460W Switch Mode PSU teardown, detailed overview and repair: Cisco 3550 PWR PoE

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monica
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Teardown and repair of a 460W switch mode power supply (SMPS) from a 24-port Cisco 3550 PWR Ethernet switch with PoE functionality. The SMPS is providing -48V DC for the PoE controller, and also 12V DC for the switch motherboard. Throughout the video a detailed overview is given, together with explanation of what is the functionality of the components, and how does the circuit operates.

The 3550 PWR switches from Cisco were introduced well over a decade ago, and they were revolutionizing the market, since they acted both as an Ethernet switch, providing at the same time power via the Ethernet cable to IP phones, IP cameras and other gadgets. Due to this early implementation, where there was not yet PoE standard available, these switches are providing so called pre-standard PoE Power Sourcing Equipment functionality. The PoE standard 802.3af was adapted years later by IEEE.

This Liteon PA-2461-1A power supply unit (PSU) has active power factor correction, which consist of an inductor pumped by two 20N60S5 FETs. The FETs are controlled by an ML4800 chip, which is used both as PFC and PWM controller made by Fairchild (now part of On Semiconductor):
http://www.mouser.com/ds/2/149/ML4800-64984.pdf

The filter caps are all high quality, rated at 105C, made by Nippon Chemi-Con corporation. The -48V DC 400W power output stage is a two-switch forward converter, and it is also using two 20N60S5 FETs feeding the primary of the chopper transformer. The magnetic core of the transformer is being demagnetized when the FETs are turned off via two fast recovery diodes.

The +12V stage (rated at 60W) is providing power for the motherboard. This supply is built based on a simple flyback configuration, where the primary inductor is pumped by a 2SK2611 FET. The switching FET is driven by a UC3844 peak current flyback controller.

On the secondary side both the -48V and the +12V rails are rectified by Schottky diodes and filtered by an LC circuit. The feedback loop is realized with the traditional TL431 integrated circuit, which contains both an error amplifier and a high precision bandgap reference diode. The TL431 is feeding the LED sender portion of the optocouplers.

In addition to the -48V and +12V power rails, the PSU also has a small stand-by circuit. This is realized by a small chopper transformer operating in a flyback configuration. To reduce part count, the PSU is using a TNY264, which is an all-in-one flyback PWM controller and FET from the TinySwitch-II product family from Power Integrations Inc.
The datasheet for this device: https://www.power.com/sites/default/files/product-docs/tny263_268.pdf
Category
Routers and Switches
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