DF04M View Datasheet(PDF) - STMicroelectronics
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DF04M Datasheet PDF : 42 Pages
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AN1262 APPLICATION NOTE
must be accounted for as well (the minimum specified value will be considered) and the turn number may result
quite high.
17 PRIMARY FEEDBACK IMPLEMENTATION
In this approach, which will be considered with regards to the L6590 and the L6590D only, the voltage generated
by the self-supply winding is sensed and regulated. This solution, shown in fig. 16, is cheap because no opto-
coupler is needed, but provides poor regulation, especially as a result of load changes.
Ideally, the voltage generated by the self supply winding and the output voltage should be related by the Naux/
Ns turn ratio only. Actually, numerous non-idealities, mainly transformer's parasitics, cause the actual ratio to
deviate from the ideal one. Line regulation is quite good, in the range of ± 2%, whereas load regulation is about
±5%. Output voltage tolerance is instead in the range of ±10%. The resulting transfer function is:
G1(jω) = -∆---V∆----VC----Oo---u-M--t--P-- = N---N--a--S-u---x- ⋅ -∆---V∆----VC----OC----MC----P-- = N---N--a---Su---x- ⋅ -R----H-----⋅---(--C-----F-1--s----+-----C-----F---p---) ⋅ j-1-ω--- ⋅ -1----+----1-j--ω--+----⋅-j--Rω----F-⋅---R⋅----C--CF-------F--⋅F----s--Cs------+--⋅-F-----C--sC-------F--F-----p--p--.
Table 21 shows how its quantities are defined. As to the selection of Vcc, the same considerations concerning
the circuit of fig. 15 apply to the circuit in fig. 16a. Such limitation is not in the circuit of fig. 16b.
Figure 16. Primary feedback: circuits
Vcc
L6590 VFB RH
L6590D
RL
Rs
22 µF
Naux
L6590
L6590D
Vcc
VFB
Rs
RH
R's
220 nF 22 µF
Naux
RL
COMP
CFs
RF
CFp
a)
COMP
CFs
RF
CFp
b)
Table 21. G1(jω) Implementation: Primary Feedback
Symbol
Definition
RL
RL > V-1---5-C---VC-- [kΩ]
RH
RH = V-----C---C-2---.-–-5----2---.--5-- ⋅ RL
CFp
CFp = N---N--a---Su---x- ⋅ ff--PZ-- ⋅ -G-----1---0--1--⋅---R-----H--
CFs
CFs
=
CFp
⋅
f-f-PZ--
–
1
RF
RF = -2----⋅---π-----⋅---f1--Z----⋅---C-----F---s-
The value of the resistor Rs (R’s for the circuit of fig. 16b) in series to the bias diode will be selected to achieve
minimum load regulation and its value may range from few units to some hundred ohm.
30/42
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