1. Introduction
With the increasing demand for high power density and efficiency, the totem-pole power factor correction (PFC) circuit has become the core solution for high-power systems of 6.6kW and above due to its advantages of low conduction loss and simple topology. However, near the zero-crossing point of the AC input, PFC circuits are prone to current spikes caused by rapid duty cycle changes of the fast switch, reverse recovery of the body diode, and parasitic capacitance effects. These issues lead to reduced system efficiency, worsened electromagnetic interference (EMI), and increased total harmonic distortion (THD). Traditional hard-switching control strategies can achieve basic power correction but struggle to balance the dynamic response at the zero-crossing point with current stress. This paper proposes a zero-crossing soft-start strategy for a 6.6kW totem-pole PFC based on the high-performance real-time control MCU, ST StellarP6. By dynamically adjusting the duty cycle timing of the fast and slow switches and incorporating a soft-start algorithm, the strategy suppresses inductor current spikes and mitigates the reverse recovery issues of the body diode during the zero-crossing period.
2. Introduction to the PFC Fast and Slow Tube Wave Timing
Based on the ST StellarP6's GTM module (Generic Timer Module), the high-precision timing capability is utilized to achieve fast switching control for totem-pole PFC. The ATOM1_0 and ATOM1_0N channels in GTM Cluster1 are used as the PWM channels for the PFC fast switch, generating complementary PWM signals. The GTM DTM module automatically inserts dead time to prevent direct short circuits between the fast switches (Q1/Q2). The center-aligned mode is adopted to ensure the symmetry of the switch conduction and cutoff. The switching frequency of the fast switch is 67 kHz.
Q3 and Q4 are directly controlled by GPIO, operating at a frequency synchronized with the power grid (50Hz), and remain either normally open or normally closed during an AC half-cycle.
Use the ATOM1_4 channel as the current sampling ADC trigger signal, and trigger the ADC at the midpoint between PWM ON and OFF to ensure the accuracy of grid current sensing.
3. Introduction to PFC Zero-Crossing Soft Start Strategy and Code Implementation
The determination of the zero-crossing point of the Vac voltage is based on the ZeroCross voltage signal obtained from the filtered AC voltage sampling. A second-order generalized integrator (SOGI) is used to filter the AC voltage signal and extract the ZeroCross zero-crossing signal free from harmonic interference.
Centered around the zero-crossing point, a 300μs dynamic window (configurable parameter) is set. By determining in the software whether the system has entered the zero-crossing soft-start window, when the ZeroCross signal switches from positive to negative, the Q2 transistor will switch from a very high duty cycle to a very low duty cycle. Within the zero-crossing soft-start range, the Q2 transistor is completely turned off, and the Q1 transistor performs the soft start. The duty cycle of Q1 will gradually increase to the duty cycle calculated by the loop during the soft-start range time. When the soft-start duty cycle matches the loop-calculated output duty cycle, the duty cycle switches to the loop-calculated output value. Once the ZeroCross signal exits the soft-start range, the Q2 transistor resumes normal operation. The Q3 and Q4 transistors remain off throughout the soft-start range. When the ZeroCross signal switches from negative to positive, the process is repeated as shown in the diagram below.
The code for zero-crossing soft start is as follows:
if (Enable_flag > 0.0F) {
OPL_Flag = ((Q3) && (Q4));
OR_p = (Q3 || Q4);
if ((!OR_p) && (DiscreteTimeIntegrator_PrevRese == 1)) {
DiscreteTimeIntegrator_DSTATE_i = 0.0F;
}
if (OPL_Flag) {
if (iL1_duty_debug > DiscreteTimeIntegrator_DSTATE_i) {
TTPL_OPLduty1 = DiscreteTimeIntegrator_DSTATE_i;
} else {
TTPL_OPLduty1 = iL1_duty_debug;
}
}
In6 = Merge;
DiscreteTimeIntegrator_DSTATE_i += 111.9403F;
DiscreteTimeIntegrator_PrevRese = (int8_T)OR_p;
}4. Experimental Phenomena
The figure below shows the soft start waveform when the AC voltage switches from positive to negative.
The figure below shows the soft start waveform when the AC voltage switches from negative to positive:
Under the 6.6kW full-load condition, it can be observed that there is no significant peak current generated at the AC voltage zero-crossing point, with an iTHD of 1.65%.
5. Conclusion
This article is based on the ST StellarP6 high-performance real-time control MCU and proposes a zero-crossing soft-start strategy for a 6.6kW totem-pole PFC. By dynamically adjusting the duty cycle timing of the fast and slow switches and incorporating a soft-start algorithm, the strategy effectively suppresses the sudden changes in inductor current and the reverse recovery issues of the body diode during zero-crossing periods. Experimental results show that under a full-load condition of 6.6kW, there is no significant current spike at the AC voltage zero-crossing point, and the total harmonic distortion (iTHD) is only 1.65%, significantly improving system efficiency and electromagnetic compatibility. This strategy has achieved notable success in addressing the challenge of balancing dynamic response and current stress at zero-crossing points, which is difficult with traditional hard-switching control strategies. It provides an effective solution for the design of totem-pole PFC circuits in high-power power supply systems.