A device that experiences a power surge might have a tough time surviving without some circuitry to absorb or divert excess electrical energy. The ability to withstand power surges is also a requirement in design for EMC and high reliability. Power surge protection components are available that can meet these reliability requirements, and these offer a simple strategy for ensuring long lifetime in your electronic systems.
Types of Surge Arrester Components
Surge arrester devices can be placed in a PCB just like other components, including placement through pick-and-place processing, or with manual placement and soldering. When a power surge occurs, the power surge must first interact with the surge protection component or circuit, so these components should be placed close to the input power terminals on the device. By suppressing the incoming power surge and its transient inrush current, the other components receiving power can be protected.
All surge arrester components withstand power surge events in three possible ways:
- Absorb the extra energy in a power surge as heat
- Divert the transient from the power surge away from other components
- Break open the circuit during the power surge
These components have simple placement rules and selection criteria for use in a PCB. Selection requires understanding something about the expected voltage surge or inrush current that might be seen in the system, or it involves exceeding the requirements in an industry standard. The list below gives an overview of the different types of surge arrester devices.
Varistors and Thermistors
These two types of surge arrester components exhibit a change in resistance during a fast transient. By offering a change in resistance during a transient event, these components can dissipate a lot of energy in a power surge during the initial fast edge, but they later drop their resistance so that they do not burn up from excessive heating.
- Thermistor: Negative (NTC) or positive (PTC) temperature coefficient thermistors are used to absorb the initial energy and convert it to heat. As these components heat up, their temperature changes and thus their resistance changes.
- Varistor: These components are similar to diodes in that they have a nonlinear resistance transfer curve. They will exhibit a resistance change as the applied voltage increases, so the initial fast edge can be absorbed or diverted during a power surge.
A variety of thermistors.
Transient Voltage Suppressor (TVS)
A TVS diode is an avalanche diode that intends to divert a power surge or transient event away from the input terminals on a component. The TVS diode operates in reverse bias; when a threshold on the applied voltage is exceeded, the diode enters an avalanche state and begins conducting the injected transient to ground. This type of component can be used on data lines or on power rails to protect a variety of circuitry.
Gas Discharge Tube
These tubes are the standard component used for surge protection in power strips connected to an AC power outlet. These components can withstand very high voltage surges reaching into the kV range. Their function is to shunt the power surge energy to ground, therefore they are often used in parallel with TVS diodes.
Although these are normally seen as large switches in residential or industrial environments, there are small circuit breakers that can be mounted on a PCB. These devices are through-hole mounted devices that include a switch extending from the side. When a power surge event occurs, typically resulting in currents of dozens of Amps, the circuit breaker will trip and will leave an open circuit. These devices are best used when high inrush currents are expected during a power surge.
Circuit breaker with through-hole mounting.
Surge Arrester Usage
The goal in using surge arresters is to ensure they perform their function quickly during a transient event. When a power surge occurs, the transient begins rising on a power rail very quickly. To ensure that the transient triggers the surge arrester component’s function, the inductance on the component path needs to be minimal.
This leads to three PCB layout guidelines for surge arrester components:
- Place the surge arrestor close to the power input to minimize trace length
- For shunting devices, make a direct connection to a ground plane or ground pour
- Use wider traces to make connections to power rails and ground connections
These two guidelines will help ensure the surge protection device can respond quickly to a transient with a given edge rate. The trace width guideline will also ensure that the device can withstand sufficiently high current without heating up too hot.
Whenever you need to prevent ESD and power surges from damaging your electronic devices, use the industry’s best CAD tools in OrCAD from Cadence to create your PCB design and layout. OrCAD is the industry’s best PCB design and analysis software with utilities covering schematic capture, PCB layout and routing, and manufacturing. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.