Varied power supplies needed

Communications infrastructure equipment employs a variety of power system components. Power-factor-corrected ac/dc power supplies with load sharing and redundancy (N+1) at the front end feed dense, high-efficiency dc/dc modules and point-of-load converters on the back end. A power-efficient design requires both the higher-voltage analog circuits and multiple tightly regulated low-voltage supplies for the high-speed digital communications ASICs and FPGAs.

More recently, diverse power supply requirements coupled with a volatile telecommunications market have forced equipment manufacturers to not only cut costs but also to provide more efficient and reliable power solutions. The challenging business environment has spawned new distributed-voltage bus standards, such as the recent +12-volt intermediate-bus architecture. The deployment of low-cost unregulated (open-loop) bricks to convert from the -48-V bus to a standard +12-V intermediate bus has allowed new low-cost point-of-load (POL) modules to be used.

Competing with these new POL modules are hybrid isolated power supply topologies, such as the cascaded current-fed or voltage-fed push-pull converters. Semiconductor suppliers are enabling power supply system designers to embed low-cost compact isolated power supplies directly onto their motherboards and line cards. New highly integrated, high-voltage (100-V) power ASICs such as the LM5041 Cascaded PWM (pulse-width modulation) and LM5030 Push-Pull PWM controllers from National Semiconductor Corp. minimize the number of external components and printed-circuit board area required. Operating directly off the -48-V bus, the cascaded converter can produce multiple low-voltage outputs with higher overall efficiency levels at a lower cost than multiple POL converters operating from a +12-V intermediate-bus converter.

The cost-vs.-design complexity and risk trade-offs are being reviewed more seriously now by information system manufacturers developing new generations of cost-reduced equipment.

Third-generation (3G) basestations, voice-over-Internet Protocol (VoIP) and digital subscriber line (DSL) require varying degrees of complexity in power supply design. We will discuss factors that influence power system design for three applications.

A VoIP dc/dc converter uses a less complex single high-power output transformer design (typically 250 W to 500 W) to buffer the main -48-V distribution bus. This minimizes the cost and the capacitance of bulk capacitors required to hold up the distribution bus voltage by narrowing the operating voltage to a range of 43 V to 57 V from the traditional 36- to 72-V range. Fault protection and safety isolation are also provided to all downstream converters or other loads on the distribution bus. Using these dc/dc converters with parallel outputs and load current sharing generates fault tolerance (N+1) and heat distribution that are conducive to cooler operation, longer life-cycles and improved reliability.

VoIP converters generally require power supply circuit topologies that are performance-driven (highly efficient with minimal conducted line current), easy to use and cost-effective with a small footprint and low profile.

A number of topologies can be designed to meet these requirements to some degree. For example, the flyback converter with its topological simplicity is often suggested. In contrast to buck-derived converters (such as the forward converter), the flyback does not require a transformer-flux resetting mechanism or an output inductor. Offsetting these advantages, especially in a high-output voltage system such as the VoIP application, is the need for expensive capacitors to filter the large ripple current at the input and output. However, the ripple-current problem of both flyback and forward converters can be mitigated by interleaving two converters in anti-phase. All things being equal, the input and output ripple currents in the interleaved system will be significantly less than those of a single converter.

A far better solution than the flyback converter for VoIP systems is the push-pull converter. Fundamentally, a push-pull converter is the equivalent of two interleaved forward converters. But the push-pull converter has only one transformer, which is self-resetting, and only one output inductor, making it only slightly more complex than a single forward converter. The input ripple current is greatly reduced because of the interleaving effect, so smaller input inductors can be used. The output inductor attenuates the output ripple current, allowing the use of less expensive capacitors with lower ripple-current ratings.

The flyback converter is often only suitable for up to about 150 watts, while the push-pull converter can perform satisfactorily in the kilowatt power range.

More complex topologies that offer better efficiency can also be used in the VoIP application, especially at the extremes of the input voltage range. One example is the cascaded-buck topology with a current-fed push-pull converter. (A dedicated PWM controller for this topology, the LM5041, is now available.) This hybrid topology is an excellent choice at higher power levels and in situations where requirements for high efficiency and performance justify the additional cost.

In a DSL application, a -48-V to multiple-output converter may be used, which incorporates a more complex, lower-power transformer (50 W to 100 W) with several outputs. The DSL power system may supply both higher-voltage analog line drivers and amplifiers (typically plus/minus 12 V) and several low-voltage supplies required by the digital ASIC (+5 V, +3.3 V, +1.8 V, +1.5 V). Selection criteria for the power supply topology in multioutput DSL converters include requirements for performance (high efficiency and tight load and line regulation), simplicity, low cost and a small footprint with a low profile.

Push and pull

A preferred power supply architecture for DSL applications is a push-pull converter used to convert the -48-V input voltage to plus/minus 12 V and to provide electrical isolation. Synchronous buck converters powered off the +12-V rail generate various low-voltage outputs. This push-pull intermediate-bus approach takes advantage of the availability of the cost-effectiveness of power-management ICs such as the LM5030 push-pull controller and the LM5642 two-channel current-mode synchronous buck controller. Each channel of the high-performance LM5642 needs only one pair of FETs, an output inductor, an output capacitor and a few resistors and capacitors.

In a 3G basestation application, two converters are used to provide the +27-V distribution-bus voltage during normal conditions and power outages. A high-voltage converter powered directly from the main ac line powers the system during normal operation, while the second converter operates off the -48-V backup batteries during power line blackouts. The -48-V backup battery converter is similar in construction and complexity to the single-output, high-power VoIP converter previously discussed. The power-factor-corrected ac/dc produces the supply voltage for the 3G basestation's RF power amplifier (typically +27 V) and the bus voltage for point-of-load converters.

A basestation power supply topology interleaves the main ac/dc converter with the battery backup converter in a single-stage dc/dc converter, thus eliminating an extra 400-V to 48-V dc/dc converter stage. This reduces costs while improving the overall system efficiency level.

A 27-V dc bus voltage is generated using a dual-FET forward converter. This forward converter has two upper FETs, each connected to a primary winding with the appropriate number of turns on the power transformer. Input-voltage sensing logic turns on the top FET Q2, which is connected to the 400-V bus, when the ac main supply voltage is within the correct range. During ac line power outages, top FET Q3 is activated to power the converter from the backup battery. The resulting 27-V distributed bus with battery backup supplies voltage to the main power transmitter and a 3.3-V brick that may be used to supply power to POL converters.

L. Haachitaba Mweene is a senior power applications engineer and Don Ashley is marketing manager in the Power Management Group at National Semiconductor Corp. (Santa Clara, Calif.).

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