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PFC APPLICATION IDEAS USING MAINS TRANSFORMER 4044 - Datasheet Archive [ST]

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AN825

PFC APPLICATION IDEAS USING MAINS TRANSFORMER

Document Number:    4044
Date Update:        22/01/99
Pages:              7
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AN825
APPLICATION NOTE
P.F.C. APPLICATION IDEAS USING MAINS TRANSFORMER
APPLICATION DESCRIPTION
We are going to evaluate two solutions,  12V and
24V for a total output power of 120W . The bene -
fits we are going to point out when using a low
voltage PFC, are listed below:
- reduction of mains harmonic content
- reduction of peak current at mains frequency
- reduction of the electrolytic bulk capacitor used
at PFC stage output
- reduced Volt/Amp requested to the mains
- reduced mains transformer size and weight
- improvement of the output regulation of the
downstream dc-dc converter (increase of the
available output power for the audio amplifier
example).
For the DC-DC converter section, two devices  of
the L4970A family have been used,  with the rela -
tive  evaluation boards. L4970A to deliver 12V
10A and L4977A to deliver 24V 5A.
The chosen switching frequency is 100KHz for
the PFC. stage and 200KHz for the dc-dc con -
verter section, for a good compromise between
efficiency and passive components size and cost.
The switching frequency affects the overall effi -
ciency due to the proportional switching losses.
A different frequency can be selected changing
the oscillators components of each stage. In any
case the synchronization between DC-DC and
PFC controller is very easy to implement.
ELECTRICAL SPECIFICATION
mains voltage:
220Vac 
+
20%
output power : 120W
Vo = 12V  10A   with  L4970A
Vo = 24V   5A   with  L4977A
The system block diagram is shown in Fig.1.
January 1999
[ 
AC MAINS TRASF
PFC 
BLOCK
DC-DC 
CONV.
LOAD
D95IN286
Figure 1:  
Converter block diagram with PFC. using mains transformer.
The typical PFC boost scheme is mainly suggested for off-line applications, with the galvanic  insula -
tion located in the downstream converter side.
There are some industrial and consumer applications in which the galvanic insulation is pr ovided by
the mains transformer( 50/60 Hz frequency).
The mains transformer guarantees a very safe insulation reducing the ac voltage at the seco ndary
winding to a profitable value for easy use.
The purpose of this note is to quantify the effect of a PFC preregulator stage, connected to  a typical
DC-DC converter in terms of efficiency, harmonic content and apparent power (VA) drawn from t he
mains.
An innovative application example, where a power audio amplifier instead of the DC-DC con verter is
used, highlights the benefit and improvement of the power section due to the input supply vol tage sta -
bilized by the PFC preregulator.
1/7
BASIC BLOCKS ANALYSIS
1)  
Mains transformer and rectifiers
rated apparent power Pa :  250VA
nominal primary voltage : 220Vac
secondary winding :  center tapped
secondary  voltage :  24+24Vac rms.
operating frequency :  50/60 Hz
BYW99P100 are the diodes used for rectification.
2) 
PFC. stage
For more details on the PFC controller, please re -
fer to the datasheet and AN628,  DESIGNING A
HIGH POWER FACTOR SWITCHING
PREREGULATOR WITH  L4981 CONTINUOUS
MODE . The topology is the standard boost type,
using  low voltage discrete components,  power
MOS and freewheeling diode.
Considering the max. operating input voltage of
the L4970A/77A of 50V, the typ. value of the out -
put voltage of the PFC stage has been fixed at
45V.
3)  
DC-DC converter stage
For the two  examples, monolithic dc-dc converter
evaluation boards  have been used.  For more
details related to the two involved devices, please
refer to the datasheets and application note
(AN557, Easy Application Design with the
L4970A, Monolithic DC-DC Converters Family).
POWER FACTOR CORRECTOR BLOCK DE -
SCRIPTION
This stage has been designed in order to be able
to deliver a max. power of 150W, at an output
voltage of 45V nominal, the overvoltage (OVP)
has been set at  52V.
The chosen switching frequency is 100kHz, to get
a good compromise between the switching losses
and the magnetic component size.
The core used for the PFC section (toroid 58071
Magnetics), is the same used for the DC-DC. The
selected power MOS device is STP60N06/FI
(V(BR)DSS = 60V and RDS(on)) = 0,06 typ 0.085
Ohm max. at Tj = 255C) and the chosen boost di -
ode is STPS20100CT (Schottky diode 100V re -
verse voltage breakdown).
The bulk capacitor is a 3300uF/63V, low ESR,
EYF (ROE). A current sense resistor has been
used considering a max. current of 6.7A.
In case of higher power or lower secondary voltage
transformer winding, two current (sense) transform -
ers can be used, improving the efficiency.
The first one sense transformer connected in se -
ries to the power MOS drain and a second one in
series to the anode of the boost diode. Fig.2
shows the schematic diagram of the PFC. stage.
