AN1049 datasheet(6/24 Pages) STMICROELECTRONICS

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If the sense resistor Rs is selected as follows:

− Vo

Ippkmax

− Vo

Pinmax

VEmin

⋅ ZE

(17).

(with ZE evaluated at fsw = fosc), the peak voltage on the current sense pin at transition will be given by:

VcspkT

= Vo + (1 − Vo) ⋅

VEmin

⋅

⋅ PinT

Pinmax

+ PinTmin

(18),

(with PinT and PinTmin evaluated at fsw = fosc). It will assume its minimum value at minimum mains volt-

age (that is, @ VE = VEmin

⇒ PinT = PinTmin):

VcspkTmin

= Vo + (1 − Vo) ⋅

Pinmax

PinTmin

(19).

Table A2 in appendix shows that in MCM systems (for which PinTmin

≤ Pinmax ≤ PinTmax) the ratio PinTmax /

PinTmin does not exceed 3.31 in practical cases. This means that also Pinmax /PinTmin will not exceed 3.31.

As a result, the transition from CCM to DCM will occur at Vcspk values that do not exceed 2 / (1+3.31) =

464 mV (when Vo = 0, and even larger values when Vo > 0).

In the end, since Vcspk1 = 367 mV, when the L5991 activates the standby frequency MCM systems are

operating in DCM. The standby input power will then be found once more from equation (8) which, ac-

counting for (17) and after some manipulations, yields:

PinSB

Pinmax

⋅







0.367

− Vo







⋅ 



Pinmax

PinTmin







⋅

PinTmin

Pinmax

(20).

Besides, all the considerations leading to equation (12), as well as equation (12), still apply. This will al-

ways be true if VcspkTmin is greater than Vcspk1, that is if the ratio Pinmax /PinTmin is such that:

Pinmax

PinTmin

≤

1.633

− Vo

0.367

− Vo

(21)

(= 4.45 for Vo = 0), which includes also a class of CCM systems. In practice, the above equations apply

to the large majority of common flyback designs.

Once the system is in standby mode, in equations (15) ZE must be evaluated for fsw = fSB, becoming

ZE’. This will modify also PinT, PinTmin and VcspkT: they all increase and become PinT’, PinTmin’ and VcspkT’

respectively.

When Vcspk =Vcspk2, that is when the input power is PinNW and the frequency is to be switched back to

fosc, the system can be working either in DCM or CCM, depending on the fosc / fSB ratio and on VE (that

is, on the input voltage). In other words, it depends on whether VcspkT’ is greater or less than Vcspk2. It is

possible to find that if the following condition:

fosc

fSB

≥

⋅

0.867

− Vo

⋅

VEmin

⋅ 



Pinmax

PinTmin







(22)

is fulfilled, then VcspkT’ >Vcspk2 and the system will be working in DCM.

The right side of (22), for Vo = 0, is top limited at 1.87 in MCM systems. Considering that in most practi-

cal cases the fosc / fSB ratio will not be less than 2, it is possible to leave out the case of CCM operation.

This makes things easier because there would be also a dependence of PinNW on VE.

In the end, PinNW will be given again by equation (13) which, rearranged more conveniently, becomes:

PinNW

Pinmax

⋅







0.867

− Vo







⋅ 



Pinmax

PinTmin







⋅

PinTmin

Pinmax

⋅

fSB

fosc

(23)

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