Lighting dimmers often suffer from MOSFET failures. Most dimmers used in low-temperature AC industrial applications are enclosed and often embedded in the wall. This can cause heat dissipation issues, and can result in heat build-up - leading to a thermal event. Usually, the MOSFET used for the lighting dimmer circuits fails in 'resistive mode'.
This generates heat, and results in a condition commonly known as thermal runaway - uncontrollable rising temperatures.
A reflow-able thermal protection or RTP from TE Connectivity provides an answer to MOSFET failure in low-temperature AC applications.
This device acts like a low-value resistor at the normal operating temperatures of the MOSFET. It is mounted almost directly on the MOSFET, and is therefore able to sense the temperature with precision. If for any reason, the MOSFET drifts into a thermal runaway condition, its temperature will rise.
This is sensed by the RTP, and at a predefined temperature, the RTP changes into a high-value resistor.
This effectively cuts off the power to the MOSFET, saving it from destruction. Thus, a lower priced resistor sacrifices itself to save a more expensive MOSFET. A similar analogy could be the use of a fuse (low-value material) in protecting more complex circuitry (e.g. a television).
One of the most interesting aspects of the RTP from TE Connectivity is its ability to withstand enormous temperatures - up to 260ºC. This is surprising since the resistance change (to protect the MOSFET) usually occurs at around 140ºC.
This miraculous feat is accomplished via innovative design by TE Connectivity. The RTP has to be activated before it starts protecting the MOSFET. The electronic activation of the RTP occurs after the flow soldering (attachment) is completed. Each RTP has to be individually armed by sending a specified current through the arming pin of the RTP for a specified time.
The time-current characteristics are part of the specifications of the RTP. Before it is armed, the value of the resistor of the RTP will follow the specified characteristics. However, once it is armed, the arming pin will become electrically open - preventing further changes.
It is very important that the layout specified by TE Connectivity be followed when designing and mounting the MOSFET and the RTP on the PCB. Since the RTP has to sense the temperature of the MOSFET, it naturally follows that the two should remain in close proximity.
The RTP resistance will allow up to 80A of current at 120V AC through the MOSFET as long as the temperature of the MOSFET remains below the Open Temperature of the RTP, which can be between 135-145ºC.