DF04M View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

DF04M

OFFLINE FLYBACK CONVERTERS DESIGN METHODOLOGY WITH THE L6590 FAMILY

STMicroelectronics 

AN1262 APPLICATION NOTE

Table 19. G1(jω) Implementation: secondary feedback (III)

Symbol

Definition

RF2; RC RF2 > 2kΩ ; RC < 2.5 ⋅ RF2

RE

RE > 1kΩ

RL

RL ≈ 0.27 to 2.7 [kΩ]

RH

RH = V-----o---u-2-t--.-–-5----2---.--5-- ⋅ RL

RB

RB

<

CTRmin

⋅

V-----o---u---t--–-----3---.--5--

2.5

⋅

RE

CF1

CF1 = -(--R----E---C--+--T---R--R--C--m--)--a--·-x---R-⋅---R-B----E-⋅--R-⋅----RH----F-⋅--2-G-----1----0-

RF1

RF1 = -2----⋅---π-----⋅---f1--Z----⋅---C-----F---1- – RH

CF2

CF2 = -2----⋅---π-----⋅---f-1-P-----⋅---R----F---2--

Figure 15 shows a special configuration, with the optocoupler connected in series to the supply pin of the IC that

provides the following benefits:

a) a large range of the voltage generated by the auxiliary winding can be allowed since the changes are

"damped" by the phototransistor and Vcc is stabilized by the error amplifier; this is useful with a poor quality

transformer or when the output voltage (tracked by the auxiliary voltage) may decrease because of constant

current regulation (e.g. battery chargers, see fig.40 on L6590’s datasheet).

b) during overload and short circuit the power throughput is automatically reduced because the operation of the

device becomes intermittent. In fact, the phototransistor carries the quiescent current IQ of the IC and, if the

output voltage is too low, there will not be enough current through the photodiode at the secondary side to

maintain IQ. The device will be switched off as it goes into UVLO.

c) despite the IC's OVP protection is bypassed by such configuration, the system is still protected against op-

tocoupler's failures: if that happens, the phototransistor will no longer be able to supply the IC, which will go

into UVLO just like in case of overload or short circuit.

The transfer function of the schematic of Fig. 15 is:

G1(jω) = -∆---V∆----VC----Oo---u-M--t--P-- = -∆---V∆----VC----OC----MC----P-- ⋅ ∆---∆-V---I-C-C---C-- ⋅ -∆∆---II--CF-- ⋅ ∆--∆--V-I--F-K-- ⋅ ∆--∆--V--V--o--K-u---t =

= CTRmax ⋅ R--R---F-B--2- ⋅ j--ω------⋅---R----H--1--1----⋅---C-----F---1- ⋅ -([--11-----++-----jj-ωω------⋅⋅---R(--R---c--H-⋅---2C----+-s---)-R--⋅--c-[--)-1---⋅--+-C----js-ω--]---⋅-⋅--(-(--1R-----+H----1-j-ω--+----⋅-R--R--F---F1---2)----⋅⋅---CC----FF----12---]).

The VCC capacitor has a significant effect on the frequency characteristic of this circuit: in particular, it introduces

a low-frequency pole that causes a phase lag noxious for the phase margin. This pole needs to be compensated

by a zero, which requires an additional resistor (RC) in series to the capacitor.

The zero (RH1+ RF1) · CF1 will be placed close to the pole due to the VCC capacitor, (RH2+RC)·CS so as to com-

pensate it. The pole at the origin and the other zero-pole pair realize a type 2 amplifier (see Table 20 to see how

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