Photovoltaic isolators (PVIs) provide an isolated gate drive for MOSFETs and IGBTs. At turn-off, however, they don't pull the gate down to drain the gate charge. Using a programmable junction transistor (PUT) provides a very fast gate pull-down, pulling as much as 5 A to discharge the gate in less than 0.2 s. The PUT is a four-layer structure, much like a silicon-controlled rectifier, but with a much more sensitive anode gate. In this application, the PUT gate is driven by the PVI to a voltage equal to or greater than that of the anode that holds the PUT in the blocking state so that no current flows through the anode-cathode channel. When the PVI is turned "off", the PUT gate voltage drops below the anode voltage, triggering the PUT and causing the anode-cathode channel to conduct. Conduction continues until the MOSFET gate voltage drops to about 0.5 V, at which time the PUT stops con-ducting and recovers for another cycle.
The PVI Photovoltaic Isolator generates an electrically isolated DC voltage upon receipt of a DC input signal. The input of the PVI is a light-emitting diode (LED) which is optically coupled to, but electrically isolated from, the output. A GaAIAs LED is used for high output and maximum stability. The infrared emission from the LED energizes, by photovoltaic action, a series connection of silicon PN junctions. A unique alloyed junction stack which is edge-illuminated is used to form the output photovoltaic generators. This novel structure produces extremely high operating efficiency. Units are available with a single 5-volt output or dual 5-volt outputs which can be series connected to produce 10 volts.
A PVI can serve as an isolator, a coupler and as an isolated voltage source. As an isolator, the PVI can serve as the key component in a solid state relay circuits. The PVI is ideally suited for driving power MOSFETs and IGBTs or sensitive gate SCRs to form solid state relays.
As a coupler, the PVI can sense a low-level DC signal and transmit a voltage signal to an electrically remote circuit. As a voltage source, the PVI can function as a 'DC transformer' by providing an isolated, low-current DC source for biasing or supplying power to low quiescent current electronic devices.
Conventional optocouplers merely modulate the resistance of an output device such as a transistor, diode or resistor. Such optocouplers require a separate voltage source to detect the presence of an input signal. In contrast, a PVI actually transmits (and transforms) energy across the isolation barrier and directly generates an output voltage. This DC voltage, available at a 2500VAC isolation level, gives circuit designers a new and uniquely useful electronic component.
Required Field 
