This rating is specified at a case temperature of 25☌ and maximum junction temperature of 150° C. The gate voltage should be limited to a much lower value to limit the collector current under fault conditions.Ĭontinuous collector current ( I C): This parameter represents the value of the dc current required to raise the junction to its maximum temperature, from a specified case temperature. The thickness and characteristics of the gate-oxide layer determine this voltage. Gate–Emitter voltage ( V GES): This parameter determines the maximum allowable gate–emitter voltage, when collector is shorted to emitter. Breakdown is specified at a specific leakage current and varies with temperature by a positive temperature coefficient.Įmitter–Collector blocking voltage ( BV ECS): This parameter specifies the reverse breakdown of the collector–base junction of the pnp transistor component of IGBT. A second pass transistor may not even be needed, if you are clever in using open collector type comparators!Ĭollector–Emitter blocking voltage ( BV CES): This parameter specifies the maximum off-state collector–emitter voltage when the gate and emitter are shorted. This may be a concern if there is a large input voltage, but the base resistance can be increased at the expense of forward voltage drop if need be.Ī similar technique could be used for overtemperature protection with the comparator set up to monitor temperature on the inverting input instead of the input voltage. The comparator output, of course, continues to rise along with the input voltage. There is a small amount of leakage, but it is inconsequential. V OUT goes to zero, and current goes to zero. As the input voltage continues to rise, however, the comparator inputs reach equality, and the comparator switches its output high, turning off the pass transistor. As the input voltage reaches 5 V, the voltage applied to the noninverting input of the comparator is still below the threshold set by the Zener diode on the inverting input, so the comparator output is still low, and the pass transistor conducts. Fine adjustments can certainly be made by adjusting resistor values and choosing a different Zener diode. A 1N4728 is a 3.3 V Zener diode, which is adequate for this circuit to function. As the input voltage continues to increase, the comparator turns on with its output in the low state (the noninverting input is lower than the inverting input). Fortunately, that is not required for the pass transistor to be “on.” All that is required is that the base be low. Looking at the DC sweep, there is, of course, a region of operation where the comparator is not under power. The output is connected through a current limiting resistor to the base of the pass transistor. The input voltage is monitored by a voltage divider on the noninverting input. Ī Zener diode is used to set a voltage reference at the inverting input.Looking at the connections to the comparator: Small package size is achieved even though the transistor is passing high current, its voltage drop is so low that the overall power dissipated in the transistor is low. The pass transistor is a PNP switching type, optimized for low collector–emitter voltage drop and high current. This, of course, is a nonrecoverable condition that may be appropriate for some applications, but why design a nonrecoverable circuit when it is just as easy to design a recoverable one? There are two types of overvoltage protection circuits-one that opens the voltage path and does not allow voltage to pass to the load, and a shorting or “crowbar” type of protection that shorts overvoltage to ground, thus blowing any fusible element on the input voltage. If both the regulator and overvoltage protection circuitry fail at the same time, the whole system should come under suspicion because something really dramatic has happened. But playing a game of odds, anything short of a lightning stroke or direct connection to line voltage will probably not produce such a failure. Granted, there are no absolutes, and there are always circumstances where the regulator and the protection circuitry fail at the same time. This is almost always catastrophic, after all, if raw unregulated voltage would not cause a problem why not use it in the first place? Fortunately, it is easy to add an overvoltage protection circuit to greatly reduce the likelihood of this happening. Bruce Carter, Ron Mancini, in Op Amps for Everyone (Fifth Edition), 2018 22.6 A Companion CircuitĬonsider the case when a voltage regulator fails- allowing unregulated input voltage to be applied to the load.

Transistor base emitter collector