ALTAIR04-900TR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'ALTAIR04-900TR' PDF : 29 Pages View PDF
Application information
ALTAIR04-900
5.5
Constant current operation
Figure 14 presents the principle used to control the average output current of a flyback
converter.
The output voltage of the auxiliary winding is used by the demagnetization block to generate
the control signal for the switch Q1. R resistor absorbs a current VC/R, where VC is the
voltage developed across the capacitor CREF.
The flip-flop output is high as long as the transformer delivers current on the secondary side.
This is shown in Figure 15.
The capacitor CREF has to be chosen so that its voltage VC can be considered as a
constant. Since it is charged and discharged by currents in the range of 10 µA (ICREF is
typically 20 µA) at the switching frequency rate, a capacitance value in the range of 4.7-10
nF suits to switching frequencies of 10 kHz.
The average output current can be expressed as follows:
Equation 2
I
OUT=
N-N----S-P--E-R--C--I ⋅ (---2G-----⋅I---R⋅---V-S---C-E---RN---ES---FE----)
where NPRI is the primary turn number.
This formula shows that the average output current does not depend neither on the input or
the output voltage, nor on transformer inductance values. The external parameters defining
the output current, are the transformer ratio n and the sense resistor RSENSE.
GI current loop gain and VCREF current reference voltage are internally defined.
Figure 14. Current control principle
.
R zcd ZCD/FB
Rfb
A ux
D EMAG
LOGI C
S
Q
R
Ir ef
Gi
To PWM Logic
CC
R
F rom R sense
Q1
IREF
C
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DocID18211 Rev 3
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