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

3.3 System Test

  1. Make sure the TPA3255 device is set up and working properly. Connect a 4-Ω resistor using amplifier configuration in BTL mode (without any transformers), and sweep frequency under different power levels.
    GUID-11D23EBB-53E5-4B82-95F2-B2A25A07FFCB-low.pngFigure 3-6 THD+N vs Frequency Results Based on TPA3255 Without Transformer
  2. As Figure 3-7 shows, build the constant voltage system using one step-up and one step-down transformer (simplify the system by applying one load to start) with a 4-Ω resistor connected to 4-Ω tap of the secondary side of the 330-040. Apply a 1-kHz signal and modify the input amplitude until RMS (root mean square) of U reaches 70.7 V (peak to peak is about 200 V).
    Note:

    Since the voltage is very high, necessary protection methods must be taken.

    For the 330-040, set 10 W, 5 W, 2.5 W, 1.25 W, and 0.62 W (all 70 V) respectively, and use an Audio Precision (AP) analyzer to measure voltage and power across resistor R0. Make sure the output voltage and power are as expected.

    GUID-66BB65E1-2953-4262-94CD-13FEACEF6676-low.gif Figure 3-7 Simplified Constant Voltage System
  3. Assemble the complete constant voltage system based on Figure 3-5 using 10 step-down transformers (N = 10) and 10 × 4-Ω resistors. Set the input to 70.7 VRMS. Start by using the 0.62-W power level for each 330-040, and measure the THD+N vs frequency as shown in Figure 3-8. Multiply the power measured on one resistor by 10 to calculate the total efficient power. Do the same thing for the 1.25-, 2.5-, 5-, and 10-W settings.
    GUID-E443FF5A-003C-4379-88C8-FAD6AD01EBE6-low.pngFigure 3-8 THD+N Versus Frequency Results Based on TPA3255 With 1 Step-Up and 10 Step-Down Transformers (1, 2 as Primary Side and A, B as Secondary Side)

Table 3-7 shows the power consumed and relevant efficiency when using the 330-040 with a 10-W configuration.

Table 3-7 System Efficiency for Ten 10-W Resistor Loads (Using One Peavey® 18737 and Ten 330-040 Transformers)
Power LevelTotal Power (W)Power for Each ResistorTotal Efficient Power (W)Efficiency
10 W186.669.73797.370.522