Electronic Design

Non-Isolated DC-DC Converters Provide Many POL Power Options

Because they're smaller and less expensive, non-isolated point-of-load (POL) dc-dc converters are supplanting venerable brick-style isolated dc-dc converters for generating the array of voltages required by today's pc boards. And because their small size allows them to be placed physically close to their load, they offer performance enhancements as well.

There are two types of POL dc-dc converters: isolated and non-isolated. In the former, a transformer in the converter circuit isolates the POL's output voltage from the system's primary distributed bus dc voltage. In contrast, the non-isolated POL has no transformer or electrical isolation, allowing it to be smaller and less expensive. An isolated POL differs from a traditional isolated brick-type converter in regard to its input voltage range. Bricks generally operate from a wide input voltage range of 36 to 75 V, which is nominally 48 V. POLs, whether isolated or non-isolated, operate from lower voltages like 12, 5, or 3.3 V. Isolated dc-dc converters are generally found in telecommunications applications as well as in computing applications, including servers and data storage equipment. They tend to offer standard brick-type footprints and pinouts. Non-isolated units, which make up the majority of POL converters, come in assorted board-friendly compact package styles and are typically used in computing applications such as memory motherboards.

Non-isolated POL converters are not intended to completely replace isolated converters. Instead, the non-isolated units complement their isolated counterparts. The isolated dc-dc converters (or bricks) step down a higher voltage bus, such as 48 V, to some board-level voltage like 12, 5, 3.3, 2.5, or 1.0 V. Non-isolated POLs are used to generate some additional voltages not generated by the bricks. For example, if your brick converts 48 V to 3.3 V, and you also require 1.5 V, then you might use a non-isolated POL to step down the brick's 3.3-V supply to 1.5 V. Non-isolated POLs offer a lower price for a given voltage and current output. Further, they're usually housed in small-size formats like single-in-line packages (SIPs), dual-in-line packages (DIPs), or surface-mount (SMT) modules. Their more compact packaging yields significant real estate savings on the pc board and allows them to be placed close to their loads. In turn, their close load placement reduces I2R losses on the board and improves transient response.

In recent years, non-isolated POL units have commonly been run off the isolated 3.3- or 5.0-V bus. However, due to rising current requirements, many designers are opting to power these POLs off of a higher voltage bus, such as 12 V. This 12-V bus then becomes an intermediate voltage bus that powers all of the POLs. Many power-supply vendors are currently developing POLs that are more powerful to help optimize the implementation of intermediate-voltage bus architectures.

A variety of non-isolated POL dc-dc converters are currently available. The table below focuses on a sampling of non-isolated POLs recently introduced by prominent vendors. One of the crucial distinguishing factors among them is input voltage. Key vendors have manufactured devices that meet 3.3-, 5.0-, and 12.0-V input requirements. For instance, the PMA series converters from Ericsson function with 3.3-, 5.0-, or 12.0-V inputs. The S007 converter developed by Celestica operates with a 5- or 12-V input. Bel Fuse's V7PC-10C unit works with a 3.3-V input, while the Austin SuperLynx converter from Tyco features an input voltage range of 3.0 to 5.5 V. Datel's LSN D5 converters accept 5.0-V inputs, and Power-One's SIS12V series converters offer a 5-V input with a voltage range of 4.5 to 5.5 V.

Ericsson's unit offers 1.2- and 5.0-V outputs, while the S007 converter features programmable output voltages from 1.0 to 5.0 V with a single resistor. The LSN D5 series units deliver 1.0-, 1.2-, 1.5-, 1.8-, 2.0-, 2.5-, and 3.3-V outputs fully rated at 10 A. The V7PC-10C series converters feature output voltages ranging from 2.5 V down to 1.0 V. Tyco's converter is capable of output voltages between 0.9 and 2.2 V dc. Output voltage for the SIS12V converters ranges from 1.2 to 3.3 V.

When dealing with non-isolated POL converters, there are a number of distinguishing factors beyond input and output voltages. Differences may exist with respect to derating and noise output. When choosing a non-isolated POL converter, efficiency and footprint obviously hold a lot of weight. Protection features are also key in the design of these units.

The units also possess other distinguishing features. Output overcurrent protection is included in the Austin SuperLynx, LSN D5, and SIS12V converters. Datel's converter features a noise specification as low as 10 mV p-p and no derating to 71°C. The Austin SuperLynx converter has a ripple and noise specification of 30 mV p-p. A remote on/off function is included in the converters from Tyco, Power-One, and Datel. Units from Bel Fuse, Datel, and Tyco have a built-in remote sense pin. The Austin SuperLynx converter has a mean time between failure (MTBF) of 10 million hours at 25°C. Most notably, the PMA series converters from Ericsson feature an MTBF of over 12 million hours.

Also worth a mention is the extremely small size of the packages in which some of the POL converters are available. The Austin SuperLynx converter offers the most compact of the SIPs, measuring 2.0 by 0.5 by 0.32 in. for a total 0.302 in3. In addition to a standard SIP, the SIS12V unit is available in an SMT package measuring 1.3 by 0.53 by 0.326 in., or 0.225 in3. The LSN D5 converter is available in a horizontal- and vertical-mount SIP measuring 2.0 by 0.55 by 0.34 in.

