Snubber Resistors In Power Electronic Applications
Snubber resistors are used in a variety of power electronic applications as circuit protection elements. They prevent damage to switching circuits and sensitive electronic components.
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.
Snubbers are regularly used in motor control circuits as a protection device 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.
THE NEED FOR SNUBBER CIRCUITS
If the current is interrupted (for example a switch opens) to an inductive load voltage transients are generated. To avoid damage to the switch, the circuit and sensitive components it is important to suppress (snub) those transients.
The purpose of a snubber circuit is to absorb energy generated by the inductive elements in a circuit on switching. A snubber circuit should reduce losses due to switching and reduce EMI that may damage sensitive electronic components.
Snubber circuits may be designed to clamp a voltage transient at a specified level or control the rate of rise of a voltage pulse. An appropriate snubber circuit should improve the performance and reliability of switching circuits.
Snubbers are often used in combinations to control both the current and the voltage of the switch. Some applications may have two or three snubbers merged into one network.
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.
RESISTOR SNUBBER CIRCUITS
The most common snubber circuit utilizes a snubber resistor and capacitor connected in series across the switch. The capacitor provides an alternative path for current generated by the inductive load at switch off. It also protects against high voltage and high rate of change of voltage (dv/dt).
The purpose of the snubber resistor is to limit the discharge current of the capacitor. The snubber resistor must have low self inductance which tends to rule out wirewound devices. Thick film snubber resistors are often the best choice as they offer low inductance, high density for a given power rating, excellent power handling capability and relatively low cost.
The components in snubber circuits are subjected to high stresses. A snubber failure can lead to damage to the switch and other circuit components. It is therefore important to select the snubber resistor and capacitor with care.
SPECIFYING A SNUBBER RESISTOR
The most widely used snubber circuit is a simple resistor, capacitor network that is suitable for many 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 these 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.
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 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.