New PFC Architecture is 90% Efficient Down to 2 Watts

Ally Winning, European Editor, PSD


Pulsiv claims to have achieved savings in size and cost, while boosting efficiency across the whole frequency spectrum in its new front-end design


Pulsiv CEO Darrel Kingham


The introduction of digital control to the power industry has gone a long way to make our designs more efficient. Being able to accurately monitor and actively control the circuit may add a little cost and complexity to some designs, but the outcome is generally worth it. Even though digital control has been implemented for well over a decade, there are still savings to be found in cost, size and efficiency. Cambridge-based startup Pulsiv claims to have achieved savings in all three of these metrics with the launch of a new front-end design intended to replace LLC solutions with flyback converters in power supplies and battery charging applications.


The company has developed a method of tightly controlling the charging and discharging of the storage capacitor to provide accurate power factor correction (PFC), removing the need for a PFC inductor. Pulsiv’s new architecture places the capacitor storage in parallel to the supply, ensuring that the power doesn’t have to pass through the storage element, as is usual in PFC applications. The power factor is kept high through smart switching controlled by the company’s OSMIUM microcontroller running a dedicated algorithm. This switching regulates the current into and out of the capacitor along with the maximum voltage, ensuring that only the energy required to compensate for the weakest point in the mains supply is stored. As the AC signal from the grid heads towards zero, the capacitor is discharged. The architecture has a separate path for charging the capacitor, which works with the supply to maximize the power factor. The discharge path is largely controlled by the follow-on DC/DC converter.


A big advantage of this design comes from the ability to control the charge path to the main storage capacitor. It basically has no inrush current and the only thing that can be seen when the solution is plugged into any power supply is the initial transient from the safety capacitors. Beyond that, there is no inrush because much smaller capacitance values are used for the storage. Instead of the traditional and expensive 400 or 450 V capacitors, lower voltage rated capacitors are used, which are significantly cheaper – giving a more cost effective solution than a boost PFC-type stage.



Explaining the new architecture’s advantages, Pulsiv CEO Darrel Kingham says, “Today's systems are typically based off a boost PFC front-end that takes the input line voltage, brings it up to around 400 volts and the converter operates from there. The problem with this technique is that at low power the losses in the boost stage become very significant and efficiency falls off. The only way to meet the fairly stringent requirements of regulations, such as ENERGY STAR, is to play a lot of tricks. We have even seen some solutions with a secondary low-power flyback converter in parallel with the flyback to give reasonable coverage across the entire range. Our approach only stores the energy necessary and the power isn't continuously going through the storage element. This means we can achieve high efficiencies all the way down to extremely low power. Even below 4 W the architecture is over 90% efficient”.


To demonstrate the viability of the design, Pulsiv has created a universal input, single switch 150W flyback power supply design that delivers 97.5% average (99.5% peak) front-end efficiency while maintaining 90% at just 2W. A 240W interleaved flyback is currently being developed and reference designs with even higher power capability are in the works.


Kingham continued, “the design has yet to be really optimised. The demonstration board was built during the pandemic and we used components that were available, instead of optimal ones for the circuit. For example, the CoilCraft inductors are meant for use around 150 kHz and the design runs around 40 kHz, so optimizing the inductors will reduce the solution size and cost. There are no GaN devices in the design either. We used fairly cheap MOSFETs, which have small losses due to on-resistance. GaN devices or better MOSFETs will push the efficiency numbers even higher. The design is also very reliable. In the development system, the only elements to reach a relatively high temperature were the magnetics. The ambient temperature was 25oC, the capacitor was measured at 36oC, and the inductor was 50oC. Capacitor manufacturers have told us it is only over 50oC that degrading really starts.”


Pulsiv will sell the products in a number of variations, from the Osmium microcontrollers themselves to full power supply designs. The OSMIUM microcontroller family and supporting components can be combined with commodity flyback DC-DC converters as a direct replacement for higher-cost LLC solutions. Pulsiv OSMIUM microcontrollers - PSV-AD-150 and PSV-AD-250, which are sampling now - can be used as a platform for applications from 1W to 10kW by adjusting three system components and connecting a suitable DC/DC converter. The technology supports Active Bridge Control, Configurable Hold-Up, X-Cap Discharge, HVDC Output Selection, a Power Consumption Indicator and Grid Failure Detection. These optional features can be selected as required.


The PSV-AD-250-DS development system can be used to evaluate Pulsiv OSMIUM technology and connecting a suitable DC-DC converter will produce a complete power supply prototype. Full reference designs are freely available from the company’s website.