A design scheme for push-pull inverter vehicle-mounted switching power supply circuit

With the increase in the types of electrical equipment used in modern cars and the increase in power levels, more and more types of power supplies are required, including AC power and DC power. These power supplies all need to use a switching converter to increase the DC voltage of +12VDC or +24VDC provided by the battery to +220VDC or +240VDC through a DC-DC converter, and then convert it into industrial frequency AC power or variable frequency voltage regulating power through a DC-AC converter. For the front-stage DC-DC converter, it includes a high-frequency DC-AC inverter part, a high-frequency transformer and an AC-DC rectifier part. Different combinations are suitable for different output power levels, and the conversion performance is also different.

The push-pull inverter circuit has been widely used for its simple structure and high transformer core utilization, especially in low-voltage and high-current input of small and medium power occasions; at the same time, the full-bridge rectifier circuit also has the characteristics of high voltage utilization and high output power support. In view of this, this paper proposes a push-pull inverter on-board switching power supply circuit design. Based on the push-pull inverter-high-frequency transformer-full-bridge rectifier design, this scheme further designs a DC-DC converter with 24VDC input-220VDC output and rated output power of 600W, and uses the AP method to design the corresponding push-pull transformer.

1 Working principle of push-pull inverter

Figure 1 shows the basic circuit topology of a push-pull inverter-high-frequency transformer-full-bridge rectifier DC-DC converter. By controlling the two switch tubes S1 and S2 to conduct alternately at the same switching frequency, and the duty cycle d of each switch tube is less than 50%, a certain dead time is reserved to avoid S1 and S2 being turned on at the same time. The input DC low voltage is inverted into AC high-frequency low voltage by the front-stage push-pull inverter, sent to the primary side of the high-frequency transformer, and through transformer coupling, AC high-frequency high voltage is obtained on the secondary side, and then the desired DC high voltage is obtained after full-bridge rectification and filtering composed of reverse fast recovery diodes FRD. Since the minimum reverse voltage that the switch tube can withstand is twice the input voltage, that is, 2UI, and the current is the rated current, the push-pull circuit is generally used in small and medium-power occasions with low input voltage.

Figure 1: Overall topology circuit diagram of the design

When S1 is turned on, its drain-source voltage uDS1 is just the conduction voltage drop of a switch tube. Ideally, it can be assumed that uDS1=0. At this time, an induced voltage will be generated in the winding, and according to the same-name terminal relationship of the primary winding of the transformer, the induced voltage will also be superimposed on the turned-off S2, so that the voltage that S2 bears when it is turned off is the sum of the input voltage and the induced voltage, which is about 2UI. In practice, the leakage inductance of the transformer will generate a large spike voltage on both ends of S2, causing a large turn-off loss. The efficiency of the converter is not very high due to the limitation of the leakage inductance of the transformer. An RC snubber circuit, also called an absorption circuit, is connected between the drains of S1 and S2 to suppress the generation of spike voltage. And in order to provide a feedback loop for energy feedback, a freewheeling diode FWD is connected in anti-parallel at both ends of S1 and S2.

2 Design of switching transformer

The area product (AP) method is used for design. For the push-pull inverter working switching power supply, the primary supply voltage UI=24V, the secondary side is a full-bridge rectifier circuit, the expected output voltage UO=220V, the output current IO=3A, the switching frequency fs=25kHz, the initial transformer efficiency η=0.9, and the working magnetic flux density Bw=0.3T.

(1) Calculate the total apparent power PT. Assume the reverse voltage drop of the fast recovery diode FRD: VDF = 0.6 * 2 = 1.2V

3 Analysis of push-pull inverter problems

3.1 Energy Feedback

During the conduction period of the main circuit, the primary current increases with time, and the conduction time is determined by the drive circuit.

