Electric vehicle
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| Figure 7.12 Circuit diagram showing the operation of a switch-mode step-down regulator In practice all these can be made very low. The efficiency of such a step-down chopper circuit should be over 90%. In higher voltage systems, about 100 V or more, efficiencies as high as 98% are possible. We should at this point briefly mention the linear regulator circuit. The principle is shown in Figure 7.14. A transistor is used again, but this time it is not switched fully on or fully off. Rather, the gate voltage is adjusted so that its resistance is at the correct value to drop the voltage to the desired value. This resistance will vary continuously, depending on the load current and the supply voltage. This type of circuit is widely used in small electronic systems, but should never be used with traction motors. The voltage is dropped by simply converting the surplus voltage into heat. Linear regulators have no place in systems where efficiency is paramount, such as an electric vehicle. 7.2.3 Step-Up or Boost Switching Regulator It is often desirable to step-up or boost a DC voltage, regenerative braking being just one example. This can also be done quite simply and efficiently using switching circuits. Voltage boost circuits are also needed for regenerative braking. The circuit of Figure 7.15 is the basis usually used. We start our explanation by assuming some charge is in the capacitor. In Figure a the switch is on, and an electric current is building up in the inductor. The load is supplied by the capacitor discharging. The diode Electric Machines and their Controllers 163 Current circulating through diode during the OFF time Current through load, being the sum of these two components On On On Off Off Off Current supplied by fuel cell during the ON time Figure 7.13 Currents in the switch-mode step-down regulator circuit Load "Electronic resistor" Control circuit Download 3.49 Mb. Share with your friends:
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