Snubber resistors are used in a variety of power electronic applications as circuit protection elements. They suppress voltage spikes that could damage sensitive electronic components.
Snubbers are used in a wide range of applications including lamp ballast, and power converters. They are regularly used in motor control circuits as a protection device at start-up / switch off.
- Eliminate voltage spikes.
- Limit current or voltage rise over time.
- Divert power dissipation from a switch to a load.
- Reduce EMI (Electromagnetic interference).
As a result snubber circuits are broadly classified as:
- Rate-of-rise control snubber.
- Voltage clamp snubber.
- Damping snubber.
In its most simple form, a snubber circuit consists of a resistor and a capacitor or, in the case of a turn off snubber circuit, a resistor, capacitor and diode. Long term reliability of the snubber circuit is vital in many applications.
Specifying Snubber Resistors
The simplest snubber circuits are based on a capacitor and snubber resistor connected in series across a switch. They 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 operations the capacitor charges and therefore controls the rate of rise and peak voltage. When steady state operation is reached the capacitor discharges through the resistor.
The value of the resistor should 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 must be balanced against resistor reliability. Given the high energy potentially stored in the capacitor, the resistor must be capable of surviving relatively high current. It must also be able to dissipate the heat generated by high transient current flow.
Snubber Resistor Material Selection
The ideal snubber resistor should have high power density, excellent thermal properties and be non-inductive. It should be as small as practical and be available in a number of standard footprints.
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. The low inductance of thick film technology makes it an ideal choice. The technology is relatively low cost and has good heat dissipation properties.
The snubber resistor designer must ensure the thick film snubber resistor device has sufficient mass to dissipate heat generated (via a heatsink if necessary). The choice of substrate material can improve thermal performance but this must be traded off against the potentially high cost of specialist materials.
Snubber Resistor Construction
Snubber resistor manufacturing process can have an impact on reliable long term, operation of the resistor device. A number of the factors causing power resistor failure can be related directly to the resistor manufacturing process.
However, careful choice of film ink and a resistor The manufacturing process must minimise defects in the resistive layer to ensure the resistor element can survive high, short term current flow.
The choice of appropriate snubber resistors for a particular application may seem trivial but for long term and reliable operation, it is important to consider both the resistor value and its construction.