Those That Are Limited

- Oct 09, 2017 -

As IEC60950 complies with safety standards - 1 (2001 version or 2001 version) of information technology equipment, in its inside and outside, should be through the adoption of appropriate materials and components as well as the appropriate risk structure to reduce the ignition and flame spreading, such as design to limit temperature of the components or limit the power output circuit to reduce the ignition danger; Use flame retardant material or sufficient space to reduce the flame spread and use fire protection enclosure when necessary. Standard that restricted the power of components, by limiting power but not installed on the V - grade 1 material of secondary circuit components, by the limited power supply of the secondary circuit components and parts with fire danger, need fire protective shell; The connector in the secondary circuit that is supplied by the restricted power supply and the components installed on the v-1 material will not need fire protection enclosure. In addition to connect additional equipment or accessories (e.g., scanner, mouse, keyboard, DVD drive, etc.) of equipment, in the casing cannot confirm these peripherals in advance whether meet the requirements of the fire cases, according to the standard IEC60950-1 (2005 edition), connect the peripheral data interface should be by the limited power supply. In the standard (2001 edition) IEC60950-1 restricted power is divided into inner restricted and the inherent limitation, and in the standard IEC60950-1 (2005) is clearly divided into no over-current protection device and have over-current protection device. For the limited power supply that is powered by ac power grid or powered by rechargeable batteries but also charged by ac power grid, the isolation transformer should be isolated from the ac grid.

Internal restricted power (no overcurrent protection device)
If a limited power supply in the absence of output over current protection device, using any of the following three schemes to limit the output current and power, and meet the requirements of table 1, you can determine for restricted within the power supply:
(1) to limit output internally and meet the requirements of table 1;
(2) use a linear or non-linear impedance (if a positive temperature coefficient device is used, the device should comply with IEC 60730-1's related requirements) to limit the output and meet the requirements of table 1;
(3) use a regulation network to limit the output and meet the requirements of table 1 under normal working conditions of the regulating network and under a single fault condition (open or short circuit).

Non-inherent restricted power (with a flow protection device)
If a limited the power use over-current protection device (should be a fuse or a cannot adjust the automatic reset of electromechanical device) to limit the output current and power and conforms to the requirements of table 2, then can be judged to be not restricted within the power supply. The current rating of the overcurrent protection device is set by the current rating of 2.1 times the rating of the current rating. In the standard IEC60950-1 (2001 edition) and such a scheme, restricted power, can use a regulation network in its normal working conditions limit output in line with the requirements of table 1, and in single fault condition (open circuit or short circuit), by an over-current protection device to limit the output and conform to the requirements of table 2. The scheme has been cancelled in the standard IEC60950-1 (2005 edition).

Test and determination of restricted power supply
As can be seen from table 1 and table 2, we determine that the restricted power supply needs to test three parameters: output voltage (Uoc), output current (Isc) and apparent power (S). First of all, we want to make sure that restricted the power of the input voltage, and consider according to the scope of the power rating of the nominal voltage or voltage tolerance values later, suggestion choice in each of the rated voltage and lower limit on tolerance and some typical voltage measurements, but in general, everybody take tolerance limit is quite recognition methods, such as an ac rated input is 100 V ~ 240 V switching power supply, then according to the standard IEC60950-1 tolerance requirements, in 90 V and 254.4 V respectively measuring ac input conditions, if the account of the differences of Chinese and will take 90 V and 264 V. The output voltage (Uoc) is the effective value of the sinusoidal ac output or non-ripple dc output voltage that is measured in the normal working condition. For the output ac voltage, the ripple ratio of the non-sine wave or the output dc voltage is greater than 10%, and the peak voltage cannot exceed 42.4 V. Output current (Isc) is the power of over current protection device bypass (if any), connected to the adjustable of capacitive load, in order to obtain maximum output current RMS, at the same time of adjusting load measurement output current to record different output current of the output voltage, in order to get the maximum output apparent power (S), because the output current of the largest state does not necessarily correspond to the apparent power output of a maximum state. To be sure, in the absence of over current protection device, if the power is through an electronic circuit or positive temperature coefficient device to restrict output, output current (Isc) and apparent power output (S) to be measured after applying load in 5 S, the other is to measure after 60 S. If the device can also be powered by a rechargeable battery, the battery must be filled and tested. Both linear power supply power frequency and high frequency switching power supply, due to the restrictions on the size and the impedance of the isolation transformer itself and circuit over-current protection device or the PPTC (polymer positive temperature coefficient thermistor) apply, most small and medium-sized power supply can meet the standards (2005 edition) of restricted IEC60950-1 power requirements, but if according to the standard IEC60950-1 (2001) may appear different determination results, such as the one without over-current protection device of the power adapter, rated output is 5 V / 6 A, the measured output voltage (Uoc) of 5.1 V, output current (Isc) is 6.5 A, apparent power (S) of 31.2 VA VA is greater than 25.5, but less than 100 VA), can be seen from table 1 the power adapter is belongs to this kind of situation. Now manufacturers in order to reduce the cost of market access, often with a series of specifications of the adapter apply for certification, the span is the output voltage and current, then paying special attention to various restricted within the scope of the power supply voltage test and determination.

Restricted the necessary conditions for power supply is not passed safety certification, but it is with power supply equipment fire protection requirements are closely linked, with limited power can to a certain extent, reduce the production cost and fire prevention requirements, so we should be when the selection and application in the actual test. One thousand test results in determining critical state, although you can use the method of evaluation of uncertainty of make a decision, but there is no guarantee that all products under the premise of consistency, and manufacturers of terminal products for special needs and must be equipped with such power, starting from the security and reliability, or treat it as a restricted power. Especially Ⅲ such equipment, if can with a variety of power adapter or rechargeable batteries, more need to confirm whether they are to meet the requirements of the limited power supply, as the user cannot change after use is not within the scope of the adapter and battery manufacturers or states, lest bring security hidden danger! With CB test report for CCC certificate restricted the power adapter, test engineers also want confirmation experiment was carried out, if the test report is based on the standard IEC60950-1 (2005 edition), and the criteria for CCC certification of the GB4943-2001 (equivalent to IEC60950:1999), the standard difference is likely to determine the result of different, the test results for the limited power supply situation, the author suggested that nameplate manufacturers restricted the power of the abbreviation "LPS" (if any) logo, avoid misleading.

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