AN1537 datasheet(17/18 Pages) STMICROELECTRONICS

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AN1537 APPLICATION NOTE

To calculate VCOMP0, Pin needs to be estimated considering the power processed by the transformer under no-

load conditions. It is possible to use the following formula:

Pin = 1.25(Vout Ioutres + Vaux Iaux) ,

(A6)

where Ioutres is the residual output current (through the output divider, through the optocoupler's LED, as well

as any residual dummy load resistor), Vaux is the voltage generated by the auxiliary winding that supplies the

L5991 (and the L6561 in the 80W adapter) and Iaux the total consumption on this winding (quiescent current of

the L5991, of the L6561 too in the 80W adapter, optocoupler's transistor current and the consumption of any

circuit supplied by the reference voltage of the L5991 or directly connected to its supply bus). The coefficient

1.25 stems from an estimated efficiency of 80% that experiments have shown many flyback transformers fea-

ture under no-load conditions, almost regardless of their size, parameters and construction.

It is obvious that the estimate of Pin is the weak point of this design procedure. Since it is essentially the asymp-

tote of the oscillator waveform that governs the oscillator frequency, even some ten mV variation of VCOMP0 may

change the frequency considerably. A small error in the assessment of VCOMP0, then, could lead to a value for

RC that gives an fmin away from the target. This is why it is recommended to use the result of the calculation as

a starting point that has to be checked on the bench and, in case, corrected after the experiments. For com-

pleteness, it must be said also that under no-load conditions VCOMP is far from being a perfect DC voltage, as

shown in the diagrams of figure A2.

45W adapter example calculation

With CT = 3.3nF, RA and RB are 12kΩ each. The residual output load is 40mW, the total consumption of the

auxiliary winding is estimated around Iaux = 10mA, that is 110mW assuming Vaux = 11V. Then, using (A6):

Pin ≈ 1.25 (0.04 + 0.11) = 0.188W .

VCOMP0 will be given by (A5), where it is assumed a perfect propagation delay compensation:

From (A3):

use standard value 5.9k

Ω.

From (A4), considering Tamb = 0°C, then VF = 0.563V:

use standard value 8.2k

Ω.

80W adapter example calculation

With CT = 3.3nF, we get RA =15kΩ and RB = 11kΩ. The residual output load is 40mW, the total estimated con-

sumption of the auxiliary winding is Iaux = 12mA, that is 120mW assuming Vaux = 10V. Then, using (A6):

Pin ≈ 1.25 (0.04 + 012) = 0.2W

VCOMP0 will be given by (A5):

From (A3):

use standard value 7.5k

Ω

From (A4) ), considering Tamb = 0 °C, then VF = 0.563 V:

use standard value 5.6k

Ω.

COM P0

1.4

3 0.47

2 0.188

⋅

510

400 10

–

⋅⋅

⋅

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

⋅

2.011V

12 10

2.011

–

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

⋅

5.934 10

⋅

5.9 10

2.011

0.563

–

2.011

–

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

⋅

8.638 10

⋅

COM P

1.4

3 0.28

20.2

⋅

510

430 10

–

⋅⋅

⋅

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

375 200 10

–

⋅⋅

430 10

–

⋅

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

–







⋅

1.616 V

11 10

1.616

–

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

⋅

7.612 10

⋅

7.5 10

1.616

0.563

–

1.616

–

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

⋅⋅

5.706 10

⋅

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