How to use the operational amplifier to test the power supply and battery?

To test the power supply and battery, it is necessary to use a current load that can absorb a large current and consume a large amount of power. The current load design described in this article is simple and accurate. It can be constructed using only an operational amplifier and a power MOSFET, as shown in Figure 1.

How to use the operational amplifier to test the power supply and battery?

Figure 1: This current load is very simple, multiple MOSFETs in parallel can achieve greater current and power consumption.

The current flowing through Q1 can be obtained by the following formula:

How to use the operational amplifier to test the power supply and battery?

This current can be easily controlled by changing the reference voltage (VREF). The operational amplifier should have a low input offset voltage and can be powered by a single power supply.

If you want the circuit to absorb large currents or consume tens of watts of power, you can use an operational amplifier to control multiple MOSFETs working in parallel. However, simply connecting MOSFETs in parallel will have two undesirable effects. On the one hand, different transistors, even if they are of the same model, usually have different turn-on thresholds, and their thresholds have a negative temperature coefficient. That is, first of all, there may be a big difference between the drain current of each transistor. Once the transistor heats up, its threshold will decrease, which will further increase the current and make it hotter.

In order to balance the currents of the transistors, a small series resistor can be added to the source of each transistor. In order to be effective, the voltage drop across the source resistor must be equal to the threshold, which will occupy a large part of 1V. In this way, the equalizing resistor will consume a lot of power, and the voltage drop across it will occupy the minimum voltage at which the circuit can work.

A better way to create a high-current, high-power load is to control each MOSFET separately, so that current imbalances caused by threshold dispersion can be avoided. The circuit shown in Figure 2 contains two such circuit blocks, but more can be added as needed. When the jumper J1 is closed and J2 is open, the circuit works in constant current mode, and the total load current is given by the following formula:

How to use the operational amplifier to test the power supply and battery?

If the values of the sense resistors are equal (R2=R5=RS), the total load current can be simplified as:

How to use the operational amplifier to test the power supply and battery?

How to use the operational amplifier to test the power supply and battery?

Figure 2: This current load schematic uses two independently controlled MOSFETs.

To measure the total load current, it is necessary to sum the current of each transistor. In this example, it can be achieved by summing the voltage drops of all the sense resistors. Usually, this is done by an inverting adder followed by an inverter, that is, using two operational amplifiers to build. The disadvantage is that voltage reversal occurs at the output of the adder, so these two operational amplifiers need to be powered by a bipolar power supply.

This design example uses a simpler method to sum the voltage drops, which is to use resistors R7 and R8 and only one operational amplifier. The principle of this addition is shown in Figure 3. Each of the N resistors is driven by a voltage source with very low impedance, which is the result obtained when a voltage drop is applied across the sense resistor in this example.

How to use the operational amplifier to test the power supply and battery?

Figure 3: This figure illustrates the voltage summation achieved at VOUT.

If there is no current flowing out of the VOUT terminal, according to Kirchhoff's law, we can get:

How to use the operational amplifier to test the power supply and battery?

therefore:

How to use the operational amplifier to test the power supply and battery?

In the case of two sense resistors, as shown in Figure 2, the voltage at the non-inverting input of U2A is half the sum of the voltage drops across R2 and R5. After passing through U2A with a gain of 2 times, the output voltage IMON is the sum of the voltages of the two detection resistors, and the total load current can be monitored with it. By adding more basic modules in parallel, the circuit can be expanded, and then using equations 3 and 5 for the number of modules, the total load current and the detection current output before U2A amplification can be calculated. For convenience, for the case of three power supply blocks, a four operational amplifier can be used.

Finally, this current load can be set to a constant resistance, which is very useful when testing certain power supplies. Its realization method is to provide a part of the load voltage VL as a reference voltage. Close the jumper J2 (open J1), the voltage at the non-inverting input terminals of U1A and U1B is determined by VL and the voltage divider formed by R9 and R10, so the load current becomes:

How to use the operational amplifier to test the power supply and battery?

Based on this, the effective load resistance RL is:

How to use the operational amplifier to test the power supply and battery?

By adjusting the voltage divider ratio or replacing R10 with a potentiometer, the load resistance can be changed from the nominal value calculated by Equation 7 (for the value of 2.55Ω in Figure 2) to near infinity when R10=0.

KonstanTInStefanov is a senior research fellow (SeniorResearchFellow) at the Electronic Imaging Center of the Open University of the United Kingdom.