DF04M View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

DF04M

OFFLINE FLYBACK CONVERTERS DESIGN METHODOLOGY WITH THE L6590 FAMILY

STMicroelectronics 

AN1262 APPLICATION NOTE

The synthesis of G1(jω) can be done by following the following step-by-step procedure:

a) Calculate gain and phase of G2(jω) at the desired crossover frequency (fc). That is:

G2c = G2(2 ⋅ π ⋅ fc)

Φ2c = 1----8π---0-- ⋅ arg [G2(2 ⋅ π ⋅ fc)] ;

G2(jω) will be calculated at maximum input voltage and maximum load, where the gain-bandwidth product is

maximum.

b) Calculate gain and phase of G1(jω) at f = fc in order for the overall open-loop gain to cross the 0 dB axis at

f = fc with the phase margin Φm:

G1c = G1(2 ⋅ π ⋅ fc) = -G---1--2---c- ;

Φ1c = 1----8π---0-- ⋅ arg [G1 (2 ⋅ π ⋅ fc)] = – 180 + Φm – Φ2c ,

c) Cancel the pole of G2(jω) by placing the zero of G1(jω) in the neighborhood:

fZ = 2--ω---⋅--Z-π-- = α ⋅ -ω2----o-⋅--u-π--t (α = 1 to 5)

d) Place the pole of G1(jω) so as to get the desired phase margin:

fP

=

2--ω---⋅--P-π--

≈

--------------------f-C---------------------

tan





1----π8---0--

⋅

Φ 1 c

,

e) Calculate the unity gain frequency G10:

G10 ≈ 2 ⋅ π ⋅ G1c ⋅ f--c---f-⋅-P--f--Z--

The synthesis of G1(jω) is completed. The following step will concern the practical implementation of such func-

tion, that is the realization of a Type 2 amplifier. This will be done considering two cases, the secondary and the

primary sensing feedback.

16 SECONDARY FEEDBACK IMPLEMENTATION

This kind of feedback, shown in fig. 12, uses a popular arrangement with a TL431 as secondary reference/error

amplifier and an optocoupler to transfer the control signal to the primary side. The error amplifier of the IC is

then used as a current source whose characteristic is shown in fig. 12 as well: the voltage VCOMP is changed

(and the duty cycle is controlled) by modulating the current Ic sunk from the pin. A change of Ic causes a change

of VCOMP corresponding to a resistance RCOMP = 9 kΩ. The resulting transfer function is:

G1 (jω) = -∆---V∆----VC----Oo---u-M--t--P-- = ∆----V----∆C---I-O-C---M----P-- ⋅ -∆∆---II--CF-- ⋅ ∆--∆--V-I--F-K-- ⋅ ∆---∆-V--V--o--K-u---t = -C----T---R-R----B-m---⋅-a--R-x----H⋅---R--⋅---CC---O-F---M-----P- ⋅ j--1ω--- ⋅ -1--1--+---+--j--ω-j--ω--⋅---⋅R---(--C-R---O-H---M--+--P---R--⋅---FC---)-C--⋅--O-C---M--F--P--

and table 16 shows how its quantities are defined

24/42

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