[013] Current Limiting and DCM Flyback Design

How inaccurate current limiting can cause problems in low-power flyback designs.  

Introduction

For very low power design, around 1 W or less, the DCM flyback converter is a very popular topology. When pushing up to higher power, however, this converter can create difficulties if you do not properly take care of the control parameters—especially current limiting.

Current Limiting and DCM Flyback Design

The discontinuous-mode (DCM) flyback converter is viewed by many designers as being the simplest converter of all. For low power design of several watts, it is often the most prudent choice. The DCM flyback converter works by charging up the primary inductance of a transformer when the power switch is on, and fully discharging the current in the secondary of the transformer when the switch is off. The inductor value, a crucial circuit element, is accurately controlled by gapping the core of the transformer.

The DCM flyback has the following features that lead designers to its selection:

Natural commutation of the output diode, minimizing switching loss and secondary noise;

Low-noise turn-on of the primary switch;

Simple control characteristics; and

Possibility of a quasi-resonant mode of operation to lower noise even further.

The disadvantage of the DCM operation is that the currents in the circuit are higher than for CCM operation.

Figure 1: DCM Flyback converter. 

In the field of power electronics, there is always a tendency towards higher power. System designers see the features they could add to new products, and inevitably increase the power requirements. Similarly, in applications like appliances, the role of power electronics is on the increase, and older mechanical parts are being replaced by electronics parts that need more power.

The same designers who worked on the low power applications are pushing to 5 W, 10W, and beyond. They are taking with them their design experience at low power, and are encouraged by silicon chip makers who are pushing the capabilities of their integrated switchers to higher and higher power levels.

This is getting a lot of designers into trouble. Design practices that they could get away with at 1 W are not so forgiving of rigorous testing at higher power levels.