# How to effectively suppress the input impulse current of power supply equipment

[Guide]The various household appliances that we use in our daily life, as well as the vehicles and trains we take, are widely used in power supplies, and power supplies are also widely used in special industries such as military and aerospace.

1. Summary

The various household appliances we use in our daily lives, as well as the vehicles and trains we ride in, are all widely used power supplies, and power supplies are also widely used in special industries such as military industry and aerospace.

In order to ensure the stable operation of the power supply equipment is not disturbed, it is necessary to connect a capacitor in parallel at the input end of the power supply equipment. At the same time, in order to realize the input overload protection of the power supply equipment, a fuse or circuit breaker will be connected in series at the input end of the power supply equipment. Due to the existence of the input capacitor, the power supply device will generate a large input inrush current during the startup process, which may cause problems such as fuse blown or circuit breaker tripping. This article will give a reliable solution from the design and application of the power supply equipment input impulse current suppression scheme.

2. Causes of input inrush current

The path of input inrush current generation is shown in Figure 1.

When the power supply system starts, the input voltage Vin rises rapidly, and the input voltage Vin charges the input equivalent large capacitance Cin through the input equivalent impedance Rin. When the input circuit does not use the input impact circuit suppression circuit, since the input equivalent impedance Rin is mainly the PCB trace impedance, the common mode/differential mode inductance impedance in the EMC circuit and the port connection impedance, the input equivalent impedance Rin is relatively small (relative to The impedance of the input impulse circuit suppression circuit can be ignored), which will result in a larger input impulse current.

At 110VDC input, the input equivalent large capacitance Cin is 100uF, when the current suppression circuit is not added, the typical waveform of the input impulse current is shown in Figure 2, and the input impulse current reaches 170A.

Three, input inrush current suppression circuit application system block diagram

The block diagram of the input impulse current suppression circuit application system is shown in Figure 3.

Excessive input inrush current will cause problems such as fusing of the input fuse or tripping of the circuit breaker. For industrial applications with high reliability requirements, input inrush current suppression is essential.

Four, circuit scheme comparison

In order to effectively suppress the input inrush current, the following schemes are often used in the industry:

Solution 1: Connect the resistor in series with the input loop of the power supply device to suppress the input inrush current. The schematic diagram of the solution after the input loop is connected in series with the resistor R1 is shown in Figure 4.

When the power supply system starts to supply power, the input equivalent large capacitor Cin voltage is small, and the input impulse current is maximum. In order to effectively suppress the input impulse current, the series resistor R1 needs a larger resistance value. When the power supply equipment is working stably, there will be continuous The current flows through the resistor R1, and the resistor R1 has the shortcomings of serious heating and affecting the efficiency of the product.

Taking the input voltage Vin=110VDC, the output power Po=100W, the product efficiency ƞ=93%, and the required input impulse current Iin≤10A as an example, the value of the resistor R1 can be calculated, and the input current of the resistor R1 during steady-state operation , The loss of resistor R1. Taking into account the size and temperature rise of the resistor, the loss of resistor R1 is generally required to be less than 1W, that is, for low-power power equipment with an output power of less than 30W, this solution can be considered. For higher-power power equipment, a better Design.

Scheme 2: Connect the thermistor with negative temperature coefficient in series in the input loop of the power supply section to suppress the input inrush current. The schematic diagram of the scheme after the thermistor RT is connected in series with the input loop is shown in Figure 5.

In order to solve the problem of serious steady-state heating of fixed resistors and affecting product efficiency, the above thermistor solutions are commonly used in the industry. When the power supply device is just started, the thermistor has not yet heated up, and its resistance is large, and can be used for input inrush current suppression. After steady-state operation, the thermistor value is reduced, the heat generation is small, and the product efficiency is less affected.

This solution has the advantages of less heat generation of the thermistor and less impact on product efficiency. However, because the characteristics of the negative temperature thermistor are that the resistance value decreases with the decrease of temperature and the resistance value recovery time is longer, the solution exists The disadvantages are:

(1) When working in a high-temperature environment, the resistance of the thermistor is small, which cannot well suppress the input inrush current;

(2) When working in a low temperature environment, the resistance of the thermistor is large, which may cause poor startup of the power supply;

(3) When the power supply equipment is started with a short switch-on interval, the thermistor’s steady-state working resistance has not recovered to a larger resistance after it has been reduced, and the input inrush current cannot be well restrained.

