Bridgeless PFC Converter: Working, Circuit & Waveforms

Q: What is a bridgeless PFC?

It is a power-factor-correction stage with no separate input diode bridge. The boost switches rectify and shape the current themselves. In the dual-boost version, two boost legs of two switches and two diodes each take one half of the line cycle, so only two semiconductors carry the current at a time.

Q: Why is a bridgeless PFC more efficient than a normal boost PFC?

A conventional boost PFC always has three semiconductors in the current path, two bridge diodes plus the switch or boost diode. A bridgeless PFC removes the bridge, leaving only two devices in the path, so conduction loss drops and efficiency rises by roughly 1 to 2 percent, which matters most at high power.

Q: How does a bridgeless PFC work over the line cycle?

In each half of the line cycle it behaves as a boost converter. During the positive half, inductor L1, switch Q1 and diode D1 do the boosting while return diode D4 completes the current path; during the negative half the mirror image uses L2, Q2 and D2 with return diode D3. The two legs alternate, which both rectifies and shapes the input current.

Q: What is a totem-pole bridgeless PFC?

It is a bridgeless PFC variant with one fast switching leg, typically GaN transistors at high frequency, and one slow leg switching at line frequency. It offers very high efficiency and low common-mode noise and is widely used in servers and EV chargers.

Q: What is the main drawback of a bridgeless PFC?

Common-mode EMI noise. Because the input is no longer tied to the DC ground, the circuit generates more common-mode noise and needs extra filtering. The control and current sensing are more complex and it uses an extra switch. It is still step-up only and non-isolated.

Introduction

A normal boost PFC starts with a four-diode bridge to rectify the mains, then a boost stage to shape the current. It works well, but look at the current path: at every instant the current has to pass through two bridge diodes plus the boost switch (or boost diode) — three semiconductors in series. Each one drops a bit of voltage, and at high power those drops add up to real heat and lost efficiency.

The bridgeless PFC asks a simple question: why keep a separate rectifier bridge at all? Instead it lets the boost switches do the rectifying as well as the current shaping. Removing the bridge means only two semiconductors ever carry the current at once, so conduction losses drop and efficiency climbs — which is exactly why bridgeless PFC is popular in high-power supplies. This page covers the most common version, the dual-boost bridgeless PFC, with its circuit, half-cycle working modes and full waveforms.

What is a Bridgeless PFC?

A bridgeless PFC is a power-factor-correction front end that has no separate input rectifier bridge. The rectifying job is merged into the boost stage. In the classic dual-boost version there are two boost “legs”: two switches (Q1, Q2) and two boost diodes (D1, D2), fed through boost inductors. Each leg takes over for one half of the line cycle, so together they both rectify and shape the input current into a sine.

The result is the same clean, near-unity power factor as an ordinary boost PFC, but with one fewer device in the conduction path at any moment — which is where the efficiency gain comes from.

Bridgeless PFC block diagram: the AC line passes through an EMI filter straight into a bridgeless PFC cell of two boost legs, with no separate diode bridge, feeding a DC bus and load; a controller shapes the current.

Figure 1: Bridgeless PFC block diagram — the diode bridge is gone; the PFC cell rectifies and shapes the current itself.

Bridgeless vs Conventional Boost PFC

The whole point of going bridgeless is the conduction path. Here is the side-by-side:

Feature	Conventional Boost PFC	Bridgeless (Dual-Boost) PFC
Input rectifier	Full diode bridge (4 diodes)	None — merged into the boost switches
Devices in the current path	3 (2 bridge diodes + switch/diode)	2 (switch + diode)
Conduction loss	Higher	Lower
Efficiency	Good	Higher (roughly 1–2 % better)
Switch count	1 switch	2 switches
EMI / control	Simpler; input grounded	Trickier (floating input, common-mode noise, current sensing)
Best for	Low-to-medium power	Higher power, where efficiency matters most

Circuit Diagram & Construction

The dual-boost bridgeless PFC is two boost legs sharing one output:

Two boost inductors (L1, L2) — one in each line wire, carrying and shaping the input current.
Two switches (Q1, Q2) — MOSFETs whose sources join at the negative DC rail. They both switch and rectify.
Two boost diodes (D1, D2) — from each switch node up to the positive DC rail.
Two return diodes (D3, D4) — provide the return path for the line current in each half-cycle (in some designs the MOSFET body diodes do this job instead). D4 returns during the positive half, D3 during the negative half.
Output capacitor (C) and load — across the DC bus.

Dual-boost bridgeless PFC power circuit: two boost inductors, two MOSFET switches with a common source, two boost diodes to the DC bus, output capacitor and load

Figure 2: Dual-boost bridgeless PFC power circuit

How It Works (Half-Cycle Working Modes)

A dual-boost bridgeless PFC has two boost paths — one per half of the line cycle — so over a full cycle the current passes through four working modes (two in each half). In the positive half (v_s > 0) the top wire drives current through L1, and Q1 / D1 do the boosting with D4 as the return diode. In the negative half (v_s < 0) it is L2 and Q2 / D2, with D3 as the return. Each mode is a boost converter in either its charging (switch on) or discharging (switch off) state:

Four working modes of a dual-boost bridgeless PFC: positive half with Q1 on and Q1 off, negative half with Q2 on and Q2 off, showing the conduction path in each

Figure 3: The four working modes of the dual-boost bridgeless PFC

Mode 1 — v_s > 0, Q1 ON: the positive line voltage is placed across L1. Current flows source → L1 → Q1 → D4 → back, so the inductor charges. The output capacitor supplies the load on its own (load current i_L).
Mode 2 — v_s > 0, Q1 & Q2 OFF: L1 now discharges through D1 into the output — charging the capacitor (i_c) and feeding the load (i_L) — and the current returns through D4.
Mode 3 — v_s < 0, Q2 ON: the mirror image. Current flows source → D3 → L2 → Q2 → back, charging L2, while the capacitor again supplies the load alone.
Mode 4 — v_s < 0, Q1 & Q2 OFF: L2 discharges through D2 into the output, and the current returns through D3.
Two devices only: in every mode just two semiconductors carry the line current (the active switch or boost diode, plus the return diode), versus three in a conventional boost PFC — that is where the efficiency comes from.

