1. Working principle of electronic load:

Electronic load, as the name implies, is a "load" function realized by electronic devices, and its output port conforms to Ohm's law. Specifically, electronic load is a device that dissipates power and electric energy by controlling the conduction flux of internal power device MOSFET or transistor.

Electronic load generally has multiple modes of constant current, constant voltage, constant resistance, constant power, short circuit and dynamic load, which can simulate various load conditions,

2. Application examples of electronic load:

(1) Computer component manufacturers such as LCD can use electronic load to realize the test and aging of their internal power supply module

(2) Manufacturer of switching power supply: generally, the electronic load can test the power supply stability, load stability, output voltage adjustment and transient characteristics of the DC power supply. For the power supply with multiple outputs, the combination can be used to attach to the test

(3) Manufacturer of adapter/charger:

It is important to test the adjustment ability of the output voltage and current of the battery adapter, which will ensure the correct power supply and charging of the equipment battery. CC and CR working modes can be used for routine performance test, and OCP and SHORT can be used to test the protection action of products. The regulating time and charging time of the charger can be measured by the load pulling time.

Schematic diagram of electronic load circuit

The schematic diagram is shown in Figure 2. The basic circuit is composed of constant voltage circuit, constant current circuit, overcurrent protection circuit and drive circuit, except for the dotted line frame ⑤ and two multimeter. V=12V input voltage, after passing through the current-limiting resistor R1 to the cathode K of the three-terminal adjustable shunt reference source U1 (TL431), the output reference voltage VR obtained from the reference terminal R is 2.5V, passing through the resistor R1 to the adjustable sliding rheostat R6, one path provides voltage for U3A through the resistor R2, and the other path provides voltage for U3C through the resistor R7.

1. Constant voltage circuit

As shown in the dashed box ① in Figure 2. When the input voltage at the load end increases, the voltage at the U3A in-phase input end increases. When the in-phase input terminal voltage is greater than the inverse input terminal voltage (reference voltage), U3A outputs a high level and generates a voltage drop on the grid G voltage VG of field-effect transistors Q1, Q2, Q3 and Q4, which reduces the voltage VDS between drain D and source S, thus achieving the purpose of constant voltage.

2. Constant current circuit

As shown in the dashed box ② in Figure 2. When the load current increases, the voltage on R19, R22, R25 and R28 increases. That is, the sampling voltage on R18, R21, R24 and R27 increases, that is, the voltage at the inverted input end of the U3C increases. When the voltage at the inverted input end of the U3C is greater than the voltage at the input end of the same phase, the U3C outputs a low level, the grid G voltage VG of the field effect transistors Q1, Q2, Q3 and Q4 decreases, the internal resistance RDS of Q1, Q2, Q3 and Q4 increases, and the load current decreases, thus achieving the goal of constant current.

3. Overcurrent protection circuit

As shown in the dashed box ③ in Figure 2. When the load current increases, the voltage on R19, R22, R25 and R28 increases, that is, the sampling voltage on R18, R21, R24 and R27 increases, and the voltage at U3B inverted input terminal increases, but the current continues to increase. When the voltage at the reverse phase end is greater than the reference voltage of the set overcurrent protection current (input voltage at the same phase end), U3B outputs a low level, the grid G voltage VG of the field-effect transistors Q1, Q2, Q3 and Q4 decreases, the internal resistance RDS of Q1, Q2, Q3 and Q4 increases, and the load current decreases, thus playing the role of overcurrent protection.

4. Drive circuit

As shown in the dashed box ④ in Figure 2. Q1, Q2, Q3 and Q4 choose high-power field-effect transistor IRF540 as the power device. However, after multi-transistor parallel connection, due to the corresponding increase of inter-pole capacitance and distributed capacitance, the high-frequency characteristics of the amplifier are deteriorated, and the high-frequency parasitic oscillation of the amplifier is easily caused by feedback. For this reason, the number of parallel composite tubes is generally not more than 4, and the anti-parasitic oscillation resistance is connected in series on the base or grid of each tube. R17, R20, R23 and R26 are driving resistors, R18, R21, R24 and R27 are sampling voltage resistors, and R19, R22, R25 and R28 are current limiting resistors. One end of C9 is connected to the drain of field effect tube IRF540, and the other end is grounded to prevent vibration.