Kieran O Malley ON Semiconductor 2000 South County Trail East Greenwich, RI 02818 APPLICATION NOTE Introduction The thermal characteristics of linear regulators depend on their operating environment and the system s load requirements. It is essential that the regulators operating temperature be minimized to reduce stress on the silicon and the package. This article reviews the basic thermal calculations for single and dual output regulators, introduces the use of heatsinks and provides guidelines for using the copper on a PCB as a heatsink. Linear regulators are a relatively inefficient method of regulating voltage for many applications. This inefficiency translates itself into additional power dissipation and heat that the design engineer must remove. The efficiency for a particular design is given by: E P OUT 100% PIN Neglecting the regulator quiescent current E V OUT IOUT VIN IOUT 100% V OUT VIN The heat generated by a semiconductor junction is inversely proportional to the efficiency of the circuit. As the junction temperature of a semiconductor rises the devices performance degrades and permanent damage may result. Most manufacturers specify the maximum operating junction temperature for semiconductors at 150 C to 180 C. While most linear regulators include thermal shutdown circuitry that will shut the circuit down to avoid permanent damage do not use this feature as an essential part of your design. Background Information Heat flows from a high temperature point to a lower temperature point. The rate of heat flow is directly proportional to the temperature difference. The thermal resistance of a material, Rθ, is a measure of how much a material opposes the flow of heat through it. This can be shown by a simple equation: R T 2 T1 Q ÌÌÌÌÌÌ ÑÑÑÑ ÌÌÌ ÌÌÌÌÌÌÌÌÌÌÌÌÌÌ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÌÌÌ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÌÌÌ ÌÌÌ Figure 1. TO 220 on a PCB Using the Copper Traces as a Heatsink. Semiconductor Components Industries, LLC, 1999 November, 2000 Rev. 0 1 Publication Order Number:
Where T 2 and T 1 are the high and low temperatures respectively and Q is the rate of heat flow in joules per second. In a semiconductor junction the heat flow is the power dissipated, P D. R T 2 T1 T 2 T1 Q PD In a linear regulator circuit the main source of heat is the pass or output transistor, although, other internal circuitry and its associated bias current will also contribute to the overall heating effect. In the case of a linear regulator the heat generated by the series pass device (and other heat generating elements) heats up the die or substrate. This heat flows through the case to the printed circuit board, to the heatsink by conduction and finally into the surrounding air by convection or radiation. In addition, a small amount flows directly into the surrounding air by conduction through the plastic molding compound. Each of these materials has a different thermal resistance. The overall heat transfer depends on the thermal resistance of all the materials between the die and ambient temperature. The thermal characteristics of molded body IC depend on the following four factors: 1. Maximum Ambient Temperature, T A ( C) 2. Power dissipation, P D (Watts) 3. Maximum junction temperature, T J ( C) 4. Thermal resistance from the junction to ambient, Rθ JA ( C/W) These four are related by the equation: TJ TA PD R JA (1) In each of these factors the lower the figure is the better, e.g. a package with a thermal resistance, Rθ JA of 25 C/W is twice as good at conducting heat as a package with a thermal resistance, Rθ JA of 50 C/W. The designer determines the maximum ambient temperature and the power dissipation while the maximum junction temperature and the thermal resistance depend on the manufacturer and the package type. The maximum junction temperature is always listed in the data sheets under the Absolute Maximum Conditions or Absolute Maximum ratings and must be adhered to. Rθ JA, is the thermal resistance between the IC junction (or die) and the ambient air. It is found in the data sheet in the Electrical Characteristics section and possibly with the Package Information section. It is a measure of how much the junction temperature will rise for each watt increase in power dissipation in still air. This parameter depends on the die size and package type. If a regulator is available in a number of packages, figures will be quoted for each package type. The lower this figure is the better power handling capability a particular package has. Another parameter quoted in the data sheet is Rθ JC this refers to the thermal resistance from junction to case, it also depends primarily on package type and die size. The power dissipation for a particular regulator is given by the equation; PD (VIN(MAX) VOUT(MAX)) (IOUT) VIN(MAX) IQ (2) where: V IN(MAX) = Maximum input voltage to the IC. V OUT(MIN) = regulator minimum output voltage. I OUT = Maximum load current. I Q = Regulator Quiescent current. The power dissipation and the value for Rθ JA are used to calculate the maximum ambient temperature, T A, or to determine if a heatsink is required for a particular application. Example 1 Consider the following example using the CS8120 in still air. V IN(MAX) = 12 V V OUT = 5.0 V +/ 4.0% I Q(MAX) = 15 ma. This is the bias current required to keep the regulator operating, it adds to the overall power dissipation, it can be quite a significant portion of the power dissipation especially in cases where the input voltage is high. I OUT = 250 ma T A Max = 35 C T J Max = 150 C The CS8120 is available in several different packages and thermal data for each package is shown in Table 1. Table 1. Package R JA R JC Units TO 220 50 3.1 C/W 14 Lead SO 125 30 C/W 8 Lead PDIP 100 52 C/W 5 Lead D 2 PAK 10 50* 3.1 C/W *Depending on the thermal properties of the substrate. Step 1. Calculate the power dissipation for the regulator using (2): PD (VIN(MAX) VOUT(MIN)) (IOUT) VIN(MAX) IQ PD (12 V 4.8 V) (0.250 A) (12 V)(0.015 A) PD 1.80 W 0.18 W PD 1.98 Watts Step 2. Calculate the maximum allowable Rθ JA using (2) to find the correct package(s). TJ TA PD R JA R JA T J TA PD R JA 150 35 1.98 R JA 58.1 C W This is the maximum allowable Rθ JA, the designer must ensure that this figure is not exceeded in the design. The only package available that has a thermal resistance, Rθ JA, of less than 58 C/W is the TO 220, it has 2
Rθ JA = 50 C/W. It will keep the junction temperature below 150 C without the need for a heatsink in this application. The operating junction temperature using (1) is: TJ TA PD R JA TJ 35 (1.98 50) 134 C In many cases there is no package available that will meet the requirements so the designer is forced to use a combination of package and heatsink to keep the junction temperature within the maximum allowable temperature range. A heatsink is simply some additional heat conductive material that is used to increase the surface area of the package and increase heat flow. The heatsink can be a specially designed component from a heatsink manufacturer, or a piece of copper on the PCB or a combination of both. Another example of a heatsink is when a package is bolted to the case of the equipment. When a heatsink is used, other factors must be considered to calculate the temperature effects accurately. Each material in the heat flow path between the die (or substrate) will have a thermal resistance. These are summed like resistors in series to arrive at a total resistance Rθ JA. 1. Thermal Resistance of the junction to case: Rθ JC ( C/W) 2. Thermal Resistance of the case to heatsink: Rθ CS ( C/W) 3. Thermal Resistance of the heatsink to the ambient air: Rθ SA ( C/W) These are connected by the equation: R JA R JC R CS R SA (3) As mentioned earlier, the value for Rθ JC is normally given on the data sheet by the manufacturer and depends on package type and die size. As usual, a lower value is better in terms of power dissipation ability. The value for Rθ SA is found on the heatsink data sheets, while Rθ CS depends on factors such as: package type, heatsink interface, (is an insulator or thermal grease used?) and how large the contact area between the heatsink and the package. Example 2 Using the previous example with the CS8120 but increasing the maximum ambient temperature to 70 C, the maximum allowable Rθ JA becomes: R JA 150 70 1.98 R JA 40 C W The heat sink must have a thermal resistance, Rθ JA, of not more than 40 C/W to keep the die temperature below 150 C. Clearly none of the packages listed in Table 1 will meet this requirement with the possible exception of the D 2 PAK so we need to use a combination of package and heatsink. In this case we will use the TO 220 package to illustrate heatsink calculations. From the CS8120 data sheet, Rθ JC for the TO 220 package is 3.1 C/W. Substituting this into equation (3) we calculate the requirements for the heatsink. 40 C W 3.1 R CS R SA R CS R SA 36.9 C W If an interface material such as thermal grease or conductive pad is not used, Rθ CS can vary between 1.0 C/W and 5.0 C/W depending on the smoothness of the case and the heatsink. Heatsink manufacturers specify various materials that are either electrical conductors or insulators. The vast majority of Linear Regulators that are available in power packages such as TO 220, D 2 PAK, etc. have their case connected to the die substrate. In most instances the substrate is also the ground connection, but there are regulators such as the CS5203A whose substrate is actually V OUT, so the case is also at a potential equal to V OUT. The user must keep this in mind so it may be necessary to electrically