This paper starts with the basic definition of safety distance, analyzes the query method of creepage distance and clearance in IEC60950 and GB4943-2011 standards and describes the working voltage test specification. Finally, it analyzes and calculates the measured voltage waveform. From theoretical analysis to case analysis, you can easily understand the safety spacing of switching power supplies in one step.
basic conceptIn the IEC60950 and GB4943-2011 standards, the minimum safety distance required for different voltage levels is specified, and the safety distance includes both electrical spacing and creepage distance. There are two aspects to the place where the switching power supply mainly needs to ensure the minimum safe distance:
1. The safety distance of the primary side circuit to the outer casing (protected ground);
2. The safety distance between the primary side circuit and the secondary side circuit.
clearanceThe clearance is the shortest distance between two conductors in air, and the minimum electrical insulation gap is determined primarily by Tables 2J, 2K and 2L. The specific method of table lookup is as follows:
1. Confirm the AC grid power supply transient voltage according to the AC grid voltage RMS and overvoltage category (determined by Appendix Z and Table 2J);
Table 2J AC grid power supply transient voltage
2. First determine the pollution level, and then determine the minimum clearance to C1 based on the measured two-point peak operating voltage B and the above-mentioned confirmed AC grid power transient voltage value (determined by Table 2K);
Table 2K Primary circuit insulation and minimum electrical clearance between primary circuit and secondary circuit (above 2000m)
The unit is mm
3. After determining the pollution level, confirm the additional clearance C2 according to the measured two-point peak operating voltage B and the grid power transient voltage (determined by Table 2L);
Table 2L Additional clearance for primary circuit (applicable to altitude below 2000m)
The unit is mm
4. If B is greater than the peak value of the AC grid, the minimum clearance is C1+C2. If B is less than or equal to the peak value of the AC grid, the minimum clearance is equal to C1.
Creepage distanceThe creepage distance is the shortest distance between two conductors along the surface of the insulating material, and the minimum creepage distance is determined only by Table 2N; the specific lookup method is as follows:
1. Determine the pollution level;
2. Determine the minimum creepage distance based on the measured rms value of the working voltage and the material group of the insulating material (determined by Table 2N).
Table 2N Minimum creepage distance
The unit is mm
If the switching power supply input has L, N and PE, the test chart is shown in Figure 1 below:
figure 1
If the switching power supply input is only L, N, the test chart is as shown in Figure 2 below:
figure 2
Case AnalysisThe measured maximum voltage waveform of the primary and secondary of the 100~240VAC input switching power supply is shown in Figure 3. The peak value of the working voltage is 500V, and the effective value of the working voltage is 265V. According to Table 2J, the transient voltage of the AC power supply is 2500V.
image 3
Clearance calculation: according to the basic insulation requirements of pollution degree 2
That is, the basic insulation of the minimum clearance is C1+C2=2.34mm (since the peak voltage is 500V is greater than the input voltage peak), and the reinforced insulation is 4.68mm;
Creepage distance calculation: According to pollution level 2, material category III, the RMS voltage of 265V is between 250V and 320V in Table 2N, and the basic insulation is 2.5mm at 250V, and the basic insulation of 320V is 3.2mm, which is 265V basic insulation. for 
That is, the reinforced insulation is 5.3mm
Altitude affects safety distanceThe safety spacing of the above query is only applicable to altitudes below 2000m, while for altitudes above 2000, the safety distance needs to be multiplied by a certain factor. Table A2 of the standard GB/T 16935.1 gives the correspondence between altitude and coefficient. For example, when the altitude is 5000mm, the minimum safety spacing needs to be multiplied by 1.48 times.
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