How are derating factors determined for multi-rail power supplies with subgroup limits?

Reference: Generalized Test Protocol for Calculating the Energy Efficiency of Internal Ac-Dc and Dc-Dc Power Supplies, section 6.1.1 Proportional allocation method for loading multiple and single-output ac- dc and dc-dc power supplies.

For this specific power supply, we have a sub-group current limited factor in the railing system

Figure 11: Multi Rail with Sub-Group Current Limits Power Supply Label

Current Ratings:

+12V1A: 18 A & +12V2A: 18A with a Max Total Current rating 18 A
+12VB: 18 A
+12VC: 13 A

Table 4: Output Variable Label with Sub-Group Current Limits and Multi-Rail Output Voltage Bus

Output Variable Label with Sub-Group Current Limits and Multi-Rail Output Voltage Bus

Step 1: Calculate derating factors for voltage rail or sub-groups rails as shown in below

Derating for 12V1A and 12V2A has a max current limitation of 18A, so you would use the following equation and for this example: Derating1 for both 12V1A & 12V2A = 0.5

step 1 equation 1

12VB has a max current of 18A, an equation you would use for this. Derating1 = 1

step1 eq2

12VC has a max current of 13A, an equation you would use for this. Derating1 = 1

step 1 equation 3

If the derating factor DS ≥ 1, then it is clear that when the subgroup is loaded to the rated dc output currents, the subgroup rated output powers will not be exceeded and there is no need for derating. However, if one or more DS factors are less than 1 then the subgroup power will be exceeded if the outputs are loaded to their full output currents and there is a need for derating

Step 2: There is also a need to check whether the sum of the subgroup maximum rated powers is greater than the total maximum power rating of the power supply (PT). If the sum of the subgroup maximum rated powers is greater than the overall power rating of the power supply then a second derating factor DT must be applied. This factor is calculated as shown below: If DT ≥ 1 then no derating is needed. If DT < 1 then the derating for each of the outputs has to be applied and is shown below.

step 2 equation 1

The total derating factor, Dt = 0.6122

12VA1 rail: 18A x 0.5 (Derating1) x 0.6122 (Dt) x 100% (load)= 5.510 A
12VA2 rail: 18A x 0.5 (Derating1) x 0.6122 (Dt) x 100% (load)= 5.510 A
12VB rail: 18A x 1 (Derating1) x 0.6122 (Dt) x 100% (load)= 11.020 A
12VC rail: 13A x 1 (Derating1) x 0.6122 (Dt) x 100% (load)= 7.959 A

Here is a screenshot of what this specific loading would be at each loading point.

Figure 12: Loading Guidelines for Multi-Rail System with Sub Group Current Limits