AN2864

Tips for reducing the standby consumption

Doc ID 15282 Rev 1

7/29

1.3

VIPER17L consumption

The VIPER17L current consumption is 1.8 mA maximum for the L version when switching.

When the device is not switching and the FB pin voltage goes below the burst mode

thresholds, the current consumption is reduced to 900

μA. Clearly, lowering device

consumption contributes to the reduction of total power consumption, but it also presents

other advantages. The control loop senses the output voltage and modulates the power

processed according to the load on the output, in order to keep the output voltage constant

at the target value. The voltage that supplies the VIPER17L is not regulated, but should

follow the output voltage according to the auxiliary to secondary windings turn ratio.

The winding coupling is never perfect in a real transformer. Even if the output voltage is well

regulated, the auxiliary voltage changes according to the load ratio between output and

auxiliary voltage. The cross-regulation problem inherent in flyback converters is well-known

in power electronics literature [1]. Even if it is not discussed here, it is important to

remember that the auxiliary voltage used for the VIPER17L device supply increases as the

ratio between output and auxiliary current increases, and decreases as this ratio decreases.

Even if no external load is present, the circuitry used for sensing the output voltage and

closing the control loop sinks some current. In an optimized converter, for very low standby

consumption this current is very low. It is then possible that the auxiliary voltage (voltage on

device shutdown. In this case, the device begins to operate in hiccup mode (continuous

shutdown and startup) and no longer guarantees the output voltage regulation. Keeping the

current consumption of the VIPER17L low, at least when operating in burst mode, means

increasing the above ratio between the output current and auxiliary current. This helps to

1.4

Transformer

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

transformer.

The primary parasitic capacitance is the capacitance measured at the primary side with the

other transformer windings open. In order to measure it using an LCR meter, the frequency

needs to be set as high as 1 MHz. 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 will be minimized.

With reference toEquation 1, it is clear that a further reduction of the switching frequency

can be obtained by increasing, as much as possible, the primary inductance of the

the power the converter must process to sustain a certain load is constant, raising the

primary inductance increases the energy processed during each switching cycle (for the

same drain peak current) and the number of switching cycles per unit of time, which means

that the average switching frequency decreases.