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


  1. I just love that way you guys explain the things, now i have bookmarked your website, thanku for this helpful article.

  2. Your post is very informative, I appreciate you for producing this great article, I want to say thanks to you for this awesome post..Readers will definitely get some useful knowledge from here..

  3. This is great Amit. I appreciate your hard work to write this blog, i believe that this is the best write-ups I have seen ever on Ratio control.
    Please also write about Power tools if some days have some time.

  4. Great post, Now I understand the difference between time ratio control. This is a must read post. I like it.
    Do you any post based on Vernier Calipers

  5. How can you use current limit strategy to control duty cycle.
    When load changes,Imax and Imin changes so required duty cycle also changes


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