Design of high voltage power supply for insulation resistance tester

0 Introduction

The predecessor of the insulation resistance tester is the megohmmeter, which is a special instrument used to measure the insulation resistance of electrical equipment such as transformers, motors, and cables. The insulation resistance measurement can determine whether the internal insulation material is damp or whether the external insulation surface is defective. The principle of measuring this insulation resistance is to add a DC high voltage to the insulation system to measure the leakage current generated, thereby calculating the insulation resistance value. This article uses a square wave with a fixed frequency, which is then boosted by a transformer, doubled and rectified into a high-voltage DC, and finally stabilized by a high-voltage regulator with overcurrent protection. The two power supplies are connected in series to realize the design of a 2500 V high-voltage voltage regulator, thereby reducing the difficulty of making the transformer.

1 Hardware structure of the insulation resistance test system

The insulation resistance test system is mainly composed of a high-voltage power supply, an AD conversion circuit, a microprocessor circuit, a display circuit, etc. Figure 1 shows the block diagram of the system. This article mainly describes the design of the high-voltage power supply part.



2 Design of DC high voltage generating circuit

2.1 Working principle of switching power supply

The power supply voltage of the system switching power supply is 12 V. It adopts a push-pull circuit. The gate of its switch tube is alternately turned on and off under the control of the excitation square wave signal. After the 12 V DC voltage is converted into a high-frequency square wave, it is alternately added to the two primary sides of the step-up transformer, which is equivalent to a half-peak 12 V alternating square wave added to the primary side of the transformer, and then converted into a high-voltage square wave on the secondary side according to the turns ratio.

2.2 Design of high voltage generating circuit

The high voltage generating circuit of this system is shown in Figure 2. The power supply voltage of each chip is unified to 12 V. The system uses CD4060 and quartz crystal to generate a 3.6864 MHz square wave, and then inputs the 115.2 kHz signal into the D flip-flop of CD4013 after 32 frequency division. Then, a 57.6 kHz signal with a phase difference of 180° is generated by two frequency division to ensure the symmetry of the driving waveform and no DC component. Finally, it is output to the parallel connected CD4049. Since the gate-source capacitance of the field effect tube is generally large, a large driving current is required to reduce the charging and discharging time to improve the ability to drive the gate of the field effect tube. The output waveforms of CD4060 and CD4049 are shown in Figure 3.


In the system design, a DC high voltage of 2500 V needs to be generated, while the maximum withstand voltage of the common field effect tube is 1500 V. Therefore, this design first generates a voltage of 1400 V, and then obtains a stable voltage of 1250 V after being stabilized by a high-voltage regulator. Finally, the two sets of circuits are connected in series to obtain the required DC high voltage of 2500 V. Therefore, by using this voltage doubling rectification method, the secondary side of the transformer only needs to output 700 V, which can reduce the difficulty of winding the secondary side of the transformer.

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In the voltage doubler rectifier circuit, since the output voltage is very large, high-voltage ceramic capacitors and fast recovery diodes are required. This system uses 103 M/3000 V high-voltage ceramic capacitors.

2.3 Transformer design

(1) Transformer coil winding method

Since the circuit uses a push-pull boost circuit, the primary side of the transformer should have a center tap. Since the primary voltage is
low, a double-wire parallel winding method is used here to connect the same-name end and the non-same-name end in series to lead out the center tap. This helps balance the two primary windings and reduce leakage inductance. In the design, the secondary side will be wound in each grid of the frame to increase the creepage distance.

(2) Transformer parameter selection

① Determination of transformer ratio

Due to the square wave output, the duty cycle is 0.5 and the switching frequency is 57.6 kHz, so when the input voltage is 12 V, the output voltage should be guaranteed to reach 700 V. Therefore, the ratio n can be formally obtained:



The transformer ratio n=N2/N1=58.3. Considering that the actual circuit will have the tube voltage drop of the power tube and the rectifier diode, the ratio n=60 can be selected.

② Selection of magnetic core

According to the formula:


j——current density, generally, 300~500A/cm2 ^; Kc

——filling factor of magnetic core, for ferrite Kc=1;

Ku——filling factor of copper, Ku is related to the wire diameter, winding process and number of windings, and is generally about 0.1~0.5.

The units of each parameter in the above formula are: P0→W, Ae→cm2 ^, Aw→cm2 ^, Bm→Gs, j→A/ ^cm2 . Take P ₀ =15 W, η=90%, and select MXO-2000 ferrite material, whose saturation flux density Bs=4000 Gs. To prevent magnetic saturation during use, the flux density Bm=2500 Gs can be taken. Its effective core cross-sectional area Ae is 0.224 cm ² , window area Aw is 0.5315 cm ² , Kc=1, Ku=0.3, j=300 A/cm ² .

