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

2.2 Basic Principle of Constant Voltage Systems

Figure 2-1 shows a typical constant-voltage system. A step-up transformer is connected to the output of the amplifier and converts the output from voltage UL to a higher voltage U (usually is 70 VRMS, 100 VRMS, or 140 VRMS). RL and R are equivalent loads of the primary and secondary side of the step-up transformer, respectively. Equation 1 shows the relationship of the voltage and resistances on each side of the transformer.

Equation 1. RLR=ULU2

On the speaker side, the high voltage is converted to a lower voltage to match the impedance of the speaker. R' and R are equivalent loads of the primary and secondary side of the step-down transformer, respectively. So,

Equation 2. P0=U02R0
Equation 3. R'R0=(U'U0)2=U'2P0R0
GUID-97124FC4-D61D-41D2-A921-D7B53CD63DCF-low.gifFigure 2-1 Typical Constant Voltage System
Equation 4. U'=U
Equation 5. R=R'N

So for the whole system, if N number of the same speaker are used is:

Equation 6. RL= R(ULU)2=R'N(ULU)2=1N(ULU)2(U'U0)2R0

UL/U and U'/U0 are used to calculate the turns ratio of the step-up and step-down transformer, respectively.

To make sure the amplifier works at certain loads, resistance must match well with the transformer. For instance, for an amplifier that can deliver 100 W with 4-Ω load, and the power goes to 10 4-Ω speakers. Then RL = 4 Ω, R0 = 4 Ω, N = 10, a turns ratio of step-up and step-down transformers should be chosen precisely according to Equation 6.