SLAA898 September   2022 TAS3251 , TPA3255

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
    1. 1.1 Power Amplifiers
    2. 1.2 Discrete Power Amplifier Implementation
    3. 1.3 Class-D Amplifier Implementation
    4. 1.4 Advantage of a Class-D Implementation
  4. 2Background
    1. 2.1 Why Use Constant Voltage Audio Systems
    2. 2.2 Basic Principle of Constant Voltage Systems
    3. 2.3 Power Loss in Transformer
    4. 2.4 Auto-Transformer
  5. 3System Test (Based on TPA3255)
    1. 3.1 Transformer Characteristics
      1. 3.1.1 Turns Ratio and Resistance Match
      2. 3.1.2 DCR of the Transformer
    2. 3.2 System Build-Up
    3. 3.3 System Test
  6. 4Efficiency Analysis and Optimization
    1. 4.1 Efficiency of Three Parts
      1. 4.1.1 Efficiency for TPA3255
      2. 4.1.2 Efficiency for Step-Up Transformer
      3. 4.1.3 Efficiency for Step-Down Transformer 330-040
    2. 4.2 Improvements on System Efficiency
      1. 4.2.1 Improve Resistance Matching
      2. 4.2.2 Apply a Transformer With Less Power Loss
  7. 5Considerations on Building a Constant Voltage System
    1. 5.1 Transformer Saturation
    2. 5.2 Low DCR
    3. 5.3 Resistance Matching

5.2 Low DCR

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.

GUID-E2E3CC23-F06B-4F6C-A979-29ED8D00CCFF-low.gifFigure 5-1 Equivalent System Circuit Including DCR for Step-Up and Step-Down Transformers

If the DCR of the step-up transformer is also taken into account, there is:

Equation 28. R'=R"+r2_P=(U"U0')2R0+r2_s_+r2_p
Equation 29. R=r1_s+R'N
Equation 30. U'=R'NR'N+r1_sU
Equation 31. RL'=(UL'U)2R
Equation 32. RL=r1_p+RL'

so

Equation 33. R=r1_s+(U"U0')2(R0+r2_s)+r2_pN
Equation 34. R'=R"+r2_p=(U"U0')2(R0+r2_s)+r2_p
Equation 35. RL'=(UL'U)2(r1_s+(U"U0')2(R0+r2_s)+r2_pN)
Equation 36. RL=r1_p+(UL'U)2R=r1_p+(UL'U)2(r1_s+(U"U')2(R0+r2_pN)

For the step up transformer:

Equation 37. ηup=RL'(RL'+r1+p)R'(R'+r1_s)

If iron loss inside transformer is neglected, the efficiency for whole system is:

Equation 38. η=ηampηupηdown=ηampRL'(RL'+r1_p)R'(R'+r1_s)R"(R"+r2_p)R0(R0+r2_s)

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:

Equation 39. RL'=112.49(0.42+124.97×(4+0.54)+27.710)=4.8 Ω
Equation 40. R'=124.97×(4+0.54)+27.7=595.06
Equation 41. ηup=4.8(4.8+0.143)595.06(595.06+0.42)=0.97

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).