User:Andy Calloway/sandbox

Active Clamp Flyback (ACF) is an improvement on classic flyback converter topology used in many Switch Mode Power Supplies (SMPS). As a result of its faster switching speeds, improved efficiency, and smaller components, ACF offers improved performance and lower power consumption while minimizing power supply size and weight.

Advantages
In contrast to conventional flyback converters, ACFs re-use power stored in the transformer’s leakage inductance that would traditionally be dissipated in a passive clamp snubber resistor. During the switching cycle, this 'recycled' energy is supplied to the load to improve converter efficiency.

It also allows a more aggressive turns ratio between the primary and secondary windings of the transformer, as well as reclaiming energy previously dissipated as heat. This facilitates the use of a lower voltage rating synchronous rectifier switch on the secondary side. In addition to reduced cost, the lower voltage rating of the switch also reduces conduction losses, resulting in a lower on-resistance.

ACF flyback converters offer several distinct advantages over conventional snubber-based converters, including substantial efficiency improvements and EMI reductions, both of which are considered crucial for high-density power supplies.

Why ACF is important
Today's engineers must continually improve the specification and performance of their products and applications while reducing size, weight, and cost. As a result of government regulations, industry standards and consumer pressure on power consumption, electronic designs have to become more efficient in order to minimize electricity consumption.

In certain applications, moving from a conventional flyback AC/DC converter topology to an active clamp flyback (ACF) topology provides engineers with a way of addressing all of these challenges in one go.

How Active Clamp Flyback works
Among the most commonly used topologies for isolated AC/DC switch mode power supplies (SMPS), the traditional flyback converter is a simple circuit requiring just a few components and capable of operating with efficiencies of around 90%. Typical designs combine a primary side FET and a transformer for isolation with a passive RCD clamp circuit, an output rectifier and components for filtering.



During operation, energy is stored in the transformer magnetizing inductance when the primary side FET is on and is transferred to the load when the FET is off. Increasing the switching speed allows the overall size of the converter to be reduced, but at a penalty of higher switching losses in the FET and transformer, reducing efficiency. In the case of the passive RCD snubber, one of the main limitations comes from the transformer’s leakage inductance. When the primary side FET switches, the energy stored in the leakage inductance is dissipated as heat in the clamp resistor. Higher switching frequency creates excessive power losses in the form of heat, imposing limitations on the maximum switching frequency, which if exceeded can cause circuit damage or catastrophic failure. This ‘active clamp’ design allows the leakage energy that was previously dissipated via the resistor to be ‘recycled’ by first storing it in the capacitor (Cclamp) and then delivering it to the load.



In addition to recycling energy that would previously have been dissipated as heat, ACF topologies can deliver additional benefits depending on the approach taken to implementation. Applying intelligent digital control to the clamp circuit, for example, allows near zero-voltage switching (ZVS) turn-on of the primary flyback FET and management of the turn-off drain voltage.

Not only do such techniques further improve efficiency but they help to reduce electromagnetic interference (EMI) and, thus, the size and number of passive components needed for filtering. Until recently this type of adaptive, intelligent control has added circuit complexity and has not been straightforward to implement. Now, however, the advent of fully integrated ACF controllers is significantly simplifying the design of small, light and high-efficiency converters.