Voltage mode
The voltage mode is a basic way of regulating a DC converter . In contrast to the current mode, the control only observes the output voltage and converts this information into the manipulated variable. The control variable in this case is the duty cycle ( duty cycle ) for the switch. The voltage chopped up by the diode and switch can be represented by a square-wave voltage source with the duty cycle d, as shown in the picture.
The mean value of the input voltage Vin appears through the filter from L and C at the output. In the picture above you can see that the controller has to set the duty cycle so that the mean value of the pulsed input voltage corresponds to the desired output voltage. The 2nd order filter, consisting of the components L and C, smooths the pulsed input voltage so that a triangular voltage appears at the output.
Due to the delaying elements L and C, the control loop is unstable, so that the controller absolutely needs a compensation circuit (F). The controller consists of the following basic components:
- an input amplifier EV
- a loop filter F
- a sawtooth generator SG
- a comparator comp
optional:
- an integrator I.
The input amplifier EV amplifies the difference between the output voltage Vout and the setpoint voltage Vref with the gain Aev. The optional integrator improves the control properties by overweighting the direct current components compared to the alternating current components. The result is a very precise mean output voltage in the steady state or the idle state. The loop filter is used to suppress the undesired middle frequency components which would otherwise lead to oscillation. The signal after the filter is compared with the comparator with a sawtooth voltage, from which the pulse duty factor is determined.
This arrangement is only required if the buck converter is at least temporarily in Continuous Current Mode (CCM, coil always carrying current). If you can ensure that the controller is always in discontinuous current mode (DCM, intermittent current in the coil), you can do without the complex filter (F).