All-Silicon RC Snubber Removes Ringing from Power MOSFETs

Silicon carbide (SiC) is rapidly being adopted in applications ranging from electric-vehicle (EV) traction inverters to solar inverters and grid-connected battery energy storage, all of which require a huge amount of power conversion at high voltages.

But the same properties that give it the edge over silicon can also create difficulties. SiC MOSFETs have inherently high slew rates, characterized by rapid di/dt and dv/dt, which often produce voltage transients and parasitic oscillations that cause ringing. These issues can generate electrical stress capable of damaging power devices as well as motor load leakage current and local heating that impacts system reliability and efficiency. The faster switching speeds of SiC may also exacerbate radiated and conducted EMI.

One way to mitigate it all is to integrate a snubber circuit close to the power switch. But traditional snubber circuits — assembled from discrete resistors and capacitors — can struggle to stay on top of the fast-switching speeds of SiC, said Billy Ye, product manager for circuit protection devices at Melexis. “Conventional discrete RC snubbers have many limitations like poor thermal dissipation capability and high parasitic inductance.”

How Melexis tries to solve these problems is with its new monolithic 1,200-V silicon RC snubber circuit, which will be on display at PCIM 2026.

Specifically designed for SiC power modules, the MLX91299 integrates a resistor and a capacitor in series, delivering voltage transient protection in a silicon die measuring only 1.5 × 5 mm. The snubber can be placed close to the switching components to reduce or eliminate ringing before it impacts efficiency and suppress voltage spikes before they impact reliability.

By effectively damping both, the silicon snubber can facilitate faster switching speeds, said Ye. As a result, it reduces critical switching losses by up to 50%, improving efficiency while lessening the need for thermal management.

The snubber circuit is designed to be deployed across each switch in a SiC power module for protection. With peak voltages of up to 1,200 V and a breakdown voltage of over 1,500 V, Melexis said it’s intended for DC-link voltages of up to 1,000 V.

Why Ringing Happens, and How Snubber Circuits Reduce It

Power designers, constantly in pursuit of power converter designs with higher efficiency and power density, are increasingly turning to the faster switching speeds of SiC and GaN to get there. These higher speeds help shrink inductor and capacitor size, but they also amplify circuit parasitics, which are unintended inductances and capacitances within the circuit. These parasitics can interact with the high frequencies of the power FETs, causing significant voltage overshoot and ringing.

In a typical half-bridge — the core building block in most SiC power modules — parasitic inductance is introduced by copper traces in the PCB and component packages, while parasitic capacitance primarily comes from the power FETs themselves.

Basically, stray inductance from the PCB traces and the FET package forms an LC tank circuit with the FET’s parasitic output capacitance (C_OSS). As a result, careful PCB layout and selection of the MOSFET are critical in power-converter designs.

During switching events, these parasitics will cause ringing at specific resonant frequencies. This ringing can diminish the efficiency of a power supply by increasing switching losses and, in some cases, trigger false turn-on of the power transistor that leads to additional conduction losses. This ringing not only places additional voltage stress on the FETs, but it can also cause unwanted electromagnetic interference (EMI) noise that could further degrade the power supply’s performance.

A snubber circuit — typically a series resistor-capacitor (RC) network — is one solution to tamp down voltage transients and parasitic oscillations in power converters while improving electromagnetic compatibility (EMC).

Snubber circuits prevent voltage overshoot, ringing, and related issues by controlling the dv/dt of the power transistor. They’re placed in parallel with the FET, acting as temporary storage for energy before it causes a voltage transient. When the switch turns off, the snubber capacitor starts to charge, absorbing energy that would otherwise cause ringing. Then the resistor dissipates this stored energy as heat, effectively damping the oscillations.

While a snubber capacitor can be used by itself to reduce the occurrence of voltage spikes, the RC snubber adds resistance to the circuit, which helps reduce peak voltages and the duration of the ringing. By slowing down dv/dt during switching transitions, the snubber protects the power FETs from overvoltage stress, enabling it to safely switch at higher frequencies. It also prevents conduction losses due to false turn-on, thus improving overall system efficiency.

However, not every snubber circuit can keep up with the higher frequencies of SiC. Traditional discrete RC snubbers add complexity, occupy more board space, and increase bill-of-materials (BOM) costs. The passive components in the snubber circuit also introduce their own parasitics, which will reduce effectiveness at high switching speeds. They also struggle with the uniquely harsh temperatures and operating conditions found in SiC power modules, said Ye.

The Specs of the All-Silicon Snubber Circuit

Melexis said its all-silicon snubber is designed to deal with SiC and other wide-bandgap semiconductors, damping oscillations caused by parasitic inductances in the converter’s commutation loop. This helps prevent localized voltage overshoots that could stress the SiC power MOSFET, enabling more reliable switching at high frequencies.

“A power module with the integration of the silicon RC snubber would allow SiC devices to switch at a much higher speed,” said Ye, “therefore the power losses from switching will be much lower.”

The 1,200-V snubber comes with a capacitance range of 0.2 to 5.8 nF and internal resistance in the range of 1 to 35 Ω.

During switching events, parasitics can produce ringing at specific frequencies. However, these frequencies may be difficult to pin down due to the unpredictable nature of parasitics, and they must be measured first to determine the correct resistance and capacitance values for the circuit. Melexis said the RC snubber values are factory-selectable for precise surge voltage suppression.

SiC power modules are now widely used in the automotive industry, including in EV traction inverters and onboard chargers (OBCs), so the all-silicon snubber is targeted in that space, too. The MLX91299 acts as a booster in these power systems by reducing voltage ringing and lowering motor leakage current, according to Melexis.

The backside of the snubber can be soldered or sintered to the direct-bond copper (DBC) substrate used in most SiC power modules. Its compact integration within the power module allows it to dissipate heat in the same direction as the SiC devices, minimizing hotspots in the module while maintaining performance at SiC junction temperatures of up to 200°C.

As a result, the MLX91299 is able to reduce cooling and material requirements by lowering heat dissipation for a given power output Alternatively, it allows for higher power density, optimizing transistor count within the same thermal budget while keeping performance the same.