A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems, the radiator (also a heat exchanger) in a car, and the SSR heat sink. Heat sinks also help to cool electronic and optoelectronic devices, such as higher-power lasers and light emitting diodes (LEDs).
For another thing, solid state relay (SSR) is an electronic switching device in which a small control signal controls a larger load current or voltage. Since there is a large load current or voltage coming through SSR, it’s significant to have a well-functioned SSR heat sink to cooling the devices. A heat sink is physically designed to increase the surface area in contact with the cooling fluid surrounding it, such as the air.
In general, a SSR heat sink performance is a function of material thermal conductivity, dimensions, fin type, heat transfer coefficient, air flow rate, and duct size. To determine the thermal performance of a heat sink, a theoretical model can be made. Alternatively, the thermal performance can be measured experimentally. Due to the complex nature of the highly 3D flow in present in applications, numerical methods or computational fluid dynamics (CFD) can also be used. This section will discuss the aforementioned methods for the determination of the heat sink thermal performance.
In solid state relay, there is an inherent substrate diode in all MOSFETs that conducts in the reverse direction. This means that a single MOSFET cannot block current in both directions. For AC (bi-directional) operation two MOSFETs are arranged back to back with their source pins tied together. Their drain pins are connected to either side of the output. The substrate diodes are alternately reverse biased in order to block current when the relay is off. When the relay is on, the common source is always riding on the instantaneous signal level and both gates are biased positive relative to the source by the photo-diode.