ATT.MAR–AT&T Microelectronics–SC–12– –##
Understanding usable power density aids power supply selection
Look beyond published density specs to get the most power in the least amount of space
BY DWAYNE VASQUEZ
AT&T Microelectronics
Power Systems
Mesquite, TX
Selecting a power supply that squeezes the most watts out of a smaller package involves more than choosing the supply with the highest published watt-density. The designer must remember that no supply is 100% efficient. Each supply dissipates part of the circuit's input power as heat, with most, though not all, of the power is lost in the power semiconductors.
Unless the heat generated from these efficiency losses is removed, the power semiconductor's junction temperature can rise high enough to cause the supply to fail. Thus, the power system's reliability depends on keeping the temperature of the switching semiconductors at a safe level.
Power supply vendors continue to reduce the package size while improving power density thanks to better power FETs and synchronous rectifiers, and pulse-width modulation (see Fig. 1). Unfortunately, as packages get smaller, power system efficiency improvements have lagged. There is less room inside the equipment to provide the needed airflow by natural convection. Consequently, users have had to supply the needed cooling either by installing an external heat sink or fan over the supply. Either way, the total amount of space that the power system occupies increases.
The key to getting the most efficient, coolest-running supply is understanding usable power density (see box, “Specifying a power supply for the most usable power density”). This is the maximum wattage a power system can deliver under a given set of operating conditions divided by its volume. The power system volume not only includes the supply, but also fans, heat sinks, and other external components used to support the power system's operation.
Limiting case temperature
In determining usable power density, the user must first consider the supply's maximum specified case temperature. This is the maximum permissible junction temperature of the power converter's semiconductor switch and the thermal impedance between it and the case's outer surface. The maximum case temperature also affects the system's thermal design because it determines the power converter's maximum permissible temperature rise.
For example, the case temperature of a power converter rated 85 degreesC max can be allowed only a 15 degreesC rise if the ambient temperature is 70 degreesC. However, if that power converter is rated 100 degreesC max, its case temperature could be allowed a rise of 30 degreesC.
A 15 degreesC additional temperature rise can be significant. It may, for example, mean that the converter's heat sink can be shallow enough to fit into the limited space available in a high-density system. The temperature rise also adds a design margin that enhances the converter's long-term reliability. For these reasons, users should select a power converter with the highest available specified case temperature.
Factors affecting the converter's case temperature include the amount of power demanded by the load, the ambient temperature, the converter's power efficiency and the cooling mechanism employed. Good engineering practice dictates including a safety margin within the design parameters.
The design of the load circuit determines both the amount of power delivered and the ambient temperature of the environment in which the power converter must operate. It also dictates the cooling mechanism used. For example, economic as well as physical factors such as the mechanical layout and board spacing inside the system may also limit or even preclude using a heat sink.
By analyzing airflow and converter surface area, the designer can easily determine the maximum power that the converter can dissipate without exceeding the maximum permissible case temperature. Assume, for example, that for a given airflow, ambient temperature and surface area, a power converter can dissipate 25 W and that the load demands 100 W. The total input power under these conditions will be 125 W. With a 100-W load, a 125-W input means the converter operates with 80% efficiency.
Now, assume the system being powered is redesigned to fit into a smaller enclosure and that the power converter's size must also be reduced. The converter's smaller case size reduces its surface area and, as a result, reduces the maximum power the converter can dissipate.
For this example, assume the maximum permissible dissipation decreases by one watt. Under worst- case conditions the power converter will now dissipate 19 W, which means it will draw 119 W from its input line. This yields a minimum acceptable converter efficiency of 84%, an increase of 4% over the previous example.
Quantifying efficiency
Data sheets usually specify efficiency in terms such as “up to” and “typically.” In some cases, only a percentage figure is provided with the designer left to guess what the stated percentage means. This partially reflects the growing use of specsmanship by manufacturers eager to sell product. However, it also reflects the complex nature of quantifying efficiency.
One way to look at efficiency is by examining the converter's inefficiency, which measures how much of the input power does not reach the output. In the previous example, the maximum acceptable inefficiency decreased from 20% to 16%. Thus a 4% increase in efficiency translates into a 20% decrease in inefficiency. This concept shows why increasing a power converter's efficiency by just a few percentage points is so difficult. Figure 2 shows the relationship between power dissipation and efficiency.
Efficiency depends on several unrelated variables, such as the input voltage and the percentage of full-rated output current actually being drawn by the load. Manufacturers usually specify efficiency at the power supply delivering its full rated output while operating on its nominal input voltage.
For any given input voltage, maximum efficiency is usually achieved when the power being drawn by the load represents about one-half the full rated output. Thus, in some situations, it makes sense to use a converter rated for twice the expected load current, thereby assuring maximum efficiency.
In many applications, load current varies as the load performs its functions. This, in turn, causes efficiency to vary. In applications where the current rises above or falls below its average value for extended periods, efficiency could drop significantly from time to time. This could cause the supply's case temperature to rise above the maximum permissible level even though the average power dissipation appeared well within design limits.
CAPTIONS:
Fig. 1. Power supply technology improvements have caused power density to rise at a faster pace than efficiency.
Fig. 2. As power supply efficiency increases, power dissipation, and the supply's inefficiency, decreases.
BOX:
Specifying a power supply for the most usable power density
How can the designer better the likelihood of obtaining the most usable power density out of his supply? First, define the system's power requirements. Determine the input voltage range, the output voltage and current range, the dimensions of the space to be occupied by the supplies and the anticipated ambient temperature within that space, as well as the available airflow. Please note that the airflow will be affected by surrounding components.
Second, review manufacturers' literature to identify power converters that appear to meet system requirements. Note incomplete or confusing specifications. An efficiency specification, for example, must include the conditions under which it was derived–in particular, the input voltage and output loading.
Because efficiency depends so much on the operating conditions, ask potential vendors to provide graphs that plot efficiency as a function of input voltage and output loading. Also, request the efficiency requirement be formulated under worst-case conditions; that is, with a low input voltage and a load current near full rated output at the maximum anticipated ambient temperature.
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