22 June 2009
Digital power gets a boost
While the profile of digitally controlled power supplies is rising, it could remain a high end option for a while.
Digital control, in all its forms, makes inherent engineering sense because of the flexibility it offers. Arguably, this is no more relevant than in switched mode power supplies (smps). Vulnerable to stray capacitances and inductances with respect to their high operating frequencies, smps are notoriously difficult to design.
The introduction of digital control could help bring simplicity, coupled with high efficiency and flexibility, to almost any application. The problem seems to be that as power supplies – specifically ac/dc power supplies – rarely differentiate a product, they default to an unwanted and additional cost for product manufacturers. With that in mind, OEMs want to lower the cost of power supply production, rather than increase it just for the sake of flexibility or design simplicity.
In order for digital control to really make a difference, it has to offer flexibility and simplicity; not only in the design of smps, but also in their production. With respect to the latter, digital control is unsurpassed, as it can compensate for tolerances and, to a large extent, remove the selection-on-test element of power supply design. So could the cost benefits associated with producing a digitally controlled power supply outweigh the potential increase in bill of materials?
It may come down to exactly how easy it is to implement and, here, the reference design is essential. As a power supply is rarely a differentiating feature, it doesn't make sense to spend too much time designing one. If a reference design can be used that fulfils all your product's requirements and offers a simpler production process, then it could be the right solution for your next application.
One area where digital power is currently seeing significant success is in solid state lighting and lcd/led tv power supplies. The increased penetration of lcd/led tvs also comes with a demand for more power; replacing a crt doesn't come without sacrifice. Balancing this increased power consumption while demonstrating compliance with energy regulations and incentives has led to many power companies targeting this application area with special 'to type' solutions.
At higher power levels, it is necessary to implement power factor correction (pfc) into the supply's design to achieve maximum efficiency. It is typically this requirement that has increased design complexity, particularly where an smps pwm controller is used in order to implement pfc and maximise efficiency. As a result, integrated solutions are appearing that tackle this complexity: OnSemi's NCP1562 is an example, sitting at the heart of its 200W Game Console ac/dc power supply GreenPoint reference design, which produces 12V at 16.6A and 5V at 1.5A.
Reference designs are often created to support dedicated controllers in conjunction with other power supply specific devices, allowing manufacturers to increase their design wins in this expanding application area. But there are also solutions built around less application specific controllers. An example is Microchip's recently announced ac/dc reference design, which the dsPIC33F GS family of digital signal controllers (DSC).
The introduction of the GS family is a further development of Microchip's DSC solutions and is claimed to integrate the features necessary for better smps pwm control for ac/dc and dc/dc applications. This includes higher processing performance (40Mips at 3.3V) and finer resolution, as well as multiple pwm modes comprising standard, complementary, push-pull, variable phase and centre aligned.
The reference design uses two DSCs: one on the primary, one on the isolated secondary (see Figure 1). The DSC on the primary side controls the pfc boost converter and phase shift zero voltage transition (ZVT) converter, while the secondary side DSC controls the multiphase and single phase buck converters, producing 3.3V dc at 69A and 5V dc at 23A respectively. These secondary buck converters are fed from a further 12V dc at 30A output, giving the reference design three regulated dc outputs with 300W of continuous power.
Power factor correction is used to improve the efficiency of a power supply, largely through reducing the total harmonic distortion introduced by any switching elements with respect to the current drawn at any instant. PFC can be implemented using either a buck, boost or buck/boost topology.
The current drawn from a buck or buck/boost converter is always discontinuous, while a boost converter operates in continuous current mode, which Microchip claims makes it an ideal choice for pfc. In this case Microchip has chosen a boost topology, possibly because it requires smaller filters than a buck circuit and lower rated mosfets than a buck/boost topology. The mosfets implemented in the pfc circuit are two Infineon CoolMOS SPP11N60CFD, connected in parallel and driven by a Microchip TC1412N gate drive ic, controlled by one of the pwms on the primary DSC.
An emi filter is also included, to comply with international standards for conducted emi. The output of the pfc stage feeds into the full bridge ZVT converter, which produces the 12V dc output, then the 3.3V and 5V output stages. Infineon CoolMOS mosfets are also used in the ZVT circuit, but instead of using a gate drive ic as before, a small transformer has been chosen, predominantly over concerns about switching frequencies causing thermal issues with a drive ic.
In the single and multiple phase buck converters, mosfets have been used in place of diodes in various places, which allows for greater control. Connecting multiple synchronous buck converters in parallel increases power achieved from a step down voltage stage and by phase shifting the pwm in each stage, a reduction in output capacitance can result. In this reference design, Microchip has implemented a three phase synchronous buck converter with a switching cycle of 120°. The mosfets (this time from International Rectifier) are controlled by a Microchip MCP1404 gate drive ic.
The control software uses a mixture of C and assembly code, with all time critical functions written in assembly and all peripheral setup routines, initialisation routines and non critical functions provided in C.
The ZVT converter uses the most significant 8bit of a voltage measurement taken on the secondary side by the second dsPIC, which is fed back to the primary through the optoisolated uart channel (see figure 1). This is right shifted to give a 10bit value, which is then compared to the reference voltage. The result is fed into the phase shifted pwm, using the phase shift capability of the dsPIC33FJ16GS504's pwm module. The phase shift can be modified by simply overwriting the value in the appropriate special function register in the PWM module. A similar voltage error routine is used on the secondary side to control the single and multiple phase synchronous buck converters, making use of the integrated analogue comparators.
Board layouts are provided for the power and signal boards, with a wide range of schematics for the various elements. Component types and values are included in the schematics, although not provided as a Bill of Materials. For more, go to www.microchip.com/SMPS.
The use of programmable controllers will undoubtedly be necessary as digital power gains acceptance. The question remaining is will it deliver commercially?
Author
Philip Ling
Supporting Information
Downloads
18881\P37-38.pdf
Websites
http://www.microchip.com
http://www.onsemi.com
Companies
Microchip Technology Inc
ON Semiconductor
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