Calculated by formula (2): AeAw=0.119 cm ⁴



of EE19 core , so EE19 core is selected in the design. ③ Design of turns Therefore, the primary single winding can be taken as 10 turns in the design, and according to the transformation ratio requirements, the secondary single winding turns are taken as 600 turns. That is, the transformer winding turns are: 10:10:600. ④ Winding design [page] Since the skin effect will occur when the alternating current flows through the wire, the effective cross-sectional area of ​​the wire will be reduced and the resistance will increase. Therefore, when the switching frequency is 57.6 kHz, the penetration depth △ can be calculated by the following formula: It can be concluded that △ is 0.2755 m, and the wire diameter should be less than twice the penetration depth. Due to the existence of leakage inductance and distributed capacitance, oscillation will occur after the two VMOS conduction states are switched, resulting in a high output voltage when the transformer is unloaded. In the design, a resistor damping can be connected in parallel to the secondary side of the transformer to reduce the output voltage when unloaded, and the turns ratio can be appropriately reduced. 3 Voltage stabilization circuit design In order to obtain stable high voltage, the system designs a voltage stabilization circuit with overcurrent protection. First, the output voltage is sampled and fed back to the positive input terminal of the op amp, and then compared with the reference voltage at the negative input terminal to control the state of the subsequent circuit. Thereby achieving the purpose of automatic voltage regulation of the system. The system electrical connection diagram is shown in Figure 4. The op amp adopts a single power supply working mode in the design and is used as a differential amplifier to compare part of the output voltage with the precise +5 V reference voltage. The +5 V reference voltage source can use LP2951, which has low quiescent current and low dropout voltage. Therefore, the quiescent current increases only slightly under dropout conditions, which can extend the service life. In addition, the 5 V reference voltage can also be used as the working power supply of the subsequent measurement circuit to achieve the unification of the power supply. In the system, if the output voltage HV is 1250 V, it is divided into 5 V by R6 and R7. In this way, the voltage of the in-phase input terminal of the op amp is the same as the output voltage of LP2951, and no current flows through the resistor R1. Therefore, the output voltage of the proportional integrator composed of the op amp remains unchanged, and the circuit is in a stable state. However, if the output voltage HV is lower than 1250 V for some reason, the voltage at the in-phase input of the op amp after voltage division is lower than the output voltage of LP2951, so the current flowing through R1 is integrated by the integrator to make its output voltage (gate voltage of Q3) lower and the drain voltage (gate voltage of Q4) higher, which causes the source voltage of Q4 (output voltage of the regulator) to increase until the output voltage reaches 1250 V. The integrator stops integrating and the circuit returns to a stable state. If the output voltage HV is higher than 1250 V for some reason, the principle is similar, and the circuit will eventually return to a stable state. In the design, D5 and Q5 play a protective role in the circuit. When Q3 quickly pulls down its drain current, in order to prevent the gate voltage of Q4 from being too much lower than the source voltage and damaging the tube, a diode D5 can be used to limit this voltage to -0.7 V. Transistor Q5 does not start when the system is working normally. When the current is too large, the voltage drop on resistor R5 is greater than the turn-on voltage, and the transistor is turned on, thereby reducing the gate-source voltage of Q4, and thus reducing the output voltage. Therefore, limiting the increase in output current can play an overcurrent protection role. In the circuit, B4 is connected in parallel with a capacitor C14 to reduce the impedance to high-frequency signals, which is equivalent to differentiation. In this way, the signal rises faster and the response speed can be improved; R8 is connected in series with capacitor C17 for filtering to improve the stability of the output voltage. Q3 and Q4 are selected as 2SK1412 field effect tubes with a withstand voltage of 1500 V, a current of 0.1 A, and a power of 20 W, which can meet the system requirements. The proportional integral link is used in the system. Since there is a static error in the simple proportional regulation, once the deviation of the regulated quantity does not exist, the output is zero, that is, the regulation effect is based on the existence of the deviation as a prerequisite, and the inertia of the proportional link is also large. Moreover, the simple integral regulation is too delayed. While improving the accuracy of the static error, it often deteriorates the dynamic quality of the system, prolongs the transition process time, and even causes system instability. Therefore, the system adopts proportional-integral regulation, thus overcoming the disadvantage of the simple proportional link with regulation error, avoiding the weakness of the integral link with slow response, and improving the stability and dynamic performance of the system. 4 Conclusion The high-voltage generating circuit designed in this paper has a simple structure and is easy to adjust. The voltage stabilizing circuit adopts negative feedback and has the function of automatically adjusting the voltage. It also has an overcurrent protection function, so it can be used as a high-voltage power supply for the insulation resistance tester.