Design of a low voltage program-controlled power supply

0 Introduction

In some automatic measurement fields, in order to meet special test conditions or measurement processes, it is often required to control the power supply to switch polarity or connect or disconnect the power supply to the measurement system during the measurement process, that is, to be able to control the power supply state at any time according to the measurement needs. With the development of power electronics technology, the switching speed, capacity and reliability of fully controlled devices have been greatly improved, making it very easy to use fully controlled devices to realize program-controllable power supplies. This paper combines an example of a measurement process to give a design of a low-voltage controllable power supply. In the method of using the DC superposition method to detect the insulation resistance of XLPE cables , in order to offset the influence of interference in the measurement, it is required to change the polarity of the power supply during the test process, and in a certain process, it is required to completely cut off the power supply. We use power electronic devices to realize a low-voltage power supply that can be controlled during the measurement process, paving the way for the realization of comprehensive automation of measurement. 1 Requirements of the measurement circuit for the power supply The main wiring diagram of the laboratory for detecting cable insulation by the DC superposition method is shown in Figure 1.








In Figure 1, the cable is modeled by a parallel circuit of a resistor and a capacitor . The 1MΩ resistor is the protective water resistor. The transformer increases the 220V voltage to 110kV and applies it to the cable. In the measurement test, the main requirement is to superimpose a 50V DC voltage on the cable to measure the insulation resistance R of the cable. In order to reduce the measurement error, it is necessary to reverse the polarity of the power supply and perform two forward and reverse measurements. In addition, in the field, since the neutral point of the transformer is often grounded through a small resistor, the resistance of this resistor is only a few Ω to more than ten Ω. In order to superimpose the DC power supply on the cable, the DC power supply must be able to provide a sufficiently large current . In the application of the DC superposition method to detect cable insulation, the DC voltage usually required is 50V. In this way, the minimum value of the neutral point grounding resistance is set to 5Ω. According to Ohm's law, we can conclude that the DC power supply must be able to provide at least 10A of current; in addition, considering the switching speed required during the measurement process, the appropriate power electronic device can be selected. After investigating the commonly used fully controlled power electronic devices, we decided to use MOSFET as the switching device, and selected IRFP460 from IR . IRFP460 is a high-speed device produced by IR. Its safe operating area is shown in Figure 2. In Figure 2, we can see that at 50V, 10A is the current it can safely shut down. IRFP460 safe operating area






2 Main circuit design

During the measurement process, it is required not only to be able to reverse the polarity of the power supply, but also to be able to completely disconnect the power supply from the measurement system. Therefore, a full-bridge circuit is used in the design to achieve polarity control and full shutdown of the power supply. The main circuit is shown in Figure 3.


As can be seen from Figure 3, the main circuit is actually a combination of a rectifier circuit and a full-bridge inverter circuit, and the switching of the power polarity is achieved through the inverter. In this way, the programmable power supply can be easily realized.

3. Drive circuit design

In the design, we did not use the commonly used DC/DC module as the drive circuit power supply, but used a simple and cheap three-terminal voltage regulator 7824 as the drive circuit power supply. The experiment shows that it reduces the cost by 3/4 without much reduction in reliability. The circuit diagram of the drive power supply is shown in Figure 4.


In Figure 4, we imitate the internal circuit of the driver IC EXB841, using resistor R1 and voltage regulator D2 to create a reference ground, so that the output voltages are +15V and -9V respectively relative to the reference ground. According to the device manual of IRFP460, these two voltages can reliably trigger and turn off the MOSFET. The drive control circuit uses TLP250 as the control circuit of the drive signal. The logic table and internal circuit of TLP250 are shown in Table 1 and Figure 5 respectively. Table 1 TLP250 logic table InputLED V1 V2




TLP250 internal circuit diagram


As can be seen from Table 1 and Figure 5, after providing the driving power supply, the interface between the driving circuit and the main circuit can be easily realized by using TLP250. When the optocoupler is turned on, V1 is turned on, VCC is approximately equal to Vo, and the gate-drain voltage output to the MOSFET is approximately 15V; when the optocoupler is turned off, V2 is turned on, Vo is approximately equal to GND, and the gate-drain voltage output to the MOSFET is approximately -9V.

4 Interface between the driving circuit and the control circuit

Since in this design, the single-chip microcomputer is used as the core of the measurement system, the core of the control circuit is also a single-chip microcomputer. In order to save the IO port of the single-chip microcomputer, a 74LS175 is used as the latch of the control signal. The interface circuit between the driving circuit and the control circuit is shown in Figure 6. In Figure 6, AD0-AD3 is the low four-bit data bus, and CLK2 is the trigger signal given by the decoder and the single-chip microcomputer read and write signal. During the measurement process, when the power supply state needs to be changed, the data is directly written into the 74LS175 and latched, and the on and off of each bridge arm can be controlled accordingly. It should be noted that during the debugging process, wrong data must not be given, causing the bridge arm to be directly connected, thus causing permanent damage to the MOSFET. Drive circuit Interface circuit with control circuit







5 Protection Circuit Design

5.1 Overvoltage Protection Circuit Design

In this design, since the power supply capacity is only 500W, a simple RC absorption circuit can be used. The circuit diagram is shown in Figure 7. RC absorption circuit




Connecting the circuit shown in Figure 7 in parallel to both ends of the MOSFET can effectively limit the impact overvoltage. The parameters of the capacitor can be calculated by actual measurement, or simply selected as twice the capacitance between the MOSFET electrodes . The parameters of the resistor are related to the switching frequency. 5.2 Overcurrent protection circuit design In this design, since the power supply capacity is not large, a transistor overcurrent protection circuit is considered, as shown in Figure 8. In Figure 8, R1-R10 is a standard resistor of 1Ω with a power of 2W. When the current exceeds the predetermined value, the voltage drop on the parallel resistor exceeds 0.7V, and the transistor is turned on. At this time, the MOSFET will be cut off due to the reverse voltage between the gate and source, thereby cutting off the main circuit; when the current value is normal, the MOSFET is turned on normally and will not affect the normal operation of the circuit. The disadvantage of this circuit is that if there is an intermittent overcurrent in the circuit, the MOSFET will continue to operate. For this reason, other protection components are also added in Figure 3. Overcurrent protection circuit










As can be seen from Figure 3, in order to prevent overcurrent damage on the rectifier side of the main circuit, an air switch is set on the secondary side of the transformer. It should be noted that this switch cannot be set on the primary side of the transformer to avoid malfunction due to excitation inrush current. A small inductor is also added to the inverter part to prevent damage caused by current changes, and a fast fuse is connected in series as a backup protection for transistor overcurrent protection.

The protection circuit between the gate and source of the MOSFET has been given in many literatures and will not be described here.

6 Conclusion

Applying MOSFET to the field of automatic measurement and using single-chip microcomputer as the core of the measurement system successfully solves the problem of needing to control the power supply state during automatic measurement. Using this circuit can not only automatically reverse the power supply polarity and realize program-controlled shutdown of the power supply, but also, under the premise that the MOSFET switching frequency allows, this circuit can be used to program and realize any SPWM waveform. This design has a compact structure, high controllability, and low cost. It has achieved satisfactory results in the measurement test and reflects the advantages of program control.