Figure 2: Push-pull inverter energy feedback equivalent circuit

Figure 2(a) is the equivalent circuit when S1 is turned on and S2 is turned off. The arrow in the figure indicates the direction of current flow, which flows out from the positive electrode of the power supply UI, flows through S1 into the negative electrode of the power supply UI, that is, the ground. At this time, FWD1 is not turned on; when S1 is turned off, before S2 is turned on, due to the storage of primary energy and leakage inductance, the terminal voltage of S1 will increase, and the terminal voltage of S2 will decrease through transformer coupling. At this time, the energy recovery diode FWD2 connected in parallel with S2 has not yet turned on, and no current flows in the circuit until an induced voltage with positive voltage on the top and negative voltage on the bottom is generated on the primary winding of the transformer. As shown in Figure 2(b); FWD2 is turned on, and the flyback energy is fed back to the power supply, as shown in Figure 2(c), and the arrow points to the direction of energy feedback. Figure 3 shows the ideal working waveform of the switching transformer circuit designed by the AP method.

Figure 3: Ideal operating waveform of the switching transformer circuit

3.2 Waveform analysis at each point

When a falling edge of a PWN signal comes, the switch element it controls is turned off. Due to the storage of primary energy and leakage inductance, a surge voltage is generated at the drain, which is greater than 2UI. Because an RC buffer circuit is added, it is finally stabilized near 2UI.

When the falling edge of the PWN signal of S1 comes, S1 is turned off, and a high impulse voltage is generated at the drain, which turns on the feedback energy diode FWD2 in parallel with S2, forming an energy feedback loop. At this time, a high impulse current is generated at the drain of S2, as shown in Figure 4.

Figure 4: S2 drain generates a high surge current

3 Experiments and Analysis

3.1 Principle design

Figure 5 is a simplified main circuit. The input 24V DC voltage is filtered by a large capacitor and connected to the middle tap of the primary side of the push-pull transformer. The other two taps of the primary side of the transformer are connected to two fully controlled switching devices IGBT, and an RC absorption circuit is added between them to form a push-pull inverter circuit. The output end of the push-pull transformer is rectified by a full bridge and filtered by a large capacitor to obtain a 220V DC voltage. The feedback voltage signal UOUT is obtained through the voltage divider branch.

Figure 5: Push-pull DC-DC converter main circuit diagram

The control circuit is constructed with the CA3524 chip as the core. The switching frequency of the fully controlled switch device is adjusted by adjusting the resistance and capacitance between pins 6 and 7. Pins 12 and 13 output PWM pulse signals, and through the drive circuit, they control the two fully controlled switch devices alternately. The voltage feedback signal is input to pin 1 of the chip, and the reference voltage of the voltage feedback signal is input to pin 2 by adjusting the potentiometer P2. Together with the COM terminal of pin 9 and the internal operational amplifier of CA3524, a PI regulator is formed to adjust the PWM pulse duty cycle to achieve the purpose of stabilizing the output voltage of 220V.

3.2 Results and Analysis

The experimental results show that the output voltage is stable at 220V, the ripple voltage is small, the maximum output power can reach nearly 600W, and the system efficiency is basically stable at 80%, achieving the expected effect. As shown in Table 1 below.

Among them, the system efficiency is low due to the large efficiency loss of IGBT. If MOSFET with smaller loss is used, the system efficiency will increase by at least 10%~15%.

Note:

(1) When the primary winding of the transformer is excited in the positive and negative directions, the corresponding volt-second products are not equal, which will cause the working magnetization curve of the core to deviate from the origin. This magnetic bias phenomenon is related to the selection of the switch tube because the different reverse recovery times of the switch tube can lead to different volt-second products.

(2) In the experiment, as the input voltage slightly increased, the system loss increased. The main reason is that the transformer core produces large eddy current losses, which reduces the system efficiency. The main measures to reduce eddy current losses are: reducing the induced potential, such as using iron powder core materials; increasing the resistivity of the core, such as using ferrite materials; lengthening the path of the eddy current, such as using silicon steel sheets or amorphous strips.

4 Conclusion

This scheme uses a DC-DC converter with 24VDC input and 220VDC output and a rated output power of 600W, and uses the AP method to design a high-frequency push-pull transformer. The experimental results show that this scheme stabilizes the output voltage at 220V and has a certain output hardness, and the efficiency reaches 80%, which is particularly suitable for low-voltage and high-current input small and medium-power occasions.