SLAA898 September 2022 TAS3251 , TPA3255
DCR is an important parameter to measure the copper loss in a transformer. For a typical constant voltage system shown in Figure 3-5, Figure 5-1 is the redrawn figure taking DCR into the consideration. The efficiency of the step-down transformer ηdown from Figure 4-5 is already known.
If the DCR of the step-up transformer is also taken into account, there is:
so
For the step up transformer:
If iron loss inside transformer is neglected, the efficiency for whole system is:
For the first experiment (18737, 1,2 as primary and A,B as secondary), r1_p = 0.143 Ω, r1_s = 0.42 Ω, r2_p = 27.7 Ω, r2_s = 0.54 Ω, and according the previous equations:
For the second experiment (18737, 1,3 as primary, and A,B as secondary), r1_p = 0.154 Ω and r1_s = 0.42 Ω, so ηup can be 0.98. It is a similar case for the third experiment with the EA300. The ηup can be 0.99 if only taking DCR into consideration. In the three experiments discussed, for both step-up transformers, the DCR is much smaller compared to the load in both sides, and copper loss can almost be neglected. Therefore, the main power loss lies in the iron loss, due to self-inductance inside the iron core of step-up transformer (generally, for a transformer with larger power, the current is higher, which requires smaller DCR to reduce copper power loss as much as possible).