In general, a conventional single-stage isolated PFC converter has the disadvantages of high voltage stress and large loss compared with a conventional DC/DC converter. In view of the above problems, this paper designs a single-stage PFC converter, which uses a Boost-Flyback topology with source clamp and soft switch to effectively limit the voltage spike of the switch. Under the soft switching condition, the main and auxiliary switch tubes of the circuit reduce their own switching losses and improve the overall output efficiency of the power system. In addition, the main and auxiliary switch tubes share a set of control circuits and have strong practicability.
1, system design 1.1, system structureA complete, independent working AC/DC switching power supply usually includes EMC (electromagneTIccompaTIbility) filter rectifier circuit, main circuit (including power converter, isolation transformer), PWM (pulsewidthmodulaTIon) generation and control circuit, drive circuit, output circuit, auxiliary Power circuit, sampling circuit. The system structure block diagram designed in this paper is shown in Figure 1. Its working principle is: input AC mains 220V, after EMC filter rectifier circuit, main circuit, output circuit, get 24V DC voltage; At the same time, use current sampling and voltage sampling to form a double closed-loop control mechanism, PWM waveform is adjusted by PWM control chip Then, the driving circuit is used to control the on and off of the power switch tube, thereby realizing a stable output of the 24V DC voltage.
Figure 2 shows the main circuit topology of a single-stage PFC converter with source clamp and soft switch. In this paper, based on the basic single-stage isolated PFC converter circuit, an auxiliary switching transistor Q and a clamping capacitor c are added. , constitute the active clamp; use the transformer leakage inductance as the resonant inductor L, the output capacitance of the main switch Q and the auxiliary switch Q: and the sum of other parasitic capacitances in the circuit as the resonant capacitor c, (dotted line), through L and The resonance of c to achieve soft switching.
As far as the whole system is concerned, the PWM generation and control circuit and the drive circuit are like the human brain, controlling the normal operation of the entire system. This article abandons the traditional PFC professional chip (such as L6561), selects the PWM control chip UC3846. This solution not only realizes real-time control of the PFC boost circuit and the flyback circuit, but also simplifies peripheral circuits and reduces costs.
UC3846 is a high-performance fixed-frequency current-mode PWM control chip produced by Texas Instruments. It uses dual complementary outputs and is often used as a driving signal source for half-bridge and full-bridge bipolar converters. The chip's advantages are full-featured, built-in high-frequency oscillator, differential current sense amplifier, precision reference voltage, under-voltage lockout circuit and soft-start circuit, with self-protection. The external control circuit is relatively simple, the working frequency is up to 500kHz, and it is suitable for the external environment of -165-150 °C, and the input voltage range is 8~40v. The block diagram of the chip is shown in Figure 3.
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