Build or buy? That question tops the list when systems designers decide how to supply dc power at the growing number of voltage and current levels present in virtually every type of electronic system. As supply voltage levels drop and currents rise, the task of powering microprocessors, ASICs, DSPs, logic, and analog elements in these systems is becoming more difficult.
Because power design has long been considered as much art as science, the first inclination may be to leave it to the experts—that is, buy power modules or bricks from established power-supply providers. But semiconductor manufacturers have developed control and driver ICs and support tools that take some of the mystery out of supply design. Now such design is accessible to engineers who want to build at least some of their system's power supplies but have little practical design experience.
The thinking in today's system power design is to use a distributed-bus architecture based on a chain of dc-dc converters that take a dc voltage generated from the ac line and break it down to the individual voltages needed to supply the system's digital elements. The first dc-dc converter in this chain is a bus converter that takes a high dc voltage, say 48 V, converted from the ac line, and distributes it to secondary supplies called point-of-load (POL) converters (Fig. 1). Each POL delivers a required voltage or voltages and current to specific elements of the system, such as microprocessors, memory, ASICs, and DSPs. With supply voltages falling below 3.3 V (2.5, 1.8, and 1.5 V are common in systems) and current demand rising (50 to 100 A or greater) in high-speed systems, it could take a large number of POLs to supply the system's loads.
A distributed-bus architecture contains two types of dc-dc converters: The bus converter must be an isolated supply, while the POLs can be non-isolated. Isolation means that the bus converter must have a transformer to separate the ac power line from the load, both for personnel and equipment safety. A discrete isolated dc-dc converter design can be quite complicated, while a non-isolated design is much simpler. This is where the build or buy decision comes into play.
"Most people buy an isolated supply, a brick or a version of one, to go from 48 V down to bus voltage," says Steve Goacher, marketing manager for Texas Instruments. "Isolated converters are complicated designs that require expertise to meet requirements such as FCC noise specifications."
Isolated designs are more complex, agrees Donald Ashley, strategic marketing manager at National Semiconductor. "The transformer is the difficult part of the design. It takes a bit of mathematics to solve the design equations, but it depends on the complexity of the design. If a novice did a few non-isolated designs, he might figure out an isolated one because there's so much knowledge and help available about non-isolated buck regulator designs," he says.
Since power-supply design can be daunting to the uninitiated, National offers an evaluation board that lets users play with a fully functional converter before venturing out on their own. The LM5030 evaluation board is designed around the LM5030 high-voltage PWM controller, mounted on a board that measures 2.4 by 2.4 by 0.5 in. It can be used for push-pull or bridge topologies, has an input voltage range of 36 to 75 V, and delivers a 3.3-V output at up to 10 A.
On the board, two alternating outputs drive two N-channel MOSFETs. These feed the two halves of the power transformer primary. A feedback path using an optocoupler (the LM3411) provides isolation and drives the COMP pin on the LM5030. This voltage controls the pulse width to the output MOSFETs. A number of built-in supervisory features, such as dual-mode current limit, soft start, sync capability, and thermal shutdown, further ease the novice designer's task.
Expertise to design an isolated converter is one part of the build or buy decision. The other is the number of systems a company plans on manufacturing. If volumes are low, say less than 100 systems per year, the consensus among semiconductor marketing managers is that it's better to buy an isolated module or brick from a cost standpoint and treat it as a component rather than attempt to design one. Yet as volumes increase, so does the rationale for doing a discrete design. Goacher believes that systems houses may start out with a brick but switch to a discrete design to get a cost reduction as volumes ramp up.
Putting it in dollar terms, Madhu Rayabhari, power management marketing director at Fairchild Semiconductor, says, "It's a tradeoff. You have to weigh the costs of investing the time to design and debug an isolated converter and save somewhere between $15 to $80 on the bill of materials versus purchasing an off-the-shelf power supply."
When it comes to build-buy decisions about the simpler-to-design non-isolated POL converters, the picture is much clearer. Semiconductor manufacturers have come up with a variety of controller and driver ICs that remove much of the complexity involved in power-supply design. The goal is to eliminate the expertise factor in supply design and allow relatively inexperienced designers to successfully implement their own designs using a building-block approach. But realizing that hardware alone can't ensure a working design, chip makers offer a comprehensive support structure that includes software design tools, reference designs, evaluation boards, schematics, component values, and access to experienced designers on their staffs.
Two of the latest controller ICs to hit the market are International Rectifier's iP1201/1202 from the company's iPOWIR family. Called "Two-Phase dc-dc Power Blocks," the devices are synchronous buck converters intended for non-isolated POL applications (Fig. 2). Each can deliver a 30-A single output or two 15-A dual outputs at output voltages that span the low-voltage range requirements of many microprocessors, ASICs, and other high-performance digital devices in today's systems. The iP1201 provides outputs from 0.8 to 2.5 V with a 3.3-V input and 0.8 to 3.3 V for a 5-V input. The iP1202 covers 0.8 to 5 V for a 12-V input and 0.8 to 3.3 V for an input below 6 V.
