On a separate page, draw a block diagram for the control of a single phase full-

Question

On a separate page, draw a block diagram for the control of a single phase full-bridge inverter with Unipolar PWM. Include scaling of an actual voltage command that can be modified by the user to the "vcontrol" signal that feeds into sine-triangle compare circuit and show a logic block that will produce gate signals. Show where the blanking time or dead time will be inserted between switching of upper and lower devices and explain why this is necessary. Include the synthesis of the trigonometric function, starting with a frequency command that can be modified by the user (note that electrical angle is the integration of the frequency). Propose how the integration function would be implemented if the controls is in a digital processor and how the integration function would be implemented if the controls are analog.

Explanation / Answer

The unipolar modulation normally requires two sinusoidal modulating waves vm and vm- which are of same magnitude and frequency but 1800 out of phase. The two modulating wave are compared with a common triangular carrier wave vcr generating two gating signals vg1 and vg3 for the upper two switches S1 and S3. It can be observed that the upper two devices do not switch simultaneously, which is distinguished from the bipolar PWM where all the four devices are switched at the same time. The inverter output voltage switches between either between zero and +Vd during positive half cycle or between zero and –Vd during negative half cycle of the fundamental frequency thus this scheme is called unipolar modulation. The unipolar switched inverter offers reduced switching losses and generates less EMI. On efficiency grounds, it appears that the unipolar switched inverter has an advantage.

Overmodulation occurs when amplitude modulation index ma is greater than unity. It causes a reduction in number of pulses in the line to line voltage waveform leading to emergence of lower order harmonics.

Moreover the notch and pulse widths near the center of positive and negative half cycle tend to vanish. To complete the switching operations of the device, minimum notch and pulse widths must be maintained. When minimum width notches and pulses are dropped, there will be some transient jump of load current.

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