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PDF AN-0003 Data sheet ( Hoja de datos )
| Número de pieza | AN-0003 | |
| Descripción | Thermal Considerations | |
| Fabricantes | ANADIGICS | |
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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
http://www.Datasheet4U.com
1 page  
AN-0003
In some applications it will be possible to mount the PCB directly to the chassis at least in the critical area
beneath the power amplifier. This is highly recommended as it provides the shortest path to the large
surface area of the chassis. If this is not possible, then a heatsink should be attached directly beneath
the amplifier. Heatsinks will be specified in degrees Celsius of temperature rise per Watt of dissipated
power. Since the amplifiers are in small packages, the heatsink must be located directly beneath the
amplifier and maintain good thermal contact with the board.
In order to maintain good thermal contact between the heatsink and the backside of the PCB, an interface
material should be used to fill any voids. Many heatsink compounds are not very good thermal
conductors but are inert, low cost and fill in the air gaps. Typical thermal conductivities of heatsink
compounds are on the order of 0.6 W/m*K. These compounds should be used sparingly, and applied in a
very thin layer. Alternative materials have been developed with better thermal properties such as
Chomerics T725 THERMFLOW ™ or Thermoset’s CoolPhase™ MPC-120. These materials are phase
change interface materials. At room temperature they are typically applied as thin sheets, and must be
heated slightly to cure and spread the material.
The Chomerics T725 exhibits a thermal impedance of 0.03 °C-in^2/W, and is available in 0.005" thick
sheets. Below is the contact information of the two companies mentioned above.
THERMFLOW is a trademark of Chomerics.
Chomerics
Div. of Parker Hannifin
77 Dragon Court
Woburn, MA 01888-4014
www.chomerics.com
Ph: 1-781-935-4850
CoolPhase is a trademark of Thermoset, Lord Chemical Products.
Thermoset, Lord Chemical Products
5101 E. 65th Street
Indianapolis, Indiana 46220-0902
www.thermoset.com
Ph: 1-800-746-8343
Conclusion
In general, it is important to keep the junction temperature of the devices low to extend the life of the part
and to improve the overall performance. Proper heat sinking of GaAs devices will ensure optimum
performance over the life of the part in your design.
1 TriQuint Semiconductor website, http://www.tqs.com/MMW/Reliability/mmw_rel_ovr.htm
05/2003
5
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| Páginas | Total 6 Páginas | |
| PDF Descargar | [ Datasheet AN-0003.PDF ] | |
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