IGBT Snubber Circuit Design

An insulated gate bipolar transistor turns off a current in a circuit that always contains some inductance, and the energy stored in that inductance has to go somewhere. A snubber circuit gives it a defined path, which limits the voltage overshoot across the device and moves part of the switching loss out of the transistor.

The Source of the voltage overshoot

The loop formed by the bus capacitor, the device and the load has a stray inductance of a few tens of nanohenries. At a turn off rate of a thousand amps per microsecond, even ten nanohenries produces a voltage spike of a hundred volts.

The overshoot adds to the bus voltage, so the device has to be rated for the sum. Reducing the stray inductance in the layout is the first and best measure, because it reduces the energy that has to be handled rather than absorbing it.

voltage overshoot is measured at the device terminals with a probe of minimal loop area. A probe with a long ground lead measures the ringing of the probe rather than the waveform of the circuit, and the reading is usually far worse than the reality.

What a Snubber Does

A snubber provides a low impedance path for the current during the transition, so the voltage rises more slowly and the peak is lower. It also absorbs energy that would otherwise be dissipated in the device, which shifts the loss to a component that is easier to cool.

The snubber introduces its own losses, and it changes the shape of the current and voltage trajectories. A well designed snubber reduces the total loss in the converter, while a badly designed one simply moves the heat to a different component and adds ringing.

The snubber also reduces the electromagnetic emission, because the slower edges radiate less. That is often the reason a snubber is fitted even where the device is comfortably within its ratings.

Types of Snubber

An RC snubber is a resistor and a capacitor in series across the device. It is simple, and the resistor dissipates the energy each cycle, which makes it suitable for low power or for damping a resonance rather than for absorbing a large turn off pulse.

A capacitor alone across the device absorbs the turn off energy and slows the voltage rise. The energy stored in it is dissipated in the device at the next turn on, which increases the switching loss of the transistor, so the benefit and the cost land on the same component.

A resistor, capacitor and diode network, sometimes combined with an inductor, gives control over where the energy goes. The diode directs the turn off current into the capacitor and the resistor discharges it, which keeps the loss out of the device during the critical edge.

Choosing the Values

The capacitor is chosen from the energy that has to be absorbed and the acceptable voltage rise. The stored energy in the loop inductance is half the inductance times the square of the current, and the capacitor voltage rise follows from the same expression.

The resistor has to discharge the capacitor in the time available between transitions, which sets an upper limit on its value, and it has to limit the current into the device at turn on, which sets a lower limit. The two requirements rarely conflict, and the window is usually comfortable.

The capacitor has to carry a pulse current of tens or hundreds of amps without degrading. A film capacitor with a low equivalent series inductance is the usual choice, and an electrolytic part is not suitable because of its inductance and its limited ripple current.

Layout of the Snubber

The snubber has to be closer to the device than the source of the stray inductance, or it will simply absorb the ringing of a longer loop. The loop formed by the snubber capacitor and the device should be the smallest loop on the board.

A snubber connected by two long leads adds its own inductance and is ineffective. The classic result is a board where the snubber is present, correct in value and useless in effect because of where it sits.

The connection should be made with wide, short traces or with a laminated bus structure, and the capacitor should be mounted directly over the device where the mechanical design allows. The general principles for containing a fast current loop are set out in our guide to EMI suppression design principles.

Interaction with the gate Drive

A snubber slows the rate of change of voltage across the device, which reduces the current fed back through the gate collector capacitance. That in turn reduces the gate voltage spike and the risk of a spurious turn on, so the gate network and the snubber interact.

Where the turn off is very fast, a gate network designed for a slower device may be inadequate, and the snubber tension with the gate resistor. Increasing the gate resistance slows both edges and reduces the overshoot, at the cost of higher switching loss in the device.

The optimum is found by measuring the device loss and the overshoot together while changing the gate resistance. A design that is optimised for one of them at the expense of the other is usually not the best compromise.

Thermal Design for the Snubber

The resistor dissipates energy on every switching cycle, and at a high frequency that amounts to a continuous power. Its rating is set from the energy per cycle and the frequency, and the calculation should be done rather than estimated.

The capacitor has a ripple current rating that is often the limit rather than its capacitance or its voltage. A film capacitor with a high ripple rating and a low dissipation factor is the type to specify, and derating for the case temperature is normal.

The thermal path should not share the device heatsink unless the coupling is intended. Heating the snubber from the device changes the resistor value and the capacitor behaviour, and the interaction makes the circuit harder to predict.

Measurement and Verification

Measure the overshoot with a probe of minimal loop area, at the device terminals, with the converter at its maximum current and minimum temperature. The worst case for overshoot is the highest current at the lowest temperature, because the device switches fastest when cold.

Measure the losses by the temperature rise of the heatsink rather than by calculation alone, since the calculation usually ignores several paths. A comparison with and without the snubber shows its real value.

Verify the snubber over a run in of several hours, because a marginal capacitor degrades. A change in the waveform over time indicates that the capacitor is losing capacitance. The release checks that keep such a board consistent are collected in our PCB design release checklist, and the assembly points are listed in judging PCB quality.

FAQ

Does every IGBT need a snubber circuit? No. Where the loop inductance is small and the device is rated well above the bus voltage, a snubber adds loss without benefit.

Should I use a capacitor alone across the device? Only for small signals. The stored energy is dissipated in the device at the next turn on and increases its switching loss.

Why has my snubber made no difference? It is probably too far from the device. The loop between the snubber capacitor and the device must be the shortest on the board.