Design of low voltage and high current soft switching power supply

1 Introduction

In the electroplating industry, the output voltage of the working power supply is generally required to be low, but the current is large. The power requirement of the power supply is also relatively high, generally from a few kilowatts to tens of kilowatts. At present, such high-power electroplating power supplies generally use thyristor phase-controlled rectification. Its disadvantages are large size, low efficiency, high noise, low power factor, large output ripple, slow dynamic response, poor stability, etc.

The switching power supply for electroplating introduced in this article has an output voltage of 0-12V and a current of 0-5000A, which can be continuously adjusted. The full-load output power is 60kW. Due to the use of ZVT soft switching technology and a better heat dissipation structure, all indicators of this power supply meet the requirements of users and have been put into production in small batches.

2 Main circuit topology

In view of such a high power output, the high-frequency inverter part adopts a full-bridge topology with IGBT as the power switching device. The entire main circuit is shown in Figure 1, including: industrial frequency three-phase AC input, diode rectifier bridge, EMI filter, filter inductor and capacitor, high-frequency full-bridge inverter, high-frequency transformer, output rectification link, output LC filter, etc.

The DC blocking capacitor Cb is used to balance the transformer volt-second value and prevent magnetic bias. Considering the efficiency issue, the resonant inductor Ls only uses the leakage inductance of the transformer itself. Because if the inductance is too large, it will cause an excessively high turn-off voltage spike, which is extremely unfavorable to the switch tube and will also increase the turn-off loss. On the other hand, it will also cause serious duty cycle loss, causing the current peak of the switch device to increase, which will reduce the performance of the system.

Figure 1 Main circuit schematic diagram

3 Zero voltage soft switching

The control mode of the high-frequency full-bridge inverter is the phase-shifted FB-ZVS control mode, and the control chip is the UC3875N produced by Unitrode. The leading bridge arm achieves zero voltage soft switching in the full load range, and the lagging bridge arm achieves zero voltage soft switching in the load range of more than 75%. Figure 2 shows the driving voltage and collector-emitter voltage waveforms of the lagging bridge arm IGBT, and it can be seen that zero voltage switching is achieved.

The switching frequency is selected as 20kHz. This design can reduce the turn-off loss of the IGBT on the one hand, and on the other hand, it can take into account high frequency and reduce the size of the power transformer and output filter.

Figure 2 IGBT drive voltage and collector-emitter voltage waveforms

4 Capacitive power busbars

In the initial experimental prototype, the busbar connecting the filter capacitor C5 _and the IGBT module is an ordinary power busbar. In the experiment, it was found that the voltage on the IGB and the current flowing through the IGBT both experienced high-frequency oscillations. Figure 3 shows the voltage and current waveforms of the transformer primary collected at full power. The reason is that the surge absorption capacitor connected in parallel to the IGBT module and the parasitic inductance of the power busbar experienced high-frequency resonance. After one hour of full-load operation, the temperature rise of the power busbar was 38°C, and the temperature rise of capacitor C5 _was 24°C.

Figure 3 Transformer primary voltage and current waveforms when using a normal power busbar

In order to eliminate resonance and reduce the temperature rise of the power busbar and filter capacitor, we finally adopted a capacitive power busbar. Figure 4 shows the voltage and current waveforms of the transformer primary collected at full power after adopting the capacitive power busbar. It can be seen from the figure that the resonance is basically eliminated. After one hour of full load operation, the temperature rise of the non-inductive power busbar is 11℃, and the temperature rise of capacitor C5 _is 10℃.

Figure 4 Transformer primary voltage and current waveforms after using capacitive power busbar

5. Use multiple transformers in series and parallel to achieve automatic current sharing between parallel output rectifier diodes.

In order to further reduce the loss, the output rectifier diodes are connected in parallel with multiple Schottky diodes with a large current of 400A and a high voltage of 80V. In addition, the secondary output of each transformer adopts a full-wave rectification method. In this way, only one group of diodes flows through the current during each conduction period. At the same time, the secondary rectifier diodes are equipped with an RC absorption network to suppress the parasitic oscillation caused by the transformer leakage inductance and the Schottky diode body capacitance. These measures minimize the output loss of the power supply and are conducive to improving efficiency.

For high current output, the output rectifier diodes are usually connected in parallel. However, since Schottky diodes are devices with negative temperature coefficients, the current sharing between them should generally be considered when connected in parallel. There are many ways to connect diodes in parallel, as shown in Figure 5. Figure a is a direct parallel connection; Figure b is a parallel connection with a resistor in series; and Figure c is a parallel connection with a dynamic current sharing transformer in series. (All take the parallel connection of four diodes as an example).

Figure a is a direct parallel connection method; Figure b is a parallel connection method with a resistor in series; Figure c is a parallel connection method with a dynamic current sharing transformer in series

For the direct parallel connection, the current sharing effect of the diode is very poor, and the output current is generally limited to tens of amperes to hundreds of amperes, and it is not easy to reach thousands of amperes. When the current is thousands of amperes, in order to achieve the purpose of current sharing, you can use the parallel connection method of series resistance or the parallel connection method of dynamic current sharing transformer. Due to the influence of proximity effect and skin effect, for the parallel connection method of series resistance, the current sharing effect of the diode changes with the size of the output current, and the current sharing effect is poor. In order to achieve a better current sharing effect, the series resistance should not be too small, which will bring greater losses. For the parallel connection method of series dynamic current sharing transformer, a better current sharing effect can be achieved, but the manufacturing process of large current transformer is complicated and the cost is high. At the same time, due to the leakage inductance and lead inductance of the dynamic current sharing transformer, the reverse peak voltage of the diode increases when it is turned off, and electromagnetic interference and loss increase accordingly.

