Building a Low-Cost Bridgeless PFC Design Using an Analog Controller

    I introduced a semi-bridgeless PFC with a standard PFC controller as a candidate for low-cost, high-efficiency PFC. Due to growing efficiency requirements, many power supply manufacturers are turning their attention to bridgeless power factor correction (PFC) topologies. Generally speaking, bridgeless PFC reduces conduction losses by reducing the number of semiconductor components in the line current path. Although the concept of bridgeless PFC has been proposed for many years, its implementation difficulties and control complexity have prevented it from becoming a mainstream topology. This article focuses on key design considerations for a semi-bridgeless PFC with an analog transition-mode PFC controller.

    Most current digital power controllers (e.g., TI's UCD30xx Fusion Digital Power Controller) offer a number of integrated power control peripherals and a power management core, such as a digital loop compensator, a fast analog-to-digital converter (ADC), a high-resolution digital pulse-width modulator (DPWM) with built-in dead time, and a low-power microcontroller. These all benefit complex, high-performance power supply designs such as bridgeless PFC.

    Standard transition-mode PFC controllers rely on current sensing and zero-current detection (ZCD) circuits as on/off triggers for the drive signal. The current sensing circuit detects the peak inductor current to turn off the switch. The ZCD circuit detects the zero-current point in the inductor to turn on the switch. As shown in Figure 1, a semi-bridgeless PFC has two switch legs, S1 and S2, instead of the single leg in a standard PFC. The key tasks become how to feed the two current sensing signals from the switch legs into a single current sense pin and how to feed the two ZCD signals into the single ZCD pin of a standard PFC controller.

    Figure 1. Power stage of a semi-bridgeless PFC circuit.

    Current sensing design

    Semi-bridgeless transition mode PFC is beneficial at higher power levels. The current transformer current sensing circuit shown in Figure 2 is recommended here. Instead of using a current sensing resistor in series with S1 and S2, using a current transformer significantly reduces the power dissipation in the sensing circuit. Furthermore, the diode in the current sensing circuit with the current transformer ensures that the peak current from the desired switching leg is detected.

    Figure 2. Current sensing circuit for a semi-bridgeless transition-mode PFC circuit.

    Zero current detection design

    In a standard transition-mode boost PFC circuit, zero-current detection is typically accomplished by sensing the voltage signal from the PFC inductor's auxiliary winding (Figure 3). An internal comparator detects the voltage polarity change on the auxiliary winding as the turn-on signal for S1. However, this circuit is impractical in a semi-bridgeless PFC. One option is to apply the ZCD circuit used in a transition-mode boost PFC to the two inductors in a semi-bridgeless PFC, with a blocking diode placed in series with the ZCD current-limiting resistor. However, the blocking diode prolongs the V ZCD drop duration and makes the ZCD pin sensitive to noise, leading to improper triggering and protection. A series RC bypasses the inductor's auxiliary winding and provides a simple detection option. When both S1 and S2 are off, one switch (typically a MOSFET) still conducts current through its body diode. This creates a voltage difference between the two switch legs. The capacitor in the ZCD circuit charges, causing V ZCD to exceed V REF. When the inductor current reaches zero, the voltage difference becomes zero, which makes V ZCD < V REF and triggers the turn-on event.

    Figure 3. Zero-current detection circuit for transition-mode boost and semi-bridgeless PFC.

    The above current sensing and ZCD circuits have been implemented in the PMP9640 – a 310W PSU, using transition-mode bridgeless PFC and LLC-SRC. The performance of the Semi-Bridgeless™ PFC in the PMP9640 is compared with the standard™ PFC design in the PMP9531 in Figure 4. In the light-to-medium load range, the Semi-Bridgeless PFC achieves nearly 1% efficiency improvement over the boost PFC.

    Figure 4. PFC efficiency of the transition-mode boost PFC in the PMP9531 and the semi-bridgeless™ PFC in TI's PMP9640 reference design.