A Comparison of a Voltage Regulator Circuit with Traditional and Passive Dielectric Fluid Cooling Dr. William Y. Bishop, P.E., PT Lecturer e Department of Electrical and Computer Engineering Dr. James E. Smith, Jr., Professor Mr. Aaron A. Howell, Undergraduate Assistant Department of Chemical and Materials Engineering The University of Alabama in Huntsville (UAH), Huntsville, 35899 y ( ) bishopwy@uah.edu, Cell 256-694-0041
The Problem Gain increased power efficiency in semiconductor power devices by removal of self generated heat. The commercial market: The semiconductor device market today is primarily driven by three industries: i Cell phone/wireless industry Automotive electronics industry Notepads, laptops, computer industry All these markets need breakthrough power semiconductor device technologies in order to gain increased power efficiency. In other words, these devices must get the self generated heat out of the device itself and away from other parts of the circuitry. it Several potential semiconductor device technologies showing great promise for gaining g increased power efficiency over the old reliable silicon based devices are: Silicon Carbide device technology Gallium Nitride (GaN) device technology Silicon on Insulator (SOI) Laterally Diffused MOSFET device technology These potential technologies remain limited by self generated heat and management and control of the removal of the heat.
National Goal for GaN, SiC Based Power Controllers for the Smart Grid is a Power Handling Gain of 3X In typical electronics systems the power converter is the hottest element in the system, and waste heat is removed by conduction, spreading, and convection to a working fluid (e.g. air, water, or a refrigerant). The Thermal Ground Plane (TGP) developed modern highperformance spreaders to replace copper alloys. The Microtechnologies for Air Cooled Exchangers (MACE) developed enhanced heat sinks with improvements that reduced thermal resistance. The Nano-Thermal Interfaces (NTI) developed lower thermal resistance interface layers. The Active Cooling Module are needed to reduce the semiconductor junction temperature.
Traditional Approaches Intel G45 Graphics Processor, an airflow speed of 1.73 m/s [341 linear ft/min] is assumed to be approaching the heatsink at a 30 angle from the processor thermal solution. The local ambient air temperature at the heatsink is assumed to be 51.6 C. The airflow can be achieved by using a processor heatsink providing omni directional airflow, such as a radial fin or X pattern heatsink. Such a heatsink can remove the heat from processor and other areas like the voltage regulator using a heat pipe mounted on the chip and heat sink. These heat sinks weighs from 80-130 grams, not including mount hardware or heat pipes.
Internal Self Heating Thermal Transient T1 max =100 o C T2 max =50 o C T ss =30 o C 56 o C Can the thermal transient at the junction be controlled at 56 o C using Passive Cooling? Can we improve the power handling of a Voltage Regulator? Why This Temperature? Example Measured Temperature rise/decay transient for a Silicon on Insulator semiconductor device T1 max = maximum transient temperature, SiO2 insulator T2 max= maximum transient se t temperature, e, Bulk Silicon T ss = Steady state temperature in degrees C Note: Bulk Silicon is not as susceptible to internal self heat effects as doped Silicon Dioxide or Doped Silicon is. Source: Arnold, Emil- Silicon on Insulator Devices for High Voltage and Power IC Applications, Journal of the Electrochemical Society, Vol. 141,No. 7, pp 1983-1988,July 1994.
Fluorinert Properties
Fluorinert Properties Compatible with most high temperature plastics and composites Can be contained in a light weight vessel
Active vs. Passive Cooling with Vaporization For Air to the exit temperature the power removed per gram of air used is cal Q 0.24 (51.6 25) C gram * C Q 6.5 cal / gram 27 watts sec / gram Using Passive with Fluoroinert FC-72 the exit temperature is 56 o C cal Q 0.263 (56 25) C 21.02 cal / gram gram* C Q 29.2 cal / gram 122 watts sec/ gram Relative Percent Increase = 352%
LM317K-T0-3(K) Package Dissection The silicon based LM317K was chosen as the experimental proof of concept device type because of availability and its wide commercial use. Heat sink (Interface) Thermal Spreader + Chip Output Nano-Thermal Interface Electrical Insulator Cover to Contain Inert Atmosphere
Equipment and Experimental Setup LM317K + in out adj HP 6813A - V
Chamber Containing and Circuit Under Fluorinert Dielectric i Fluid
Experimental Results 40 LM317K Performance With and Without FC72 Cooling LM317K LM317K with FC72 Cooling 70 LM317 Temperature Profile With and Without FC72 Cooling LM317K LM317K with FC72 Cooling 30 60 Pow wer (W) 20 Tempe erature (C) 50 40 10 30V 20V 20V 50% 30 0 0 10 20 30 40 Applied Voltage 20 0 10 20 30 40 Applied Voltage The Power handling of the LM317K was increased by being cooled with the FC72. The temperature profile shows that when the traditionally cooled chip heated to above 60 o C, the chip could no longer perform efficiently.
Experimental Results 1.4 LM317K Control Current With and Without FC72 Cooling 70 LM317 Temperature Profile With and Without FC72 Cooling LM317K LM317K with FC72 Cooling 1.2 60 Control Curre ent 1 0.8 0.6 25V 15V 15V 66.667 % Temperature (C C) 50 40 30 LM317K LM317K with FC72 Cooling 0.4 0 10 20 30 40 20 0 10 20 30 40 Applied Voltage Applied Voltage The control current was sustained to 66.7% higher voltages by using the FC72 Fluoroinert coolant.
Possible Package Modifications Assumes Compressed Copper Powder in Cap to Act as Heat Pipe to Top of Cap (a) (b) (c) (a) (b) (c) (a) Existing chip fitted with an active cooling cap. (b) Existing circuit board fitted with an active cooling cap to locally remove heat. (c) Cap fitted to an exposed circuit with fluid injection followed by sealing of the injection port for active cooling.
Passive Heat Extraction FC-72 Liquid from Onboard Storage Porous Frit Vapor/Liquid Separation FC-72 Vapor to Non-Propulsive Vent Heat Source T BP Q m c dt m H Q overall p vap envir. T s
Conclusions and Recommendations Performance and Control of the LM317K exceeded 50% Under This Preliminary Feasibility Experiment. The majority of the chip heating was removed from the junction by boiling heat transfer of the dielectric fluid. Voltage Regulatory circuits should be tested to >40V. Power Amplifiers such as VU932R where breakdown and distortion are a function of temperature.