When measuring the output energy or energy consumption of AC-DC and DC-DC power supplies, power devices, batteries, battery chargers, etc., a load is required. The traditional method is to use fixed resistors and variable resistors to act as the load to be measured. An emerging electronic instrument and test equipment, the electronic load, came into being. It uses power devices to simulate resistors and has strong operational flexibility. At present, the development of electronic load technology is relatively mature. In terms of its type, it generally has

working modes such as constant current (CC), constant resistance (CR), constant voltage (CV), and constant power (CP). Researching and developing new low-cost electronic loads has also become a meaningful task.

1 Introduction to the structural block diagram of the constant current (CC) electronic load

The constant current (CC) electronic load is a special device used to test the various performances of the voltage source. This article introduces a new solution for a constant current electronic load. It is based on feedback control theory and uses an analog PI regulator to control the conduction strength of the N-channel high-power MOSFETDE to achieve zero-static control of the measured current. It has high control accuracy, simple circuit and low cost. Figure 1 is a block diagram of the constant current electronic load. The DC regulated power supply frame is a DC regulated power supply circuit. It provides a constant current setting voltage, a PI regulator working voltage, and a current detection and conversion circuit working power supply. It requires a high power output and a high voltage stability index. The constant current setting circuit can provide a linear adjustable negative polarity voltage output. The PI regulator is composed of an ordinary operational amplifier, and the PI parameters are adjusted to the best by experimental methods. The actuator is an N-channel MOS tube or a MOS tube group.


2 Control circuit design and experimental research

To achieve a zero-static-error regulation control, the proportional-integral (PI) differential control law must be adopted. For this control object, the proportional-integral (PI)

control can meet the requirements. The hardware circuit is shown in Figure 2. The circuit is mainly composed of an inverter, a PI regulator, a MOS tube and a Hall current sensor. When designing, it is generally carried out from the control object or the execution unit. The first thing to determine is the transfer function of the execution unit, that is, the determination of the amplification factor Ks of the MOS tube . 2.1 Determination of the amplification factor Ks

of the MOS tube

The test circuit is shown in Figure 3. The power of the voltage source under test is large enough, and the output current meets the test requirements. Adjust the given potentiometer W, measure the voltage UG at the G pole of the MOS tube and the current IO flowing through the DS pole of the MOS tube to obtain a set of experimental data recorded in Table 1. It can be seen from the table that when UG≤2.5 V, the MOS tube is not turned on, IO = 0, which is called the dead zone. After UG >2.5 V, the MOS tube begins to turn on, and when UG > 3.3 V, the relationship changes linearly. Calculate KS in the linear section .

 

2.2 Determination of current feedback coefficient β

That is to say, the conversion coefficient of the Hall current sensor is determined. The current sensor used in the design is a Hall sensor, the input is current, and the output is voltage. After testing, the Hall coefficient K = 0.8 V/A is determined, that is, when the input current of the sensor is 1 A, the output voltage is 0.8V. β = K = 0.8 V/A

 

2.3 Determination of the static gain factor KP of the PI regulator

According to the negative feedback closed-loop control principle: K = βK P K S = 1, we get: K P = 1/ βK S △0. 875V/A. Based on this value, select the input and output resistance values ​​of the regulator to satisfy R F /R I = K P .

 

2.4 Determination of voltage polarity and a brief description of control principles

The polarity of each voltage is generally derived from the back to the front. The control voltage UG of the MOS tube is positive (+), which requires the output of the PI regulator to be positive (+). Considering that the output of the Hall current sensor is always positive (+), in order to form negative feedback control, the given voltage of the PI regulator should be negative (-). Therefore, the PI regulator uses a negative phase input. The inverter of the previous stage converts the adjustable positive voltage generated by the potentiometer W into an adjustable negative voltage, which is added to the input pole of the regulator as the constant current setting value. It is compared with the current feedback voltage provided by the Hall current sensor. The PI regulator implements proportional integral regulation according to the deviation and positive and negative polarity to achieve constant current (IO ) . Change the size of the integral capacitor to meet the requirements of rapid response and stability.

3 Experimental research results

Table 2 is the experimental measured data. From the data pattern, U GD (potentiometer W) and the MOS tube drain and source current I O have a good linear relationship. And I O / U GD = 1/ β = 1/ 0. 8 = 1. 25. The rapidity of the adjustment response and the anti-interference performance in the experiment can be adjusted to the best.

 

4. Some explanations and improvement measures

 

(1) Due to the use of PI regulator, the dead zone of MOS tube does not need to be eliminated by specially designed circuits. The nonlinearity of MOS tube is eliminated by itself in the closed loop.

(2) According to the properties of the device under test (resistance, inductance, capacitance), the PI parameters can always be adjusted to ensure its rapidity, stability and anti-interference requirements.

(3) The system has strong scalability. Digital setting and regulation control can be realized.

(4) When the test current is required to be large, the parallel combination of multiple MOS tubes can be considered to expand the load capacity. As shown in Figure 4, a parallel combination of two MOS tubes is shown. In this case, K `S = mK S ; K `P = K P /m, where m is the number of MOS tubes in parallel.

(5) After the capacity is expanded, it is necessary to consider adding a MOS tube drive circuit.

(6) There are no energy-consuming components in this circuit. All load power consumption is borne only by the components in the loop, mainly the self-consumption of the MOS tube. Due to the power consumption performance of the MOS tube itself, the current allowed to flow cannot be too large, even if expansion measures such as parallel circuits are adopted. Therefore, the heat dissipation of the MOS tube must be fully considered during normal use.