Constant Pulse Width Variable Frequency of Chopper (Frequency Modulation)

Constant Pulse Width Variable Frequency of Chopper (Frequency Modulation)

Q. What is frequency modulation chopper control ? State two methods for this control. List out any two drawbacks of this method.

Q. Explain with waveforms the control techniques used for chopper control.

Working Principle of Constant Pulse Width Variable Frequency

• In this method the on time of the chopper is kept constant and the variation in the average load voltage is obtained by varying the chopping frequency f.
• In the PWM strategy low values of output voltage are obtained by reducing the on time T_ON, of the chopper. In practical switches T_ON cannot be reduced below a minimum value, T_min.
• Under these conditions the ratio (T_ON/ T) is reduced by keeping T_ON Constant at T_min and by increasing T (or reducing f).
• This is also called as frequency modulation. The frequency has to be varied over a wide range to obtain full output voltage range. The frequency control is illustrated in Figure.
  

Demerits of Constant Pulse Width Variable Frequency

• This type of control generates harmonics at unpredictable frequencies and the filter design would be difficult.