Snubber resistors are used in a variety of power electronic applications as circuit protection elements. 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. In this post, we consider the key resistor construction issues that should be considered.
Typical applications for snubber circuits include
• Elimination of 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.
Snubbers are regularly used in motor control circuits as a protection devices at start-up / switch off. They are also used in lamp ballast, power converters, to limit voltage spikes in switching applications and a wide range of applications where inductive elements may generate voltage spikes.
Specifying Snubber Resistors
The most widely used snubber circuit is a simple resistor, capacitor network that is suitable for most rate of rise control and damping applications. It is therefore important to carefully consider both the resistor and capacitor value to ensure they are matched appropriately.
The value of the snubber resistor should be chosen to be as close as possible to the impedance of the parasitic resonance that it is intended to damp. The tradeoff is to choose a resistor that both meets this criteria and keeps power dissipation to a minimum.
Wherever possible the resistor value should be minimised both to reduce the size of the required component and to prevent heat generated affecting surrounding components and the system as a whole.
Snubber Resistor Construction
The ideal snubber resistor should have high power density, excellent thermal properties and be non-inductive. It should be as small as practical, available with a range of resistance values and be available in a number of standard footprints. As always there are compromises to be made based on components are available from the major manufacturers. Where compromises are out of the question only a custom design may suffice.
Thick film technology is an ideal choice to dissipate high power in a small area. However, careful choice of film ink and a resistor manufacturing process designed to minimise defects in the resistive layer are required to ensure long term, reliable operation. Resistors should be low inductance to minimise ringing effects and effective overshoots which rule out the use of wire wound resistor technology in most applications.
Suitable heat sinking and an appropriate choice of substrate material can improve thermal performance. Substrate materials such as aluminium nitride and beryllium oxide are attractive for their thermal properties but the high cost and limited availability of these products often make alumina the substrate of first choice.
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.