DFS04M datasheet(7/42 Pages) STMICROELECTRONICS

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AN1262 APPLICATION NOTE

Table 5. Cin values for 1W input power

The actual values of Vinmin and VDCmin need to be recalculated with the actual capacitance value. Since the evaluation

of Vinmin involves an equation having no closed form solution, an iterative cycle needs to be established:

;

(1)

where TC is the recharging time of Cin, that is the time while the bridge diodes are conducting, which can be

initially assumed equal to zero. After few iterations both Vinmin and TC will converge to their respective values.

In case of holdup requirement the cycle should be executed twice. The first time with NH = 1 to find Vinmin after

one mains cycle missing (which will be used to check for maximum duty cycle and maximum peak current) the

second one with NH = 0 to find Vinmin in normal operation (to be used for steady state and thermal calculations).

VDCmin will be simply the average of Vinmin (calculated with NH = 0 anyway) and VPKmin:

(2)

The voltage rating of Cin is selected depending on VPKmax: it is usually 200 V for 110 VAC applications and 400V

for 220 VAC or WRM applications.

PRELIMINARY CALCULATIONS (STEP 2)

The next step is to check for not exceeding the limits imposed by the IC. Prior to this, the power processed by

the transformer (PinT) and the average voltage drop across the ON-resistance of the internal MOSFET

(VDS(on)x) will be evaluated. VDS(on)x is subtracted to Vinmin and the resulting value is the voltage actually applied

to the primary winding of the transformer. The RDS(on) used must take temperature into account. Use the max-

imum value defined at 125°C.

The first limit to be checked is the maximum duty cycle DX. If it exceeds 62-64%, either the reflected voltage VR

should be lowered or the minimum input DC voltage Vinmin should be increased by selecting a larger input ca-

pacitance.

The second limit to be checked is the maximum drain voltage during the OFF-state of the MOSFET. At least

50V margin should be ensured. The overvoltage spike can be reduced to allow more reflected voltage if neces-

sary, keeping in mind that it cannot be much lower than VR not to hurt the primary-to-secondary energy transfer.

The last check concerns the peak primary current that must not exceed the minimum guaranteed OCP threshold

(0.55A). If this is exceeded, a higher maximum duty cycle DX should be used, if possible. Also a higher Vinmin

is beneficial. Some iterations, involving a recheck of the first two points, may be necessary to find the optimum

compromise. If no solution can be found, either CCM operation should be considered or the power handled by

the converter should be derated.

110 VAC or WRM

220 VAC

NH=0

2.0 µF/W

3.0 µF/W

0.55 µF/W

0.8 µF/W

Vinmin = 90V

VDCmin = 105V

Vinmin = 100V

VDCmin = 110V

Vinmin = 180V

VDCmin = 210V

Vinmin = 200V

VDCmin =220V

NH=1

7.2 µF/W

10.4 µF/W

1.8 µF/W

2.8 µF/W

Vinmin = 90V

VDCmin = 116V

Vinmin = 100V

VDCmin = 117V

Vinmin = 180V

VDCmin = 236V

Vinmin = 200V

VDCmin =239V

inm i n

PKm i n

--------

–

12 N

⋅

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

–

⋅





⋅

arccos

inmin

PKm i n

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





π f

⋅⋅

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

DC m i n

---

PK m in

inm i n

()

⋅

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