Snubber Resistor Design For Motor Control Applications

The purpose of snubber circuits is the suppression of voltage spikes that otherwise may damage sensitive electronic components. They are also used to reduce (damp) electrical oscillations (ringing) caused by the voltage pulse (spikes) and for controlling the rise rate of a voltage pulse. In this post we consider the key snubber resistor design and construction issues that should be considered to ensure reliable, long term operation.

The switching of inductive loads tends to cause significant voltage transients and oscillations in motor control circuits. The simplest snubber circuits, based on a capacitor and snubber resistor connected in series across the switch, are suitable for most voltage transient control and damping applications, particularly for low to medium power applications where the loss in the resistor is not excessive.

During switching the capacitor charges and therefore controls the rate of rise and peak voltage, while during steady state operation it discharges through the resistor. The value of the resistor should be be as close as possible to the impedance of the parasitic resonance that it is intended to damp. This must be balanced against minimising power dissipation which means the resistor value should be as low as possible.

A low resistance value minimises power losses, and heat generation, but given the potentially high energy stored in the capacitor the snubber resistor designer must ensure the resistor can survive relatively high current flow and rapidly dissipate the heat generated. Failure of the snubber circuit components may have severe consequences in many motor control applications.

Thick film is an ideal choice when cost is an issue and it is important to dissipate high power in a small given area. Thick film technology is also low inductance which minimises ringing effects and simplifies the snubber circuit design.

The snubber resistor designer must ensure the thick film snubber resistor device has sufficient mass to dissipate the heat generated (via a heatsink if necessary) and the resistor element can survive the high, short term current flow. The choice of materials,  including both the resistor and substrate materials, is crucial if a reliable device is to be produced in the smallest possible footprint. This must be traded off against the potentially high cost of those materials.

Snubber resistor manufacturing is also important for reliable, long term, operation. A number of the factors causing power resistor failure can be related back directly to the resistor manufacturing process.