CADDOCK Applications Engineering Note: AEN-0102 Release Date: 9/20/02, Rev. A, Rev. Date: 9/20/02 Page 1 of 2 Understanding the Power Rating of Caddock MP9100 Resistors Understanding the Power Rating The maximum power rating of the Caddock MP9100 (TO-247 type) resistor is specified with the ceramic surface maintained at 25°C using the same method established and proven by power semiconductor manufacturers over many years. Caddock’s careful design practices and thorough reliability testing assure the user that the part will perform to all published specifications as long as its case temperature (TC) does not exceed that specified in Figure 1. Surge ratings must be observed. terial characteristics and ambient temperature, (TA). The maximum continuous power rating (100 Watts) for the Caddock MP9100 resistor is based on a case temperature (TC) of 25°C measured at the center of the exposed ceramic surface. The Caddock MP9100 resistor has a thermal resistance of 1.5°C/watt (RØJC), from the junction (resistor film) to the case (exposed ceramic case interface). For a case temperature (TC) above 25°C, derating is necessary. Allowable power dissipation (PD, watts) may be determined by the following equation. Equation 1 PD = MP9100 Derating Curve g Figure 1 % Rated Power 100 % 80 % 60 % 40 % 20 % 0 25 100 Case Temperature (T , °C) 175 C In high power applications, the actual power dissipation capability of the resistor will be greatly dependent on the heat sink, mounting method, interface ma- Mounting Screw TJ - T A (RØJC + RØCS + RØSA) Using the information provided in Fig 2, allowable power dissipation and temperature gradients can be estimated. Typical thermal resistance of various materials can be found in Table 1 on the next page. It should be noted that material characteristics are those provided by the manufacturers and these can vary greatly if careful manufacturing processes are not observed. Caddock suggests that actual temperature measurements should be made under worst case conditions to assure maximum reliability of a design. Figure 2 MP9100 with Mounting Screw and Belleville Washer Center Line through Center of Ceramic Element Belleville Washer TJ Resistor Film Temperature (Max. 175°C) R ØJC Thermal Resistance - Resistor Film to Exposed Ceramic Case Interface Thermal Grease or Pad Heat Sink (mounting surface) Detail of Cross Section TC Case Temperature (Exposed Ceramic Surface) PD R ØCS Thermal Resistance - Case to Heat Sink Surface (Pad or Thermal Grease) TS Heat Sink Temperature R ØSA Thermal Resistance - Heat Sink to Ambient Air or Reference Temperature TA Ambient or Reference Temperature CADDOCK © Caddock Electronics, 2002 A copy of this Application Note can be obtained at www.caddock.com Applications Engineering Note: AEN-0102 Page 2 of 2 - Rev. A Calculation Examples Mounting Configurations Using Equation 1 and data from Table 1, the allowable power can be determined for given ambient conditions. Assuming a maximum ambient air temperature of 50°C with the part mounted using thermal grease on an aluminum chassis, the allowable power can be calculated from Equation 1 as follows: Mounting pressure, surface cleanliness, surface flatness and surface finish are just a few of the variables which can have considerable effect on temperature rise and power dissipation. Caddock recommends an ideal spring mounting force of 4 to 15 pounds, or a typical mounting screw torque of 6 to 8 in - lbs which should provide 150 to 300 pounds of force. If a screw is used, Caddock recommends the use of a good thermal grease in conjunction with a #6 Belleville washer compressed to 50% of its normal height. These washers (p/n 04CSB52200F004) are available from Arizona Hydrogen, phone number 602-275-4126. PD = TJ - TA (RØJC + RØCS + RØSA) PD = 175°C - 50°C (1.5°C/W + 0.5°C/W + 2°C/W) PD = 125°C 4°C/W PD = 31.25 watts The same part, mounted on a water cooled heat sink providing a constant 50°C reference temperature, will dissipate more than twice this amount of power. PD = 175°C - 50°C (1.5°C/W + 0.5°C/W) PD = 62.5 watts Table 1 Typical Thermal Resistance of Various Materials Used for High-Power Designs with Caddock MP9100 Resistors Resistor Type Caddock MP9100 (junction to case) Interface Material (thickness) 1.5 R ØCS (°C/W) 0.1 Epoxy (0.010”) 10 Thermal Grease (0.003”) Q-Pad II (0.006” ) Surge or pulse voltages can be a factor in design reliability. The following guidelines should be followed. (°C/W) ØJC Solder (0.010”) Filled Epoxy (0.010”) Surge and Pulse Ratings R 2.5 0.5 to 2 0.9 SIL-PAD 400 (0.007”) 3.1 SIL-PAD 2000 (0.015”) 1.7 1. Do not exceed 750 volts peak. Thermal Clad ® dielectric (0.003”) 0.4 2. Use the following guidelines for maximum pulse energy of a single pulse. FR-4 / G-10 printed-circuit board (0.063”) Pulse Width 10 µsec 100 µsec 1 msec 10 msec 100 msec Energy (1 J = 1 Watt second) 0.07 J 0.25 J 0.90 J 3.5 J 15 J Aluminum (0.063”) Copper (0.063”) Steel (0.063”) 96% Alumina (0.030”) Aluminum Nitride (0.030”) Beryllia (0.030”) Typical Mounting Surfaces to Ambient Air 3. Accumulated pulse energy delivered in each second should not exceed allowable average power. Derate the pulse energy using the Derating Curve (Figure 1) based on estimated case temperature. 7" X 5" X 2" 0.040" Aluminum chassis 4. Above 100 msec to 5 seconds, the overload rating is 1.5 times the allowable power of the resistor in the application. 20 to 65 0.1 0.06 0.5 0.36 0.1 0.05 R ØSA (°C/W) 2 Small Finned Sheet Aluminum heat sink (convection) 20 Small Finned Sheet Aluminum heat sink (200 LFM Airflow) 10 4.5" X 3" X 3" Mult-Finned Extruded Aluminum (convection) 1.5 4.5" X 3" X 3" Mult-Finned Extruded Aluminum (200 LFM) 0.8 CADDOCK © Caddock Electronics, 2002 A copy of this Application Note can be obtained at www.caddock.com