Power quality events are a common occurrence in industrial facilities and standard “industrial” power supplies typically provide a level of immunity to these events. The most common events are voltage sags (nearly 50%) followed by voltage transients and short duration overvoltage events or “swells” (about 10% each). Other, less common events include sustained over- or under-voltage, harmonics and frequency variations. Advanced switching power supplies are designed to eliminate or reduce to an acceptable degree the effect these conditions have on the electronic systems they supply.
Some manufacturing processes are particularly susceptible to voltage sags. The ability of a power supply to perform properly under defined voltage sag conditions may be characterized in a number of industry-accepted ways that vary in degrees of rigor or stringency of requirements, including ITIC (CBEMA), IEC-61000-4-11 and SEMI F47.
Comparing the standards
As early as 1970, an organization known as the Computer and Business Equipment Manufacturers Association (CBEMA) developed a benchmark for power supply operation under voltage sag conditions. In the late 1990s, this standard was refined as what is known today as the Information Technology Industry Council (ITIC) curve. The International Electrotechnical Commission (IEC) standard is a further refinement of the ITIC curve, as shown in Figure 1.
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Figure 1: Comparing Voltage Standards for Power Supply Voltage Sag Immunity
Developed by Semiconductor Equipment Materials International, SEMI F47 specifies the required voltage sag tolerance for semiconductor processing equipment. Intending to strike a balance between voltage sag immunity and increased equipment cost, SEMI F47 is important because semiconductor plants require high levels of power quality due to the sensitivity of equipment and process controls which are especially susceptible to voltage sags.
A closer look at SEMI F47
The SEMI F47-0706 defines minimum voltage sag immunity requirements for equipment to be used in the semiconductor industry. These immunity requirements state that the equipment must operate without interruption during voltage sags that are specified by sag depth (in percent of nominal voltage remaining during the sag) and voltage sag duration (in cycles or seconds).
Semiconductor process equipment manufactures can use this standard to ensure that the voltage sag immunity requirements that they adhere to are met by their subsystem and component suppliers.
SEMI F47-0706 does not address safety issues, nor does include requirements or recommendations for overvoltage conditions (voltage swells), high frequency impulse events, and other power disturbances. This specification also does not pre-empt or override international, national or local codes.
SEMI F47-0706 power supply test summary
In order for a power supply to meet the SEMI F47-0706 standard, the supply must provide the required output voltage and current without interruption when subjected to the voltage sag levels below nominal and durations listed in Table 1 below, or in other words, continuously operate without interruption during the conditions above the bold line in Figure 1.
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Table 1: Required Voltage Sag Immunity
(The power supply should be tested for voltage sag immunity according to the methods set forth in IEC 6100-4-11 for equipment rated at 16 A per phase or less).
In addition to the above requirements, the supply is recommended, but not required, to be immune to the voltage sags below nominal detailed in Table 2 below (again the supply should continuously operate without interruption during the conditions above the bold line in Figure 2). In the absence of alternative instructions or requirements, the Pass/Fail criteria for semiconductor processing equipment (end application) should be no interrupts (a failure or an unplanned stoppage that requires external intervention).
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Figure 2: Recommended Voltage Sag Immunity
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Table 2: Recommended Voltage Sag Immunity
The requirements of the subsystems and components can then be specified to meet one of the following criteria:
a. Performs at full rated operation
b. May not perform at full rated operation but recovers operation without operator and/or host controller intervention. Must not send error signals to the equipment host controller indicating when full rated operation is not achieved.
c. May not perform at full rated operation but recovers operation without operator and/or host controller intervention. May send signals to the equipment host controller indicating when full rated operation is not achieved.
The above voltage sag tests should be carried out under load that is representative of the load required by the end application. It is not necessary to test multiple model types if reasonable judgement indicates similar performances under voltage sag conditions.
Excelsys power application note
Excelsys Technologies has published Application Note AN1401, which covers the information in this article. It also provides information about the specific Excelsys power supplies that have been designed to comply with the recommended voltage sag immunity criteria as depicted in Figure 2.
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