AN3093 datasheet(13/29 Pages) STMICROELECTRONICS

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AN3093

Efficiency and no-load measurements

Doc ID16539 Rev 1

13/29

significantly reduces the average switching frequency of the converter, when light loaded or

completely no-loaded.

As the output voltage is sensed through the auxiliary winding just at the end of the

secondary demagnetization, rendering the IC is blind to the output voltage between one

switching cycle and the next, the switching frequency cannot be reduced below the

minimum restart time, TRESTART, fixed at 500 µs (typical value).

Let's calculate the minimum input power consumption when the IC works at minimum restart

frequency:

Equation 1

where (VSENSE_M/RSENSE) is the peak drain current during burst mode (VSENSE_M =

140 mV internally defined).

Neglecting the losses through the input bridge rectifier, the input capacitor leakage current

losses and the conduction losses, the primary side losses can be calculated as follows:

Equation 2

where PBIAS = VCC x IQ is the consumption of the IC (the quiescent current, IQ, is 1 mA

typical value), PSW represents the switching losses and PSNUBBER are the losses through

the RCD snubber connected to the drain.

If PIN(MIN) > PPRI, the residual energy is transferred to the secondary side and it must be

dissipated through a dummy load in order to avoid an increase in the output voltage. In this

case, the bleeder resistor can be calculated as following:

Equation 3

However, if PIN(MIN) = PPRI the dummy load can be removed.

Since most of the time PPRI is lower than PIN(MIN), the above consideration shows that input

power reduction can be achieved by reducing Equation 1 so that the RBLEDEER can be

increased in accordance with Equation 3.

Design optimization

The sense resistor RSENSE sets the constant current value together with transformer's

primary-to-secondary turn ratio (see Section A.2), but also defines the minimum input power

consumption according to Equation 4.

IN MIN

()

---

SENSE M

SENSE

---------------------------

⎝⎠

⎜⎟

⎛⎞

RESTART

--------------------------

PRI

BIAS

SNUBBER

BLEEDER

OUT

---

SENSE M

SENSE

---------------------------

⎝⎠

⎜⎟

⎛⎞

RESTART

--------------------------

PRI

–

-------------------------------------------------------------------------------------------------------------------

≤

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