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PFC APPLICATION IDEAS USING MAINS TRANSFORMER 4044 - Datasheet Archive
Application Note
[ST] PFC APPLICATION IDEAS USING MAINS TRANSFORMER
AN825
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PDF Portable Document Format 4044 22/01/1999 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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