DC-DC CONVERTER BLOCK DESCRIPTION
The dc-dc converter schematic used is very simi -
lar to the test circuit schematic suggested in the
datasheets. Fig.3 shows the schematic for 12V
10A, and Fig.4 shows the schematic for 24V 5A,
DC-DC converters.
R17 
100K 
5%
4
C1 
2.2 m F 
100V
L4981A
15
16
R15 
150 
5W
19
D3 
1N4150
C11 
100 m F 
25V
L = core turns L = 40 m H COGEMM 949178
13 14
C9 220nF
R12 100K 5%
R13
20
R8 
4.7K 
5%
R11 
390 
5%
R4 
4.7K 
5%
C4 
1nF
R16 
24K 
1%
C6 
1 m F 
16V
C5 
1 m F 
16V
R2 
13K 
1%
R10 
13k 
1%
R1 
100K 
1%
R9 
120K 
1%
D1 STPS20100CT
12 17 7 18 9 5
8
2
FUSE
MAINS 
Vi 
(176V to 265V)
C2 
3300 m F 
63V 
EYF 
ROE
+
-
Vo=45V
D95IN234B
D2 1N4150
R
S
R21 5.1K 1%
D
S
BYW99P100 250VA, 
220Vac/24Vac
Dz 
18V
15 5%
Q1 
STP60N06
R21
C3
5.1K 1%
220pF
.015 2W
3
1
11
6
10
Po=150W
L
Figure 2:
PFC. stage schematic diagram.
AN825 APPLICATION NOTE
2/7
L4970A
3
14
15
12
D
7
R5
C10
6 9
L
Vo=12V 
10A
Vi=45V
GND
D95IN287
C4 C3 C5
13
SYNC
TP2
8
C7
TP3
C8
R3
10 1
R4
2
C12
TP4
C11
11
C13 C14 C15
R8
R10
C9
GND
Figure 3:
L4970A DC-DC converter stage schematic diagram.
Typical Performances: 
h
= 84%; V
o
= 12V; I
o
= 10A; f
SW 
= 200kHz
Part List
R3
R4
R5
R8
R10
C1, C2
C3, C4, C5
C7
C8
C9
C10
C11
C12,C13,C14 (*)
C15
D
L1
15k
W
16k
W
22
W  
0.5W
6.2k
W
4.7k
W
3300
m
F 63VL EYF (ROE)
2.2
m
F
390pF Film
22nF MKT 1817 (ERO)
2.2nF KP1830
220nF MKT
2.2nF MP1830
220
m
F 40V
L
EKR
1
m
F Film 
MBR 1560CT (or 16A/60V or equivalent)
40
m
H core 58071 MAGNETICS 27 turns 
j
1.3mm (AWG 16)
(*) 3 capacitors in parallel to reduce total output ESR
L4977A
3
14
15
12
D
7
R5
C10
6 9
L
Vo=24V 
5A
Vi=45V
GND
D95IN288
C4 C3 C5
13
SYNC
TP2
8
C7
TP3
C8
R3
10 1
R4
2
C12
TP4
C11
11
C13 C14 C15
R8
R10
C9
GND
Figure 4:
L4977A DC-DC converter stage schematic diagram.
Typical Performances: 
h
= 92.3%; V
o
= 24V; I
o
= 5A; f
SW 
= 200kHz
AN825 APPLICATION NOTE
3/7
Table 2:
V
O
= 12V; I
O
= 10A (L4970A); with PFC preregulator.
WITH PFC
Mains rms
Voltage (V)
Mains rms
Current (A)
Mains app.
power (VA)
Power
factor (%)
Harmonic Distortion (%)
Efficiency
(%)
THD AH3 AH5 AH7
176 1.01 178.3 99.6 4.1 3.5 1.1 0.6 67.6
220 0.80 176.3 96.9 10.8 10.4 2.3 0.5 70.2
257 0.79 203.6 84.0 25.6 24.3 7.5 2.5 70.2
MEASUREMENT RESULTS.
In the following tables are summarized  the results of the evaluation for both the solution s (12V-10A and
24V-5A), without and with  active PFC stage.
APPLICATION WITH 12V 10A DC-DC CONVERTER
In 
Table 1
are reported the measurement results of the whole system, from the mains to the output DC
regulated voltage, without the PFC block.
Table 1:
V
O
= 12V; I
O
= 10A (L4970A); no PFC.
WITHOUT PFC
Mains rms
Voltage (V)
Mains rms
Current (A)
Mains app.
power (VA)
Power
factor (%)
Harmonic Distortion (%)
Efficiency
(%)
THD AH3 AH5 AH7
176 1.32 232.1 76.4 83.4 74.2 35.8 9.15 67.7
220 1.07 236.6 71.7 95.0 79.5 47.0 20.5 70.7
257 1.03 264.7 64.2 93.9 76.5 46.6 26.3 70.6
Table 3:
V
O
= 24V; I
O
= 5A (L4977A)
WITHOUT PFC
Mains rms
Voltage (V)
Mains rms
Current (A)
Mains app.
power (VA)
Power
factor (%)
Harmonic Distortion (%)
Efficiency
(%)
THD AH3 AH5 AH7
257 0.99 253.7 64.8 93 76 47 23 73
In 
Table 4
the measurement results of the whole system are reported , from the mains  to the output dc
regulated voltage, with the PFC. block. 
