Thermal Considerations

In order to ensure the performance and longevity of the controller board, components must be examined for thermal output. These output temperatures must be kept below the maximum junction temperature of the component to ensure safe operation and maximum life of the board. According to Brian Dean, and expert at BD Micro LLC, our chosen development board, BD Micro Mavric IIb, has no appreciable heat generation. Therefore the heat generated by our designed voltage regulators and MOSFET drivers will be of primary concern.

Voltage Regulators:

LT1129 w/ TO-220 Package

From Linear Technologies micropower low-dropout regulator data sheet, we obtained a set of equations relating voltage and current to the junction temperature of the device.

…………………………………………………………...….…...……...…..…(1)

…………………………………………………………………………………………………………………………………….(2)

…...……………………...………………………………………...…………………...…………………...……………….(3)

Where the input and output values are given as IoutMAX, Ignd, VinMAX and Vout. When applied to constantsRt, the thermal resistance between the junction and ambient airand Tamb, the ambient temperature, Pd, the power dissipated and Tj, the junction temperature can be obtained.

For logic power the voltage needs to be regulated from 7.4 V to 5 V with a maximum output current of approximately 500mA. The ground current for the chosen regulator package, Linear Technologies LT1129-5 w/TO-220 package, can be obtained from the following chart.

Figure 1: Ground pin current for Linear Technologies LT1129-5

From Figure 1 an approximate ground pin current of 0.3mA was obtained. The input and output values can be used in equation (1) to obtain the power dissipated.

The power distributed can then be used in conjunction with the thermal resistance (50oC/W for TO-220 package) to find the junction temperature rise above ambient. This temperature can then be used with the ambient temp (90oF /32.22oC) to calculate the total junction temperature.

The maximum junction temperature calculated is well below the maximum junction temperature listed in the datasheet of 125°C.

LT1083 w/TO-3P package

For servo power the voltage needs to be regulated from 8.4 V to 6 V with a maximum output current of approximately 1A. The data sheet for our second chosen voltage regulator, Linear Technologies LT1083 w/TO-3P package, provides an equation for calculating the power dissipated in both the control section and power transistor.

………………………………………………………………..……………………………………………………..(4)…………………………………………………………………………………..(5)

Coupling equation (4) with equation (5) the junction temperatures can be obtained and compared to the maximum allowable junction temperature (125oC), using a thermal resistance of 0.2°C/W for Case to Heat Sink (with thermal paste) and 1.0°C/W for heat sink.

- Control Section

- Power Transistor

LT1965 w/TO-220 Package

After the detailed design review, the voltage regulation for the servo was changed from utilizing the LT1083 to the LT1965. This regulator is better suited for our application however, it will require heat analysis in order to maintain a desired junction temperature of <125oC.

From the LT1965 data sheet, the regulator with a TO-220 package has a thermal resistance from junction to ambient of 50oC/W. By using equation (1), also valid for LT1965, the power dissipation can be found.

This power can be used to obtain the temperature rise above ambient by applying it to equation (2).

Therefore the total maximum junction temperature can be found by applying equation (3)

Since this temperature is above the maximum allowable junction temperature of 125oC, a heat sink will need to be applied to this component to ensure safe and efficient operation.

MOSFET Drivers (H-bridge):

Another board component deemed to produce a significant amount of heat are the MOFSET drivers used in the H-bridge. From the MOSFET driver analysis, the maximum power dissipated at steady state will be 40mW per transistor with two transistors per driver.The data sheet of the chosen MOSFET driver, Microchip TC4424 8-pin PDIP, provides a thermal resistance, from junction to ambient, of 125oC/W for 4.5V<VDD<18V and a maximum junction temperature of 150oC. From equations (6) and (7) the junction temperature can be obtained.

……………………………………………………………………………………………………………………………..….(6)

……………………………………………………………………………………………………………………………………(7)

Since this temperature is much lower than the maximum junction temperature, there is no need for additional heat dissipation on this device.

Another board component deemed to produce a significant amount of heat are the MOFSET drivers used in the H-bridge. The data sheet of the chosen MOFSET driver, Microchip TC4424A 8-pin PDIP, provides a thermal resistance, from junction to ambient, of 84.6oC/W for 4.5V<VDD<18V and a maximum junction temperature of 150oC. From equations (6) and (7) the junction temperature can be obtained.

Comparing TJMAX to the maximum allowable junction temperature, it is apparent that no further thermal management should be required.

Heat Due to Coating:

Typical silicone conformal coatings have a thermal conductivity (the inverse of thermal resistance) of 0.04 and 0.12 W/oC-m. The meter unit comes from the thermal conductivities dependency on the exposed area as well as the coating thickness.

For example for the LT1129 TO-220 package, the exposed area is 2500 mm2 and a maximum coating thickness of 210um. The maximum thermal conductivity of the coating is 1.43e6 W/oC which corresponds to a thermal resistance of approximately 7e-7 oC/W. This thermal resistivity, for all intensive purposes, will add no significant increase in heat to the components.

Conclusions:

After completing this thermal analysis, it is clear that at least one component on the controller board will require some additional thermal management. However, we would like to do additional testing on each of these devices to prove these calculations and quantify the potential for burn injuries.

Due to these calculations alone, we recommend the use of an exposed (uncoated) heat sink on both the LT1129 and LT1965 regulators to prevent potential failure and human harm.

Possible Heat Sinks for TO-220 Package:

Manufacturer: CTS Thermal Management Products

Part Number: 7-321-BA

Supplier: Digikey

Part Number: 294-1017-ND

Price: $0.57

Manufacturer: Heatsink Pwr Horz

Part Number: 7-345-1PP-BA

Supplier: Digikey

Part Number: 294-1067-ND

Price: $3.66

Manufacturer: CTS Thermal Management Products

Part Number: 7-342-2PP-BA

Supplier: Digikey

Part Number: 294-1086-ND

Price: $4.47

Heat Sink Calculations:

As stated, it is clear that some form of thermal management is required. To ensure the reliability of the voltage regulators, further calculations taking into account the chosen heat sinks are required. With knowledge of the anticipated power dissipation, the junction-to-case thermal resistance and the thermal resistance of the heat sinks, the junction temperature can be determined.

For the LT1965 TO-220 package, the junction-to-case thermal resistance is 3 °C/W. The chosen heat sink (Aavid Thermalloy HS112-ND) has a thermal resistance of 15.6 °C/W. Pd, the power dissipated by the voltage regulator is 2.53 W.

For the LT1129 TO-220 package, the junction-to-case thermal resistance is 5 °C/W. The chosen heat sink (Aavid Thermalloy HS112-ND) has a thermal resistance of 15.6 °C/W. Pd, the power dissipated by the voltage regulator is 1.70 W.