A handbook for quickly querying different control methods. It is useful for the review of High Frequency course of Prof. LIU.

Basic Concept

Main application domains of PE

  • Industrial
  • Residential / commercial
  • Transportation
  • Scientific research
  • Military
  • ...

Main applications of PE

  • Power supply
  • Motor devices
  • Utilities

Issues the engineers should consider

  • Efficiency
  • Reliability
  • Perturbation suppression

Why do we need models?

  • Sometimes objects are not available or too expensive
  • Models resemble the original objects in some aspects

Two categories of models

  • Physical models
  • Mathematical models

Multi-level modeling of power electronics circuits

  • Device behavior level
  • Circuit operation level
  • System control level

The objectives of control

  • Let the circuit performs as we want, in other words, improve the performance
  • Overcome the disturbance

The difference between two main kinds of control: the Feedback control and the Feedforward control

  • Whether there exists a closed control loop
  • Whether the mesured signal follows the given reference directly

Feedforward Control

Advantages (Compared with Feedback)

  • Stability - No stability issue. Feedforward can prevent disturbances amplification.
  • Accuracy - No static errors. The output of feedforward should be predictable, so it is precise.
  • Dynamic response - Feedforward is proactive, which means it can react before error actually occurs. Once an upstream disturbance is identified Feed Forward immediately seizes the initiative and prescribes an appropriate change. Thus, it is faster.

Drawbacks hard to overcome (Feedforward)

  • Unable to accurately reject the perturbations for the switching errors exist
  • Unable to accurately reject the perturbations which is not considered in the design. This is common because the practical condition contains many sources of perturbations. The more source of perturbations we consider, the more complex the controller will be.

Feedback Control

Advantages (Compared with Feedforward)

  • Perturbation suppression - it has a strong ability of automatic correction. It can handle the potential errors caused by internal and external disturbance. It gets a strong anti-interference ability.

Why do we need compensator (Drawbacks to overcome)

The potential disturbance may make the system fail to work, because the feedback control inherently exists the stability issue. Also, the static error and dynamic response may not be excellent.

The objectives of a compensator design (in feedback)

  • Stability
  • Accuracy
  • Dynamic response

Control of the Voltage

Control of the Output Voltage

Control of the voltage mostly refers to control of the output voltage.

Voltage Mode Control

  • Simple, easy to realize

Drawbacks Hard to Overcome (VM)

  • Compensator may hard to design, sometimes a good design is impossible
  • Dynamic response - A large number of switching cycles is required before the steady-state is regained. What's more, in non-linear converters, the bandwidth of the loop has to be set very low to avoid the instability.

One Cycle Control

  • Significantly improve the dynamic response performance of VM - Automatically correct the switching errors within one switching cycle
  • Perturbation suppression - Reject the perturbations in one switching cycle

Drawbacks Hard to Overcome (One-Cycle)

Control of the Current

Benefits of current control

  • Current limiting (peak current protection without soft-start mechanism)
  • Current sharing

Control of the Inductor Current

Peak Current Mode Control

  • Benefits of current control
  • Easier compensator design (compared with VM)
  • Good dynamic performance - in non-linear converters, the bandwidth can be set very high

Limitations of PCC

  • Cannot accurately control average inductor current
  • Low DC gain of the current loop
  • Loop gain sensitive to line variation
  • Switching noise problem

Average Current Mode Control

Control of the Input Current

Suitable Application Range

Control of the input current usually means the control of the input switching current of buck or flyback converters. In my opinion, it also includes the buck-boost converters. Input current of these converters are pulsating, so the filtering is usually useless. If the input current is the inductor current, i.e. the boost converter, we can refer to the section Control of the Inductor Current

Advantages

  • Good input current sharing
  • Switching noise immunity

Drawbacks to overcome

The application of tradition Average Current Mode Control in the input current control has these drawbacks listed below:

  • Input current is too pulsating to be filtered
  • Too much filtering will render the current loop useless
  • Too much filtering will result in loop instability

The application of tradition Peak Current Mode Control in the input current control has these drawbacks listed below:

  • Cannot control average input current
  • Switching noise problem

Charge Control

Drawbacks hard to overcome

  • Subharmonic oscillation