Difference between Time Ratio Control and Current Limit Control

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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 (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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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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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, 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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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
Disadvantages of Control Strategy

• 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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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 f1/T.

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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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