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AN-7503 Datasheet(PDF) 2 Page - Fairchild Semiconductor |
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AN-7503 Datasheet(HTML) 2 Page - Fairchild Semiconductor |
2 / 6 page ©2002 Fairchild Semiconductor Corporation Application Note 7503 Rev. A1 turns off, unlike the thyristor family of power semiconductors, which must be either externally or naturally (internally) com- mutated. The on-state voltage drop or resistance characteristic of a conductivity-modulated FET is markedly different from that of a standard power MOSFET, and is similar to that of a thyristor family member, the SCR. There is an offset voltage component (typically 0.6V) due to the p-n junction on the drain side, and a somewhat nonlinear resistive component, both of which are in series between the drain and source terminals. This series arrangement results in a highly nonlinear equivalent resistance, unlike the linear resistive characteristic of VDS(ON) of a standard FET. The structure of the conductivity-modulated FET operates during its turn on just as a standard FET does, hence its turn-on speed is very similar to that of a standard FET. With its high input impedance and its short propagation delay, the turn-on transition of the conductivity-modulated FET, as well as the standard power FET, is easily controlled by the gate driving circuit. This characteristic allows the designer the ability to control EMI and RPI generation easily. With other power semiconductors, it may be necessary to employ elab- orate circuit schemes to limit rapidly rising in-rush currents. A significant characteristic that must be considered in power switching applications is that of turn-off speed. The internal action that makes the conductivity-modulated FET such a silicon-efficient device also makes it an inherently slower device during turn-off. The injection of the minority carriers during the on-state conduction of current results in these carriers being present at the moment of turn-off. Without any way of removing these carriers by external means, they must recombine within the structure itself before the device can revert to its fully off-state condition. The quantity of these carriers and how fast they can deplete themselves determines the turn-off switching speed of the conductivity- modulated FET. This process of recombination is considerably slower than the simple discontinuance of majority carrier flow by which the standard power FET turns off. Hence, again, the conductivity-modulated FET is an inherently slower device. Its turn-off speed lies somewhere between the performance of a thyristor and that of a bipolar transistor. The final characteristic that makes the conductivity- modulated FET different from a conventional FET is the variance of on-state voltage with temperature. The characteristic of the conductivity-modulated FET is similar to that of an SCR, varying about -0.6mV/oC. The conventional FET has a positive temperature coefficient such that on high- voltage devices the RDS(ON) will double from its +25 oC value when the junction temperature reaches +150oC. The system designer must take this characteristic into consider- ation when the heat sink is being designed for the system. It is these similarities and differences that make the conduc- tivity-modulated FET a unique member of the family of power-semiconductor switching devices. Applications of this alternative power switching device invariably make use of one or more of its unique characteristics. Applications Automotive Ignition An application that can take advantage of the low drive- power capability of the conductivity-modulated FET is the electronic automotive ignition system. In Figure 2, the control IC takes the signal from the pickup coil located in the distributor and regulates the current through the ignition coil. At the proper time, the IC removes base drive from the bipolar transistor, which all systems currently employ as their coil driver. This removal of base drive allows the transistor to shut off which, in turn, causes a rapid decrease in the ignition-coil primary current. As the primary current decreases to zero, the energy stored in the field surrounding the primary is transferred to the secondary coil. The secondary coil, consisting of many more turns than the primary, transforms this energy into a higher voltage, resulting in a spark being generated in the cylinder. The control IC determines when this spark occurs, so as to derive usable power. With the use of a bipolar transistor, it is estimated that approximately two-thirds of the power dissipation that occurs in the control IC is the result of the need to be able to drive the required base current of the ignition output transistor. The high-impedance input of the conductivity-modulated FET virtually eliminates the base- current drive dissipation of the control IC. With improved silicon usage, the conductivity-modulated FET brings to power semiconductor switching devices the die size necessary to attain the required voltage and current- handling capabilities of the electronic ignition. This smaller- sized die makes possible smaller modules, whether they be hybrid or standard PC-based systems, than those currently implemented with bipolar-transistor technology. Brushless DC Motors Another emerging application that can make use of conductivity-modulated FETs is the emerging field of brushless DC motors. In this class of application, the solid- state devices are used to electronically switch the voltage to the multiplicity of windings that are employed. The motor consists of an armature that has a number of N and S poles consisting of high-strength permanent magnets. The stator is made up of the multiplicity of windings that were TABLE 1. CONDUCTIVITY-MODULATED FET CHARACTERISTICS Voltage Gated Small gate power required. Similar to standard power MOSFET. Turn Off When gate drive is removed... Unlike an SCR! Nonlinear On-State Voltage drop Like that of an SCR. Turn On Speed Fast! Comparable to a standard power MOSFET. Turn-Off Speed Slow! Comparable to a bipolar transistor. Temperature Independent On-State Voltage Drop Unlike the typical 2x variation of a power MOSFET. Application Note 7503 |
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