In order to overcome the shortcomings of the above parallel connection method, so that the output rectifier diode can achieve automatic current sharing, reduce losses, and reduce the complexity of the manufacturing process, we designed a novel high-frequency power transformer, as shown in Figure 1. This transformer is composed of 8 identical small transformers with a transformation ratio of 4:1. Their primary is connected in series, while the secondary adopts a parallel structure. The transformer adopts a cooling method that combines primary self-cooling and secondary water cooling. This is mainly due to the different heat losses and can greatly simplify the manufacturing process of the transformer.

The following takes two transformer groups as an example (as shown in Figure 6) to illustrate the current sharing between diodes.

Figure 6 Schematic diagram of connection of multiple transformers

When u _in is positive, u ₁ and u ₃ are positive, diodes D ₁ and D ₃ are turned on, and D ₂ and D ₄ are turned off. At this time, it can be concluded that:

(1)
(2)
(3)
(4)
(5)
(6)

When the diode voltage drops u _D1 and u _D3 are not equal, it can be concluded from formulas (3), (4), (5), and (6) that the voltages u _A and u _B on the primary sides of the two transformers are also not equal. The voltage on the primary side of the transformer with a higher diode voltage drop is higher, and vice versa. From formulas (1) and (2), we can get: U ₁ =U ₃ , that is, the currents flowing through diodes D ₁ and D ₃ are always equal, achieving automatic current sharing. It can be seen that this connection method of the transformer achieves automatic current sharing of the output rectifier diode by adjusting the voltage on the primary side of a single transformer.

多个变压器的这种连接方式,不仅可以使得输出整流二极管实现自动均流,还可以使得变压器的设计模块化,简化变压器的制作工艺,降低了损耗。与一只单个变压器相比,多个变压器的这种连接方式,减小了变压器的变比,增强了变压器原副边的磁耦合性,减小了漏感(实际测量8个变压器原边串联后的漏感为6uH)，减小了占空比的丢失。图7为满载时变压器初级电压波形V _P 和次级电压波形V _S ,从图中可以看到占空比丢失不多(大约为5%),使得系统的性能显著提高。

Figure 7 Transformer primary and secondary voltage waveforms

6. Design of control circuit

Since the overload bearing capacity of the switch element used in this power supply is limited, the output current must be limited. Therefore, the control circuit adopts a voltage and current dual loop structure (the inner loop is the current loop, and the outer loop is the voltage loop), and the regulator is PID. Figure 8 is a block diagram of the control circuit. After adding the current inner loop, not only can the output current be limited, but also the dynamic response of the output can be improved, which is conducive to reducing the ripple of the output voltage.

Figure 8 is the principle block diagram of the control circuit

In the actual control circuit, the automatic conversion mode of voltage stabilization and current stabilization is adopted. Figure 9 is the automatic conversion circuit of voltage stabilization and current stabilization. Its working principle is: when the current stabilization works, the voltage loop is saturated, the voltage loop output is greater than the current setting, so the voltage loop does not work, and only the current loop works; when the voltage stabilization works, the voltage loop is desaturated, the current setting is greater than the output of the voltage loop, the current setting operational amplifier is saturated, the current setting does not work, the voltage loop and the current loop work at the same time, and the controller at this time is a dual-loop structure. This control method limits the output voltage and output current to a given range, and the specific working mode is determined by the given voltage, given current and load.

Figure 9 is a voltage and current stabilization automatic conversion circuit

Since the capacity of this power supply is 60kW, in order to improve efficiency, reduce volume and improve reliability, soft switching technology is adopted. The control mode of the high-frequency full-bridge inverter is the phase-shifted FB-ZVS control mode, which uses the leakage inductance of the transformer and the parasitic capacitance resonance of the tube to achieve ZVS. The control chip uses the UC3875N produced by Unitrode. Through phase-shift control, the leading bridge arm achieves zero voltage soft switching in the full load range, and the lagging bridge arm achieves zero voltage soft switching in the load range of more than 75%. Figure 2 shows the driving voltage and collector-emitter voltage waveforms of the lagging bridge arm IGBT, which shows that
zero voltage switching is achieved.

7 Conclusion

In this power supply device, the phase-shifted full-bridge soft switching technology is used to enable the power device to achieve zero-voltage soft switching, reduce switching loss and switching noise, and improve efficiency; a novel high-frequency power transformer is designed and used, and the output rectifier diode is automatically current-balanced by adjusting the primary voltage of a single transformer; a capacitive power busbar is designed and used to reduce the oscillation in the system and the heating of the power busbar. The control circuit adopts a voltage-stabilized and current-stabilized automatic conversion scheme to achieve automatic switching of output voltage-stabilized and current-stabilized, improve the reliability of the power supply and the dynamic response of the output, and reduce the ripple of the output voltage. The experiment has achieved satisfactory results, among which the power factor can reach 0.92, the full-load efficiency is 87%, and the output voltage ripple is less than 25mV. Not only that, all indicators have met or even exceeded user requirements, and have passed the technical appraisal of relevant departments, and have now been put into mass production.

References:

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