Power Electronic Converters for Motor Drives
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use of a ‘freewheel diode’ (sometimes called a Xywheel diode), as shown in Figure 2.4. A diode is a one-way valve as far as current is concerned: it oVers very little resistance to current Xowing from anode to cathode (i.e. in the direction of the broad arrow in the symbol for a diode), but blocks current Xow from cathode to anode. Actually, when a power diode conducts in the forward direction, the voltage drop across it is usually not all that dependent on the current Xowing through it, so the reference above to the diode ‘oVering little resistance’ is not strictly accurate because it does not obey Ohm’s law. In practice the volt-drop of power diodes (most of which are made from silicon) is around 0.7 V, regardless of the current rating. In the circuit of Figure 2.4(a), when the transistor is on, current (I) Xows through the load, but not through the diode, which is said to be reverse-biased (i.e. the applied voltage is trying unsuccessfully to push current down through the diode). When the transistor is turned oV, the current through it and the battery drops very quickly to zero. But the stored energy in the inductance means that its current cannot suddenly disappear. So, since there is no longer a path through the transistor, the current diverts into the only
R (a)
Current
L
(b)
Inductive load
(c)
Figure 2.4 Operation of chopper-type voltage regulator
Resistive load
T