AN-110| Application Note

Fast IC Power Transistor with Thermal Protection

Fast IC Power Transistor with Thermal Protection
Introduction
Overload protection is perhaps most necessary in power circuitry. This is shown by recent trends in power transistor technology. Safe-area, voltage and current handling capability have been increased to limits far in excess of package power dissipation. In RF transistors, devices are now available and able to withstand badly mismatched loads without destruction. However, for anyone working with power transistors, they are still easily destroyed. Since power circuitry, in many cases, drives other low level circuitry -- such as a voltage regulator -- protection is doubly important. Overloads that cause power transistor failure can result in the destruction of the entire circuit. This is because the common failure mode for power transistors is a short from collector to emitter -- applying full voltage to the load. In the case of a voltage regulator, the raw supply voltage would be applied to the low level circuitry. A new monolithic power transistor provides virtually absolute protection against any type of ove
rload. Included on the chip are current limiting, safe area protection and thermal limiting. Current limiting controls the peak current through the chip to a safe level below the fuzing current of the aluminum metalization. At high collector to emitter voltage the safe area limiting reduces the peak current to further protect the power transistor. If, under prolonged overload, power dissipation causes chip temperature to rise toward destructive levels, thermal limiting turns off the device keeping the devices at a safe temperature. The inclusion of thermal limiting, a feature not easily available in discrete circuitry makes this device especially attractive in applications where normal protective schemes are ineffective. The device's high gain and fast response further reduce requirements of surrounding circuitry. As well as being used in linear applications, the IC can interface transistor-transistor logic or complementary-MOS logic to power loads without external devices. In fact, the input-current require
ment of 3 microamperes is small enough for one CMOS gate to drive over 400 LM195's. Besides high dc current gain, the IC has low input capacitance so it can be easily driven from high impedance sources -- even at high frequencies. In a standard TO-3 power package, the monolithic structure ties the emitter, rather than the collector, to the case effectively boot-strapping the base-to-package capacitance. Additionally, connecting the emitter to the package is especially convenient for grounded emitter circuits. The device is fully protected against any overload condition when it is used below the maximum voltage rating. The current-limiting circuitry restricts the power dissipation to 35 watts, 1.8 amperes are available at collector-to-emitter voltage of 17V decreasing to about 0.8 amperes at 40V. In reality, however, like standard transistors, power dissipation in actual use is limited by the size of the external heat sink. Switching time is fast also. At 40V 25 Ohm load can be switched on or off in a relativ
ely fast 500 ns. The internal planar double diffused monolithic transistors have an ft of 200 MHz to 400 MHz. The limiting factor on overall speed is

National Semiconductor Application Note 110 April 1998

the protective and biasing circuitry around the output transistors. An important performance point is that no more than the normal 3 uA base current is needed for fast switching. To the designer, the LM195 acts like an ordinary power transistor, and its operation is almost identical to that of a standard power device. However, it provides almost absolute protection against any type of overload. And, since it is manufactured with standard seven-mask IC technology, the device is produceable in large quantities at reasonable cost.

Circuit Design
Besides the protective features, the monolithic power transistor should function as closely to a discrete transistor as possible. Of course, due to the circuitry on the chip, there will be some differences. Figure 1 shows a simplified schematic of the power transistor. A power NPN Darlington is driven by an input PNP. The PNP and output NPN's are biased by internal current source I1. The composite three transistors yield a total current gain in excess of 106 making it easy to drive the power transistors from high impedance sources. Unlike normal power transistors, the base current is negative, flowing out of the PNP. However, in most cases this is not a problem.

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FIGURE 1. Simplified Circuit of the LM195 The input PNP transistor is made with standard IC processing and has a reverse base-emitter breakdown voltage in excess of 40V. This allows the power transistor to be driven from a stiff voltage source without damage due to excessive base current. At input voltages in excess of about 1V the input PNP becomes reverse biased and no current is drawn from the base lead. In fact it is possible for the base of the monolithic transistor to be driven with up to 40V even though the collector to emitter voltage is low. Further, the input PNP isolates the base drive from the protective circuitry insuring

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