There are six important gate parameters, classified in terms of current and voltage. The following four are maximum gate parameters. When any of these are exceeded the SCR may be destroyed.
- Maximum Gate Peak Inverse Voltage
VGRMis the maximum value of negative DC voltage that can be applied without damaging the gate cathode junction
- Maximum Gate Trigger Current (IGTM)
IGTMis the maximum DC gate current allowed to turn on the device.
- Maximum Gate Trigger Voltage (VGTM
VGTM is the DC voltage necessary to produce IGTM.
- Maximum Gate Power Dissipation (PGM)
PGM is the maximum instantaneous product of gate current and gate voltage that can exist during forward-bias. If VGTM and IGTM are used at their extreme limits simultaneously, PGM is certain to be exceeded.
The last two gate ratings are the smallest voltage and current needed to trigger the SCR. The gate trigger must exceed both to fire the SCR.
- Minimum gate trigger voltage VGT
VGT is the minimum DC gate to cathode voltage required to trigger the SCR. The voltage applied between the gate and the cathode must exceed this value while providing adequate gate current to turn the SCR on.
- Minimum Gate Trigger Current (IGT
IGTis the minimum DC gate current necessary to turn the SCR on. Most SCR’s require a gate current of 0.1 - 50 mA to fire. The magnitudes of the gate current and voltage required to trigger an SCR vary inversely with the temperature. At higher ambient temperatures, both gate trigger requirements decrease. At lower temperatures, these requirements increase. Worst case triggering conditions occur, therefore, at the minimum operating temperature.
Junction Temperature Rating
In all semiconductor devices, the most important consideration is junction temperature (T1). It not only defines the maximum and minimum limits but also determines whether the device can withstand prolonged operation: if the junction temperature in an SCR exceeds its maximum rating, the break over voltage drops noticeably and the off-state current and reverse leakage current increase rapidly. The turnoff time also increases significantly. On the other hand, if the junction temperature falls below its minimum limit, the SCR may not trigger at all.
Increasing SCR Ratings
To increase that SCR maximum ratings are not exceeded. An SCR with sufficient ratings must be chosen. We can increase SCR rating by using external cooling to remove the heat produced by losses in the SCR. Adding external circuitry can also increase the voltage and current handling capabilities, and ratings can be extended by connecting SCR’s in series and in parallel.
The reliability and life of semiconductor devices often depend on how well they are cooled. The power wasted as heat in the device also lowers efficiency. Usually SCR’s dissipate about 1% of the total power. SCR’s should be in well ventilated, cool places away from the heat generating devices.
In most situations, the SCR case is not effective in carrying away the heat from the junction, so suitable heat dissipation devices like heat sinks should be provided. A heat sink is made of a metal, normally copper or aluminum, that is a good conductor of heat. It is quite thick where it contacts the SCR and thin where it contacts the air. This design provides a large surface area from which the heat can pass by convection and radiation to the surrounding air. To help heat conduction, silicon grease infused with metallic oxides is normally used between the adjoining surfaces of the SCR and the heat sink. The outside surface normally has parallel fins to let convection currents of air flow freely. For cooling larger SCR’s, the convection flow can be further improved by using a fan or forced air. Water cooling is also used with very high-power dissipating devices.
SCR ratings can be extended by adding external circuitry. For example, using an RC snubber circuit to extend the dv/dt capability and series inductance to extend the di/dt rating have already been mentioned. The VDRM of an SCR can be increased by placing a resistor across the gate and cathode, this also decreases IH and IB however, the gate drive requirement increases. VRSM can be increased by inserting a diode of equal current rating in series with the SCR.