Part # Application Note Power Supply And Power Management L6561 AN1007 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: 6561 - based switcher replaces MAG AMPS in silver boxes


Part Details:

AN1007 ® APPLICATION NOTE L6561 - BASED SWITCHER REPLACES MAG AMPS IN SILVER BOXES by Claudio Adragna Mag amps (a contraction of "Magnetic Amplifier") are widely used in multi-output switching powersupplies to get auxiliary regulated power rails. However, they are expensive, bulky, and require ahigh level of design expertise.ST s L6561, an 8-pin Transition Mode PFC (Power Factor Corrector) controller, is surprisingly suit-able for implementing a switch-mode architecture as an alternative to mag amps. Much better per-formance, a dramatic reduction of parts count, cost and design effort are the benefits of such an ap-proach. Drawbacks? None. And once more the L6561 turns out to be a really versatile device. IntroductionDesktop computer power supplies provide two or more low-voltage, high-current, isolated power rails,typically a 5V rail and a 12V rail. More an more often, it also provides a 3.3V auxiliary rail with high-cur-rent capability. The power is generated by an off-line forward switching converter (inside the so-called "silver box") thatregulates only one power rail through an isolated feedback loop. The other power rails are usually post-regulated to meet the specifications on the output voltage tolerance and regulation. A typical architectureis shown in fig. 1. Figure 1. Typical architecture of an SMPS for desktop computer ("silver box"). (ISOLATED) FEEDBACK MAIN RECTIFIER FILTER OUTPUT R ORME DC Input F PWM CONTROLLER S & POWER SWITCH NA RECTIFIER MAG AMP R + REGULATOR T FILTER IN AUXILIARY A DC OUTPUTS M RECTIFIER LINEAR + REGULATOR FILTER Many power supply manufacturers use magnetic amplifiers (in short, "mag amps") to achieve secondarypost-regulation. Mag amps regulate the output voltage with a saturable core reactor that exhibits asquare B-H loop: when not saturated, the core material has a very high permeability and the reactor avery high impedance, then it abruptly saturates thus the permeability drops to a very low value and sodoes the impedance of the reactor.By varying the magnetic DC flux inside the core, mag amps control the time needed for the core to satu-rate under a given input voltage pulse. Therefore, the reactor acts basically as a delayed switch and per-form PWM by modulating the leading-edge of the input voltage pulse applied to the output section. October 2003 1/6 AN1007 APPLICATION NOTE Mag amps have a number of drawbacks: they are expensive, bulky, slow, inefficient and, last but notleast, their design is not easy.In this paper a switch-mode architecture is proposed as an alternative to mag amps. Basically, the satu-rable core reactor is replaced by a power MOSFET. The control circuitry is based on a well-known 8-pincontroller IC, in this context used in a not conventional way: the PFC controller L6561.The benefits of this architecture, which takes advantage of the current mode control offered by theL6561, will be detailed in this paper and can be summarized as follows: much better performance at amuch lower cost. Mag amps as secondary post-regulatorsFigure 2 shows a block diagram of a mag amp regulator. It looks simple but there are several aspects ofthe design that must be accounted for in order to ensure a proper operation. Figure 2. Mag amp regulator block diagram. MAG AMP REGULATOR Vout REMR IN OF MA S BIAS AND


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Application Note Power Supply And Power Management L6561 AN1007.pdf Datasheet