Difference between ‘Time Ratio Control’ and ‘Current Limit Control’

In DC-DC converters, the average output voltage is controlled by varying the alpha (α) value. This is achieved by varying the Duty Cycle of the switching pulses. Duty cycle can be varied usually in 2 ways:

  1. Time Ratio Control
  2. Current Limit Control

In this post we shall look upon both the ways of varying the duty cycle.

Before moving on, I would like to tell you that, Duty Cycle is the ratio of ‘On Time’ to ‘Time Period of a pulse’.

Time Ratio Control:
As the name suggest, here the time ratio (i.e. the duty cycle ratio Ton/T) is varied.

This kind of control can be achieved using 2 ways:

  • Pulse Width Modulation (PWM)
  • Frequency Modulation Control (FMC)

A.    Pulse Width Modulation (PWM):
In this technique, the time period is kept constant, but the ‘On Time’ or the ‘OFF Time’ is varied. Using this, the duty cycle ratio can be varied. Since the ON time or the ‘pulse width’ is getting changed in this method, so it is popularly known as Pulse width modulation.

Pulse Width Modulation

B.    Frequency Modulation Control (FMC)
In this control method, the ‘Time Period’ is varied while keeping either of ‘On Time’ or ‘OFF time’ as constant. In this method, since the time period gets changed, so the frequency also changes accordingly, so this method is known as frequency modulation control.

Frequency Modulation Control

Current Limit Control:
As is obvious from its name, in this control strategy, a specific limit is applied on the current variation.

In this method, current is allowed to fluctuate or change only between 2 values i.e. maximum current (I max) and minimum current (I min). When the current is at minimum value, the chopper is switched ON. After this instance, the current starts increasing, and when it reaches up to maximum value, the chopper is switched off allowing the current to fall back to minimum value. This cycle continues again and again.

Current Limit Control

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