With the increase in the types of electrical equipment used in modern automobiles and the increase in power levels, more and more types of power supply are required, including AC power supplies and DC power supplies. These power supplies need to use a switching converter to increase the +12VDC or +24VDC DC voltage provided by the Battery to +220VDC or +240VDC through a DC-DC converter, and then convert the DC-DC converter to a commercial frequency AC power supply or a later stage. Frequency regulation power supply. For the pre-stage DC-DC converters, it also includes high-frequency DC-AC inverters, high-frequency transformers, and AC-DC rectifiers. Different combinations adapt to 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 of the transformer core, especially in the low-power high-current input medium and small power applications. At the same time, the full-bridge rectifier circuit also has a high voltage utilization and supports output power. Advanced features. In view of this, this paper proposes a push-pull inverter automotive switching power supply circuit design. Based on the design of push-pull inverter-high frequency transformer-full-bridge rectifier, the design of DC-DC converter with 24VDC input-220VDC output and rated output power of 600W is further designed. The corresponding push-pull transformer is designed using AP method. . 1 push-pull inverter working principle 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 alternately conduct at the same switching frequency, and the duty cycle d of each switch tube is less than 50%, a certain dead time is allowed to avoid simultaneous conduction of S1 and S2. From the pre-push-pull inverter, the input DC low voltage is inverted to AC high-frequency low voltage, sent to the primary side of the high-frequency transformer, and coupled through the transformer, AC high-frequency high voltage is obtained at the secondary side, and then through the reverse rapid The full-bridge rectification and filtering of the recovery diode FRD yields the desired DC high voltage. Since the switch can withstand a backpressure of at least twice the input voltage, ie, 2UI, and the current is the rated current, the push-pull circuit is generally used in low and medium power applications where the input voltage is low. When S1 is turned on, its drain-source voltage uDS1 is just the turn-on voltage drop of a switch. In an ideal case, uDS1 can be assumed to be zero. At this time, an induced voltage will be generated in the winding and the same name according to the primary winding of the transformer. In the end relationship, the induced voltage is also superimposed on the turned-off S2 so that the voltage that the S2 withstands when it is turned off is the sum of the input voltage and the sensed voltage is about 2UI. In practice, the leakage inductance of the transformer will produce a large The spike voltage is applied to both ends of S2, resulting in a large turn-off loss, and the efficiency of the converter due to the leakage inductance of the transformer is not very high. An RC buffer circuit, also referred to as an absorption circuit, is connected between the drains of S1 and S2 to suppress the generation of a spike voltage. And in order to provide a feedback loop for energy feedback, the freewheeling diode FWD is connected in anti-parallel across S1 and S2. 2 switch transformer design Design using area product (AP) method. For push-pull inverter switching power supply, the primary supply voltage UI = 24V, the secondary side is a full-bridge rectifier circuit, the desired output voltage UO = 220V, the output current IO = 3A, the switching frequency fs = 25kHz, the initial transformer efficiency η = 0.9, working flux density Bw=0.3T. (1) Calculate the total apparent power PT. Set the voltage drop of the reverse fast recovery diode FRD: VDF = 0.6 * 2 = 1.2V 3.1 Energy feedback While the main circuit is on, the primary current increases with time, and the on-time is determined by the driver circuit. Figure 2 (a) is S1 conduction, S2 off when the equivalent circuit, the arrow in the figure for the current flow, from the power supply UI positive outflow, after S1 into the power UI negative, that is, at this time FWD1 does not turn on; When S1 turns off, before S2 is turned off, due to storage of the primary energy and leakage inductance, the terminal voltage of S1 will increase, and the voltage at the terminal of S2 will decrease due to the transformer coupling. At this time, the energy recovered in parallel with S2 is restored. The diode FWD2 has not yet been turned on, and no current flows in the circuit until an induced negative voltage 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), 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.
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