How do snubber circuits offer circuit protection in power electronics?

Snubber circuits are commonly connected across semiconductor switches for protection. Common ways of protection include suppressing V-I spikes and ringing, complying with SOA, resonance suppression, and reducing switching loss. This FAQ will provide insights into each of these ways they are addressed.

Commonly employed snubber circuit configurations

Snubber circuits come in various forms, the most common of which are shown in Figure 1. The simple C snubber circuit (a) consists of only a capacitor and is primarily used to reduce surge voltage by absorbing energy in stray inductance.

The RC snubber circuit (b) adds a series resistor that helps reduce voltage spikes and ringing. However, this configuration dissipates heat because of the resistor, which can limit efficiency, especially at higher switching frequencies.

Figure 1. Four common configurations of snubber circuits (a) C snubber (b) RC snubber (c) discharge RCD snubber (d) non-discharge RCD snubber (Image: ROHM Semiconductor)

The discharge RCD snubber circuit (c) adds a diode parallel to the resistor for enhanced energy dissipation and surge absorption. The primary function of the diode is to handle surge currents; however, the reverse recovery characteristics should be considered.

The non-discharge RCD snubber circuit (d) provides a criss-cross connection of the resistor-diode combination, which leads to lower power dissipation. This is because the configuration only discharges energy absorbed during overvoltage conditions. It is also helpful for better efficiency and surge voltage suppression in high-frequency operations.

Having understood a few basics of the configurations, we will now see how snubber circuits help protect power electronics circuits.

Suppression of V-I spikes and ringing

Snubber circuits help suppress voltage and current spikes during switching transitions in power semiconductor devices, as shown in Figure 2. As observed from the figure, without a snubber, the circuit experiences severe voltage and current oscillations, with drain-source voltage (vds) spiking to approximately 850 V and drain current (id) showing violent oscillations between +15 A and -15 A.

Figure 2. Use of a snubber circuit to suppress voltage and current spikes/ringing. (Image: IET)

These high-frequency oscillations persist for several microseconds, which can cause damage to the device and generate electromagnetic interference. When equipped with snubber capacitors (middle and right panels), these spikes are significantly reduced as the capacitors provide alternative current paths during transitions, absorbing the energy that would otherwise cause ringing.

Shaping the load line within the safe operating area (SOA)

The SOA of a power semiconductor device defines the voltage and current conditions under which the device can operate without damage. Snubber circuits can shape the trajectory of voltage and current across the switch during transitions, ensuring that it stays within the safe operating area (SOA) boundaries.

As illustrated in Figure 3, the switching trajectory without a snubber (labeled “PWM switching without snubber”) shows a more rectangular path, indicating higher switching losses. The switching trajectory with a snubber (labeled “PWM switching with snubber”) shows a more triangular path with a reduced area, indicating lower switching losses. The “SOA” is marked with a dotted rectangle, showing the voltage and current limits the switch must operate to avoid damage.

Figure 3. Switching trajectories of a power semiconductor device with and without snubber circuits, illustrating the impact on SOA compliance. (Image: ScienceDirect)

By controlling dv/dt and di/dt, snubbers prevent the simultaneous occurrence of high voltage and high current, a significant cause of SOA violations and device failure, particularly during hard switching.

Resonance suppression and phase stabilization

Resonance occurs when the frequency of an external source matches the natural frequency of a circuit, leading to amplified voltages or currents. Such a phenomenon can cause noise interference, overheating, or even damage components. Phase stabilization refers to maintaining a consistent phase relationship between signals or components in a system.

Figure 4 presents a frequency response analysis across a dc bus capacitor of a buck converter with and without a snubber network across frequencies ranging from 10 kHz to 100 MHz.

Figure 4. Frequency response comparison showing how snubber networks dampen resonance peaks and stabilize phase transitions in power electronic circuits. (Image: Springer Nature)

The top displays impedance magnitude (|Z|) on a logarithmic scale, revealing how the snubber network (red line) dampens the resonance peaks visible in the unprotected circuit (blue line) around 100 MHz.

The middle graph tracks the phase angle transitions, showing how the snubber creates more gradual phase shifts than the abrupt changes in the unprotected circuit. The bottom panel illustrates the real component of impedance, demonstrating how the snubber increases resistive elements at higher frequencies.

Without protection, the sharp resonance peaks and abrupt phase transitions would manifest as voltage spikes, ringing effects, and electromagnetic interference — all of which can damage components and create compliance issues. The snubber reshapes the frequency response to maintain more consistent impedance characteristics throughout the operating range, particularly at higher frequencies, where problems often arise.

Reduction of switching losses

Snubber circuits are also known to reduce switching losses across semiconductor switches. This is shown in the graph in Figure 5, which illustrates how snubber circuits and gate resistance (Rg) optimization reduce turn-off energy losses (Eoff) under zero-voltage-switching (ZVS) conditions.

Figure 5. Reduction in turn-off switching losses with the use of a snubber circuit in ZVS conditions. (Image: Power Electronics Tips)

The plot compares three configurations across increasing drain currents (ID) up to 100 A: a high-resistance gate drive without snubber (gray line) showing steep energy loss increase, a snubber with low gate resistance (blue line) achieving 52% energy reduction, and a snubber with further reduced gate resistance (green line) delivering a 74% energy savings.

Summary

As power electronics circuits become smaller and more power-dense, voltage and current spikes or ringing can cause significant electromagnetic interference. Snubber circuits can help protect the circuit by absorbing excess voltage charges across the semiconductor switches and diverting current spikes. Commercial snubber circuits are commonly available and can be used as plug-and-play modules in power electronics circuits.