Part # AN825 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics


Part Details:

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 insula-tion located in the downstream converter side.There are some industrial and consumer applications in which the galvanic insulation is provided bythe mains transformer( 50/60 Hz frequency).The mains transformer guarantees a very safe insulation reducing the ac voltage at the secondarywinding 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 typicalDC-DC converter in terms of efficiency, harmonic content and apparent power (VA) drawn from themains.An innovative application example, where a power audio amplifier instead of the DC-DC converter isused, highlights the benefit and improvement of the power section due to the input supply voltage sta-bilized by the PFC preregulator. APPLICATION DESCRIPTION The chosen switching frequency is 100KHz for We are going to evaluate two solutions, 12V and the PFC. stage and 200KHz for the dc-dc con- 24V for a total output power of 120W . The bene- verter section, for a good compromise between fits we are going to point out when using a low efficiency and passive components size and cost. voltage PFC, are listed below: The switching frequency affects the overall effi-ciency due to the proportional switching losses. ­ reduction of mains harmonic content A different frequency can be selected changing ­ reduction of peak current at mains frequency the oscillators components of each stage. In any ­ reduction of the electrolytic bulk capacitor used case the synchronization between DC-DC and at PFC stage output PFC controller is very easy to implement. ­ reduced Volt/Amp requested to the mains­ reduced mains transformer size and weight ELECTRICAL SPECIFICATION ­ improvement of the output regulation of the mains voltage: downstream dc-dc converter (increase of the 220Vac ±20% available output power for the audio amplifierexample). output power : 120W For the DC-DC converter section, two devices of Vo = 12V 10A with L4970A the L4970A family have been used, with the rela- Vo = 24V 5A with L4977A tive evaluation boards. L4970A to deliver 12V The system block diagram is shown in Fig.1. 10A and L4977A to deliver 24V 5A. Figure 1: Converter block diagram with PFC. using mains transformer. PFC AC MAINS TRASF BLOCK DC-DC LOAD CONV. D95IN286 January 1999 1/7 AN825 APPLICATION NOTE BASIC BLOCKS ANALYSIS POWER FACTOR CORRECTOR BLOCK DE- 1) Mains transformer and rectifiers SCRIPTION

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