LR726 battery!A push-pull inverter vehicle switching power supply circuit design scheme

This article adopts the push-pull inverter-high-frequency transformer-full-bridge rectification scheme to design 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.

With the increase in the types of electrical equipment and power levels in modern automobiles, more and more types of power supplies are required, including AC power supplies and DC power supplies. These power supplies 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. Frequency conversion and voltage regulation power supply. For the front-stage DC-DC converter, it also 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 due to its simple structure and high utilization rate of the transformer core, especially in small and medium power situations with low voltage and large current input. At the same time, the full-bridge rectifier circuit also has the advantages of high voltage utilization rate and supports high output power. Therefore, this article adopts the push-pull inverter-high-frequency transformer-full-bridge rectification scheme to design 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.

How push-pull inverter works

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 switching tubes S1 and S2 to alternately conduct at the same switching frequency, and the duty cycle d of each switching tube is less than 50%, a certain dead time is left to prevent S1 and S2 from being turned on at the same time. The push-pull inverter at the front stage inverts the input DC low voltage into AC high-frequency low voltage, which is sent to the primary side of the high-frequency transformer, and coupled through the transformer, the AC high-frequency high voltage is obtained on the secondary side, and then through the reverse fast The full bridge composed of the recovery diode FRD is rectified and filtered to obtain the desired DC high voltage. Since the back pressure that the switching tube can withstand is at least twice the input voltage, that is, 2UI, and the current is the rated current, push-pull circuits are generally used in small and medium power applications with low input voltages.

Figure 1: Push-pull inverter-high-frequency transformer-full-bridge rectifier circuit diagram

When S1 is turned on, its drain-source voltage uDS1 is just the conduction voltage drop of a switching tube. Under ideal circumstances, uDS1=0 can be assumed. At this time, an induced voltage will be generated in the winding, and according to the name of the primary winding of the transformer, terminal relationship, the induced voltage will also be superimposed on the turned-off S2, so that the voltage S2 withstands when 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 produce a large A peak voltage is applied to both ends of S2, causing 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 buffer circuit, also called an absorption circuit, is connected between the drains of S1 and S2 to suppress the generation of peak voltages. And in order to provide a feedback loop for energy feedback, freewheeling diodes FWD are connected in anti-parallel at both ends of S1 and S2.

Design of switching transformer

The area product (AP) method is used for design. For 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, and the initial transformer efficiency η= 0.9, working magnetic flux density Bw=0.3T.

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

Design of switching transformer using AP method

Problem analysis of push-pull inverter

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) shows the equivalent circuit when S1 is on and S2 is off. The arrow in the figure is the direction of current flow. It flows out from the positive terminal of the power supply UI, passes through S1 and flows into the negative terminal of the power supply UI, that is, ground. At this time, FWD1 is not conducting; when 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 in parallel with S2 is restored. The diode FWD2 is not yet conducting, and no current flows in the circuit until an upper positive and lower negative induced voltage is generated on the primary winding of the transformer. As shown in Figure 2(b); FWD2 is turned on, feeding back the flyback energy into the power supply, as shown in Figure 2(c), the arrow points in the direction of energy feedback.