Design of a Low Voltage Programmable Power Supply

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Design of a Low Voltage Programmable Power Supply [Copy link]

Abstract: With the development of power electronics technology, the combination of power electronics technology and automatic measurement technology can make the design of programmable power supply simple and feasible. This paper introduces a low-voltage programmable power supply for automatic measurement, which realizes real-time control of the power supply.

Keywords: program-controlled power supply; single-chip microcomputer

　

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 article 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 [1], in order to offset the influence of interference in the measurement, it is required to be able to change the polarity of the power supply during the test process, and in a certain period of the process, it is required to be able to completely cut off the power supply. We use power electronic devices to realize a controllable low-voltage power supply during the measurement process, paving the way for the realization of full automation of the measurement.

1 Measurement circuit power supply requirements

The main wiring diagram of the laboratory for testing cable insulation using the DC superposition method is shown in Figure 1.

Figure 1 Measurement main wiring diagram

In Figure 1, the cable is modeled as a parallel circuit of a resistor and a capacitor. The 1MΩ resistor is the protective water resistor. The transformer increases the voltage of 220V 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, on site, 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. When applying the DC superposition method

When testing 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, we can choose a suitable power electronic device. 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 [3].

Figure 2 Safe operating area of IRFP460

2 Main circuit design

Since the measurement process requires not only the ability to reverse the polarity of the power supply, but also the ability to completely disconnect the power supply from the measurement system, a full-bridge circuit is used in the design to achieve polarity control and full shutdown of the power supply [2]. The main circuit is shown in Figure 3.

Figure 3 Main power circuit

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. Driving circuit design

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

Figure 4 Single-channel drive circuit power supply

In Figure 4, we imitate the internal circuit of the driver integrated circuit EXB841, using resistor R1 and voltage regulator D2 to create a reference ground, so that the output voltage relative to the reference ground is +15V and -9V respectively. 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 [4]. The logic table and internal circuit of TLP250 are shown in Table 1 and Figure 5 respectively.

Table 1 TLP250 logic table

InputLED	V 1	V 2
ON	ON	OFF
OFF	OFF	ON

Figure 5 TLP250 internal circuit diagram

It can be seen from Table 1 and Figure 5 that after providing the driving power supply, the interface between the driving circuit and the main circuit can be easily realized 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 drive circuit and control circuit

Since the single-chip microcomputer is used as the core of the measurement system in this design, 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 drive circuit and the control circuit is shown in Figure 6.

In Figure 6, AD0-AD3 is the lower four-bit data bus, and CLK2 is the trigger signal given by the decoder and the microcontroller read and write signal. During the measurement process, when the power supply state needs to be changed, the data is directly written into 74LS175 and latched, and the on and off of each bridge arm can be controlled accordingly. It should be noted here that in the debugging process, wrong data must not be given, causing the bridge arm to be directly turned on, thereby permanently damaging the MOSFET.

Figure 6 Interface circuit between drive circuit and control circuit

5. Protection circuit design

5.1 Overvoltage protection circuit design

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

Figure 7 RC absorption circuit

Connecting the circuit shown in Figure 7 in parallel to both ends of the MOSFET can effectively limit the impulse overvoltage. The parameters of the capacitor can be calculated through actual measurement, or simply selected as twice the inter-electrode capacitance of the MOSFET. 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 the 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.

Figure 8 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 MOSFET has been given in many literatures and will not be elaborated here [3].

6 Conclusion

By applying MOSFET to the field of automatic measurement and using a single-chip microcomputer as the core of the measurement system, the problem of controlling the power supply state during the automatic measurement process has been successfully solved. This circuit can not only automatically switch 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 measurement tests and reflects the advantages of program control.

　

About the Author

Zhang Yadi (1981-), male, master student, his research interests are power system overvoltage and the application of power electronics in power systems.

Yao Hongyu (1981-), male, master student, his research direction is the application of power electronics in power systems.

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Nice, thanks for sharing!