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.
Pd  I outMAX VinMAX  Vout   ( I gnd  VinMAX ) …………………………………………………………...….…...……...…..…(1)
…………………………………………………………………………………………………………………………………….(2)
…...……………………...………………………………………...…………………...…………………...……………….(3)
Where the input and output values are given as IoutMAX, Ignd, VinMAX and Vout. When applied to constants Rt,
the thermal resistance between the junction and ambient air and 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.
Pd  500mA8.4v  5v   0.3mA  7.4v   1.70W
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.
C
 85.11C
W
 85.11C  32.22C  117.33C
t  1.70W  50
TJMAX
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.
Pd  (Vin  Vout )( I out ) ………………………………………………………………..……………………………………………………..(4)
TJ  TA  Pd  HeatSink   CaseTo  HeatSink   JC  …………………………………………………………………………………..(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.
Pd  (8.4V  6V )(1A)  2.4W
C
 36.3C - Control Section
W
C
TJ  32.22C  2.4W 1.0  0.2  1.6
 38.94C - Power Transistor
W
TJ  32.22C  2.4W 1.0  0.2  0.5
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.
Pd  1A8.4v  6v   16mA  8.4v   2.53W
This power can be used to obtain the temperature rise above ambient by applying it to equation (2).
t  2.53W  50
C
 126.5C
W
Therefore the total maximum junction temperature can be found by applying equation (3)
TJMAX  126.5C  32.22C  158.7C
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.
t  Pd max   Rt ……………………………………………………………………………………………………………………………..….(6)
TJMAX  t  TA ……………………………………………………………………………………………………………………………………(7)
C
 10C
W
 10C  32.22C  42.22C
t  80mW  125
TJMAX
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.
t  80mW  84.6
C
 6.768C
W
TJMAX  6.768C  32.22C  38.99C
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.
 j  a   j c   c  dis
 j  a  3C / W  15.6C / W  18.6C / W
T j  Ta   j  a  Pd
T j  32.22C  18.6C / W  2.53W
T j  79.28C
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.
 j  a   j c   c  dis
 j  a  5C / W  15.6C / W  20.6C / W
T j  Ta   j  a  Pd
T j  32.22C  20.6C / W  1.70W
T j  67.24C