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Hi-pot Type Testing Powe Supplies Application Note |
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| By ddarrow |
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Hi-pot Type Testing Powe Supplies Application Note |
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Hi-pot Type Testing Application Note
The complete article including figures and charts can be found at this link:
http://www.xppower.com/pdfs/High-Pot_Testing.pdf
Foreward
This Application Note was written to provide insight into the one time requirement for ‘type’ testing of power supplies by the safety agency or manufacturer. It will assist in the interpretation of requirements for this testing and provide broad guidelines on exactly what is required of the manufacturer in this test procedure.
This application note is written primarily with regards to Class I power supplies that have a safety earth ground. At the end of the document there is a section that outlines the differences for Class II power supplies.
Insulation Types
Figure 1 below represents a typical power supply from the standpoint of insulation systems.
• Between Primary and Secondary, reinforced insulation is required
• Between Primary and Chassis Ground, basic insulation is required
• Between Secondary and Chassis Ground, operational insulation is required
R – Location of typical Reinforced Insulation
B – Location of typical Basic Insulation
O – Location of typical Operational Insulation
Figure 1 - Typical Power Supply Insulation
Safety Agency Testing
Testing requirements are categorized into two groups:
• Type Testing (Design Verification)
• Production Testing
Type Tests
These are the tests which are done by the safety agencies, and are intended to prove that the construction of the power supply meets the requirements dictated by the relevant safety standard.
For IEC/UL60950-1 (ITE) and UL60601-1 (Medical) the requirements are as follows:
60950-1 Primary to Secondary - 3000VAC, or the equivalent DC voltage
Primary to Grounded chassis - 1500VAC, or the equivalent DC voltage
Secondary to Grounded chassis - No requirement provided the secondary voltage is less than 42.4VAC or 60Vdc
60601-1 Primary to Secondary - 4000VAC, or the equivalent DC voltage
Primary to Grounded chassis - 1500VAC, or the equivalent DC voltage
Secondary to Grounded chassis - No requirement provided the secondary voltage is less than 42.4VAC or 60Vdc
Figure 2 - Insulation Type Testing Levels
Note: Information for IEC/UL60950-1 is based on 250Vrms as specified in 2nd Edition, Table 5B. Information for IEC/UL60601-1 is from 2nd Edition Table V and is based on a 250Vrms working voltage.
Production Testing
These tests are done as part of the manufacturing process and are intended to ensure integrity of safety critical insulation. Production line testing will be conducted on Basic insulation and on Reinforced insulation during manufacture.
Reinforced (primary to secondary) insulation cannot be tested without over-stressing basic insulation on the end product. (Note: see UL60950-1, C5.2.2 or UL60601-1 2nd Edition Section 20.4 or IEC60601-1 3rd Edition Section 8.8.3 for more information.) Because of this the agencies allow manufacturers to test reinforced insulation separately. This means they are allowed to test transformers and other primary to secondary isolation barriers separately before components are incorporated into the product.
Figure 3 - UL60601-1 2nd Edition Section 20.4
Only primary to chassis ground (basic insulation) testing is required in production on the final assembly prior to shipping each product.
Type Test (Design Verification Test)
Often times a customer or safety agency engineer wants to verify the type tests on a finished product in the field. This can be done, however special precautions must be taken to ensure a correct result is achieved.
If basic (primary to chassis ground) insulation is to be verified then no special considerations need to be taken. Simply apply a 1500VAC voltage from Primary AC input to chassis ground.
However, if a primary to secondary insulation is to be verified special precautions need to be taken.
Because only basic insulation exists between primary and chassis ground and only operational insulation exists between secondary and chassis ground, any attempt to apply 3000VAC directly from primary to secondary on the finished product will over stress the primary to chassis ground and secondary to chassis insulation resulting in a failure.
To properly test reinforced insulation the power supply needs to be removed from the chassis. In addition, all paths to chassis ground, as far as practical, need to be removed so as not to over-stress basic and operational insulation during the test. This usually entails removal of all Y-capacitors and gas discharge tubes and will need interaction with XP Power Engineering to ensure the correct disassembly. (See Figure 4)
Figure 4 - Power Supply disassembly may be required for type testing
Figure 5 - UL60601-1 2nd Edition Section 20.4 Rationale
Unfortunately on some products not all paths can be removed. The reason for this is that some printed circuit boards have ground barriers or grounded traces laying between primary and secondary circuitry. Although ‘creepage and clearance’ requirements are met as specified in the safety standards, these traces, due to proximity, make up small capacitive dividers between secondary traces, floating ground traces and live primary traces. In some instances when applying the primary to secondary hi-pot voltage, some arcing will be observed on the printed circuit boards. In all cases this arcing has been limited to the secondary to grounded trace locations. In some instances this arcing can cause component failure, rendering the power supply inoperable.
This is a breakdown of operational insulation (secondary to chassis ground) only. It does not indicate a failure of primary to secondary insulation that is the focus of the test. Provided this ‘fails’ in a safe manner, the test is considered successful for safety purposes.
Differences for Class II Power Supplies
The previous sections of this document all dealt with power supplies that were of Class I and so had a safety earth ground. Because of this they required hi-pot testing from primary to ground. In the case of Class II power supplies there is no safety ground and so there is no need, nor ability, to test from primary to chassis. Because of the lack of any grounding we also do not have to worry about over stressing any components from the primary to ground side or from the output to ground side.
The user is able to simply test from the input to the output on the power supply at 3000VAC (or 4121Vdc) for ITE devices or 4000VAC (5656Vdc) for medical devices to verify the insulation in the supply.
Care must be made when doing this test that there is not a “functional earth” used for EMI filtering for the supply. If there is a functional earth present the user must make sure not to connect up the ground to the test or they will over stress the components tied to ground and damage the supply.
Conclusion
Type testing may result in component failure. It is extremely important to isolate the power supply components that are involved in the testing and measure the results on those areas only, taking into account the letter and intent of the Safety Agency requirements and specifications. Over stressing components during these tests cannot always be avoided and will usually cause the failure of components in neighboring parts of the circuit that are not intended to be tested with a given procedure.
Even standard hi-pot testing will eventually start to degrade the insulation within a system. Care should be made to not repeatedly test a system as that will potentially cause excessive insulation break down.
For further assistance please contact XP Power via our website at http://www.xppower.com
Additional technical articles are available at: http://www.xppower.com/page.php?pagename=TechnicalArticles |
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