Electric vehicle
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| Figure 7.14 Linear regulator circuit prevents the charge from the capacitor flowing back through the switch. In Figure b the switch is off. The inductor voltage rises sharply, because the current is falling. As soon as the voltage rises above that of the capacitor (plus about 0.6 V for the diode) the current willow through the diode, and charge up the capacitor and flow through the load. This will continue as long as there is still energy in the inductor. The switch is then closed again, as in Figure a, and the inductor re-energised while the capacitor supplies the load. 164 Electric Vehicle Technology Explained, Second Edition (a) Inductor Load On Current paths while switch is ON V 1 Fuel cell + − Inductor Load Off Current flow while switch is OFF (b) V 1 Fuel cell + − Figure 7.15 Circuit diagram to show the operation of a switch-mode boost regulator Higher voltages are achieved by having the switch off fora short time. It can be shown that for an ideal convertor with no losses V 2 = t ON + t OFF t OFF V 1 (7.20) In practice the output voltage is somewhat less than this. As with the step-down (buck) switcher, control circuits for such boost or step-up switching regulators are readily available from many manufacturers. The losses in this circuit come from the same sources as for the step-down regulator. However, because the currents through the inductor and switch are higher than the output current, the losses are higher. Also, all the charge passes through the diode this time, and so is subject to the 0.6 V drop and hence energy loss. The result is that the efficiency of these boost regulators is somewhat less than for the buck. Nevertheless, over 80% should normally be obtained, and in systems where the initial voltage is higher (over 100 V), efficiencies of 95% or more are possible. For the regulation of fuel cell voltages, in cases where a small variation in output voltage can be tolerated, an up-chopper circuit is used at higher currents only. This is illustrated in Figure 7.16. At lower currents the voltage is not regulated. The circuit of Figure is used, with the switch permanently off. However, the converter starts operating when the fuel cell voltage falls below a set value. Since the voltage shift is quite small, the efficiency would be higher. |