In high-power switching systems, some of the most damaging electrical events occur within only a few microseconds.
When an IGBT turns off, its collector current decreases rapidly. However, the energy stored in the parasitic inductance of the DC-link, busbar, module terminals and connecting conductors cannot disappear immediately. This energy produces a transient voltage that is added to the DC bus voltage, creating a collector-emitter voltage overshoot.
If the peak voltage exceeds the IGBT’s blocking-voltage limit, the device may suffer avalanche stress, insulation degradation or immediate failure. Even when the voltage remains below the absolute maximum rating, repeated spikes can gradually shorten the service life of the power module.
Why Does the Voltage Spike Occur?
The turn-off voltage overshoot can be approximated by:
ΔV = Lσ × |di/dt|
where:
Lσ is the total stray inductance in the switching loop.
di/dt is the rate at which the IGBT current decreases.
The voltage stress becomes more severe when the switching current is higher, the IGBT turns off faster or the current loop contains greater parasitic inductance.
Modern IGBT modules are designed for faster switching to improve conversion efficiency. However, higher switching speed also increases di/dt, making the system more sensitive to busbar layout, terminal structure and connection distance.
How Does a Snubber Capacitor Work?
A snubber capacitor is installed close to the IGBT module or across the relevant DC bus terminals. During turn-off, it provides a short, low-impedance path for the high-frequency transient current.
This helps to:
Reduce turn-off voltage overshoot.
Suppress high-frequency ringing.
Limit semiconductor electrical stress.
Improve switching stability and electromagnetic compatibility.
Extend the operating life of the power module.
The snubber capacitor should be used together with a low-inductance busbar and an optimized switching-loop layout. It cannot compensate for an excessively long or poorly designed current path.
Key Selection Factors
When selecting an IGBT snubber capacitor, engineers should evaluate:
Continuous DC-link voltage.
Repetitive and non-repetitive peak voltage.
Required capacitance.
Equivalent series inductance.
Peak and RMS current.
Switching frequency.
Ambient and hotspot temperature.
Terminal spacing and mounting method.
Available installation space.
The capacitor should also be positioned as close as possible to the switching module. Long wires and narrow PCB traces increase parasitic inductance and reduce the effectiveness of the snubber network.
Verification Under Real Operating Conditions
The final capacitor selection should be verified under actual operating conditions.
A double-pulse test can be used to compare the IGBT collector-emitter voltage before and after installing the snubber capacitor. Testing should cover the maximum DC-link voltage, switching current, temperature and expected overload conditions.
The voltage probe connection should be kept very short. A large probe loop can introduce additional inductance and display a voltage spike that does not accurately represent the voltage across the IGBT terminals.
Inside the SPB/STA Series Capacitor: CABO's IGBT Snubber Capacitor
CABO’s SPB and STA Series polypropylene film capacitors are specifically designed for IGBT snubber and semiconductor power circuits. Connected in parallel with power semiconductors, they help suppress turn-off voltage peaks and high-frequency ringing, reducing transient stress on IGBT modules.
Both series feature low ESR, high dV/dt capability, self-healing performance and excellent high-frequency characteristics, enabling them to withstand the repetitive peak currents generated during inverter switching.
The SPB Series uses a plastic-box construction with lug terminals and includes low-inductance direct-mount configurations for installation close to the IGBT module. Axial and radial configurations are also available to accommodate different circuit and busbar layouts.
The STA Series features an axial-leaded construction with polypropylene film and dual metallized electrodes, providing reliable self-healing performance and high peak-current carrying capability.
With multiple terminal structures and customizable electrical specifications, the SPB and STA Series are suitable for inverters, motor drives, power converters and other high-frequency semiconductor switching systems.
An effective snubber design depends on the coordination of the semiconductor, busbar, operating waveform, capacitor structure and physical installation.
In the microsecond when an IGBT turns off, these details determine whether the transient voltage is safely controlled or transferred directly to the power semiconductor.
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