Hysteretic mode
The hysteresis mode is a basic way of regulating a DC chopper. This rule is also known as two-step control ( English Bang-bang control ;) and is the simplest of all possible regulations and the control of a DC chopper . The two-position controller only ensures that the output voltage is within the specified limits and switches the switch on or off accordingly.
The mean value of the input voltage V in appears through the coil and capacitor (L, 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, with components L and C, smooths the pulsed input voltage so that a smoothed triangular voltage appears at the output.
The 2nd order filter is also the reason for the fundamentally unstable behavior of this structure. This can prevent a series resistor from being present at the output in addition to the capacitance . If the resistance is high enough, the integral effect of the capacitance takes a back seat and the only delaying element is the inductance , so that the circuit is stable.
The hysteresis controller only needs the following basic components:
- a hysteresis or two-point controller
- a resistor in series with the output capacitor
This controller works both with an uninterrupted current flow in the coil ( Continuous Current Mode , CCM for short) and with discontinuous current ( Discontinuous Current Mode , DCM for short).
The advantage of the simplest structure is offset by the following disadvantages:
- The frequency depends on the set hysteresis and the resistance.
- The frequency can therefore move in a very large range, which makes the design of the input filter that is often required very difficult.