AN1049 Datasheet, PDF (1/24 Pages) STMicroelectronics – MINIMIZE POWER LOSSES OF LIGHTLY LOADED FLYBACK

AN1049
®
APPLICATION NOTE
MINIMIZE POWER LOSSES OF LIGHTLY LOADED FLYBACK
CONVERTERS WITH THE L5991 PWM CONTROLLER
by Claudio Adragna
The L5991 PWM controller is particularly suitable for SMPS of equipment that must comply with
standards concerning energy saving. The device, optimized for flyback topology, monitors the power
demanded by the load and changes the operating frequency of the converter accordingly: high fre-
quency at heavy load, low frequency at light load.
In this way, power losses dependent on frequency are reduced at light load maintaining, at the same
time, the advantages offered by a high switching frequency at heavy load.
The frequency reduction is very helpful but is not the only means needed to minimize power losses.
This note surveys the above mentioned functionality of the L5991 (called "Standby" function) as well
as the most significant points to consider in order to achieve the goal of a very efficient lightly loaded
flyback.
INTRODUCTION
The minimization of the power drawn from the mains under light load conditions (Standby, Suspend or
some other idle mode) is an issue that is recently becoming of great interest, above all else because
new and more severe standards are coming into force.
This is already well-established in the area of computer monitors, where norms define precisely the vari-
ous idle modes and the relevant maximum consumption admitted, but more and more often power sup-
plies for other pieces of office equipment (i.e. printers, photocopiers, fax machines, AC-DC adaptors,
etc.), are required to accomplish with specifications concerning energy saving.
Figure 1. L5991 Internal Block Diagram
RCT
DC
DIS
ST-BY
ISEN
SS
2
+
3
-
14
-
+
2.5V
STANDBY
16
SYNC
1
TIMING
DC-LIM
15
T
DIS
VREF
BLANKING
OVER
CURRENT
13
+
-
1.2V
7
1V
PWM
FAULT
SOFT-START
2R
R
12
SGND
VCC
8
21V
+
10V -
PWM
UVLO
SQ
R
VREF
OK
CLK
DIS
VREF
4
Vref
9
VC
17V
10
OUT
11
PGND
6
COMP
-
E/A 2.5V
+
+
2.5/4.0V _-
5
VFB
STANDBY
Anyway, minimizing the power wasted by a lightly loaded switch-mode converter is a demanding chal-
lenge for power supply designers and, to achieve the goal, an appropriate design strategy is required.
March 2000
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