AN1049 Datasheet, PDF (16/24 Pages) STMicroelectronics – MINIMIZE POWER LOSSES OF LIGHTLY LOADED FLYBACK

AN1049 APPLICATION NOTE
Experimental results
In order to validate the above considerations, an experimental example will be given. It concerns a 40W,
wide-range mains power supply for an inkjet printer, whose design has been optimized following some of
the guidelines here presented, and evaluated on the bench.
Fig. 13 shows the schematic with indication of the relevant parts. The 28V output powers the stepper
motors while the 12V output supplies the printhead. When the printer is idle these two outputs are not
loaded. The 5V section supplies the logic circuits as well as the µcontroller that must be operating also
when the system is idle.
The system operates at 100 kHz at nominal load. This value is set by the parallel of the 22 kΩ and the
5.6 kΩ resistors connected at pin 2, along with the 3.3 nF capacitor placed between pin 2 and ground.
When the output load is decreased so that the input power falls below about 8W, the output of the error
amplifier crosses the lower threshold (VT1) of the internal comparator. The L5991 now disconnects inter-
nally the 5.6 kΩ resistor, so that the capacitor is charged through the 22 kΩ resistor only and the oscilla-
tor frequency is changed to about 20 kHz.
Figure 13. 40W Power Supply for inkjet printers. Electrical schematic
4700pF 4KV 4700pF 4KV
F01 AC 250V T1A
LF01
BD01
2.2
85 to C01
C02
265 Vac
100µF
400V
1N4148 56K 56K
BZW50-100
STTA106
BAT46
4.7M
N1
4.7M
BYW100-200
N2
BYW98-100
N3
2 x 470µF
16V
BYW100-50
2 x 330µF
35V
0.1µF
22K
3.3 nF
330 nF
5.6K
5.6K
47K
5.6K
4
15
14
3
2
16
L5991
1
7
5
330K
22
98
10
13
12
11
6
33 µF / 25V
22
1K
470pF
470
470pF
TRANSFORMER SPECS.
CORE:
Prim. Induct:
N1:
N2:
N3:
N4:
Naux:
EDT29x16x10, 3c85 material, 1.4mm air gap
260µH
51 T, AWG26
11 T, AWG25
10 T, AWG20
4 T, AWG25
10 T, AWG32
Naux
N4
470µF
16V
STP4NA60
0.47
1/2 W
220
1K
4N35
TL431
0.022µF
3.9K 5.1K
2.7K
270K
28V / 0.7A
12V / 1.5A
GND
5V / 0.5A
The system works in standby at 20 kHz as long as the input power does not exceed about 9 W. When
the load current increases and this power is exceeded, the output of the error amplifier overcomes the
upper threshold (VT2) and the L5991 connects again the 5.6kΩ resistor, thus switching the oscillator fre-
quency back to 100 kHz.
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