New cycle-by-cycle PFC circuit design

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One-piece inductor

Overview:
Power factor correction technology is a research hotspot in the field of power supply design. In this paper, a new design method is proposed for the PFC circuit without multiplier in the power factor correction circuit, which simplifies the circuit and proposes a corresponding improvement method for the instability of the PFC circuit.

In order to improve the efficiency of switching power supply and reduce grid pollution, power factor correction technology has become a research hotspot in the field of power supply design. The current common power factor correction circuits can be divided into two categories - PFC with multiplier and PFC without multiplier. The PFC circuit without a multiplier uses a cycle-by-cycle control technique that controls the switch at the same time using the overcurrent detection signal and the feedback output voltage in one cycle. Compared to multiplier-based systems, PFC systems without multipliers do not require AC input detection techniques. They are compact and require all the information needed to correct current waveforms (to ensure high power factor) from DC bus voltage and loop current. .

In this paper, the Boost PFC circuit without multiplier and working in critical current control mode is taken as an example to introduce the working principle of the cycle-by-cycle PFC circuit. Aiming at the instability of the cycle-by-cycle PFC circuit, a corresponding improvement design method is proposed.

Basic principle of cycle-by-cycle PFC circuit The cycle-by-cycle PFC circuit does not contain a multiplier internally. In order to force the input current to follow the waveform change of the input voltage, the power factor correction function is completed. In one cycle, the error control signal, the overcurrent detection signal, and the The zero-crossing detection signal controls the turn-on and turn-off of the switch. At the initial moment, the current flowing through the external inductor is zero. The switch is opened by the zero-crossing detection signal, the power supply voltage charges the inductor, and the inductor current rises. The output voltage feedback signal passes through the error amplifier and is compared with the internal sawtooth wave to obtain a certain duty. Than the PWM wave. The PWM signal is coupled to the overcurrent detection signal to control the turn-off of the switching transistor. At this time, if the external inductor current exceeds the current limit value, the zero-crossing detection signal jumps to low, and the switch tube is turned off; otherwise, the switch tube is controlled by the output voltage feedback signal. Since the switching frequency is very high relative to the grid voltage frequency, the feedback voltage can be considered constant in one cycle, whereby a PWM signal of a fixed duty cycle can be obtained, and the switching transistor is turned off after reaching the maximum on-time. When the switch is turned off, the inductor current drops until it reaches zero, and the next conduction period is triggered by the zero-crossing detection signal.

Cycle-by-cycle technology can significantly improve the performance of the power supply, with good linearity and fast input and output dynamic response; inherent pulse-by-pulse current limit, simplifying overload and short-circuit protection; eliminating the poles of the output filter inductor, making the power supply The system is reduced from second order to first order, and there is no conditional loop stability problem in the system.
However, when the switching duty ratio of the PFC circuit is greater than 50%, the error generated by the disturbance signal is gradually amplified, which will cause instability of the system and deteriorate the anti-interference performance of the power supply. In addition, when the output is light or no load, It can also cause the power supply to go out of control. In this paper, multi-vector error op amps, slope compensation circuits, dynamic adjustment circuits and timing self-starting circuits are proposed to improve the performance of cycle-by-cycle PFC circuits.
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