 |
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
| Datasheet File OCR Text: |
| (R) AN825 APPLICATION NOTE P.F.C. APPLICATION IDEAS USING MAINS TRANSFORMER The typical PFC boost scheme is mainly suggested for off-line applications, with the galvanic insulation located in the downstream converter side. There are some industrial and consumer applications in which the galvanic insulation is provided by the mains transformer( 50/60 Hz frequency). The mains transformer guarantees a very safe insulation reducing the ac voltage at the secondary 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 the mains. An innovative application example, where a power audio amplifier instead of the DC-DC converter is used, highlights the benefit and improvement of the power section due to the input supply voltage stabilized by the PFC preregulator. APPLICATION DESCRIPTION We are going to evaluate two solutions, 12V and 24V for a total output power of 120W . The benefits 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 relative 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 converter section, for a good compromise between efficiency and passive components size and cost. The switching frequency affects the overall efficiency 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. Figure 1: Converter block diagram with PFC. using mains transformer. AC MAINS TRASF PFC BLOCK DC-DC CONV. LOAD D95IN286 January 1999 1/7 AN825 APPLICATION NOTE 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 refer 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 output 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). Figure 2: PFC. stage schematic diagram. 250VA, 220Vac/24Vac DS BYW99P100 POWER FACTOR CORRECTOR BLOCK DESCRIPTION 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 = 25C) and the chosen boost diode is STPS20100CT (Schottky diode 100V reverse 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) transformers can be used, improving the efficiency. The first one sense transformer connected in series 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 similar 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. L R15 150 5W D3 1N4150 C11 100F 25V 4 19 Dz 18V R12 100K 5% 13 14 3 20 D2 1N4150 R13 15 5% C9 220nF D1 STPS20100CT + Vo=45V R1 100K 1% R9 120K 1% Po=150W MAINS Vi (176V to 265V) R17 100K 5% FUSE C1 2.2F 100V 15 16 Q1 STP60N06 L4981A 1 10 6 C2 3300F 63V EYF ROE 8 2 5 R21 5.1K 1% C3 9 18 17 12 11 7 R2 13K 1% R10 13k 1% 220pF R21 5.1K 1% R8 4.7K 5% R11 390 5% R4 4.7K 5% C4 1nF R16 24K 1% C6 1F 16V C5 1F 16V D95IN234B RS .015 2W L = core turns L = 40H COGEMM 949178 2/7 AN825 APPLICATION NOTE Figure 3: L4970A DC-DC converter stage schematic diagram. Typical Performances: = 84%; Vo = 12V; Io = 10A; fSW = 200kHz SYNC TP2 Vi=45V C10 9 15 13 6 L L4970A 3 14 12 8 10 TP3 C8 C3 C4 C5 C7 R4 R3 C9 1 TP4 7 Vo=12V 10A C11 D R5 C12 C13 C14 C15 R8 2 11 R10 GND D95IN287 GND Figure 4: L4977A DC-DC converter stage schematic diagram. Typical Performances: = 92.3%; Vo = 24V; Io = 5A; fSW = 200kHz SYNC TP2 Vi=45V C10 9 15 13 6 L L4977A 3 14 12 8 10 TP3 C8 C3 C4 C5 C7 R4 R3 C9 1 TP4 7 Vo=24V 5A C11 D R5 C12 C13 C14 C15 R8 2 11 R10 GND D95IN288 GND Part List R3 R4 R5 R8 R10 C1, C2 C3, C4, C5 C7 C8 C9 C10 C11 C12,C13,C14 (*) C15 D L1 15k 16k 22 0.5W 6.2k 4.7k 3300F 63VL EYF (ROE) 2.2F 390pF Film 22nF MKT 1817 (ERO) 2.2nF KP1830 220nF MKT 2.2nF MP1830 220F 40VL EKR 1F Film MBR 1560CT (or 16A/60V or equivalent) 40H core 58071 MAGNETICS 27 turns 1.3mm (AWG 16) (*) 3 capacitors in parallel to reduce total output ESR 3/7 AN825 APPLICATION NOTE MEASUREMENT RESULTS. In the following tables are summarized the results of the evaluation for both the solutions (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: VO = 12V; IO = 10A (L4970A); no PFC. WITHOUT PFC Mains rms Mains rms Mains app. Power Voltage (V) Current (A) power (VA) factor (%) 176 220 257 1.32 1.07 1.03 232.1 236.6 264.7 76.4 71.7 64.2 Harmonic Distortion (%) THD 83.4 95.0 93.9 AH3 74.2 79.5 76.5 AH5 35.8 47.0 46.6 AH7 9.15 20.5 26.3 Efficiency (%) 67.7 70.7 70.6 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. Table 2: VO = 12V; IO = 10A (L4970A); with PFC preregulator. WITH PFC Mains rms Mains rms Mains app. Power Voltage (V) Current (A) power (VA) factor (%) 176 220 257 1.01 0.80 0.79 178.3 176.3 203.6 99.6 96.9 84.0 Harmonic Distortion (%) THD 4.1 10.8 25.6 AH3 3.5 10.4 24.3 AH5 1.1 2.3 7.5 AH7 0.6 0.5 2.5 Efficiency (%) 67.6 70.2 70.2 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. Table 3: VO = 24V; IO = 5A (L4977A) WITHOUT PFC Mains rms Mains rms Mains app. Power Voltage (V) Current (A) power (VA) factor (%) 257 0.99 253.7 64.8 Harmonic Distortion (%) THD 93 AH3 76 AH5 47 AH7 23 Efficiency (%) 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: VO = 24V; IO = 5A (L4977A) WITH PFC Mains rms Mains rms Mains app. Power Voltage (V) Current (A) power (VA) factor (%) 176 220 257 4/7 0.92 0.73 0.74 162.4 161.3 189.0 99.5 96.3 82.2 Harmonic Distortion (%) THD 4.1 10.8 27.2 AH3 3.5 10.4 25.8 AH5 1.1 2.3 8.04 AH7 0.63 0.5 1.56 Efficiency (%) 74.3 77.3 77.3 AN825 APPLICATION NOTE Measurement results of the Transformer and Diodes rectifier block. Table 5a : Transformer and diodes without PFC (Vo = 12V, Io = 10A). WITHOUT PFC Mains rms Voltage (V) 176 220 257 Mains rms Current (A) 1.32 1.07 1.03 Sec. peak Current (A) 26 23 21 Sec. rms. Current (A) 14.4 10.7 9.3 Mains App. power (VA) 232.1 236.6 264.7 Efficiency (%) 85.5 83.4 82.8 Table 5b : Transformer and diodes without PFC (Vo = 24V, Io = 5A). WITHOUT PFC Mains rms Voltage (V) 257 Mains rms Current (A) 0.99 Sec. peak Current (A) 20 Sec. rms. Current (A) 9.1 Mains App. power (VA) 253.7 Efficiency (%) 82.9 Table 6 : Transformer end diodes using PFC. WITH PFC Mains rms Voltage (V) 176 220 257 Mains rms Current (A) 1.01 0.80 0.79 Sec. peak Current (A) 11.4 8.9 7.3 Sec. rms. Current (A) 8.1 6.3 5.2 Mains App. power (VA) 178.3 176.3 203.6 Efficiency (%) 86.5 88.4 88.1 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 provided with an input (bulk) capacitive filter (see fig. 5) using 2 3300F according with the standard L4970/77 evaluation board circuits (see AN557). To be noted that when the PFC. block is used, Table 7: PFC in solution 12V/10A (L4970A). Mains rms Voltage (V) 176 220 257 Sec. rms Voltage (V) 19 24 29 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 voltage 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. PFC output voltage (V) 46 46 46 Output Power (W) 143 143 143 Efficiency (%) 93 94.6 94.8 Table 8 : PFC in solution 24V/5A (L4977A). Mains rms Voltage (V) 176 220 257 Sec. rms Voltage (V) 19 24 29 PFC output voltage (V) 46 46 46 Output Power (W) 130 130 130 Efficiency (%) 93 94.7 95 5/7 AN825 APPLICATION NOTE Figure 5a. MAINS 3300F DC-DC CONVER 3300F LOAD +45V -120W D95IN289 Figure 5b. DC-DC CONVER MAINS 3300F PFC BLOCK LOAD +45V -120W D95IN290 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 capacitor, while is providing for high peak current request, increases a lot the harmonic content. The Figure 6. use of the above described architecture (PFC) represents an innovative solution in audio amplifier applications because of the additional advantages offered by the preregulation. In fact a preregulated supply allows to optimize the dynamic and the efficiency of the audio amplifier system. In fig. 6 the block schematic of this application example is shown. LOUDSPEAKER PFC BLOCK 1mA + AUDIO POWER AMPLIFIER D95IN291 6/7 AN825 APPLICATION NOTE Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specification mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics (c) 1999 STMicroelectronics - Printed in Italy - All Rights Reserved STMicroelectronics GROUP OF COMPANIES Australia - Brazil - Canada - China - France - Germany - Italy - Japan - Korea - Malaysia - Malta - Mexico - Morocco - The Netherlands Singapore - Spain - Sweden - Switzerland - Taiwan - Thailand - United Kingdom - U.S.A. http://www.st.com 7/7 |
Price & Availability of AN825
 |
|
|
|
|
All Rights Reserved © IC-ON-LINE 2003 - 2022 |
| [Add Bookmark] [Contact Us] [Link exchange] [Privacy policy] |
|
Mirror Sites : [www.datasheet.hk]
[www.maxim4u.com] [www.ic-on-line.cn]
[www.ic-on-line.com] [www.ic-on-line.net]
[www.alldatasheet.com.cn]
[www.gdcy.com]
[www.gdcy.net] |