Current mode
The current mode is a basic way of regulating a DC chopper or a switched-mode power supply . In contrast to the voltage mode , the control monitors the coil current or switch current in addition to the output voltage. The coil or switch current (whichever is easier to observe) is regulated to a setpoint by a fast, internal control circuit.
This current setpoint is specified by an outer loop. Only the inner loop determines the duty cycle .
The mean value of the current appears through the filter C at the output.
Since only one delaying element (the C) exists, the closed loop is stable under the condition that the inner loop is very fast.
The controller consists of the following basic components:
- an input amplifier EV
- a comparator comp
- an oscillator (like the sawtooth generator)
optional:
- an integrator I.
- a loop filter F
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 fed to the current regulator.
If the requirements placed on the output voltage are not too high, this principle can be used to implement very simple and thus inexpensive step-up or step-down converters without a loop filter and integrator. The loop filter F can only be implemented in integrated circuits with external capacitors and means additional component costs.
This arrangement is only required when the buck converter or boost converter is at least temporarily in Continuous Current Mode (CCM for short, current flow through the coil is not interrupted). If you can ensure that the controller is always in discontinuous current mode (DCM for short, intermittent current in the coil), you can do without the filter (F).