Efficiency and no-load measurements

AN3093

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A proper design can help reduce the energy per switching cycle transferred to the

secondary and, consequently, no-load consumption. This can be done by increasing

voltage, in order to achieve the CC setpoint according to Equation 4. Other design

considerations should be taken into account in designing the transformer.

For the purpose of reducing the switching losses, some consideration should also be given

to the transformer. The primary parasitic capacitance causes switch-on losses as it is fast-

charged each time the MOSFET is turned on and the charging current dissipates energy

within the MOSFET itself. Reducing this capacitance as low as possible, this type of loss is

minimized.

other design considerations also).

In this case the energy per cycle sent to the secondary side is reduced, which means that

the bleeder resistor value increases, thus reducing the dissipation of the dummy load.

3.2

Five star charger energy rating

To help save energy, in November 2008 the world's five largest mobile phone makers

announced their own five star energy rating system to help consumers more easily identify

the most energy-efficient chargers.

The new rating system shows how much energy each charger uses when left plugged in.

Chargers are labeled with ratings from five stars for the most efficient chargers (

≤0.03 W), to

zero stars for those using the most energy in no-load mode (> 0.5 W).

Figure 10.

No-load consumption score chart

As shown in Table 6, the charger presented here meets the “four star rating”.

By optimizing the design, based on suggestions provided in Section 3.1, the input power

consumption can be reduced. Also, if a higher output voltage in no-load can be accepted,

the bleeder resistor value can be increased a bit more than the minimum value in Equation

3, thus achieving a further reduction in no-load consumption and helping to meet the “five

star rating”.