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AN1955 Datasheet(PDF) 1 Page - Freescale Semiconductor, Inc |
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AN1955 Datasheet(HTML) 1 Page - Freescale Semiconductor, Inc |
1 / 5 page AN1955 1 RF Application Information Freescale Semiconductor Thermal Measurement Methodology of RF Power Amplifiers By: Mali Mahalingam and Edward Mares INTRODUCTION This document explains the methodology used by Freescale for thermal measurement of high power RF (Radio Frequency) power amplifiers (RFPA). Semiconductor device reliability heavily depends on device operating temperature so the accurate thermal characterization of these high power devices is crucial in establishing the reliability of the systems that use such devices. DIE SURFACE TEMPERATURE (TJ ) MEASUREMENT Infrared (IR) microscopy is used to determine the die surface temperature (TJ) during amplifier operation. Because this IR measurement method requires a direct view of the die, the protective ceramic lid is removed and replaced with a modified lid that has an opening to view the die. In the case of overmolded plastic packages, the center portion of the mold compound is etched away until the die is sufficiently exposed. Because the heat flow from the device to the heatsink is dominated by conduction, the measurement error caused by the removal of the lid or removal of the mold compound around the die surface is negligible. The exposed die is coated with a high emissivity coating (see Appendix) to obtain a fixed emissivity value for IR thermal measurement. This coating greatly improves the accuracy of the IR measurement because it eliminates the need for any emissivity correction procedure used by the IR microscope. The emissivity correction procedure recommended by IR microscope manufacturers is ineffective at compensating for the translucent nature of silicon [1]. With the IR microscopy procedure, the maximum die surface temperature (“hot spot”) in the measurement field can be located. The hot spot temperature is selected as the die temperature (TJ) for thermal resistance (θJC). The thermal resistance calculations are described later. CASE TEMPERATURE (TC) MEASUREMENT The case temperature (TC) of the package is measured by a 0.020″ diameter stainless steel sheath thermocouple (Type J; Omega part # JMQSS-020G-12) that is mounted within the heatsink of the RF circuit. It is mounted from the bottom and protrudes through the mounting interface to contact with the bottom surface of the package (Figure 1). A 0.032″ diameter hole is drilled through the circuit heatsink to permit thermocouple passage. This small hole provides minimal disturbance to the heat flow path and interface integrity. The thermocouple model is selected based on its sensitivity combined with excellent durability. A spring mechanism is added to the thermocouple to guarantee constant mechanical contact with the bottom side of the flange. The placement for this thermocouple is centered relative to the centermost active transistor in the package (Figure 2). Die Package Temperature of Case (TC) Measured with Spring− Loaded Thermocouple Making Direct Contact Heatsink Die Temperature (TJ) Measured with IR Microscope 4 Active Die Thermocouple Centered Relative to Centermost Active Die in Package Figure 1. Case Temperature Measurement Figure 2. Positioning of Thermocouple AN1955 Rev. 0, 1/2004 Freescale Semiconductor Application Note © Freescale Semiconductor, Inc., 2004, 2006. All rights reserved. |
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