Power Electronics - Control Methods

In a converter, there are two basic methods of control used to vary the output voltage. These are −

• Time ratio control
• Current limit control

Time Ratio Control

In time ratio control, a constant k given by $\frac{T_{ON}}{T}$ is varied. The constant k is called duty ratio. Time ratio control can be achieved in two ways −

Constant Frequency

In this control method, the frequency (f = 1/T_0N) is kept constant while the ON time T is varied. This is referred to as pulse width modulation (PWM).

Variable Frequency

In variable frequency technique, the frequency (f = 1/T) is varied while the ON time T is kept constant. This is referred to as the frequency modulation control.

Current Limit Control

In a DC to DC converter, the value of the current varies between the maximum as well as the minimum level for continuous voltage. In this technique, the chopper (switch in a DC to DC converter) is switched ON and then OFF to ensure that current is kept constant between the upper and lower limits. When the current goes beyond the maximum point, the chopper goes OFF.

While the switch is at its OFF state, current freewheels via the diode and drops in an exponential manner. The chopper is switched ON when the current reaches the minimum level. This method can be used either when the ON time T is constant or when the frequency (f=1/T).