input of U3.
Op-amp U3 drives the n-channel MOSFET N1 to achieve a load current value of ILOAD = VISET/RSENSE. The value of this sense resistance depends on which tap the load connects to. The taps are sized to allow this circuit to operate over a four-decade range, handling currents as great as 10 A (see the table).
One additional consideration is heat dissipation in the circuit. The sense resistors must be at least 1 W to handle the load current, and you may need to mount MOSFET N1 on a heatsink to protect it from thermal damage.