Free-wheeling diodes

Finally, Fig. 8.54 shows that during the resonance phase, the current flows through free wheel diode Di and resistor R whenever the voltage ml, is negative. The same... 

Fig. 8.54 Waveforms of current iDi through free wheel diode D ...

The principle of pulse-width modulation is shown in Figure 10.11. The same circuit as shown in Figure 10.9 is used. In the positive cycle, only switch D is on all the time, and switch A is on intermittently. When A is on, current builds up in the load. When A is off, the current continues to flow, because of the load inductance, through switch D and the free-wheeling diode in parallel with switch C, around the bottom right loop of the circuit. 

The basic operation of the inverter is quite simple. First switches A and D are turned on and a current flows to the right through the load. These two switches are then turned off at this point we see the need for the diodes. The load will probably have some inductance, and so the current will not be able to stop immediately, but will continue to flow in the same direction, through the diodes across switches B and C, back into the supply. The switches B and C are then turned on, and a current flows in the opposite direction, to the left. When these switches turn off, the current free wheels on through the diodes in parallel with switches A and D. 

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