28 Sep 2012
Integrated Power Considerations
Hans-Peter Ludeke, Field Application Engineer, Murata Europe investigates the options for board mounted power for embedded applications
Engineers are now opting for integrating power conversion on-board in their designs.
There are many advantages to this migration when one considers the cabling losses and increasing complexity of systems plus the higher current requirements due to the lower voltage operation of high performance components.
In the past, devices such as MCUs, MPUs and FPGAs tended to be driven by (higher) voltage rails typically of +5, +12. But now, MCUs and MPUs are increasingly driven at voltages of +3.3 or less.
Reducing voltage in these components of course results in a significant increase in the current needed. This results in increased voltage drops along the power connection route. Traditionally a multiple output AC/DC power supply would have been used and distributed by cable harness in industrial automation equipment for example.
However, doing the same with a lower voltage runs the risk of the voltage drop being too high at the load. The designer now has to consider another power distribution approach. This is now more likely to be a single output AC/DC unit supplying 12, 24 or 48 volts and then another stage of power conversion at the application board. This approach is certainly proving to be popular with the ‘blade’-based approach to servers used in telecomm and high end computing today.
Another consideration is that of heat. Utilizing a single power conversion stage creates a hot-spot around the PSU; however, putting a conversion stage on-board allows the designer to convert the amount of power required for that board and thereby spreading the heat losses around the system and making it easier to cool.
Figure 1: Footprint comparison between discrete & module solution
There are many options now open to the designer; power modules provide the conversion necessary with all thermal, safety, and EMI (Electro Magnetic Interference) considerations taken care of, saving valuable design time and re-spins of discrete solutions. Point of Load (PoL) solutions also are of great value in minimizing loss-making long transmission paths by placing the power conversion next to the device. Additionally, with the advent and general acceptance of digital power control, the designer is presented with the task of selecting the optimum solution within the performance, cost and time-to-market requirements.
Power requirement specification
Power requirements need to be specified as precisely as possible in terms of the actual needs. This includes the nominal values of input and output voltage, and output current as well as the tolerance ranges, ambient temperature and expected dynamics such as load fluctuations and changes on the input voltage.
Micro Controller or FPGA power requirements depend on the implemented software. At the time the project starts and power requirements need to be specified, the related software is normally not ready and the hardware designer has to estimate power consumption. Deviation of 50% or more from the ‘real’ power need, measured when hardware and software are ready, is not uncommon.
Next thing to consider is the Power Architecture. Should all output voltages come directly from the input source? Or will a multi-stage conversion with an intermediate bus voltage lead to better overall performance; considering the often conflicting requirements for high efficiency, minimum board space, and excellent thermal performance at minimum cost? These are typical questions a designer has to decide on. In any case there is no universal answer to that question as the best power architecture depends on the application.
Once power architecture has fixed, the designer has to decide on ‘make or buy, i.e. a discrete solution or a standard, qualified and tested module. A power converter needs to perform efficiently in typically providing a constant output voltage under all conditions; at light load as well as at full load, at -20°C as well as at highly elevated temperatures, in steady state as well when dynamic load or input voltage changes apply without affecting the output voltage stability.
The converter needs to operate with minimum losses, thermal stability, without the introduction of noise, and remain safe even in the case of abnormal conditions such as a short on any system or converter component.
Power companies, designing power supplies professionally, take care of all these aspects. But it takes even these professional designers weeks, if not even months, to turn the first concept into a product that is finalized and ready for mass production, fully tested, approved, and documented.
Discrete design considerations
For DC/DC converters with electrical isolation one can chose from several topologies; simple fly-back with lowest component count which would be of minimum bill of materials (BOM), which would please the purchasing department, but would introduce higher ripple on the output voltage, 1- or 2-switch forward, bridge topology, the options open to the designer can be daunting.
