What is the principle of average current control? A brief introduction to the design of average current buck circuit

Brief description: ****

The current control framework of the buck converter is shown in the figure. It adopts dual-loop control. The outer loop is the voltage loop, which is used to adjust the voltage error and generate the current error and the current command of the inner current loop. The error between the current loop command and the inductor current is then used to generate the drive signal of the switch tube through PWM. The current mode control methods mainly include: peak current control, valley current control, fixed on-time control, fixed off-time control, hysteresis current control, average current control and other methods. This experiment mainly focuses on average current control.

Average current control principle:

The block diagram of average current control is as follows. The external circuit voltage error amplifier acts as a voltage regulator to generate the inductor current command signal. The error between the inductor current and the current signal is then used to generate the control voltage required for PWM through a current error amplifier. Finally, the control voltage is compared with the triangular wave to generate the drive signal for the switch tube.

Current loop design:

According to the state average method:

Ignore the disturbance of input voltage and output voltage:

Considering the sensing attenuation of the inductor current and the gain of the PWM, we know that:

For the first-order system, the current error amplifier can be designed using a second-class error amplifier. Since the PWM control voltage can only intersect with its triangular wave signal once in one cycle, the maximum bandwidth of the current loop is limited by the rising slope of the control signal being less than the rising slope of the sawtooth wave. From the above limitations, it can be seen that:

Re-arrangement shows:

Therefore:

Rearrange:

If the design is based on the rising slope limit of the control voltage, the theoretical maximum current loop bandwidth may be higher than or close to the switching frequency, so it is impossible to design with this value. Generally, the current loop bandwidth is selected to be one-fourth to one-eighth of the switching frequency of the switch tube. Therefore, the error amplifier parameters of the current loop can be determined based on the current loop bandwidth.

Voltage loop gain error amplifier design:

Generally, the response speed of the voltage loop is lower than that of the current loop. Therefore, when designing the voltage loop, the current loop can be regarded as an ideal state, that is:

Based on this assumption, the voltage loop equivalent circuit of average current control can be simplified as shown below:

According to the above formula, the control block diagram of the voltage loop can be written as follows:

The voltage error amplifier can be designed using a type II error amplifier, and the bandwidth of the voltage loop is generally one-third to one-fifth of the current loop.

The specifications of the average current controlled buck converter in this experiment are as follows:

This design is done using matlab, and its source code is as follows:

The simulation results are as follows:

Experimental verification:

Summarize:

Compared with single voltage control, average current control has faster response speed and better anti-interference ability, and is a typical application of dual-loop control in power electronic control theory.

Simulation effect diagram: