ST AN1686 Application note
AN1686
APPLICATION NOTE
AN L5991-BASED CONVERTER WITH
TEMPORARY EXTRA POWER CAPABILITY
by C. Adragna
In some applications the SMPS, normally supposed to deliver a certain amount of power, from time to
time undergoes load peaks that can be even tw o or more times as much. Such peaks are often too l ong
to be properly handled by oversizing the output capacitors, but short if compared to the thermal time
constants of the power components. Typical examples of such loads are motors and audio systems.
In this case, designing the SMPS for the peak power demand from the load would lead to a poorly used
and more expensive s ystem. It is more cost- effecti ve to desig n for the maximum continuous power and
allow the peak power to pass. However, if for any reason the load demands such peak power level for
a long time, some of the power components, not sized for withstanding this, wil l definitely fail unless the
system is stopped somehow.
In this application note a design example of such a sys tem, based on the S T's advanced PWM contr oller L5991, is carried out.
Introduction
Purpose of this note is to provide a few brief design guidelines on the switch-mode power supply whose electrical specification is summarized in the following table.
Table 1. Design Electrical Specification
Symbol Parameter Value Unit
V
in
f
L
P
outx
P
outpk
V
out
∆V
out
F
osc
F
SB
η Target Efficiency (@P
Notes:
(*) If P
out
P
shall cause converter’s shutdown (latch mode) within 10÷100 ms.
outpk
Input Voltage Range 88 to 264 V
Mains Frequency 50/60 Hz
Maximum Continuous Output Power 45 W
Peak Output Power (t ≤ 0.5 s) (*) 75 W
Regulated Output Voltage (@ P
Output Voltage Ripple (@P
Normal Operation Switching Frequency 70 kHz
Light Load Switching Frequency 18 kHz
= P
out
Maximum Input Power (@P
Maximum Input Power (Open load, V
is such that P
outx
< P
out
≤ P
outpk
=0 ÷ P
out
= P
out
outx
out
the converter shall be shutdown (latch mode) within 1÷2 s; a load exceeding
, Vin =88÷264 VAC)2%
outx
, Vin =88÷264 VAC) 80 %
= 0.5 W, Vin =88÷264 VAC) ≤ 2W
=88÷264 VAC)
in
outpk
, V
= 88÷264 VAC)
in
18V ± 2% V
≤ 1W
ACrms
The special requirement for this converter is the ability to cope with a peak power demand from the load that
exceeds the maximum conti nuous power by over 66% for a limited ti me (
≤
0.5 s), still maintaining output v oltage
regulation, and to automatically shut down in case this overload lasts more than a specified time. Furthermore,
March 2003
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AN1686 APPLICATION NOTE
in case of anomalous power demand fr om the load (exceeding the p eak power, e.g. d ue to a short circ uit) , converter's shutdown must occur within 10
A power peak lasting 0.5 s is too long to be handled with just a reinforced output capacitor bank: as a matter of
fact, after few ms the conver ter's cont rol loop has alr eady reacted to maintain the output v oltage regulated. Thus
a power demand lasting 0.5 s can be considered as a steady-state operating condition from the electrical point
of view. On the other hand the thermal time constants of the power components are such that the heat generated during a power peak of even a couple of seconds will not cause an excessive temperature rise: their thermal
impedance
is to be invoked rather than their thermal
From these considerations, it turns out that the converter can be thermally designed just considering the maximum continuous output power P
demand P
. There are, however some points that one should consider for the electrical design:
outpk
outx
1) the overcurrent li miting c ircuit must all ow P
responding peak primary current Ip
to P
outx
;
2) the transformer and/or inductor must not saturate with this higher peak current;
3) there is a tradeoff between the input bulk capacitor size (which determines the minimum input DC voltage)
and the value of Ip
: a low capacitance leads to a high input voltage r ipple, then to a lower minimum input
pklim
DC voltage and to a higher Ip
sulting minimum DC input voltage.
4) As envisaged by the spec in table 1, designing the converter for a given power but allowing a much higher
level calls for a means to make the system work safely during abnormal operating conditions.
Except for the special r equirement di scuss ed so far, the spec ificati on is identic al to that of the converter considered in [1], which will be then used as the starting point for the present design. In this context, only the modifications needed for fully complying with the spec of table 1 will be discussed. For reader's convenience figure 1
reproduces the electrical schematic of the original converter.
Additional circuitry will be needed for discriminating a peak power demand from a normal load condition and
detecting a short circuit as well as realizing the required double time-out.
÷
100 ms.
resistance
.
(and the related RMS currents) that it has to deliver and not the peak p ower
to pass, hence i ts setpoi nt needs to be greater than the cor-
outpk
and, possibly, much greater than the peak primary current related
pklim
. Also the maximum duty cycle of the converter must account for the re-
pklim
Figure 1. 45W, wide-range mains AC-DC adapter: electrical schematic (original design)
T1
N1 N2
N3
4.3 kΩ
IC2
PC917
7
6
R22 C17
D5
BYW29-200
330 µF
C12
4.7 nF
1kV
R17
3
25 V
C10
C11
C9
330 µF
25 V
1
R19
1.2 kΩ
2
C13
4
330 µF
25 V
C14
470 nF
R21
348Ω
C15
100 nF
2.2 kΩ
R18
R12
24 kΩ
C4
100 nF
R13
8.2 kΩ
C5
3.3 nF
C3 100 nF
F1 T2A250V
J1
88 to 264
Vac
VREF
ST-BY
16
RCT
2
DC-LIM SGND
NTC1
R5 47 kΩ
R8 5.6 kΩ
R9 6.8 kΩ
DCC
43
L5991
15
12
5
IC1
56 nF
C6
R6
330 kΩ
SS
BD1
DF04M
C1
100 µF
400 V
R10
22Ω
814
9
6
7
COMPVFB
1N4148
VCVCCDIS
C7
220 pF
R1
R3
56 kΩ
2.2 MΩ
R4
R2
2.2 MΩ
56 kΩ
D3
R7 4.7 Ω
C2
47 µF
25 V
R11
OUT
10
ISEN
13
11
PGND
10 Ω
R14
1 kΩ
C8
100 pF
R16 100 Ω
D1
BZW06-154
STTA106
D4 1N4148
D2
Q1
STP7NB6 0
R15
Ω
0.47
1/2 W
R20
5.6 kΩ
Vout
J2
GND
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AN1686 APPLICATION NOTE
Adaptations and modifications of the original design
With reference to the schematic of fi gur e 1, here follow s a step- by-step dis cussi on on the modi fications needed
for fulfilling the spec of table 1.
Input Capacitor.
Looking at the evaluation results presented in [1], it is reasonable to assume that with 75 W
load the input power will be around 90 W. Estimating about 5 W power loss before the transformer, the power
managed by the transformer will be 85 W. With the worst-case spr ead (-20 %) of C1, that is with C1=80 µF, the
valley voltage across C1 is expected to be around 54 V. Under these conditions the maximum peak primary
current will be around 3.24A, exceeding the saturation current of the transformer (2.84A).
Moreover, the maximum duty cycle (@ Vin = 54V) will be around 59%: since the system is working in CCM,
slope compensation will be needed to avoid unconditional instability of the current loop and the resulting subharmonic oscillations.
To avoid remaking the transformer and adding the slope compensation circuit, an attempt can be done using a
larger C1. Assuming there is no change in pow er levels, w ith 150 µF capaci tance (120 µF, worst ca se), the valley voltage across C 1 will be 78 V, the maximum peak primar y current 2.88 A and the maximum duty cyc le 50%.
Although necessary, this step is not enough to guarantee that the transfor mer will not saturate and then the system is still very close to the instabi lity limit. The transformer need s then to be modified anyw ay (see the follow ing
points).
Sense resistor.
value should be 0.92 V / 2.88 A = 0.319
to the existing 0.47
Transformer.
The sense resistor R15 needs reducing, to allow a peak primary current of 2.88 A. The maximum
Ω
. This will be achieved by paralleling a 1Ω resistor (1%, metallic film)
Ω
. The worst-case peak current, will be 1.08 V / 0.319Ω = 3.39 A.
The changes to the transformer aim at meeting two specific requirements: 1) not to saturate with
3.39A primary current; 2) to reduce the maximum duty cycle below 50%, to avoid the use of a slope compensation circuit. Design constraint is the cor e size , which must be unchanged. The increase of total losses should
be as low as possible.
Starting from point 2), to maintain the same maximum duty cycle as in the original design, the turn ratio should
be reduced proportionally to the reduction of the valley volta ge across C1. Formerly, with 45 W load, the worstcase valley voltage was 83 V and now is 78 V, then the new turn ratio is (50/12)·(78/83) = 3.916.
Table 2. Modified transformer specification
Core Philips EFD30x15x9, 3C85 Material or equivalent
Bobbin Horizontal mounting, 12 pins
Air gap ≈1.4 mm for an inductance 2-6 of 360 µH
Leakage inductance < 10 µH
Windings
Spec & Build
Winding Wire S-F Turns Notes
Pri1 AWG27 2-4 30
Sec (a) AWG25 11-7 16 Bifilar with Sec (b)
Sec (b) AWG25 12-8 16 Bifilar with Sec (a)
Pri2 AWG27 4-6 30
Aux AWG32 3-1 14 Evenly spaced
Note: sec (a) and sec (b) are paralleled on the PCB
To make sure that the core will not satur ate with 3.39A, keep ing the same inductance value, the number of turns
of the primary winding should be raised from 50 to 66 and the air gap from 0.7 to 1.5 mm. To reduce the copper,
it is acceptable to reduce the primary inductance by 10%: in this way the primary winding turn number will be
60 and the air gap 1.4 mm.
The secondary turn number will be 60/3.916=15.321, rounded off to 16. Keeping the same wire diameters, the
primary resistance will be increased by a factor 60/50, that is 20%, and the secondary resistance by 33% and
so will be the respective conduction losses (the RMS currents change very little). However, the flux swing is
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