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

3.2 System Build-Up

To use a 70-V constant voltage system to deliver power to different smaller speakers, the voltage must be converted to 70.7 VRMS first through one step-up transformer, and then down to the proper voltage by the Peavey step-down transformers (see Figure 3-5; suppose N speakers are used, so N step-down transformers). The TPA3255 (along with a LC filter) is used as the amplifier (51 V, BTL mode, same condition in all of the following experiments). A Peavey step-up transformer (abbreviated as 18737) and a step-down transformer (abbreviated as 330-040) were used. In the first experiment, the configuration for the 18737 is 1,2 as primary and A,B as secondary, while the configuration for the 330-040 can vary between 10 W, 5 W, 2.5 W, 1.25 W, and 0.62 W in the primary side (secondary is fixed with a 4 Ω load).

GUID-D7D57F15-9551-4839-A613-5B410CD92CA2-low.gif Figure 3-5 Full System for Constant Voltage System Based on TPA3255
Equation 11. R L R = ( U L   U ) 2
Equation 12. U ' = U
Equation 13. P 0 = U 0 2 R 0
Equation 14. R ' R 0 = ( U ' U 0 ) 2 = U 2 P 0 R 0
Equation 15. R = R ' N
Equation 16. R L = R U L 2 U 2 = R ' N U L 2 U 2 = 1 N U L 2 U 2 U ' 2 U 0 2 R 0

To best match the resistance, RL should be more than 4 Ω. If the 18737 configuration is fixed as 20 V to 70 V and 330-040 is fixed as 10 W, 4 Ω:

Equation 17. N ≤ U L 2 U 2 U ' 2 U 0 2 R 0 R L = 1 12 . 496225 × 124 . 97 × 4   Ω 4   Ω ≈ 10

Note that RL is not always 4 Ω; it gets higher if using a lower power level for the 330-040. So N = 10, and ten 4-Ω resistors are used to simulate 4-Ω speakers.