# Difference between Time Ratio Control and Current Limit Control - PowerPoint PPT Presentation

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## Difference between Time Ratio Control and Current Limit Control

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### This article discusses the main difference between time ratio control current limit control includes constant frequency and variable frequency operation. – PowerPoint PPT presentation

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Title: Difference between Time Ratio Control and Current Limit Control

1
Difference Between Time Ratio Control and Current
Limit Control
2
Difference Between Time Ratio Control and Current
Limit Control
Introduction
• In DC-DC converters, the average output voltage
is controlled by varying the alpha (a) value.
This is achieved by varying the Duty Cycle of the
switching pulses. Duty cycle can be varied
usually in 2 ways Time Ratio Control and Current
Limit Control.

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3
Difference Between Time Ratio Control and Current
Limit Control
Basic Methods Of Control
• There are two kinds of control strategies used in
DC choppers namely
• Time Ratio Control and Current
• Limit Control.
• The average value of the o/p voltage can be
changed.

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4
Difference Between Time Ratio Control and Current
Limit Control
Time Ratio Control
• In the time ratio control the value of the duty
ratio, K TON /T is changed.
• Here K is called the duty cycle.
• There are two ways to achieve the time ration
control, namely
• Variable Frequency
• Constant Frequency Operation.

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5
Difference Between Time Ratio Control and Current
Limit Control
Constant Frequency Operation
• In constant frequency control strategy operation.
• The ON time TON is changed.
• Keeping the frequency, i.e, f1/T, or time period
T constant.
• This operation is also named as PWM (pulse width
modulation control).
• Hence, the output voltage can be varied by
varying ON time.

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6
Difference Between Time Ratio Control and Current
Limit Control
Constant Frequency Operation
Constant Frequency Operation
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7
Difference Between Time Ratio Control and Current
Limit Control
Variable Frequency Operation
• In variable frequency control strategy operation.
• The frequency (f 1/T) is changed, then the time
period T is also changed.
• This is also named as a frequency
modulation control.
• In both cases, the o/p voltage can be changed
with the change in duty ratio.

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8
Difference Between Time Ratio Control and Current
Limit Control
Variable Frequency Operation
Variable Frequency Operation
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9
Difference between Time Ratio Control and Current
Limit Control
• The frequency has to be changed over an extensive
range of the o/p voltage control in FM (frequency
modulation).
• The designing of the filter for wide frequency
change is quite difficult.
• For the duty cycle ration control. Frequency
change would be varied.
• As such, there is a possibility of intrusion with
systems by positive frequencies like telephone
lines and signaling in frequency modulation (FM)
technique.

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10
Difference between Time Ratio Control and Current
Limit Control
• The huge OFF time in FM (frequency modulation)
technique may make the load current irregular,
which is unwanted.
• Therefore, the constant frequency system with
pulse width modulation is preferred for choppers
or DC-DC converters.

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11
Difference between Time Ratio Control and Current
Limit Control
Current Limit Control
• In a DC to DC converter, the current value varies
between the maximum as well as the minimum level
of constant voltage.
• In this method, the DC to DC converter is turned
ON then OFF to confirm.
• That current is preserved constantly between the
upper limits and also lower limits.
• When the current energies beyond the extreme
point, the DC-DC converter goes OFF.

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12
Difference Between Time Ratio Control and Current
Limit Control
Current Limit Control
Current Limit Control
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13
Difference Between Time Ratio Control and Current
Limit Control
Current Limit Control
• While the switch is in its OFF state, current
freewheels through the diode and falls in an
exponential manner.
• Chopper is turned ON when the flow of current
• This technique can be utilized either when the ON
time T is endless or when the frequency f1/T.

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14
Difference Between Time Ratio Control and Current
Limit Control
Conclusion
• We can conclude that DC-DC converters or choppers
are presented along with the operation and its
wave forms in each case. The different control
strategies used in DC choppers are discussed.. As
the name suggest, here the time ratio (i.e. the
duty cycle ratio Ton/T) is varied. As is obvious
from its name, in this control strategy, a
specific limit is applied on the current
variation. This is all about the differences
between time ratio control and current limit
control.

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