Table 4:
V
O
= 24V; I
O
= 5A (L4977A)
WITH PFC
Mains rms
Voltage (V)
Mains rms
Current (A)
Mains app.
power (VA)
Power
factor (%)
Harmonic Distortion (%)
Efficiency
(%)
THD AH3 AH5 AH7
176 0.92 162.4 99.5 4.1 3.5 1.1 0.63 74.3
220 0.73 161.3 96.3 10.8 10.4 2.3 0.5 77.3
257 0.74 189.0 82.2 27.2 25.8 8.04 1.56 77.3
In 
Table 2
the measurement results of the whole system are reported , from the mains to the output dc
regulated voltage, with the PFC block.
APPLICATION WITH 24V 5A DC-DC CONVERTER
In 
Table 3
are reported the measurement results of the whole system, from the mains to the output dc
regulated voltage, without the PFC. block.
AN825 APPLICATION NOTE
4/7
Measurement results of the PFC. block.
For DC-DC converters blocks efficiency,  please
refer to the L4970A and L4977A datasheets.
Comments to the evaluation:
a)  In this evaluation, whenever the PFC block is
not used, the DC-DC converter has been pro -
vided with an input (bulk) capacitive filter (see
fig. 5) using 2 
V
3300
m
F according with the
standard L4970/77 evaluation board circuits
(see AN557). 
To be noted that when the PFC. block is used,
it is possible a significant  reduction of this bulk
capacitor (the output capacitor of PFC is the
input filter for DC-DC converter). 
b) Since all the evaluations have been done using
the same transformer, in  table 3 (TAB3)  the
reported data are related to high mains volt -
age only. In fact it is necessary to  ensure  the
input voltage, at the DC-DC converter section,
higher then  the regulated output voltage (24V
in this case) in any condition.
Measurement results of the Transformer and Diodes rectifier block.
Table 5a :
Transformer and diodes without  PFC (V
o
= 12V, I
o
= 10A).
WITHOUT PFC
Mains rms
Voltage (V)
Mains rms
Current (A)
Sec. peak
Current (A)
Sec. rms.
Current (A)
Mains App.
power (VA)
Efficiency
(%)
176 1.32 26 14.4 232.1 85.5
220 1.07 23 10.7 236.6 83.4
257 1.03 21 9.3 264.7 82.8
Table 5b :
Transformer and diodes without  PFC (V
o
= 24V, I
o
= 5A).
WITHOUT PFC
Mains rms
Voltage (V)
Mains rms
Current (A)
Sec. peak
Current (A)
Sec. rms.
Current (A)
Mains App.
power (VA)
Efficiency
(%)
257 0.99 20 9.1 253.7 82.9
Table 6 :
Transformer end diodes using PFC.
WITH PFC
Mains rms
Voltage (V)
Mains rms
Current (A)
Sec. peak
Current (A)
Sec. rms.
Current (A)
Mains App.
power (VA)
Efficiency
(%)
176 1.01 11.4 8.1 178.3 86.5
220 0.80 8.9 6.3 176.3 88.4
257 0.79 7.3 5.2 203.6 88.1
Table 7:
PFC  in solution 12V/10A  (L4970A).
Mains rms
Voltage (V)
Sec. rms
Voltage (V)
PFC output
voltage (V)
Output
Power (W)
Efficiency
(%)
176 19 46 143 93
220 24 46 143 94.6
257 29 46 143 94.8
Table 8 :
PFC in solution 24V/5A (L4977A).
Mains rms
Voltage (V)
Sec. rms
Voltage (V)
PFC output
voltage (V)
Output
Power (W)
Efficiency
(%)
176 19 46 130 93
220 24 46 130 94.7
257 29 46 130 95
AN825 APPLICATION NOTE
5/7
USING POWER AUDIO AMPLIFIER.
The Audio Power Amplifiers used in home stereo
equipment, are typically supplied using a mains
transformer for very safe insulation, followed by a
rectifier and a large electrolytic capacitor. This ca -
pacitor, while is providing for high peak current re -
quest, increases a lot the harmonic content. The
use of the above described architecture (PFC)
represents an innovative solution in audio ampli -
fier applications because of the additional advan -
tages offered by the preregulation. In fact a pre -
regulated supply allows to optimize the dynamic
and the efficiency of the audio amplifier system.
In fig. 6 the block schematic of this application ex -
ample is shown.
DC-DC 
CONVER
3300 m F 3300 m F
LOAD 
+45V -120W
MAINS
D95IN289
Figure 5a.
DC-DC 
CONVER
3300 m F
PFC BLOCK
LOAD 
+45V -120W
MAINS
D95IN290
Figure 5b.
PFC 
BLOCK
AUDIO 
POWER 
AMPLIFIER
1mA
D95IN291
LOUDSPEAKER
+
-
Figure 6.
AN825 APPLICATION NOTE
6/7
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AN825 APPLICATION NOTE
7/7

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