A design of a switching power supply suitable for single-phase and three-phase power supply-EEWORLD

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1. Introduction

A 28V/100A three-phase power supply switching power supply was developed for auxiliary operation training of a certain military equipment, and its working performance was good. However, because the troops sometimes go to the field for training, they rent civilian power supplies and only have single-phase power supply. At this time, the switching power supply cannot play the role of auxiliary training. For this reason, it is proposed to design a switching power supply that is suitable for both single-phase and three-phase power supply.

2. Power supply description

2.1 Main performance

of the power supply The maximum output power P0 of the power supply is 2.8KW, the output rated voltage U0 is 28V, and the designed working switching frequency is 20KHZ.
2.2 Input circuit structure

In order to simplify the circuit as much as possible, on the basis of the three-phase power supply control scheme, the method of external transformer is adopted to realize single-phase power supply. Its principle circuit is shown in Figure 1. When powered by a three-phase power supply, A, B, and C are connected to three live wires respectively; when powered by a single-phase power supply, an auxiliary single-phase step-up transformer is added, and its primary side is connected to the power supply. Any two ends of A, B, and C are connected to the secondary side of the single-phase transformer. The two pairs of diodes in the original rectifier circuit are used for single-phase full-wave rectification, and the other pair of diodes is disconnected from the EMI circuit.

2.3 Main Circuit Structure

Considering the power capacity and minimizing the loss and manufacturing cost of the switching power supply device, the main circuit adopts a full-bridge PWM conversion circuit with IGBT as the main switching element. The circuit is shown in Figure 2.

[page]2.4 Control Circuit

The PWM control circuit uses UC3846. The main part of its application circuit is shown in Figure 3 (a). Pin 1 is connected to R1 and R2, which determine the primary current limit value and determine whether the device is locked or restarted when overcurrent occurs. CS+ and CS- are connected to overcurrent signals to implement automatic overcurrent and overvoltage protection. EA+ and EA- are the input terminals of the internal error amplifier, which receive error signals from the output voltage and output current to implement on/short-on time control to achieve the purpose of PWM duty cycle control. Pin 16 SHUTDOWN is the terminal for blocking the output pulse, receiving overcurrent and overvoltage blocking signals. Pins 8 and 9 are connected to resistor RT and capacitor CT that determine the switching frequency. The control output terminals Bout (pin 14) and Aout (pin 11) are connected to the set "1" terminal and the set "0" terminal of the D flip-flop respectively. Through the delayed flip of the flip-flop, a switch signal that lags behind the switch signal of the leading arm by some time is obtained on the lagging bridge arm, and ZVS and ZCS are realized through the soft switching circuit of the main circuit. S1-S4 are control signals output to the IGBT drive circuit. As shown in Figure 3(b), it is an IGBT drive circuit.

[page]3. Circuit parameter calculation

Calculate the components and parameters. The switching frequency and PWM control pulse width (duty cycle) are the key to the output stability, and the IGBT and high-frequency rectifier fast recovery diode are the key to the power supply recovery.

3.1 Calculation of switching frequency and duty cycle

In order to calculate these two parameters, the high-frequency transformer is designed to have a turn ratio of 10:1. Because the power supply output voltage U0 is 28V, the average voltage US' at the input end of the high-frequency transformer should be 280V. From the DC-DC conversion principle, we know that: Us, / Ud = D/T, and Ud = 1.35UL, where: UL---- three-phase power supply line effective value (380V), so, D/T=280/513=0.545=0.55, because it is a full-bridge conversion, so the duty cycle of each switch group Dp=D/2*T=0.2757T Figure 4-a shows a schematic diagram of the working waveform of a group of switches.

Considering the high requirements for power reliability and the high requirements for power volume, the switching power frequency is determined to be 20KHZ, and the minimum dead time is easily calculated to be 11.25µs.
It can be seen that with such a large dead time, the output can be guaranteed to be stable when the input voltage fluctuates greatly.

