Voltage Mode Control and One Cycle Control

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Voltage mode control is regulating scheme for controlling switching mode power supply. This controlling method has only one feedback loop over output voltage.

vmc_block

Control is achieved by pulse width modulation (PWM), where constant amplitude voltage sawtooth signal is compared with PI or PID signal from the error regulator. Such approach has some advantages and some disadvantages compared with current mode control. Advantages are:

  • Simplicity
  • Smaller design since there is no need for sensing resistor
  • Voltage mode control is applicable for wide range of inductor (or load) current

Disadvantages are:

  • Much slower response compared to current mode control
  • No over current protection
  • Not applicable to parallel connected converters since VMC has no possibility for equal current sharing.

Voltage mode control scheme applied on boost dc-dc converter is shown in figure bellow.

voltage-mode-control

Main switch will be in “on” state as long as error signal (ePI) is higher then sawtooth signal. Since, on every clock pulse, sawtooth signal starts from zero, there is no bifurcation.

vmc_sawtooth

Response of the control for outer disturbances is much slower comparing to current mode control. This slowness comes from the fact that any change on input voltage and/or the load, must affect output voltage first, and only after that, voltage feedback will start to change error signal (ePI) and correct the error. This slow response can also affect circuit stability if sawtooth signal has small amplitude, and high error signal appears. In that case, Hopf bifurcation appears.

vmc_hopf

Similarly to current mode control, overall gain of control system is higher if amplitude of sawtooth signal is smaller. Error signal (ePI) is adjustable, so with smaller sawtooth signal, ePI reaches optimal height for appropriate duty cycle sooner, then for high sawtooth signal. Since there is only feedback over output voltage, load current is not limitation factor for voltage mode control applicability. But for the same reason, voltage mode control is not applicable for control of parallel connected dc-dc converters.

One cycle control is improved version of voltage mode control. It also has no sensing resistor but sawtooth slope and height are not constant. Similarly to current mode control, one cycle control introduces another feedback loop by integrating input and output voltage. Since sawtooth signal is integral of input and output voltage, any change in input/output voltage will affect sawtooth sooner then the change would affect error signal (ePI).

vmc_one_cycle

For this reason, one cycle control is better then voltage mode control because it has better response time.

Read about current mode control.

External links:

Voltage Mode Control on Powerelectronics
Voltage Mode Control on Spellmanhv
Voltage Mode Control on Electronicdesign
One Cycle Control on Onecyclecontrol

 

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