Power transistors

Figure 6.15 Circuit symbols and electrical representation of a basic triode or power transistor (BJT)...

The development of this hybrid combination in a bipolar transistor has greatly enhanced the application of power transistors in the field of power conversion and variable-speed drives. It possesses the qualities of both the power bipolar transistor (BJT) and the power MOSFET. Like a power MOSFET, it is a voltagc-eontrollcd switching device... 

Another eonsideration is the type of pass unit to be used. From a headroom loss standpoint, it makes absolutely no differenee whether a bipolar power transistor or a power MOSFET is used. The differenee eomes in the drive eireuitry. If the headroom voltage is high, the eontroller (usually a ground-oriented eireuit) must pull eurrent from the input or output voltage to ground. For a single bipolar pass transistor this eurrent is... 

This drive loss ean beeome signifieant. A driver transistor ean be added to the pass transistor to inerease the effeetive gain of the pass unit and thus deerease the drive eurrent, or a power MOSFET ean be used as a pass unit that uses magnitudes less de drive eurrent than the bipolar power transistor. Unfortunately, the MOSFET requires up to 10 VDC to drive the gate. This ean drasti-eally inerease the dropout voltage. In the vast majority of linear regulator applieations, there is little differenee in operation between a buffered pass unit and a MOSFET insofar as effieieney is eoneerned. Bipolar transistors are mueh less expensive than power MOSFET and have less propensity to oseillate. 

The operation of switehing power supplies ean be relatively easy to understand. Unlike linear regulators whieh operate the power transistor in the linear mode, the PWM switehing power supply operates the power transistors in both the saturated and eutoff states. In these states, the volt-ampere produet aeross the power transistor is always kept low (saturated, low-U/high-/ and eutoff, Hi-T/No-T). This El produet within the power deviee is the loss within all the power semieonduetors. 

There are two major types of power switches used today the bipolar power transistor (BJT) and the power MOSFET. The IGBT (integrated gate bipolar transistor) is used in the higher power industrial applications, such as 1 kW power supplies and electronic motor drives. The IBGT has a slower turn-off than does the MOSFET, so it is typically used for switching frequencies of less than 20 kHz. 

The bipolar power transistor is a current driven device. To guarantee a switchlike operation, it must operate close or within its saturated state. For this to occur, the on base current must satisfy (also refer to Figure 3-29). 

Figure 3-32 Waveforms for a bipolar power transistor within a PWM switching power supply.

Power MOSFE B can be driven more efficiently than bipolar power transistors. The gate of a MOSFET has two equivalent capacitors connected to the terminal, the gate-to-source capacitor (Ciss) and the drain-to-source capacitance (Crss). The loss exhibited by the gate drive of the MOSFET is created by charg-... 

The traditional snubber has been the approaeh used to keep bipolar power transistors away from seeond breakdown eonditions. It is also useful in the redue-tion of radiated EMI by eontrolling the dv/dt of reetifiers with abrupt reverse... 

These heatsinks will help a marginal heat situation. The alternative is to use a reetifier in a power transistor paekage sueh as a TO-220, TO-218, ete., and plaee it on a heatsink or to investigate a different teehnology of diode that exhibits a lower forward voltage drop sueh as a Sehottky. 

The most recent development in the starting of squirrel-cage induction motors is the introduction of the electronic soft-start. This principle has been derived from variable-frequency speed controllers using switched Thyristor or power transistor bridges. The supply sine wave is chopped so that a reduced voltage and frequency is applied to the motor.These are gradually increased so that the motor speed rises in a controlled manner, with the starting current limited to any chosen value. 

Semiconductor High-power transistors High-power microwave Photovoltaic elements Field-effect transistors UV sensors... 

Many years later in Singapore, we were using a specially formulated thermally conductive glue to fix the overtemperature sensing thermistor smack on to the very plastic body of the TO-220 power transistor. We had empirically ascertained that in this way, the junction temperature and the adjacent temperature as seen by the thermistor were less than 10°C apart, even during an abnormal event. So if, for example, we wanted to have the transistor turned off just before it hit 150°C, we simply needed to set the trip temperature (of the thermistor-based circuit) at about 140°C. In that way, we could also be sure that we wouldn t encounter nuisance tripping on a particularly hot day, when the temperature inside the enclosure would also be much higher. 

We can never hope to converge to a valid and optimum solution if we are unwilling to do what was initially suggested develop an ability to look at any practical problem through the eyes of its supporting theory and vice versa. I think that s how I eventually succeeded. First I created a rather textbook base drive circuit (not expensive). But to do that I had to go back to first principles and educate myself further on what is the best way to drive NPN power transistors, the exact waveshape required, and so on. Then I created a balun drive, which was, I still think, a rather clever trick to produce symmetrical drive waveforms for both... 

Finally, a transistor-heated hotplate will be described (Sect. 4.4), which offers the advantage of lower power consumption, since there is no additional power transistor needed on the chip. Moreover, a transistor hotplate can be digitally controlled and addressed so that new operation modes can be realized (Sect. 4.5). The integration of the transistor hotplate with accompanying, mostly digital circuitry will be described in Sect. 6.3. 

Fig.4.16. Heating approaches for monolithicaUy integrated microhotplates (pHP) (a) shows a resistive heater with power transistor and (b) shows a PMOS transistor heater fiheat denotes the heating resistor R is the metal-oxide chemiresistor, and Rj is a resistor used as temperaturesensor (see Fig. 4.4)...

Schottky diodes is that the invention of the very fast Si CoolMOS transistor is helping the introduction of the SiC Schottky diode. The power transistors always have a freewheeling diode in antiparallel, however, there has been no diode fast enough to match these fine Si transistors until the introduction of the SiC Schottky diode. 

Yuan, H. -T., et al., A 2-Watt X-Band Silicon Power Transistor, IEEE Trans, on Electron Devices, ED-25,1978, pp. 731-736. 

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