Renesas Electronics has announced the development of an optical-coupled MOSFET (metal-oxide-semiconductor field-effect transistor), the PS7901D-1A, which achieves complete electrical isolation within the package by using light for signal transmission. The MOSFET device features an ultra-compact 4-pin flat-lead package measuring 2.9×2.3mm, which is 40% smaller than that of the company's existing PS78 series products, while retaining an assured isolation voltage of 500V rms, equivalent to the company's existing products. The MOSFET chip inside the package combines low output capacitance and low leakage current. The low leakage current when in the off state makes the MOSFET suitable for high-frequency signal control in applications such as IC testers. An optical-coupled MOSFET device combines in a single package three different elements: on the input side an LED that converts an electrical signal into light, on the output side a photovoltaic diode that converts light into an electrical signal, and an output MOSFET. The use of light to transmit the signal means that the input and output sides are completely isolated from each other electrically. This type of device is also referred to as a solid state relay. In contrast to mechanical relays, semiconductor relays are not subject to malfunction due to worn contacts or contamination by foreign matter. For this reason, mechanical relays are being replaced by solid state relay in a range of applications, including IC testers, factory automation equipments, electric household appliances, and communication equipments. Changing the structure inside the package from one in which the right and left lead frames are of different heights to one in which the lead frames face each other made it possible to achieve a smaller package size while maintaining the necessary distance and angle between the light-emitting and light-receiving elements. The MOSFET used as an output element employs the company's silicon-on-insulator technology. This enables an output capacitance of only 0.75pF. An issue with MOSFETs is signal component leakage via the output capacitance when in the current-off state. The output capacitance of the MOSFET is about 25% lower than the 1pF of the company's existing compact-package products. This makes it possible to suppress signal component leakage and makes the MOSFET device suitable for high-frequency operation.
Through optimisation of the gate film structure of the MOSFET, leakage current is reduced by 70% from the 100pA of the company's existing products to 30pA. This contributes to equipment with high measuring precision.
The device is suitable for a range of applications where compactness and thin form factor are desirable, including measuring equipment such as IC testers, factory automation equipment, and office equipment.
