Practical Techniques to Increase Power Converter Efficiency with Resonant Topologies

When a factory’s power bill spikes, the first thing most managers do is blame the machines. In reality, a lot of that waste lives inside the converter itself. Getting those losses down can mean a healthier bottom line, cooler equipment, and a smaller carbon footprint – all reasons to pay attention right now.

Why resonant topologies matter today

The world is moving faster, and the demand for compact, high‑power devices is only growing. Traditional hard‑switching converters are simple, but they waste energy every time a transistor turns on or off. Resonant topologies, by contrast, let the switch move through a “soft” region where voltage or current is already low. The result is less heat, higher efficiency, and longer component life. In my work at Resonance Engineering, I’ve seen a 5‑10 % jump in efficiency just by swapping a conventional buck for a resonant version.

The basic idea of resonance

Resonance is a natural phenomenon you probably met in a physics class: a system that stores energy in two forms – usually magnetic (inductors) and electric (capacitors) – and swaps it back and forth. In a power converter, we arrange an inductor and a capacitor so that their energy exchange creates a sinusoidal voltage or current waveform. When the switch turns on or off at the right moment, the voltage across it is near zero (zero‑voltage switching, ZVS) or the current through it is near zero (zero‑current switching, ZCS). Those are the sweet spots that keep loss low.

Key techniques to boost efficiency

Below are the practical tricks I rely on when I design a resonant converter. They are simple enough to try on a bench, yet powerful enough to make a real difference.

1. Soft switching – the foundation

Soft switching means the transistor never sees a high voltage while a large current is flowing, and vice‑versa. To achieve this, you typically add a series resonant tank (an inductor Lr and a capacitor Cr) between the switch and the load. The tank forces the voltage and current to cross zero at the switching instant. In practice, you tune Lr and Cr so that the resonant frequency is a little higher than the switching frequency. That way the waveforms naturally settle into the soft region.

2. Zero‑voltage switching (ZVS)

ZVS is a special case of soft switching where the voltage across the switch is zero when it turns on. The trick is to let the resonant capacitor discharge a little before the gate signal arrives. A small “pre‑charge” current flows through the switch, pulling the voltage down. The result is a painless turn‑on with almost no loss. In my lab, I often use a gate driver that adds a short lead‑time pulse to the main drive – a tiny timing tweak that pays off in a few percent efficiency gain.

3. Zero‑current switching (ZCS)

ZCS flips the script: the current through the switch is zero when it turns off. This is useful when the switch has a high reverse‑recovery charge, such as a diode‑based MOSFET. By letting the resonant inductor carry the current to zero before the gate signal goes low, you avoid the painful “shoot‑through” that burns energy. ZCS works best with a series‑resonant tank and a slightly lower resonant frequency than the switching frequency.

4. Optimized magnetic design

Inductors and transformers are the heart of any resonant converter. A poorly wound coil can add core loss, stray capacitance, and unwanted parasitic resistance. I always start with a ferrite core that has low loss at the intended frequency, then wind the coil tightly and evenly. Using Litz wire for high‑frequency applications reduces skin‑effect loss. A quick tip: measure the DC resistance of the winding before you assemble the tank – a high reading usually means a bad turn or a loose connection that will sap efficiency later.

5. Loss‑aware component selection

Even with perfect resonance, the rest of the circuit can drag you down. Choose MOSFETs with low gate charge and low on‑resistance (RDS(on)). Look at the total gate drive loss, not just the static on‑resistance. For diodes, a Schottky device often beats a regular silicon diode in forward drop, but be sure its reverse recovery time fits the switching speed. Capacitors matter too: a low‑ESR (equivalent series resistance) film capacitor will keep the resonant tank’s Q factor high, meaning less energy is wasted each cycle.

Putting it together – a quick design checklist

Define the power level and frequency range. This sets the size of Lr and Cr.
Pick a core material that stays low‑loss at the chosen frequency.
Calculate resonant frequency: f₀ = 1/(2π√(Lr·Cr)). Aim for f₀ ≈ 1.2 × switching frequency for ZVS, or a bit lower for ZCS.
Select the switch with low RDS(on) and suitable gate charge. Verify the driver can provide the pre‑charge pulse for ZVS.
Design the gate driver to add a few nanoseconds of lead‑time for ZVS, or a trailing edge delay for ZCS.
Lay out the PCB with short, wide traces for the resonant tank, and keep the loop area small to reduce stray inductance.
Validate with a scope. Look for voltage and current crossing zero at the switching edges. Adjust Lr or Cr if the waveforms are too skewed.
Measure efficiency at full load and at 50 % load. Resonant converters often keep efficiency high across a wide range, but it’s good to confirm.

My recent project – a lesson learned

Last quarter I was asked to improve the efficiency of a 2 kW RF power supply used in a plasma etching line. The original design used a hard‑switching buck converter with a 92 % peak efficiency. By swapping in a series‑resonant topology, adding a small pre‑charge gate driver, and re‑winding the inductor with Litz wire, we nudged the efficiency up to 96 % at full load. The biggest surprise? The temperature of the MOSFET dropped by 30 °C, which meant we could reduce the heatsink size and free up space on the panel. It was a reminder that sometimes the simplest change – a few nanoseconds of timing – can have a big impact.

At Resonance Engineering, I love watching those numbers climb. It feels a bit like tuning a musical instrument; you listen, you adjust, and when the right note rings out, you know you’ve done something right.