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.
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).
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.
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.
resistor of the RTP will follow the
specified characteristics. However, once it is armed, the arming pin will
become electrically open - preventing further changes.
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.
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