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Daniel engineer tells you: Switching power supply 'Y capacitor' is calculated like this!

by:Shenmao     2021-06-13
The basic principle diagram of the switching power supply 1. Primary Circuit: directly connected to the external grid power supply. 2. Secondary Circuit: the part of the circuit that is located in the device and isolated from the primary side. 3. Y-capacitor (Y-Capacitor): A high-voltage capacitor connected across the primary circuit and the ground or between the primary and secondary circuits. Switching power supply grounding, leakage current, withstand voltage test (safety) 1. Ground Continuity Test (Ground Continuity Test) : A. Definition: Pass the current from the Inlet PG terminal to the user's accessible grounding terminal to ensure that its resistance is less than the specified value to achieve the function of grounding protection. B. Standard: 1. The input current is not more than 25A, (DC or AC) The voltage should not exceed 12V, and the time should be at least 3 seconds (TUV requirement). 2. Test result: The resistance value should not be greater than 100 mΩ. 2. Earth Leakage Current Test: A. Definition: Pass a The 'human body impedance simulation circuit' approved by regulatory units (UL, TUV, CSA...) measures the amount of current flowing through the human body between the touchable metal part and the ground when the object under test (SPS) is turned on. B Standard: 1. The input voltage is 106% of the upper limit of the rated voltage. 2. Test results: Class I≦3.5mA; Class II≦0.25mA. 3. Dielectric Withstand Voltage Test: A. Definition: again It is called high-voltage dielectric test, namely Hi-pot (High Potential) Test, which implements high voltage from the primary side to the secondary side (or the primary side to ground) to determine that the internal insulating layer has the function of isolating dangerous voltages. B. Standard: 1. The input voltage is as follows: 2. Test result: No insulation breakdown phenomenon (Breakdown). The difference between AC and DC withstand voltage test Leakage current calculation method for withstand voltage test 1. Leakage current for DC test Setting: The DC test current is very small (μA). Generally, high DC voltage is implemented between the primary side and the secondary side. The leakage current setting is 0μA~100μA. 2. The theoretical calculation of the leakage current of the AC test: calculation formula: I u003d 2π*f*V*Cy Where: f—Test voltage frequency (50Hz or 60Hz) V—Test voltage (unit: volt)Cy—The sum of Y capacitance across the primary side and ground or between the primary and secondary sides. So: Iminu003d 2π*f*V*CyminImaxu003d 2π*f*V*CymaxCy capacitance calculation:: Cy u003dCy1Cy2Cy3... If a Y capacitor (Cy0) is connected between the primary side ground and the secondary side ground, then: The tolerance of Y capacitance is generally: /-20% OR /-10% 3. Actual setting of AC leakage current test needs to be considered Consider the following factors: 1. Consider the initial leakage current: the initial leakage current is the measured leakage current when there is no DUT. 2. Consider the Y capacitor tolerance: When the power engineer chooses the Y capacitor with the same capacity, There are often several models, but their tolerances are different (some are /-10%; some are /-20%), which brings trouble to the actual leakage current setting, so we should set it according to the /-20% tolerance. Otherwise, we must follow /-10% tolerance to set. 3. Consider the scattered capacitance existing in the actual circuit, so the leakage current range setting: (lower limit rounded: upper limit rounded)
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