Restricted Power Sources for Information Technology Equipment

Preface

As information technology equipment that complies with the safety standard IEC60950-1 (2001 or 2005 edition), the ignition hazard and flame spread should be reduced by using appropriate materials, components and structures inside and outside. For example, circuits that can limit component temperature or power output should be designed to reduce the ignition hazard; flame retardant materials or sufficient space should be used to reduce the spread of flames, and fireproof enclosures should be used when necessary. The standard considers that components within restricted power supplies, components on secondary circuits powered by restricted power supplies but not installed on V-1 grade materials, and components on secondary circuits powered by non-restricted power supplies are fire hazards and require fireproof enclosures; however, connectors in secondary circuits powered by restricted power supplies and components installed on V-1 grade materials do not require fireproof enclosures. In addition, for devices that can be connected to additional devices or accessories (such as scanners, mice, keyboards, DVD drives, etc.), if it is impossible to confirm in advance whether the shells of these peripherals meet the fire protection requirements, according to the standard IEC60950-1 (2005 edition), the data interface connecting these peripherals should be powered by a restricted power supply. In the standard IEC60950-1 (2001 edition), restricted power supplies are divided into two categories: intrinsically restricted and non-intrinsically restricted, while in the standard IEC60950-1 (2005 edition), they are clearly divided into two categories: no overcurrent protection device and overcurrent protection device. For restricted power supplies powered by the AC power grid or powered by rechargeable batteries but charged by the AC power grid at the same time, they should be isolated from the AC power grid by an isolation transformer.

Intrinsically limited power supply (no overcurrent protection)

If a limited power supply uses any of the following three schemes to limit output current and power without an output overcurrent protection device and meets the requirements of Table 1, it can be determined as an inherently limited power supply:

(1) Intrinsically limit output and comply with the requirements of Table 1;

(2) Use a linear or nonlinear impedance (if a positive temperature coefficient device is used, the device shall comply with the relevant requirements of IEC 60730-1) to limit the output and meet the requirements of Table 1;

(3) Use a regulation network that limits the output and meets the requirements of Table 1 under normal operating conditions of the regulation network and single fault conditions (open circuit or short circuit).

Not inherently limited power supply (with overcurrent protection device)

If a limited power supply uses an overcurrent protection device (which should be a fuse or an unadjustable non-automatic reset electromechanical device) to limit the output current and power and meet the requirements of Table 2, it can be judged as a non-inherently limited power supply. The current rating of the overcurrent protection device is set according to the 2.1 times rated current that it cuts off within 120S. There is also such a scheme in the standard IEC60950-1 (2001 edition) that the limited power supply can use a regulating network to limit the output under its normal operating conditions to meet the requirements of Table 1, and under single fault conditions (open circuit or short circuit), the output is limited by an overcurrent protection device and meets the requirements of Table 2. This scheme has been cancelled in the standard IEC60950-1 (2005 edition).

Testing and determination of limited power supply

As can be seen from Table 1 and Table 2, we need to test three parameters to determine whether it is a limited power supply: output voltage (Uoc), output current (Isc) and apparent power (S). First, we need to determine the input voltage of the limited power supply. According to the rated voltage or range of the power supply and considering the voltage tolerance, we can then take the value. It is recommended to measure at each rated voltage and tolerance upper and lower limits and some typical voltage conditions. However, in general, taking the tolerance upper and lower limits is a method that everyone recognizes. For example, a switching power supply with an AC rated input of 100V~240V, then according to the tolerance requirements of standard IEC60950-1, it is measured at 90V and 254.4V AC input respectively. If the difference in China is taken into account, 90V and 264V are taken. The output voltage (Uoc) is the effective value of the sinusoidal AC output or ripple-free DC output voltage measured under normal working conditions after disconnecting all load circuits of the power supply. For power supplies with non-sinusoidal output AC voltage or output DC voltage ripple greater than 10%, the peak voltage cannot exceed 42.4V. The output current (Isc) is measured by bypassing the overcurrent protection device of the power supply (if any) and connecting an adjustable non-capacitive load to obtain the maximum output current effective value. When adjusting the load to measure the output current, the output voltage corresponding to different output currents should be recorded in order to obtain the maximum output apparent power (S), because the state of maximum output current does not necessarily correspond to the state of maximum output apparent power. It should be noted that when there is no overcurrent protection device, if the power supply is limited by an electronic circuit or a positive temperature coefficient device to limit the output, the output current (Isc) and output apparent power (S) should be measured 5s after the load is applied, and in other cases, after 60s. If the device can also be powered by a rechargeable battery, the battery must be fully charged before testing. Whether it is a power frequency linear power supply or a high frequency switching power supply, due to the size and impedance limitations of the isolation transformer itself and the use of overcurrent protection devices or PPTC (polymer positive temperature coefficient thermistor) in the circuit, most small and medium power supplies can meet the limited power requirements of the standard IEC60950-1 (2005 version), but if it is based on the standard IEC60950-1 (2001 version), different judgment results may appear. For example, a power adapter without an overcurrent protection device has a rated output of 5V/6A, a measured output voltage (Uoc) of 5.1V, an output current (Isc) of 6.5A, and an apparent power (S) of 31.2VA (greater than 25.5VA, but less than 100VA). It can be seen from Table 1 that this power adapter belongs to this situation. Now, in order to reduce the cost of market access, manufacturers often apply for certification with a series of adapters with a wide range of output voltage and current. At this time, special attention should be paid to the testing and judgment of limited power in each voltage range.

Conclusion

Restricted power supply is not a necessary condition for passing safety certification, but it is closely related to the fire protection requirements of the powered equipment. The use of restricted power supply can reduce production costs and fire protection requirements to a certain extent, so we must test it when actually selecting and applying it. In case the test results are in a critical state, although the uncertainty assessment method can be used to make a judgment, under the premise that the consistency of all products cannot be guaranteed, if the terminal product manufacturer must select such a power supply for special needs, it is still treated as a non-restricted power supply from the perspective of safety and reliability. Especially for Class III equipment, if it can be equipped with a variety of power adapters or rechargeable batteries, it is even more necessary to confirm whether they all meet the requirements of restricted power supply. As a user, you cannot replace and use adapters and batteries that are not provided or declared by the manufacturer without authorization, so as to avoid safety hazards! For restricted power adapters that obtain CCC certificates with CB test reports, test engineers must also conduct confirmation tests. If the standard based on the test report is IEC60950-1 (2005 edition), and the standard based on the current CCC certification is GB4943-2001 (equivalent to IEC60950:1999), the difference in standards may lead to different judgment results. In the case of non-restricted power supply, the author recommends that manufacturers remove the "LPS" (if any) logo on the nameplate for restricted power supply to avoid misleading.