Building a Low-Cost Bridgeless PFC Using an Analog Controller

    In PC and server applications, a bridgeless power factor correction circuit (PFC) can be selected, as shown in Figure 1, to meet 80 Plus Platinum or Titanium requirements.

    Figure 1. Power stage of a bridgeless PFC circuit.

    Compared to the traditional boost PFC in Figure 2, a bridgeless PFC eliminates the bridge rectifier and its power losses. For a 400W power supply, at 120VAC/60Hz input, the bridge rectifier power losses can reach as high as 6W. This efficiency reduction of 1.5% due to the bridge rectifier power loss clearly demonstrates why bridgeless PFC is considered when high efficiency is a concern.

    Figure 2. Power stage of a traditional boost PFC circuit.

    In the bridgeless PFC circuit shown in Figure 1, switches S1 and S2 cannot be on simultaneously, requiring complex drive strategies. Digital controllers such as the C2000 Piccolo MCU or the UCD3138 are commonly used as controllers for bridgeless PFC circuits.

    Digital controllers can be used for nearly any control strategy you might want to apply to a power supply. They also offer the ability to read real-time information about the power supply's operation. However, using a digital controller implies a higher bill of materials (BOM) cost. Those who desire high converter efficiency and don't require a communications interface may still want to look for low-cost analog controllers for bridgeless PFC. The semi-bridgeless PFC circuit in Figure 3 is an option for a high-efficiency, low-cost PFC circuit.

    Figure 3. Power stage of a semi-bridgeless PFC circuit.

    A semi-bridgeless PFC combines two boost converters with return diodes to perform power factor correction. The current paths during the positive and negative AC half-cycles are shown in Figure 4. With the help of return diodes DR1 and DR2, switches S1 and S2 can now be turned on and off simultaneously. We can now apply any of the following low-cost, standard analog boost PFC controllers to a semi-bridgeless PFC:

    TI Analog Boost PFC Controller:

    UCC38051 – Transition-mode PFC controller.

    UCC28019 – Continuous Conduction Mode PFC controller, fixed frequency.

    UCC28180 – Continuous Conduction Mode PFC controller with programmable frequency.

    (one)

    (two)

    Figure 4. Bridgeless PFC operating with return diode: during the positive half of the AC cycle (a); during the negative half of the AC cycle (b).

    The UCC38050 and UCC38051 are PFC controllers for low- and medium-power applications requiring compliance with the IEC 1000-3-2 harmonic mitigation standard. These controllers are designed for boost pre-regulators operating in transition mode (also known as boundary conduction mode or critical conduction mode operation). They feature a transconductance voltage amplifier for feedback error processing, a simple multiplier for generating a current command proportional to the input voltage, a current sense (PWM) comparator, PWM logic, and a totem-pole driver for driving an external FET. In transition mode operation, the PWM circuit is self-oscillating, with turn-on controlled by the inductor zero current detector (ZCD pin) and turn-off controlled by the current sense comparator. The controllers also provide peak current limiting, a default timer, overvoltage protection (OVP), and enable functions.

    The UCC28019A 8-pin active power factor correction (PFC) controller uses a boost topology operating in continuous conduction mode (CCM). This controller is suitable for systems ranging from 100 W to >2 kW with a wide-range universal AC line input. Startup current during undervoltage lockout is less than 200 μA. A user-controlled low-power standby mode can be activated by pulling the VSENSE pin below 0.77 V. Average current-mode control is used to achieve low-distortion waveform shaping of the input current, eliminating the need for input line sensing and reducing external component count. A simple external network allows flexible compensation of the current and voltage control loops. The switching frequency is internally fixed and trimmed to an accuracy of better than ±5% at 25°C. A fast 1.5 A peak gate current drives the external switch.

    Numerous system-level protection features include peak current limiting, soft overcurrent, open-loop detection, input loss, and output over/undervoltage. Soft-start limits boost current during startup. A trimmed internal reference provides accurate protection thresholds and regulation setpoints. An internal clamp limits the gate drive voltage to 12.5V.

    The UCC28180 is a flexible and easy-to-use 8-pin active power factor correction (PFC) controller that operates in continuous conduction mode (CCM) to achieve high power factor, low current distortion, and excellent voltage regulation for AC-DC front-end boost pre-regulators. The controller is suitable for universal AC input systems operating in the 100-W to several kW range, with a switching frequency programmable between 18 kHz and 250 kHz to easily support power MOSFET and IGBT switches. Integrated 1.5-A and 2-A (SRC-SNK) peak gate drive outputs, internally clamped to 15.2 V (typical), enable fast turn-on, turn-off, and easy management of external power switches without the need for snubber circuitry.

    Average current-mode control is used to achieve low-distortion waveform shaping of the input current, eliminating the need for input line sensing and reducing external component count. Furthermore, the controller features a lower current-sense threshold, facilitating the use of small-value shunt resistors to minimize power dissipation, particularly important in high-power systems. To achieve low current distortion, the controller also features internal current-loop regulation circuitry to eliminate associated inaccuracies.

    A simple external network allows for flexible compensation of the current and voltage control loops. Furthermore, the UCC28180 provides an enhanced dynamic response circuit based on the voltage feedback signal to provide better response during fast load transients, whether during output overvoltage or undervoltage conditions. When the voltage feedback signal exceeds V-OVP_L, the unique VCOMP discharge circuit provided in the UCC28180 is activated, giving the control loop an opportunity to quickly stabilize and avoid encountering overvoltage protection, which typically results in audible noise due to PWM shutdown. The controller also features soft-start, which gradually regulates the input current during startup and reduces stress on the power switch. The controller incorporates numerous system-level protection features, including VCC UVLO, peak current limit, soft overcurrent, output open-loop detection, output overvoltage protection, and open pin detection (VISNS). A trimmed internal reference provides accurate protection thresholds and regulation setpoints. Users can control a low-power standby mode by pulling the VSENSE pin below 0.82 V.