Determining Power Supply Specifications

As was explained in the section on [Design Procedure], before a design process can be started, power supply specifications must be decided upon, such as the specific performance characteristics and properties that the power supply unit must exhibit.

In actuality, power supply specifications are not something that the designer of a power supply device can dream up from a blank sheet. Beyond the input power to be used and the accuracy of voltage and the type of current required by the load for which power is supplied, there are several verification items that must be addressed, including efficiency and operating temperature range. These parameters are governed by the overall system specifications and the specifications for the board to which power must be supplied.

In reality, those specifications are not always clearly established at the start of the design process. This is not because of the fault of the designer of the circuit for which power must be supplied, but because of the fact that in many cases what the power-hungry board requires cannot be known until the design process has moved forward to some extent.

That said, the length of time allotted to the design process in the overall developmental project is short, and waiting until all parameters have been fixed is not a luxury that you can afford. Consequently, at some point in time the design process must be undertaken based on available, albeit incomplete, information, with the understanding that specifications are subject to change and that an adequate amount of latitude and flexibility must be provided as you go along.

The items below describe decisions that must be made before a design process can commence, and a minimum set of parameters that must be established at the start of the design process.

We now discuss the topic

[Minimum specifications to be fixed before starting the design phase]

in concrete terms.

・Input voltage range

Given that the converter of interest is an AC-DC converter, the input is naturally from an AC power supply. Fortunately, the AC power for homes and offices is governed by established nominal voltages. While in Japan it is 100VAC, worldwide we must address at least a range encompassing 100VAC to 240VAC. Moreover, since these numbers are nominal values, including tolerances, in many cases a lower limit of -15%, which is 85VAC, and an upper limit of +10%, or 264VAC may have to be accommodated. Since in some countries electrical power is poorly regulated or delivered, establishing adequate tolerances requires considerable experience and an understanding of the actual conditions that prevail in a given country. In a nutshell, the input voltage range for the power supply device being designed is determined by the conditions in the country to which the systems are shipped incorporating the power supply device.

Worldwide principal residential power supply voltages (nominal value):
Japan: 100VAC
U.S.: 120VAC
Canada: 120VAC/240VAC
England: 230VAC/240VAC
Russia: 127VAC/220VAC
China: 110VAC/220VAC

・Output voltage/accuracy/current

The output voltage from the AC-DC converter must be set to the DC voltage required by the system and the circuit board for which the converter is intended. In the case of an industrial device, for example, common standard voltages such as 24DC and 12DC are predominant. Nowadays, however, it is not unusual that the converter output voltage is set to 5VCD, 3.3VDC, and other direct-drive voltages. At any rate, a ±5% accuracy requirement must be met by output voltages, subject to the requirements imposed by the particular device to which power is supplied. In the design process, the parts and methods to meet the required voltage accuracy need to be evaluated.

An even more critical requirement in output specifications is the level of output current, capable of supplying the current required by the circuit for which power is to be supplied and at a level sufficient to maintain the regulated output voltage. Because providing a large latitude broadens the tolerance range at the expense of increased parts cost and size, information on the maximum load current that must be supported is of critical importance. Moreover, for situations where load transients are likely to occur, the response characteristics need to be evaluated. Inadequate response properties can potentially result in critical system faults, such as unexpected system resets.

In addition to evaluating the required output in terms of currents, if the power supply system is to be configured using individual switching regulators based upon the output from an AC-DC converter, the output current requirements must be considered based on output power requirements. Because switching regulators perform power conversions, if a 12VDC output is produced by an AC-DC converter and if the next-stage switching regulator that takes this output as an input has an 80% efficiency, producing 5V/0.A, the input power will be 5W. In simple terms, fulfilling this power requirement means that since the 12VDC output needs to be only 5W, an output current of 0.42A will be good enough. In many cases, the output capability of a power supply unit that performs power conversions is indicated in terms of wattage.

・Output ripple voltage

“Ripple” refers to pulsation. Converted DC voltages contain pulsation that is related to the frequency of the input AC power supply or the frequency of the switching conversion. Although naturally the conversion process includes smoothing/filtering, that does not reduce the pulsation to zero. If there is a 400mVp-p ripple centered on a 5VDC output, for example, the maximum value will be 5.2V, with a minimum of 4.8V. This amounts to 5V±4%, barely satisfying the general accuracy requirement of ±5%. A 400mVp-p ripple on a 3.3V output, however, results in 3.3V±6%.

Image of an output ripple voltage stemming from switching operations

AC-DC converters produce a voltage, such as 12VCD, which is referred to as a bus voltage. In a configuration where this is used as an input voltage and the voltages required by circuits are provided by individual voltage regulators, the ripple requirements on an AC-DC converter may be relaxed. In situations where power is directly supplied to a low-voltage device, as described above, the presence of ripple voltage may present a problem. At any rate, the ripple voltage should be made as small as possible, and the acceptable tolerance levels must be set by considering the amount of footprint and cost for filters.

・Insulation voltage tolerance

Some system specifications require insulation for the AC-DC converter. Industrial equipment and medical devices basically require insulation, and in some cases the identification of specific insulation level is provided. The insulation for an AC-DC converter refers to the electric non-conduction between the primary (AC input) and secondary (DC output) sides, a feature which is basically provided by the transformer. Insulation is specified in terms of an insulation structure, insulation class, and other standard-based evaluation items, as well as voltage levels such as 3kVAC. The design of a transformer requires knowledge of standards and components. For further details on this topic, documents on applicable standards should be consulted.

・Operating temperature range

Systems and devices for which a power supply unit is to be designed may be subject to specifications on operating temperature range. An AC-DC converter must be composed of control ICs and components that are capable of meeting those requirements. Also, although device specifications are mostly represented in terms of ambient temperatures, if an AD/DC converter is mounted in a case, operating temperature ranges must be established in terms of in-the-case temperatures. AC-DC converters generate considerable amounts of heat. If the temperature exceeds the ratings for the components used in the device, critical problems may be generated. Accordingly, extensive validation in regard to temperatures must be performed.

・Efficiency

Efficiency refers to the ratio of input power to output power, expressed in percentage figures. An 80% efficiency means 20% losses, which basically turn into heat. Given that nowadays efficiency improvement is an essential requirement, we need to have a good understanding of relationship between efficiency and heat.

Improving efficiency requires studying the specific conversion method, control ICs, and external parts that are employed.

・No-load input power

“No-load input power” refers to the amount of input power that is consumed when no output current is flowing, that is, the amount of self-consumed electric power in the absence of a load. Energy savings being a mandatory requirement, self-losses, which amount to a complete waste of energy, must be minimized, as exemplified by EnergyStar ratings. For reducing self-consumption, circuit configuration and the types of control ICs play critical roles.

Despite the use of the expression “to minimize” in the above paragraph, the fact of the matter is that depending on the circumstances under which a designer has to operate, there may be cases where it is difficult to obtain the information necessary to minimize power consumption. In this area, a rule-of-thumb approach could be your guide in estimating what performance characteristics are needed in the power supply unit in order to accommodate a broad range of conditions. It is important to start the design process by clearly defining what items can be modified as you go along and what parts would have to be redone from scratch if a given strategy did not pan out as planned.