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Номер произв AN-0003
Описание Thermal Considerations
Производители ANADIGICS
логотип ANADIGICS логотип 
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Thermal Considerations for PAs
AN-0003
Thermal Considerations for Power Amplifiers
Overview
Proper heatsinking to control junction temperature is an extremely important consideration for use of all
power amplifiers. Gallium Arsenide (GaAs) devices can tolerate considerably higher junction
temperatures than Silicon (Si), but due to its lower thermal conductivity, it requires more consideration
than Si to remove heat. The thermal conductivity of GaAs is only about one third that of Si, and it has a
nonlinear relationship with temperature (the conductivity worsens with increasing temperature).
Although many factors inside and outside the package influence the junction temperature, the end user of
the power amplifier can control the implementation of the connection and layout on the printed circuit
board (PCB). The choice of the board material and thickness, the number of thermal vias placed beneath
the part and the design of the heatsink are all important factors in properly using the part under difficult
thermal requirements.
Application Requirements
ANADIGICS power amplifiers are typically packaged in LPCC (Leadless Plastic Chip Carrier) packages.
These packages offer excellent thermal characteristics due to the thin copper paddle used for mounting
the chip. The part itself (bare die) is also very thin and enables very good heat dissipation. A cross
section of the package is shown in Figure 1.
Gold Wire
Lead
Mold Compound
DIE
0.9mm
Solder Plating
Copper Lead Frame Die Attach Epoxy
Figure 1. Cross section of an LPCC package.
The thermal design of such a part involves maintaining the junction temperature below a certain level,
defined as Tmax. The junction temperature depends directly on the case temperature, defined as the
temperature at the bottom of the copper lead frame. Although each application is different, junction
temperatures of 150°C are considered desirable with an almost infinite device life. Junction temperatures
of 180°C are typical in many applications and yield device MTTFs (Median-Time-To-Failures) better than
1 million hours (114 years). Figure 2 indicates the MTTF curves1 for the fabrication processes used in the
ANADIGICS MESFET power amplifiers. Various applications may require different maximum junction
temperatures depending upon the MTTF requirements.
05/2003
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AN-0003 Даташит, Описание, Даташиты
AN-0003
Figure 2. Typical MTTF data for TriQuint MESFET, HFET, and PHEMT.
In some applications, the device performance needs to be de-rated to ensure that the maximum
acceptable junction temperature (Tmax) is not exceeded. Figure 3 shows a generic de-rating curve. The
device can dissipate the maximum power for case temperatures less than Td. For case temperatures
greater than Td, the dissipated power is limited as indicated in Figure 3 in order to keep the junction
temperature below Tmax. ANADIGICS performs detailed thermal analysis of the junction to case
temperature using finite difference software. These analysis results have been verified through the use of
infrared microscope measurements, and are used to develop the de-rating curves.
Pmax
Td Tma x
Case tmep erature
Figure 3. Generic de-rating curve.
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AN-0003
ANADIGICS has developed a generic thermal analysis for power amplifiers. Although each design has
unique implementation requirements, the typical dimensions of a 4x4 mm part are presented in Figure 4.
0.1 mm
0.01 mm
0.2 mm
Thermal vias
0.9 mm
PCB
board
2.8 mm 4 mm
2.8 mm
4 mm
Figure 4. Typical dimensions of a 4x4 mm part.
PCB Thermal Capability
In typical applications the user should place a large number of thermal vias beneath the mounting pad for
the paddle. For a 4x4 mm part mounted on a 0.032" (0.8 mm) PC board, it is possible to place
approximately 16 vias under the part with each fabricated using a 0.014" (0.35 mm) drill. This drill size is
typically used for holes of 0.010" (0.25 mm) finished size. Typically the plating thickness will be 0.001"
(0.025mm). Thicker copper plating will lower the thermal resistance, helping to reduce the junction
temperature.
The thermal resistance of a single copper via can be calculated as:
*RTH = σ
L
π (R) O 2 RI 2
05/2003
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