According to Carl Smith, marketing manager for network and communications products, dc-dc sector, "If you try to lay out discrete devices and passives on a pc board, it becomes quite involved. With the iP1201/1202, you get guaranteed power losses which encompass all of the effects of switching and conduction losses, including the driver losses and second-order losses associated with the layout." This allows such controllers to operate at efficiencies of greater than 90%, an important consideration to limit dissipation in the power system.
"Whether you want to build or buy, we believe we have an appropriate solution," says Tony Armstrong, product marketing manager, Power Business Unit, Linear Technology Corp. The company offers a full array of power management products that run the gamut from a wide variety of switching regulators to hot-swap controllers to dc-dc converters to power controllers. "In particular," Armstrong notes, "the breadth of our PolyPhase products addresses numerous applications."
A pair of Maxim step-down buck controller ICs, the MAX5033/5035, includes an internal low-RDS(ON), high-voltage DMOS transistor to provide high efficiency and reduced system cost. "Putting the switches inside the controller IC gives you simplicity and cost savings and reduces layout problems," says Nitin Kalje, corporate applications engineer.
Built-in FETs are generally for lower-power applications up to around 3 A: The MAX5033 is rated for 500 mA of output current. The MAX5035 is a 1-A output controller. Both can handle inputs from 7.5 to 76 V and use just 350 mA of quiescent current at no-load conditions. Employing pulse-width modulation (PWM), the converters operate at a fixed 125-kHz switching frequency at heavy loads. However, they automatically switch to a pulse-skipping mode at light loads to reduce the quiescent current and improve efficiency. Both come in fixed output voltage (3.3, 5, and 12 V) and adjustable (1.25 to 13.2 V) output versions.
Analog Devices' entry for do-it-yourselfers is the ADP3050, a 200-kHz step-down switching-regulator controller that integrates a 1-A power switch and all of the control, logic, and protection functions necessary for standalone operation. The controller accepts input voltages from 3.6 to 30 V and offers two output options—adjustable and fixed at 3.3 and 5 V. A 2.5-V on-chip regulator provides the internal operating current to enhance efficiency. The internal compensation scheme allows the use of any type of output capacitor—tantalum, ceramic, or electrolytic—and equivalent-series-resistance (ESR) value. A complete regulator design requires only a few external components that can be standard off-the-shelf devices.
Control features include a shutdown input that puts the controller in a low-power mode, reducing the supply current to under 20 mA. Thermal shutdown and cycle-by-cycle current limiting for the power-switch supply complete device protection under fault conditions.
For high-current POL applications, a multiphase converter is needed, such as Fairchild Semiconductor's FAN5019, a two- to four-phase synchronous buck controller. "To get high current," explains Madhu Rayabhari, "you need multiple units of power conversion connected in parallel. Each unit is called a phase. To get 90-A capability, for example, you need a three-phase dc-dc converter with each phase supplying 30 A."
Such a three-phase arrangement uses the FAN5019 driving three FAN5009 synchronous dc-dc MOSFET drivers (Drill Deeper: See a schematic of this arrangement online at www.elecdesign.com.) Intended for powering high-current, low-voltage CPU cores, the circuit's output voltage ranges from 0.8375 to 1.6 V and can supply 74 A dc, 93 A peak. Because of the high current capability, all of the parts are individual components mounted on a pc board. In such high-current, multiphase applications, the drivers generate so much heat, they must be external to the controller and the MOSFETs.
One advantage of a multiphase design is that the output's ripple voltage rides at a much higher frequency than in a single-phase converter. For example, if each phase operates at 1 MHz, the ripple is around 3 MHz, and its magnitude is reduced. This makes it easier to filter, resulting in fewer and smaller filter components, lower component cost, reduced power consumption, and less area occupied on a pc board.
For the converter in the online schematic, targeted for inexperienced power designers, Fairchild provides Excel spreadsheets to assist in the design, demonstration boards (because layout is very important), schematics, a preferred layout, and engineering help from the staff.
TI offers a controller family for high-current dc-dc converter POL applications. The TPS4009x dc-dc buck controllers use PWM to manage two-, three-, or four-phase designs that support applications from 40 to 120 A. Each phase can provide an output current from 20 to 30 A.
A typical high-current, two-phase design (Fig. 3) includes a TPS4009PW controller. It drives a pair of external synchronous buck drivers that can be TPS2830 or TPS2834 adaptive gate drivers. The gate drivers, in turn, control the power MOSFETs, which are also external due to the high current to be provided. In a four-phase design operating at a switching frequency of 4 MHz, the ripple frequency will be 4 MHz, making filtering easier, more efficient, and less costly than at lower frequencies. An output voltage droop (pins 7 and 8) can be programmed to improve the transient window and reduce the size of the output filter.
The TPS4009x controllers provide system protection features, including current-sense fault, programmable overcurrent, and individual phase-current detection. An adjustable voltage threshold sets current limit up to 200 A. The company claims an efficiency of greater than 90% in step-down conversion application.
|Need More Information?|
Advanced Monolithic Systems
Analog Devices Inc.
Champion Microelectronic Corp
Fairchild Semiconductor Corp.
Linear Technology Corp.
Maxim Integrated Products.
Microchip Technology Inc.
National Semiconductor Corp.
ON Semiconductor Inc.
Vishay Intertechnology Inc.