This article discusses the main difference between time ratio control current limit control includes constant frequency and variable frequency operation.

1. Difference Between Time Ratio
Control and Current Limit
Control

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Difference Between Time Ratio Control
and Current Limit Control
Introduction:
 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: Time Ratio
Control and Current Limit Control.

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

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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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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, f=1/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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Difference Between Time Ratio Control
and Current Limit Control
Constant Frequency Operation
Constant Frequency Operation

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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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Difference Between Time Ratio Control
and Current Limit Control
Variable Frequency Operation
Variable Frequency Operation

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Difference between Time Ratio Control
and Current Limit Control
Disadvantages of Control Strategy
 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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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.
Disadvantages of Control Strategy

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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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Difference Between Time Ratio Control
and Current Limit Control
Current Limit Control
Current Limit Control

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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 spreads the
minimum level.
 This technique can be utilized either when the ON time ‘T’ is endless
or when the frequency f=1/T.

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Difference Between Time Ratio Control
and Current Limit Control
 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.
Conclusion

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