6 min.

The Evolution of Resonant Power Converter Topologies

Designers of electrical products used in the home and office, such as televisions, large appliances, and heaters, are required to maximise the energy efficiency of their designs to satisfy legislative and market demands. Complying with electromagnetic compatibility (EMC) and safety norms is also a key concern. Resonant power converters can help meet both requirements, thanks to inherently low energy losses and switching noise.

Simple converter topologies, such as buck, boost, and flyback, use pulse-width modulation to regulate the output voltage, with “hard” switching: at the time the modulating switch is turned on, the maximum voltage is present across the main channel of the device.

Hard switching voltage and current waveforms

Hard switching imposes several disadvantages, including high energy losses due to the presence of voltage across the switch, and its finite fall time, at the same time current begins to rise in the switch after turn-on. Losses are also incurred as the switch’s output capacitance (MOSFET Coss) is discharged.

Energy loss in the switch due to voltage and current overlap

In addition, hard switching places stress on semiconductor devices and the drain-source voltage and drain current waveforms can exhibit ringing and transients that produce EMI emissions.

Switching with Resonant Converters

To alleviate the energy losses associated with hard switching, a soft switching strategy operates the switch when the voltage across the main channel and/or the current passing is low or zero. In addition, soft switching also eliminates losses associated with discharging the switch output capacitance and the gate-drive circuit can be simplified due to the absence of Miller effects.

In soft switching, the device is turned on when the applied voltage is very close to zero

Resonant converters achieve zero-voltage switching (ZVS) and zero-current switching (ZCS) by using resonant components, typically inductors and capacitors to form a "resonant tank". This tank is tuned to resonate at a specific frequency, or at multiple frequencies, which allows for a smoother transition of energy between the components. This results in lower electromagnetic interference (EMI) and reduces stress on the semiconductor devices. On the other hand, the tuning requirements add to design complexity and only limited frequency ranges can be used to ensure optimal performance.

Alternatively, quasi-resonant converters operate with characteristics similar to resonant converters, achieving ZVS or ZCS by using resonant elements, but without a strict resonance condition. They combine the benefits of resonant and non-resonant switching to achieve ZVS or ZCS but may not achieve the same level of efficiency as pure resonant converters.

In a two-quadrant converter, designed to supply current in both forward and reverse directions, ZVS is traditionally achieved using an active commutated resonant pole (ACRP) converter. The ACRP uses auxiliary switches to charge and discharge the main switches through an inductor. The auxiliary switches handle freewheeling current that would otherwise pass through the main MOSFETs’ body diodes during reverse recovery, potentially damaging the devices.

Two-quadrant ACRP converter and equivalent circuit

LLC Resonant Converter

Unlike conventional (typically hard-switching) converters, which regulate the output voltage using pulse-width modulation, resonant converters rely on frequency modulation. Changing the frequency of the driving voltage changes the impedance of the resonant circuit to split the input voltage between the impedance and the load.

There are two basic types of resonant converters. These are the series resonant converter (SRC) and parallel resonant converter (PRC). In the SRC case, the resonant circuit works as a voltage divider between the input and the load. One drawback of this circuit is that regulation becomes difficult under light-load conditions when the impedance of the load is much larger than that of the resonant circuit. The frequency required to maintain regulation approaches infinity as the load approaches zero.

In the PRC, which connects the load in parallel with the resonant circuit, a high circulating current is needed. This precludes use where high power density is required or large load variations are expected.

The LLC resonant converter overcomes the limitations of the SRC and PRC by introducing extra components that result in additional resonant frequencies. This permits the converter to regulate the output over wide line and load variations, while maintaining high efficiency.

The resonant circuit could also be built using an LCC configuration. However, the requirement for two capacitors results in a relatively expensive solution. In the LLC circuit, the two inductors can be combined into a single physical component that effectively comprises the series resonant inductance and the transformer magnetising inductance.

LLC resonant half-bridge converter

Resonant Converter with Boost PFC

When designing converters for applications above 75W, power-factor correction is also mandatory. A popular solution is to combine a boost PFC input stage with an LLC converter that provides isolation and controls the output voltage.

A two-stage arrangement like this can satisfy the requirement for high efficiency, EMC compliance and mandatory PFC. However, the BOM count and associated cost of the control and ancillary components can be relatively high. Also, EMI from the two stages can be difficult to predict and manage; the boost PFC can be fixed or variable frequency depending on operating mode, while the LLC is inherently variable frequency. Moreover, ZVS in the LLC converter can be lost at light loads and the topology can only be optimized for fixed output voltages.

Boost PFC and LLC resonant converter for applications above 75W

For applications rated from about 120W to 500W - typical of some home appliances and battery chargers for equipment such as e-bikes, drones, and robots - a novel topology using forced ZVS can deliver advantages. This approach can be seen in the Eggtronic SmartEgg, a single-stage converter that combines PFC and an isolated regulator. One of the most important advantages is the fact that, because the ZVS is forced rather than coming from the reactive network, it is possible to achieve ZVS in every load conditions, ensuring high efficiency in every working point.

In addition, SmartEgg halves the BOM, reducing the cost significantly, increasing power density and - because the number of stages in series is reduced - increasing efficiency compared to traditional PFC + LLC converters.

SmartEgg topology with forced ZVS by EPIC101AGSE ASIC

Also, compared to the traditional PFC + asymmetric half bridge approach, SmartEgg has the intrinsic advantage of behaving like a forward converter, reducing significantly the size of the transformer and removing the boost inductor (which is substituted by the leakage inductance of the transformer).

This converter implements forced ZVS switching with the EPIC101AGSE primary-side AC/DC controller ASIC.

Applications such as power supplies for TV panels, PCs, servers and large home appliances, as well as power conversion in electric and hybrid vehicles and solar panel inverters, require power levels from 500W to 1kW or more. By using similar principles to those applied to SmartEgg, Eggtronic has created the ClassEgg architecture, which allows engineers to address these requirements with a boost PFC + ZVS LLC resonant converter capable of high efficiency over a wide load range.

Designed to offer dual, well regulated outputs, this architecture once again ensures high power density, achieving total peak efficiencies greater than 95% and an extremely flat efficiency curve in every working point, on both outputs.

In comparison with traditional LLC, a single transformer and the proprietary rectifier are able to substitute two LLC in parallel (one for each output), significantly reducing BOM and size of the converter.

ClassEgg topology comprising PFC and resonant converter
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