Line-Cycle Waveforms

Over a full line cycle you can see the two legs hand off to each other and the input current come out as a clean sine.

Figure 4: Over a line cycle, Q1 chops during the positive half and Q2 during the negative half, together shaping a sinusoidal input current in phase with the voltage, while the output bus stays nearly constant.

Input (source) current i_s: a smooth sine in phase with the voltage — near-unity power factor, with only the fast switching ripple on top.
Q1 & Q2 gates: each switch is the active boost device for one half-cycle and rests (its body diode conducting) in the other — that alternation is what replaces the diode bridge.
Output V_out: held near constant by the capacitor, with a small ripple at twice the line frequency.

Switching-Cycle Waveforms

Zooming into a few switching periods (inside one half-cycle), the active boost cell behaves exactly like a boost converter:

Figure 5: Inside a half-cycle — the inductor charges when the switch is on and discharges through the boost diode when off; the switch and diode currents are the two halves of the inductor current.

Inductor current i_L: a triangular ripple that rises with the switch on and falls with it off. In CCM it never reaches zero.
Switch and diode currents: the switch carries the rising part of the ramp; the boost diode carries the falling part and charges the output. Together they equal the inductor current.

Conduction Modes (CCM, CrCM, DCM)

CCM (Continuous Conduction Mode): the inductor current never falls to zero — low ripple and low peak current, so it suits higher power. Most high-power bridgeless PFCs run in CCM.
CrCM / BCM (Critical / Boundary Mode): the next cycle starts exactly as the current reaches zero, giving soft switching — popular at low-to-medium power, with a variable switching frequency.
DCM (Discontinuous Conduction Mode): the current rests at zero for part of each cycle — simple but with high peak currents, used only at low power.

Types of Bridgeless PFC

Dual-boost bridgeless PFC (covered here) — two boost legs with two switches and two boost diodes. The classic, robust choice.
Totem-pole bridgeless PFC — a fast “totem-pole” leg (usually GaN switches) plus a slow line-frequency leg. Very high efficiency and low common-mode noise; now common in servers and EV chargers.
Semi-bridgeless PFC — adds two slow diodes to the dual-boost cell to tame the common-mode noise, trading a little efficiency for easier EMI.

Key Parameters & Formulas

Because each half-cycle is a boost converter, the same boost relations apply (V_in is the instantaneous rectified input):

Quantity	Formula & Value
Boost voltage relation	V_out = V_in / (1 − D)
Duty over the line cycle	D(t) = 1 − \|v_in(t)\| / V_out
Devices in conduction path	2 (vs 3 in a conventional boost PFC)
Typical DC bus	V_out ≈ 390–400 V (above the line peak)
Output ripple frequency	2f (100 Hz on a 50 Hz line)
Typical result	PF > 0.99, THD < 5 %, ~1–2 % higher efficiency

Advantages & Disadvantages

Advantages

Higher efficiency — only two semiconductors conduct at a time, so conduction losses drop.
Less heat to remove, which helps power density in high-power supplies.
Keeps all the PFC benefits: near-unity power factor and low THD.

Disadvantages

More common-mode (EMI) noise because the input is no longer tied to the DC ground — needs extra filtering.
Trickier control and current sensing, and an extra switch and gate driver.
Still step-up only and non-isolated — like any boost PFC, a following isolated DC-DC stage is needed.

Applications

High-power server and telecom power supplies where every efficiency point counts.
EV on-board and off-board chargers.
Industrial SMPS and welding supplies.
High-end adapters and lighting drivers that must hit tight efficiency targets.

Frequently Asked Questions – FAQs

It is a power-factor-correction stage with no separate input diode bridge. The boost switches rectify and shape the current themselves. In the dual-boost version, two boost legs (two switches, two diodes) each take one half of the line cycle, so only two semiconductors carry the current at a time.

A conventional boost PFC always has three semiconductors in the current path (two bridge diodes plus the switch or boost diode). A bridgeless PFC removes the bridge, leaving only two devices in the path. Fewer conducting devices means lower conduction loss and roughly 1 to 2 percent higher efficiency, which matters most at high power.

In each half of the line cycle it behaves as a boost converter. During the positive half, inductor L1, switch Q1 and diode D1 do the boosting while return diode D4 completes the current path; during the negative half the mirror image uses L2, Q2, D2 with return diode D3. The two legs alternate, which both rectifies and shapes the input current.

It is a bridgeless PFC variant with one fast switching leg (typically GaN transistors running at high frequency) and one slow leg switching at line frequency. It offers very high efficiency and low common-mode noise, and is widely used in servers and EV chargers.

Common-mode EMI noise. Because the input is no longer tied to the DC ground, the circuit generates more common-mode noise and needs extra filtering. The control and current sensing are also more complex, and it uses an extra switch. It is still step-up only and non-isolated.

Bridgeless PFC Converter