onsemi Reimagines Bridgeless Totem Pole PFC Design to Boost Efficiency

March 21, 2022 by Jake Hertz

A system's complexity and cost tend to be two major design considerations. Hoping to ease this stress, a new mixed-signal controller from onsemi aims to achieve high efficiency at low complexity.

In many cases, one of the largest sources of inefficiency in an electronic system is the power conversion circuitry. 

One historical pain-point is AC/DC conversion, where the traditional bridge rectifier-based AC/DC converters are no longer power efficient enough to meet many product requirements.

Instead, engineers have turned to new alternatives to the full-bridge rectifier, such as totem pole bridgeless converters and power factor correction (PFC) systems.


An example conventional boost PFC circuit (left) and a bridgeless totem pole PFC circuit. Screenshot used courtesy of onsemi


Today, onsemi released a brand new mixed-signal controller which aims to enable high-efficiency PFC systems while significantly reducing system complexity.

This article will discuss totem pole PFC converters and how onsemi’s newest controller seeks to improve the technology. 


Sources of Inefficiency in AC/DC Converters

In traditional AC/DC converters, there are two major sources of inefficiency.

The first significant source of inefficiency is the diode bridge itself. Specifically, the forward voltage of standard rectifying diodes means that, when conducting, the diodes are a glaring source of power loss. 

Research has shown that a rectifying bridge will consume ~2% of the input power at the low line of a wide mains application—a non-trivial amount of loss when designing high-efficiency systems.


Output voltage and current waveforms of an AC/DC converter.

Output voltage and current waveforms of an AC/DC converter. Image used courtesy of Monolithic Power


AC/DC converters tend to have low power factors (i.e., less than 1.0), the ratio between deliverable power (kW) and total power (kVA), due to waveform distortions that result from output ripple voltages. 

Beyond this, AC/DC converters tend to suffer losses due to poor power factors. When looking at the waveforms of an AC/DC converter, it becomes clear that the output capacitor only charges for a short period where the voltage at the capacitor’s input is greater than the voltage on the capacitor. 

This feature means that the capacitor undergoes current spikes during voltage peaks, and this nonlinear behavior results in distortion, harmonics, and ultimately low power factor.


Bridgeless Totem Pole PFC

To address the losses related to the diode bridge, engineers instead opt for what is known as a bridgeless totem pole architecture. 

In this architecture, the diodes are replaced with metal–oxide–semiconductor field-effect transistors (MOSFETs), which are precisely controlled to switch in the same way the diode bridge would. This way, the losses of the diodes are mitigated.


A bridgeless rectifier with PFC implemented with SiC transistors.

A bridgeless rectifier with PFC implemented with SiC transistors. Image used courtesy of Texas Instruments


To address power factor challenges, engineers will couple the bridgeless totem pole architecture with PFC circuitry. The most popular form of PFC is active PFC, which integrates a boost topology into the converter circuit to increase the voltage being fed to the output capacitor. 

In this way, active PFC ensures a continuous flow of current from input to output which removes waveform distortions and “corrects” the power factor, increasing efficiency.

However, the challenge with bridgeless totem pole PFC circuitry is the level of complexity it adds to the circuit. Basically, accurately controlling the bridgeless totem pole and the boosting circuitry in an active PFC circuit requires an MCU, which has the undesirable effect of adding complexity, cost, and the need for firmware development to the system.


onsemi’s PFC Controller Reduces Complexity and Cost

As mentioned, onsemi made headlines as it released its new NCP1681 mixed-signal PFC controller. The new IC is a dedicated PFC controller for driving FETs in a bridgeless totem pole PFC topology. 

In the follow-up to their NPC1680 controller, the NCP1681 extends power capabilities into the kilowatt range, operating on mains from 90-265 Vac. Using a dedicated controller IC, onsemi hopes to allow engineers to reduce system complexity and cost by removing the need for control units like MCUs. 

Altogether, the NPC1681 enables topologies to achieve up to 99% efficiency at the high line and help achieve efficiency standards such as 80 Plus or CoC (Code of Conduct) Tier 2 that require high efficiency over a wide load range. 

On top of this, the IC offers other features such as continuous conduction mode (CCM), multi-mode, and digital voltage loop compensation.

All in all, this device can help make using totem pole typography easier and more cost-efficient.