Hard switching has been well known and used over decades with a lot of control ICs available on the market making a design easier and more predictable. Soft switching on the other hand is more complex, needs additional components and often special control methods but in return offers the ability to achieve higher efficiency and less noise generation. Fixed switching frequency makes EMI filtering easier.
Other considerations such as whether to use continuous (CCM) or discontinuous conduction mode (DCM); Digital or Analogue control and even the selection of the transformer, if required, can cause the designer several challenges.
Figure 2: EMI reduction using Micro DC/DC converter solution
Component placement can be a crucial factor. General rules like ‘keep the loops as small as possible’ are often difficult to put into practice due to other restrictions. A poor layout can turn the best-known topology into a noisy, unstable power supply, which performs at an efficiency level well below target.
Switch mode power supplies are still analogue circuits where ‘parasitics’ like stray inductance of traces or leads can have a high impact on the performance. This is something that needs to be considered carefully, even when the rest of the customer board is purely digital.
Once hardware is available for testing, the design needs to be verified. Here, special equipment is required; equipment that may not be standard in many test labs.
To make sure that a power supply will not be the cause of any safety related aspect, safety engineers (e.g. from UL, TUV, VDE) will be checking for spacing on the board (leakage and creepage distance between components and/or traces) and all safety critical components.
The UL safety inspector is testing for abnormal conditions of the discrete power supply while being part of the final customer board, often worth $1000 or more. By nature, an abnormal condition means simulating a fault that can damage many components on the board. Manufactures of standard power modules often spend around $10k for a safety approval to relieve this critical mission from their customers.
Figure 3: Murata’s 1/32nd brick 30W dc/dc converter
Using commercially available power modules as building blocks is a much more straightforward approach to implement power architecture into a real customer- acceptable solution which obviates the need for the several fundamental design steps necessary for a discrete design.
Murata Power Solutions provides a whole family of converters, with and without electrical isolation, for almost every power range in a broad offering in packaging (e.g. SIP4, DIP24, 1”x1”, 1/32 brick, 1/4 brick) enabling engineers to have a wide range of options to achieve space efficient designs.
Thermal performance can be checked using the thermal de-rating curves, given in the data sheet to make sure that the selected module will be able to deliver the required power at maximum expected ambient temperature. Otherwise a module rated for the next higher power range, or additional ways to improve cooling need to be considered. For example, Murata Power Solutions offer optional heat-plate versions for several of the higher rated modules to allow easy heat-sink mounting.
To guarantee proper operation, a certain amount of input and output capacitance often needs to be added as external components. Murata data sheets contain the necessary information allowing an estimation of the expected level of ripple & noise, both on the input as well as on the output side. EMI filtering and input fusing are other aspects that need to be added externally allowing the designer to go for one solution for the entire board.
There are many ways to design a power supply, but many have many unforeseen penalties that can show up later in the design process and result in costly delays, poor performance and reworking of designs due to approval problems. Using board mount modules such as those from Murata provides a viable alternative to taking a discrete approach to power conversion and avoids the bottle-neck often experienced when the power supply needs to be finalized and required to work ‘right first time’. Few developments today can afford the luxury of time and budget to design a discrete solution from scratch – even in the historically longer-term designs for defence and military applications.
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About the author
Hans-Peter Lüdeke is a Field Applications Engineer with Murata Europe and is responsible for Power Suppliers. He joined Murata in 2011 in a technical sales role. He previous roles with Lineage Power and Tyco Electronics have all involved responsible for power electronics. At Lucent Technologies he was responsible for designing power supplies. Hans-Peter graduated from the University of Paderborn with a PhD in electrical engineering.
Murata was founded by Akira Murata as a personal venture in Nakagyo-ku, Kyoto-shi, Japan in October 1944. Since then the company has grown to be one of the world’s largest component manufacturers, producing everything from ceramic capacitors through to piezo-electric, ceramic and SAW filters, connectors, isolators, inductors and sensors. The company also produces a he volume of modules including power supplies in which they have gained significant experience and as a result have a major percentage of the global market.
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