S007 Converter
The S007 converter is a 5- or 12-V input, nonisolated dc-dc converter designed for use in server and enterprise communications equipment. Offered in an SIP-style package, this unit significantly improves system motherboard real estate over traditional form-factor POL SIPs. Available in a 1.2- by 1.1- by 0.39-in. form factor, the S007 converter delivers an 8-A output current. The 12-V input converter provides a range of programmable output voltages from 1.0 to 5.0 V with a single resistor. MTBF falls within the range of 3.6 million and 5.4 million hours. In addition, the unit provides PowerGood and Enable signals. The converter aims at computer, communication, and storage OEMs. Pricing for the S007 converter starts at $8.50 each with an annual purchase of 1000 units.
Celestica Inc.
www.celestica.com/power; (888) 899-9998

See associated figure

LSN D5 Series
With its 4.5- to 5.5-V input range, the LSN D5 series of SIP dc-dc converters accepts 5-V inputs and delivers 1.0-, 1.2-, 1.5-, 1.8-, 2.0-, 2.5-, and 3.3-V outputs fully rated at 10 A. The units require no external I/O filtering to achieve either specified noise levels or FCC-compliant radiated/conducted emissions. They're designed with on-board I/O capacitors fully rated to handle the required input ripple current. The LSN D5's SIP measures 2.0 by 0.5 by 0.34 in. and can be configured in either vertical or horizontal orientations. The vertical model occupies 0.7 in.2, while the latter measures only 0.34 in. high. The units offer tight accuracy of ±1% and high efficiency between 89% and 96%. Full power is guaranteed to 65°C with no supplemental airflow. The LSN D5 converter's power density is achieved with a fully synchronous, fixed-frequency (300-kHz) buck topology. Pricing starts at $9.95 each in OEM quantities.
Datel Inc.
www.datel.com; (800) 233-2765

See associated figure

PMA Family
The PMA family of surface-mountable dc-dc regulators offers 1.2- and 5-V outputs. The 3.3-VIN/2.5-VOUT and 5-VIN/3.3-VOUT versions are rated for an output of 6 to 8 A. All other models can deliver up to 12 A. The units operate from 3.3-, 5-, or 12-V dc inputs. Thanks to efficient thermal transference, the high-reliability devices offer an MTBF of over 12 million hours. The units' construction also minimizes the need for external cooling components. The modules, which are up to 95% efficient, feature integrated input filtering, I/O synchronization, remote control, output voltage trimming, and output sensing. Built on ceramic substrates, PMA series modules are housed in compact, transfer-molded packages. They can operate from -30°C to 85°C with limited airflow. PMA converters run $13.90 each for a full-featured version or $12.00 each with limited filtering, both in OEM quantities.
Ericsson Power Modules
www.ericsson.com/powermodules; +46-8-568 69620

See associated figure

SIS12V Series
Power-One's SIS12V series dc-dc converters provide POL conversion of a single 5-V input into a regulated low output voltage with up to 12 A of output current. Output voltage ranges from 1.2 to 3.3 V, while efficiency is rated from 82% to 86%. Available in a through-hole mounting configuration, the units provide output overcurrent protection. In addition, the SIS12V converters feature remote on/off and output voltage trim functions. The trim feature lets users adjust each output voltage by ±10% from the nominal. A vertical mounting option helps save space. The converters are particularly suited for powering ASICs in a distributed power architecture. Output ripple and noise is 100 mV p-p maximum. The converters are designed for natural or forced convection cooling. They operate from -40°C to 55°C. The SIS12V POL converter costs $15.00 each in 1000-unit quantities.
www.power-one.com; (978) 725-5974

See associated figure

Austin SuperLynx
With its single output of 16 A, the Austin SuperLynx offers designers a cost-efficient open-frame design. It delivers output voltages from 0.9 to 3.3 V dc, with a full-load efficiency of 94% at 3.3 V. Input voltages range from 3.0 to 5.5 V dc. The module is available in a variety of voltages. A key characteristic is its tightly regulated outputs (typically ±1%). Additional features include remote on/off, overtemperature protection, and auto-reset output overcurrent protection. While the SMT version measures 1.3 by 0.53 by 0.326 in., the SIP version is 2.0 by 0.50 by 0.32 in. The converter's small footprint enables it to be located near the load. The converter operates from -40°C to 85°C. Austin SuperLynx costs $19.50 each in 1000-unit quantities.
Tyco Electronics
www.power.tycoelectronics.com; (800) 526-7819

See associated figure

V7PC-10C Series
A 3.3-V input non-isolated dc-dc converter from Bel Fuse is designed to provide up to 10 A of output current. The V7PC-10C series comprises devices with output voltages ranging from 2.5 down to 1.0 V, including a 2.5-V unit that features 92% efficiency at full load. The units' high-efficiency design and robust thermal management features enable these SIP converters to provide 10 A of output current in the same footprint as competing products that deliver only 6 A. Less than one inch of pc-board real estate is occupied with the series. The converters operate to 70°C with no derating over a 3.0- to 3.6-V input range. Also, an optional remote sense pin is offered for applications demanding precision regulation of low output voltages. Pricing for the V7PC-10C series units starts at $9.75 each in 10,000-unit quantities.
Bel Fuse Inc.
www.belfuse.com; (201) 432-0463

See associated figure

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