3 Noise Suppression for TP-PFC

One method to minimize Common-mode noise is to slowly charge the Y-capacitors before exiting the AC zero-crossing region. This can be achieved by allowing the high-frequency PWM to produce a few short pulses. However, this approach does not completely suppress the Common-mode current, as the current spikes remain large enough to saturate the EMI Common-mode choke and cause EMI failures, as shown in Figure 3-1.

Figure 3-1. Controlled Y-Capacitor Charge Redistribution Scheme with Active Pulses

In this regard, the current source scheme is introduced for Y-capacitors charge redistribution.

Figure 3-2. Controlled Y-Capacitor Charge Redistribution with Current Source Scheme

Figure 3-2 shows the simplified PFC circuit with the current source scheme added. A current source is formed by a Zener diode, a resistor (R1/R2) and a MOSFET. In this scheme, the MOSFET is operated in its linear region, functioning as a standard current source. This configuration controls the rate of charge redistribution between the Y-capacitors at the AC zero crossing. The duration for which this current source needs to be activated depends mainly on the Y-capacitor on the output side, as the process is focused on charge redistribution and is influenced by the smallest Y-capacitor. With the given Y-capacitors, and assuming the PFC is driven with a transformer, a fixed charging time can be achieved. If an AC source is used instead of a transformer, the Y-capacitor on the input side will be larger and will require an extended activation time for the current source. Note that excessive current source activation can lead to burnout of the resistor or MOSFET.

The firmware monitors V (N, PGND) to determine how long the current source needs to be enabled. Figure 3-3 depicts the turn-on time of the current source circuit to suppress the current spike and Common-mode noise.

Figure 3-3. Suppressed Common-Mode Noise with Current Source Circuit

Figure 3-4 shows that when GPIO_Y_H is active, the Y-capacitors are charged by the current source and are discharged to VOUT before the PWM_H_L1 and PWM_H_N switches become active. With PWM_H_N, the potential at AC_N is connected to VOUT, bringing this potential close to VOUT.

Figure 3-4. AC Input Going From Zero Crossing to Negative Halfwave

When GPIO_Y_L is active, the Y-capacitors (which are charged to VOUT) are discharged by the current source controlled by GPIO_Y_L before the PWM_L_L1 and PWM_L_N switches become active. With PWM_L_N, the potential at AC_N is connected to PGND, causing this potential to approach 0V, as depicted in Figure 3-5.

Figure 3-5. AC Input Going From Zero Crossing to Positive Halfwave