Since its inception in 1947, the silicon transistor has become a staple of modern electronics. Transistor-based components provide unmatched power and signal control to nearly every industry, application, and low-voltage circuit. As transistor capabilities grow, circuit optimizations are critical to protecting the downstream hardware.
Snubber capacitors for IGBT
In some industries where high-voltage transistors are utilized, specific precautions need to be taken to ensure related peripheral circuits are efficient and protected from harmful effects commonly seen at high-voltage, high-frequency operations. Let's explore how to use a snubber capacitor to reduce parasitic inductance of electrical wiring, which is a result of high-frequency system effects commonly found in high-voltage IGBT circuitry.
What is an IGBT?
Insulated Gate Bipolar Transistors (IGBTs) are predominantly used in medium- to high-power, high-frequency switching applications given their competitive speed and control of higher source-voltages with significantly lower signal voltages. Unlike other power transistors that rely on current flowing into their base, current flows to an IGBT through its gate to act as the signal of the transistor. This setup allows for characteristics such as low on-stage voltage drop and on-state current density, which is advantageous in high-power control systems. IGBTs are notable for their use in applications where high voltages and high switching speeds are critical, such as:
- • Electric vehicles
- • Air conditioners
- • Variable frequency drives
- • Industrial equipment
With their high power and high-frequency capabilities, IGBTs are surprisingly prone to surges at switch-off ― which is often caused by parasitic inductance of the peripheral circuit's wiring.
Capacitor parasitic inductance: high power IGBT module
Most high-power circuit applications involve a large volume of wiring that is used for distributing power across a system. This wiring has an inductance value of its own, which is called parasitic inductance. This is not just true for large wiring systems but actually all components including even the smallest resistor.
However, the system impact of this inductance is compounded in high-power systems that contain large amounts of mass and wiring. If you imagine the amount of wiring required in an EV's powertrain, miles of control wires and large masses of high-voltage, high-current wiring are required to distribute power around the battery system to the controls system and ultimately the drivetrain. When combined, the large volume of wiring is prone to creating large values of parasitic inductance. If the system cannot effectively mitigate the negative effects of this inductance, it can damage other components in the circuit, such as an off-state IGBT.
IGBTs are especially prone to this over time as they switch at very high frequencies, ultimately leading to a large amount of both on- and off-states over the life of the component. For example, if device (A) switches at a 10 Hz frequency for 1 year and device (B) switches at a frequency of 100 Hz frequency for 1 year, device (B) will have been in both an on-state and off-state 10 times as much as device (A). In the case of IGBTs, their high-frequency exposes them to significantly more on-states and off-states than standard transistors, making them prone to degradation and failure as a result of parasitic inductance-caused surges over the life of the part.
What is a snubber capacitor?
A carefully placed snubber capacitor mitigates the impact of parasitic inductance in IGBT systems and eliminates some of the risks associated with utilizing inductance-prone materials in high-power systems. A design that uses a properly sized snubber capacitor ― or even a snubber capacitor array ― can drastically reduce the magnitude of a parasitic inductance surge and reduce ‘long-term’ harmonic ringing of the IGBT system. Using a larger-value capacitor as the snubber capacitor will better reduce the overall surge value, but may result in undesirable ringing given system harmonic. Regardless, using a snubber capacitor can significantly reduce the chance of ruining your IGBT gate during an off-state parasitic inductance-caused surge.
Snubber capacitor types
A snubber capacitor is designed to increase the circuit's performance, but it also increases the IGBT's ability to exploit high-speed switching capabilities more efficiently, potentially allowing for even high-frequency control. A high-performing capacitor is ideal for use in a high-performance, high-power circuit as it must be able to also cope with its challenging system effects as well. When Tech Web analyzed snubber capacitor placement, they focused on how various capacitor configurations and types performed, specifically testing various series and parallel configurations of snubber capacitor designs. In comparing two large Nippon Chemi-Con film capacitors against an array of five parallel sets of two ceramic Murata capacitors, the ceramic capacitors boasted a surge suppression effect that was nearly twice as effective as the large film capacitors.
IGBT snubber capacitor selection & placement
When you design a snubber capacitor into your IGBT circuit, location relative to the IGBT is critical. Because a snubber capacitor seeks to minimize system-wide parasitic inductance, minimizing the amount of "system" between the snubber capacitor and the IGBT is very important as well. If your snubber capacitor is far from your IGBT, the material between the two component assemblies may form unwanted parasitic inductance and even further harmonic surging, ultimately reducing the snubber's surge suppression effect and risking harm to the IGBT. The closer the snubber capacitor and the IGBT sit, the less parasitic inductance-caused surging will be noticed by the IGBT.
IGBT snubber capacitor design
Test your snubber capacitor design to avoid damaging the overall circuit. Tech Web's analysis demonstrated that different snubber capacitor components and configurations can drastically influence the surge impact during a switching event. While some parameters such as capacitor size and quality can be controlled, the eventual system effects of your circuit may be less controllable. Significant testing is desirable for choosing the best snubber capacitor design. Ultimately, choose your capacitors carefully based on how you need your circuit to perform and how tolerant your IGBT is to surging.
