Most overvoltage protection of AC power lines is cost driven and typically employs the bare minimum of protection strategies. But there are plenty of power designs that require solid protection to avoid costly warranty returns or to meet total cost of ownership goals. And, failure is not an option in certain applications that are either difficult to repair or replace. That’s where new hybrid devices that combine a GDT and MOV into single-package solutions and Power TVS diodes shine. The enhanced protection capabilities of these devices offer increased reliability that can substantially offset the higher initial protection device cost compared to after-sale maintenance and warranty costs.
To meet the enhanced protection requirements of such applications, Bourns has introduced a novel protection circuit. Commonly, overvoltage protection strategies for AC power lines specify conventional clamping devices. Unfortunately, there will always be some level of surge current that may result in damaging clamping voltages. And, there are competing requirements with clamping devices that many times force the designer to select line input components with very high voltage ratings that come with a corresponding increase in cost for protection components.
The solution? Electronic Current Limiters (ECLs) are a good answer. ECLs are able to switch “OFF” in about a microsecond when the current exceeds the rated trip current. Once tripped, the internal Field-Effect Transistor (FET) devices remain in the “OFF” state until the next voltage zero crossing, at which point it reverts back to the low resistance “ON” state. In the “OFF” state, the ECL can withstand several hundred volts.
Often an ECL, such as a Bourns® TBU® High-Speed Protector (HSP), is coupled with a small TVS diode. In this solution, the TVS device conducts when its threshold voltage is exceeded causing the current to flow in the TVS device which, in turn, trips the ECL. This protection voltage level is independent of the magnitude of the surge voltage. The advantage of implementing this type of exact and assured voltage limit to downstream AC input components allows a designer to minimize the voltage ratings of those components, helping to decrease costs while providing the effective protection necessary.
An ECL-based solution can also be used for load overcurrent protection. In this scenario, the ECL acts as an automatically resetting line fuse. It can also function as a soft-start controller, limiting the peak inrush current during power supply start-up. This helps to reduce the stress on rectifier diodes, filter capacitors and other AC input components.
For voltage swell protection, the combination of a TVS diode and ECL is an effective solution. Just as it would during a transient event, this solution prevents excessive voltages from reaching the protected circuit. As soon as the swell condition ends, the ECL will remain in the “ON” state until the next AC swell or transient event occurs. Different than simple voltage clamp protection circuits, this circuit will not only tolerate AC line voltage swells, but will also protect downstream components from the higher voltages. In many cases, this circuit design approach will even allow the application to function normally during the voltage swell event.
The circuit protection design also delves into “primary protection” that protects the ECL device from voltages that exceed its data sheet limits and without interfering with the protection provided by the ECL and TVS device combination. In this case, a Bourns® TBU® HSP rated to withstand 850 V is ideal, and is combined with the TVS diode to set the lower limit of the primary protection breakdown voltage. This helps ensure that primary protection will be inactive at acceptable voltage levels while still providing needed protection for the ECL device.
For more information, check out the article, “A Novel Protection Circuit Design for AC Power Lines” and the corresponding white paper. The goal is to be a useful resource in guiding designers on a viable AC power line protection solution that can handle real world voltage swells or surges, up to rated limits.