Feature Power Electronics Smaller size or more functionality?


Chris Leek at Charcroft Electronics, explains how improvements in power-conversion efficiency and power density are giving designers the opportunity to reduce system size or increase end-system functionality


here was a time when a diode was the engineer’s friend but, now, this simple yet robust silicon component is increasingly being replaced by low RDS(on) Field Effect Transistors (FETs) and digital control circuitry. The simple truth is that, every design iteration of a power supply employs even more silicon content. For the end-user, this means a reduced size, combined with higher efficiency and flexibility, and a lower operational cost. Original industrial-grade linear power supplies were perhaps 45 per- cent efficient, with a power factor of 0.65, and a power density of just 0.47 Watts/in3. Migrating to a switching power supply topology improved the efficiency into the 70 percent range. The addition of power factor correcting (PFC) circuitry improved the power factor of the supply to above 0.95, and in many cases to 0.99.


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Following the lead of high-efficiency 80Plus front-end power supplies for the server market, current-generation indus- trial power-supply design provides the user with an entirely new experience. For example, a high-density 3 by 5 inch, open-frame power supply such as Power-One’s ABC400–1012G provides 360W of power in a 1U form factor. This power density is a 30 times improvement over the old linear sup- plies, and nearly three times the density of that which was available just a few years ago. Packaging techniques, such as using the 1U channel chassis as a heat-sink and integrated magnetics, are part of the equation that results in this high-density package.


Energy-efficient circuit design pro- vides the second half of the equation. This high efficiency reduces thermal dissipation in customers’ systems which will improve reliability and reduce size.


Power supplies of greater than 65W are required to have power factor cor- rection. High-efficiency designs, such as the ABC400–1012G, employ an interleaved power factor correction circuit for efficiency and packaging density. The PFC circuits, operating at 180


out of phase, allow the input ripple current to be lower and of higher frequency, reducing input filter requirements. At lighter loads, one phase may be turned off to reduce system losses and maintain efficiency.


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Chris Leek is the power specialist at Charcroft Electronics


A significant improvement in effi- ciency occurs when the output diode is replaced with a synchronous recti- fier. In this case, the diode is replaced with a FET and control circuitry. Take the example of the ABC400-1012G power supply. The output current of this 12V power supply is rated at 30A.


Fig. 1 Fig. 1:


Conventional diode rectification (left) versus higher- efficiency synchronous


rectification (right)


Using conventional diode rectifica- tion, with the rated VF of the diode at 1.1 volts (a common value for power rectifiers), the output dissipation is quite significant. Replacing the output diode with synchronous rectification employs a FET with RDS(on) of just


3.2mΩs. Even though the loss in that FET is a function of I2R, the loss in the output circuit is reduced by approxi- mately 80 percent (see Figure 1).


50% 55% 60% 65% 70% 75% 80% 85% 90% 95% 100%


10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Fig. 2:


Comparison of efficiencies by generation and application


Figure 3 left: The FCP400-12G


Figure 4 right: Power One’s ABC400- 1012G


To properly drive synchronous rec- tifiers to offer this efficiency requires the use of sophisticated control circuitry. In the current and emerging generations of power supplies, it is not uncommon to find the use of micro- controllers and DSP devices. These sophisticated control devices allow a greater control of the supply over the parameters of input voltage, output current and temperature.


Although circuit techniques such as synchronous rectification significantly add to the peak efficiency of the power supply, there is more to be considered.


Last Gen 400W ABC400-1012G FCP400-12G


Fig. 2


Careful control of synchronous rectifi- cation, in addition to sophisticated control of the primary side converter and the PFC circuitry, leads to real- world efficiency gains in the applica- tion of the supply. Part of the Total Cost of Ownership (TCO) of the power supply is the energy cost associated with the opera- tion of the device. As the work per- formed by the power supply changes over time and task, the power supply rarely operates at any one load point indefinitely. Typically, when load decreases, power supplies become less efficient. This creates unwanted loss such as heat dissipation in the assem- bly. The use of synchronous rectifica- tion control and other regulating devices in the current generation of power supplies enables an extended efficiency curve at lighter loads. The real thoroughbreds of the effi- ciency race are the power supplies designed for use in information tech- nology servers. Attempts to attain high efficiency, Energy Star certification, and 80Plus certification have led to a classification known as Platinum effi- ciency. An example of a power supply with an equivalent power rating is the FCP400-12G, which has been tested by 80Plus to have Platinum efficiency. To attain this efficiency level, a single output, redundant-capable power supply must have efficiency of greater than 90 percent at 20 percent load; 94 percent efficiency at 50 per- cent load; and 91 percent efficiency at 100 percent load. The requirements for this include testing at 230V AC and with the fan disconnected. Even at 10 percent load, the FCP400-12G achieves 87 percent efficiency that is higher than peak efficiency of many current designs. Figure 2 shows the efficiency of the FCP400-12G when the fan is connected.


Once again the diode takes a back- seat to the FETs used to increase power-supply efficiency to the Platinum level. The OR-ing diodes that make redundancy possible in server supplies have been replaced with low RDS(on) FETs. It has been said that the server power supply is nothing more than an industrial supply with OR-ing diodes. While there are some salient differences between an industrial supply and an information technology supply, the reality is that efficiency gains in one sector provide commensurate effi- ciency gains in the other. Charcroft www.charcroft.com


Enter 204 NOVEMBER 2013 Electronics


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