TIDUEW8 August   2022

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
    1. 1.1 Key System Specifications
  7. 2System Overview
    1. 2.1 Schematic Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 THS3491 Current Feedback Amplifier Specifications
    3. 2.3 System Design Theory
      1. 2.3.1 Theory of Operation
        1. 2.3.1.1 Concept of Power Supply Range Extension
      2. 2.3.2 Stability Considerations
        1. 2.3.2.1 Inclusion of Series Isolation Resistance (RS)
      3. 2.3.3 Power Dissipation
        1. 2.3.3.1 DC Internal Power Dissipation of Driver Amplifier for a Purely Resistive Output Load
        2. 2.3.3.2 AC Average Internal Power Dissipation of Driver Amplifier for a Purely Resistive Output Load
        3. 2.3.3.3 Internal Average Power Dissipation of Driver Amplifier for RC Output Load
      4. 2.3.4 Thermal Performance
        1. 2.3.4.1 Linear Safe Operating Area (SOA)
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Required Hardware
    2. 3.2 Test Setup
    3. 3.3 Test Results
  9. 4Design Files
    1. 4.1 Schematics
    2. 4.2 Bill of Materials
    3. 4.3 PCB Layout Recommendations
      1. 4.3.1 Layout Prints
    4. 4.4 Altium Project
    5. 4.5 Gerber Files
    6. 4.6 Assembly Drawings
  10. 5Related Documentation
    1. 5.1 Trademarks

2.3.4 Thermal Performance

The device package and the PCB material are responsible for thermal performance and conduction of heat out of the device die. The internal power dissipation of the device increases the internal die junction temperature, and the topic is extensively discussed in Section 2.3.3.

For an RGT package with an exposed thermal pad, the heat sink at the PCB bottom plane allows the least resistive path for heat transfer, and the majority of thermal energy is dissipated through the thermal pad to the heat sink. Because of the higher thermal handling capability, the inclusion of a heat sink at the bottom of the device allows for higher internal device power dissipation and subsequently allows for driving higher output current from the device.

For a heat sink design, Figure 2-12 shows the various sources of thermal resistances. Because power dissipation causes a rise in junction temperature that is similar to the voltage drop across a resistor resulting from current flow, a simplified thermal model (see Figure 2-13) can be developed that is analogous to an electrical circuit. The temperature, power dissipation, and thermal resistance are represented as voltage, current, and resistors, respectively. These parameters, as described in Equation 25, allow the maximum internal power dissipation to be solved for by using a simple KCL equation.

GUID-8C6BF6C5-CA58-4334-AC2D-CB2A1A82ACEE-low.gifFigure 2-12 TIDA-060033 PCB Cross Section of Different Thermal Resistances
Figure 2-13 Simplified Thermal Model for Heat Sink Design
Equation 25. GUID-EDDD6FC2-901B-4BA9-9F7B-D5166547714D-low.gif

where:

  • PD(MAX) (W) = Maximum internal power dissipation of the amplifier
  • TJ(MAX) (°C) = Absolute maximum junction temperature
  • TA (°C) = Free-air operating ambient temperature
  • θJC(BOT) (°C/W) = Bottom junction-to-case thermal resistance of the device package from the data sheet
  • θC-VIA (°C/W) = Case to via thermal resistance
  • θTMC (°C/W) = Thermal mold compound resistance between the PCB and heat sink
  • θHA (°C/W) = Heat sink to ambient thermal resistance

To calculate the maximum power dissipation allowed for a heat-sink design, each of the individual thermal resistance parameters in Equation 25 must be known. For the TIDA-060033 EVM, an estimate can be made of the thermal resistances that should hold true for most practical applications:

  1. The bottom junction-to-case thermal resistance (θJC(BOT)) is usually provided in the device data sheet. For the THS3491RGT package, this value is 7.8°C/W.
  2. The PCB thermal resistance is the thermal resistance of the flooded vias (θC-VIA), and is estimated from the via pad diameter, the via height, and PCB material of the via. The EVM has two ounces of copper on each layer with a via diameter of 7.87 mil and a PCB thickness of 62.2 mil that results in a single via thermal resistance of 180°C/W. This thermal resistance estimation is based on the Saturn PCB toolkit for an FR-4 dielectric. The PCB is flooded with 80 vias in and around the bottom of the device, which results in an effective θC-VIA of approximately 3°C/W to 3.5°C/W. For a more accurate PCB thermal resistance estimation, finite element software tools can be used from vendors such as ANSYS or Keysight for thermal modeling of the PCB.
  3. The thermal resistance of the heat-sink adhesive or the thermal-mold compound (θTMC) is approximately 0.3°C/W to 0.5°C/W for a maximum heat transfer from the PCB to the heat sink.
  4. BDN14-3CB/A01 is the heat sink selected for this design. The BDN14-3CB/A01 has a natural convection free-air standing thermal resistance of θHA at approximately 16.2°C/W.

Plugging in these individual estimated thermal resistances into Equation 25 results in a combined thermal resistance of 27.5°C/W. For a maximum TJ of 150°C and a free-air operating ambient temperature (TA) of 25°C, the resulting maximum internal power dissipation allowed is 4.54 W with the BDN14-3CB/A01 heat sink applied.

Considering Equation 24, driving a 1-MHz, 50-Vpp sinusoidal output into a 1-nF capacitive load with 50 Ω of isolation resistance, will result in 3.1 W of average internal power dissipation in the driver amplifier. Comparing this to the previous result proves that the heat sink chosen is thermally sufficient for these output conditions at room temperature. Further analysis of the safe operating area is discussed in Section 2.3.4.1.