Like the bus converters discussed in last month's “Analog Feedback,” nonisolated point-of-load converters (POLs) are modular building blocks for intermediate voltage bus architectures (IBAs). The development of the POLs is closely linked to that of the bus converters because the output voltage specification of the bus converter and the input voltage specification of the POL must be compatible.
In the past, system designers implementing IBAs were limited in their choice of intermediate bus voltages because of the limited options among POLs. Initially, power-supply vendors were primarily offering standard POLs with 3.3-V and 5-V inputs and some POLs with 12-V input. Because the input voltage range on these converters was fairly narrow, the choices for the intermediate voltage bus were limited. Many designers opted for use of a 12-V bus because it limited distribution losses on the pc board versus a 5-V or lower bus.
According to Mark Masera, director of engineering at Bel Power (Westboro, Mass.), the widespread use of the 12-V intermediate bus accounts for the current popularity of bus converters with the 4-to-1 ratio of voltage transformation.
However, 12 V was never considered an ideal intermediate voltage because of the low output voltages required of the POLs. Since POLs are simply buck converters, the wide input-to-output voltage ratio associated with 12-V input forces them to operate with very low duty cycles, which lowers their efficiency. When the intermediate bus voltage is reduced, the POLs' duty cycles increase, raising their efficiency.
Naturally, the improvement in POL efficiency comes at the expense of an increase in board-level distribution losses. Designers can optimize the overall efficiency of their IBA by choosing an intermediate bus voltage that trades off these two conflicting factors, and often this leads to the selection of an intermediate bus voltage between 6 V and 12 V. In the past, most off-the-shelf POLs with 12-V input were not very flexible in this regard, because their input voltage was limited to a range of about ±10%.
But the demand for lower-value bus voltages has encouraged the recent development of many POLs capable of operating over wide input voltage ranges. Some recently introduced examples appear in the table; these represent just a sampling of the dc-dc converter modules with wide input range that are currently available. Note that there are many additional POL modules with narrow input range, and with input ranges designed for lower voltages such as 3.3 V to 5 V. In addition to the modules, there are many dc-dc converter ICs available for building buck converters. These ICs offer different levels of integration, different voltage and current ratings, and different feature sets. (These chips are beyond the scope of this article.)
As for the POL modules discussed here, many of these devices can accommodate an intermediate voltage bus of 12 V, 9.6 V or 8 V. These values are noted because they represent step-down voltage transformations of 4-to-1, 5-to-1 and 6-to-1 from the 48-V bus that feeds the bus converter.
As the POLs with wide input range have proliferated, more customers have opted for a lower intermediate bus voltage such as 9.6 V. This migration to that particular value accounts for the rising popularity of bus converters with the 5-to-1 ratio, says Masera.
In time there may be continued migration to lower voltages. Lou Pechi, director of market development at Power-One (Camarillo, Calif.), suggests that as IC-supply voltage requirements continue to drop to levels below 1 V, in systems with lower power usage, IBA designers may migrate to 5 V for the intermediate bus voltage to maintain reasonable duty cycles, and hence efficiency, in their POLs. Higher power systems will still remain at the present IBA voltages because of the current-handling capability required of pc board traces at the lower voltage levels.
In effect, a migration to a 5-V intermediate voltage bus would help bring the IBA trend back to where it started. Before the terms “IBA” came into fashion, designers routinely used POLs to step down the 5-V output from their well-regulated bricks to power lower voltage logic devices. Currently, there are many POLs available for operation from either 3.3 V or 5 V.
The introduction of POLs with wide input voltage range is only one aspect of POL development. As with the isolated dc-dc converters, the current ratings and current density of the POLs have been steadily rising to accommodate more power-hungry applications. Some of the recently introduced POLs deliver as much as 60 A in a 2-in2 footprint. At full load, most of the POLs shown in the table specify typical efficiencies as percentages in the low to mid-90s.
In most cases, wide input voltage range is accompanied by resistor-programmable output voltage. In addition, many features have been added such as an enable function with positive or negative logic; tracking and sequencing; margining; power good output; and monotonic startup into prebiased loads. In addition, many POLs offer protection features such as input UVLO, output overcurrent protection and overtemperature protection. Some POLs automatically current share.
The introduction of digital control in POLs is expanding the set of programmable functions and adding diagnostic capabilities. An example is Power-One's Z-POL converters, digitally controlled PWMs that are part of the company's Z-One digital IBA product offerings.
In the company's original Z7000 models, which are I2C programmable through the company's power-management controller, there are numerous programmable features. These include output voltage, turn-on and turn-off delays, output voltage slew rates, feedback loop compensation, thresholds for various protection features and limits for the power good output.
It's expected that more vendors soon will begin offering digitally programmable POLs. Already some vendors have announced their plans to introduce digitally controlled POLs that leverage the PMBus communications protocol.
|Company/Model Number||Input Voltage (V)||Output Voltage (V)||Max Output Current or Power||Dimensions||Pricing|
|Bel Power S7KS-25D Keystone series||3 to 15||0.7 to 5.4 (digitally programmable)||25 A||1.7 in. × 1.09 in. × 0.45 in. SMT||$22.50 each in quantities of 1000|
|C&D Technologies Volant NFA series||6 to 14||0.75 to 5||10 A, 16 A or 20 A||43.2 mm × 13.46 mm × 9.30 SMT (20 A)||Not Available|
|Delta Electronics Delphi Series IPM24S||8 to 36||1.2 to 2.5, 3.3 to 6.5, or 8 to 15||3 A or 20 W||0.7 × 0.591 in. × 0.308 in. SMT or SIP||$5.85 each in quantities of 1000|
|Power-One Z1000 No-Bus||3 to 13.2||0.5 to 5.5||5 A, 7 A, 10 A, 15 A or 20 A||8 mm × 32 mm (15 A) vertical-mount SIP||$11 each in quantities of 1000 (15 A)|
|SynQor NiQor family||9.6 to 14.4||0.85 to 5||16 A||1.3-in. × 0.53-in. × 0.29-in. SMT||$18.90 each in quantities of 1000|
|Texas Instruments PTH12040||8 to 14||0.8 to 5.5||50 A||2.05-in. × 1.05-in. SMT or PHT||$35 each in quantities of 1000|
|Tyco Electronics Power Systems AUSTIN MEGALYNX ATS version||6 to 14||0.8 to 5.5||30 A||2-in. × 0.5-in. × 0.55-in. SIP||Starts at $18 each in quantities of 1000|
|V-Infinity The VPOLXXX-12W series||6 to 14||0.75 to 5||10 A, 16 A or 20 A||20.3-mm × 11.43-mm × 6.1-mm SMT (10 A)||Starts at $10.50 each in OEM quantity|
|Wall Industries DOSA Alliance-Compatible POL series||8.3 to 14||0.75 to 5||6 A, 10 A or 16 A||0.8-in. × 0.45-in. × 0.258-in. SMT (6 A)||$7.55 to $13.30 in OEM quantity|
|• Table does not include POLs designed specifically for 3-V to 5-V input.|
|• Although dimensions cited here are mainly for surface-mount packaging, most of these models are also offered in through-hole SIPs or horizontal-mount versions, which may have slightly different dimensions.|
|• Depending on how the vendor chooses to assign model numbers, a series of devices may contain devices with just one current rating or multiple versions with different current ratings. See vendors' online selection guides for complete lists of current ratings and other options.|
|• TI's PTH12040 is being second sourced by Artesyn and by other POLA members.|
|• SynQor plans to introduce POLs with expanded input range in the future.|