3.2 Selection of IGBT

The selection of IGBT mainly considers the effective value of the current flowing through the IGBT during normal operation, the average current and the reverse voltage Uces. Because factors such as switching loss heating and harsh working conditions cannot be ignored, the parameters of the components should be taken as more than 2 times the safety factor when selecting. Since it is a full-bridge circuit and the high-frequency transformer ratio is 10:1, the secondary output current is a continuous 100A current, so the average value IL (av) of the primary current flowing through the transformer should be 10A. The stable current waveform flowing through each IGBT is shown in Figure 4-b, and the current is calculated as follows:
Because

Therefore, the steady-state amplitude current of the IGBT is:

The effective value of the IGBT current is:

In the device manual, the rated current of the IGBT component parameters given is the DC continuous average current, while in actual work, the overcurrent damage of the IGBT is generally caused by thermal effects. Therefore, considering the cost and reliability issues, the rated current of the IGBT is often selected to be more than twice the effective value of the working current IT. In the circuit, the working voltage of each switching element is the power supply voltage Ud, that is, Uces=Ud=513V, so the withstand voltage of the IGBT should be selected to be more than 1000V.

3.3 Selection of secondary rectifier diodes

The analysis and calculation method is the same as the selection of IGBT. The peak current flowing through the diode is 180A, the average current IDav=50A, and the effective value current ID=95A, so a fast recovery diode with IFav=200A and VRRM.≥100V is selected.

3.4 Determination of the input rectifier circuit and the additional single-phase step-up transformer

Because when powered by three phases, Ud=1.35UL=1.35×1.707×UP. In order to ensure that the output of the switching power supply remains unchanged when the single-phase power supply is supplied, it is obvious that Ud'= Ud=1.35×1.707×UP must be met at this time. According to the principle of single-phase full-wave rectification, the rectified output voltage Ud' is 1.1~1.2 times the effective value of the rectified output voltage UX. Therefore, from the above formula: (1.1~1.2)UX=1.35×1.707×UP, where UP is the phase voltage.
Therefore, when selecting a rectifier circuit, its withstand voltage value should be higher than that of the three-phase circuit. At the same time, the rectified current value should also be larger than that of the three-phase circuit. Generally, it can be selected to be one-third larger. The transformer turns ratio is designed to be 1:2 to meet the requirements.

4. Selection of PWM circuit

The article selects UC3846 dual-output current-type PWM controller integrated circuit. When the switching frequency of this circuit is 20KHZ, its maximum duty cycle can reach 50%, so its control is effective and stable.

[page]5. IGBT drive and protection

The operation of IGBT is controlled by the positive and negative gate voltage. When UGE>0, IGBT is turned on. When UGE<0 (generally UGS≤-5V), IGBT is turned off. Considering its characteristics and working principle, the M57962L type special drive integrated circuit powered by positive and negative dual power supplies is selected. The circuit has a built-in timing logic short-circuit protection circuit and a protection delay function. The drive protection principle circuit is shown in Figure 3 (b). In addition to the short-circuit (overcurrent) protection function provided by the drive circuit itself, the parallel connection of the RC buffer branch on the IGBT and the parallel connection of the absorption capacitor at both ends of the power supply are also effective methods.

6. Conclusion

This switching power supply is more suitable for practical applications. Because the parameter design of the rectifier circuit is based on single-phase considerations, there is sufficient margin when used in three-phase power supply, so the system is more stable and reliable; it can also work normally when used in single-phase power supply. This solves the contradiction of the switching power supply due to the limitation of the power supply. This method can also be used to transform the existing switching power supply to make it a single-phase or three-phase switching power supply.

References

1. Zhang Zhansong and Cai Xuansan, Principle and Design of Switching Power Supply, 1998.05, Electronic Industry Press
2. Zhao Xiaomin, Design and Application of Switching Power Supply, 1994.09, Shanghai Science Popularization Press
3. Zhang Jin et al., 1500A High Frequency Switching Type Starting Power Supply, 2002.04, Second Issue of Power Electronics Technology

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