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

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

frequency:

Equation 1

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

the RCD snubber connected to the drain.

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

increased in accordance with Equation 3.

Design optimization

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

consumption according to Equation 4.

P

IN MIN

()

1

2

---

L

P

V

SENSE M

R

SENSE

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

⎝⎠

⎜⎟

⎛⎞

2

1

T

RESTART

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

×

×

×

=

P

PRI

P

BIAS

P

SW

P

SNUBBER

++

=

R

BLEEDER

V

OUT

2

1

2

---

L

P

V

SENSE M

R

SENSE

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

⎝⎠

⎜⎟

⎛⎞

2

1

T

RESTART

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

×

×

×

P

PRI

–

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≤