Scheme 3: Connect the resistor in parallel with the Electronic switch and then connect it in series with the input circuit of the power supply device, and use an automatic control circuit to control whether the electronic switch is connected to the input circuit of the power supply device to suppress the input impulse current.

The schematic diagram of the scheme after the input loop series resistor, electronic switch and control circuit is shown in Figure 6.

In order to solve the shortcomings of solution 1 and solution 2, the above solution 3 can be adopted. When the solution starts, the electronic switch S1 is opened, the resistor R1 performs input inrush current suppression, and the electronic switch S1 is closed during steady-state operation. The advantages of this solution are :

(1) The input inrush current can be effectively suppressed. At the same time, since the resistor R1, electronic switch S1 and the control circuit are less affected by temperature, the input inrush current suppression effect is also less affected by temperature;

(2) Resistors and electronic switches have low loss and low heat generation, which has a small impact on the efficiency of the power supply equipment during normal operation;

(3) The recovery time of the control circuit is fast, and the input inrush current can be effectively suppressed even when the power equipment is switched on and off in a short time.

Solution 4: Connect the electronic switch in series in the input circuit of the power supply device, and use the automatic control circuit to control the impedance of the electronic switch for constant current control of the input impulse current. The schematic diagram of the input circuit after the electronic switch and control circuit are connected in series is shown in Figure 7. Show.

In order to solve the shortcomings of solution 1 and solution 2, the above solution 4 can also be adopted. In addition to the advantages of solution 3, this solution also has the following advantages:

(1) Resistor R1 is not used, which can reduce the size and cost of system equipment.

(2) The impedance of the electronic switch adopts constant current control, which is less affected by the input voltage transformation.

In summary, for applications requiring higher performance and reliability, you can choose option 3 or 4 for design.

5. Reliability design of power supply equipment

The design of power supply equipment requires high reliability, and is affected by environmental impact (such as cooling, dry heat, alternating damp heat, vibration, impact, etc.), electromagnetic compatibility (conducted disturbance immunity, radiated disturbance immunity, electrical fast transient Under variable pulse group, power supply overvoltage, surge, electrostatic discharge, etc.) and other influencing conditions, circuit characteristics and life design have application specifications.

The reliability of power supply equipment is mainly considered from the following three aspects:

1. Design reliability: Reliability design for circuit scheme selection, derating design, thermal design, safety design, EMC design and PCB design;

2. Reliability of raw materials: selection of materials for reliability indicators such as failure rate, MTBF, and service life;

3. Manufacturing reliability: manufacturability, production environment, production personnel, etc. guarantee manufacturing reliability.

Six, power equipment application program

Take Jinshengyang power module applied to the electric traction control unit of the railway system as an example, as shown in Figure 8.

The railway 110VDC power supply system is used to supply power to the DC/DC power supply module through the input inrush current suppression circuit, the protection circuit and the EMC circuit. The DC/DC power supply module converts the voltage into 3 different output voltages, and the output voltage is then output EMI The circuit supplies power to loads with different needs.

The board power supply integrates input impulse current suppression circuit, EMC circuit, and protection circuit (anti-reverse connection, over-under voltage, over-current, short circuit, etc.). The board power supply output impulse current can be suppressed to less than 10A, with complete functions, and meets the iron standard EN50155 certification requirements, etc., can meet the needs of system applications. The power supply of Jinshengyang board card is shown in Figure 9.

The input surge current waveform after the board power supply uses the input surge suppression circuit is shown in Figure 10.

Seven, summary

Power equipment is widely used in various industries. For high-reliability power equipment, input inrush current suppression circuit design is inseparable. Our company can provide customers with supporting solutions, as well as system solutions for railway and other industry applications. The designed board power supply can better meet customer application requirements, simplify customer product development and testing cycles, and comprehensively improve system stability, safety and reliability.

Source: Jin Shengyang

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