insulate the heatsink from the package. In the case of the CS8120 the case is connected to ground. If we do not use any thermal grease or a conductive pad and assume worst case conditions, i.e., Rθ CS = 5.0 C/W 5.0 C W R SA 36.9 C W R SA 31.9 C W Figure 2 shows thermal characteristics for the 504222 heatsink from AAVID Thermal Technologies. To select a heatsink, we take the power dissipation (in this case 1.98 Watts) on the X axis. The Y axis shows the mounting Surface Temperature Rise Above Ambient. This is simply how much the heatsink temperature will rise for a given power dissipation. Dividing this figure by the power dissipation gives a value for Rθ SA. In this example we see that the temperature rise will be approximately 20 C. Mounting Surface Temperature Rise Above Ambient C 100 90 80 70 60 50 40 30 20 10 0 0 1 2 3 4 5 6 7 8 9 Heat Dissipated Watts Figure 2. Thermal Characteristics for the 504222 from AAVID Thermal Technologies We can now calculate the actual die temperature in the application. First we calculate the Rθ JA with the heatsink. R JA R JC R CS R SA R JA 3.1 C W 5.0 C W 20 C W R JA 28.1 C W 10 Semiconductor Components Industries, LLC, 1999 November, 2000 Rev. 0 3 Publication Order Number:
V IN V OUT2 ENABLE + Pre Regulator + Anti saturation and Current Limit V OUT1 GND Bandgap Reference Overvoltage Shutdown + Anti saturation and Current Limit Thermal Shutdown Figure 3. Block Diagram of CS8156 This means that the die temperature will be: TJ TA PD R JA TJ 70 (1.98 28.1) 125.6 C Dual Regulators To calculate power dissipation for a dual regulator such as the CS8156 we modify equation to take the second output into account. PD (VIN(MAX) VOUT1(MIN)) (IOUT1) (VIN(MAX) VOUT2(MIN)) (IOUT2) VIN(MAX) IQ In this case the quiescent current is the total quiescent current when both outputs are on. Surface Mount Packages. Heat sinks for surface mount packages are becoming increasingly popular with the emphasis on circuit miniaturization. For low to medium power needs however it is possible to use the printed circuit board as a heatsink by providing an area of copper to provide a heat flow path from the package. The shape and size of this copper area is adjusted to fit within the PCB dimensions. If a double sided board is being used copper foil can also be used on both sides with through holes to provide a path for the heat flow as shown in Figure 4. For a high power application using a heatsink in addition to the PCB area can offer some additional cooling effects but a change of package may be the only choice in some cases. It is difficult to produce a universal graph that will give an accurate indication of PCB copper area required for all applications. While most IC companies including ON Semiconductor produce a graph it is usually on the conservative side since it tries to factor in the many operating environments in which the regulator may be used. The best advice we can give here is to use these graphs as a starting point but be sure to back this up with temperature measurements of your own. Figure 5 shows a graph that will provide an adequate guideline for copper area for regulators in a D 2 PAK. Example 3 Using the previous example with the CS8120, but in this case we choose the D 2 PAK, and will use the copper area on the PCB as the heatsink. Ambient temperature is still 70 C. The maximum allowable Rθ JA remains: R JA 150 70 1.98 R JA 40 C W The copper area on the PCB must be sized so that the CS8120 has a thermal resistance, Rθ JA, of not more than 40 C/W to keep the die temperature below 150 C. 4
ÑÑÑÑÑ ÑÑÑÑÑÑÑ ÑÑÑÑÑÑÑÑ ÑÑÑÑÑÑ ÑÑÑÑÑ ÑÑÑÑÑÑÑ ÑÑÑÑÑÑÑ ÑÑÑÑÑÑ ÑÑÑÑÑÑÑÑÑÑÑÑÑ ÑÑÑÑÑÑÑÑÑ ÑÑÑÑÑÑÑÑÑ ÑÑÑÑÑÑÑÑÑ Figure 4. PCB Layouts for SO 16 Package with Fused Leadframe R JA, Thermal Resistance Junction to Air ( C/W) 80 70 60 50 40 30 0 Free Air Mounted Vertically Minimum Size Pad Rθ JA P D(MAX) for T A = 50C 1.0 5.0 10 15 20 25 30 L, Length of Copper (mm) 2.0 oz. Copper ÇÇÇÇ L ÇÇÇÇ L ÇÇÇÇ 3.5 3.0 2.5 2.0 1.5 P D, Maximum Power Dissapation (W) Figure 5. Thermal Resistance Junction to Ambient vs. Copper Area for D 2 PAK. Semiconductor Components Industries, LLC, 1999 November, 2000 Rev. 0 5 Publication Order Number:
From Table 1, the D 2 PAK has an Rθ JA of between 10 C/W and 50 C/W depending on the thermal properties of the PCB. Since the package will be soldered directly to the PCB we will assume that Rθ CS = 0 C/W, i.e., they will both be at the same temperature. (In practice there will be some small thermal resistance but we will ignore it for this example.) From the CS8120 data sheet, Rθ JC for the TO 220 package is 3.1 C/W. Substituting this into equation (3) we calculate the requirements for the heatsink. 40 C W 3.1 0 R SA R SA 36.9 C W Rθ SA must be less than 37 C/W so we size the heatsink area to meet this requirement. Using Figure 5 above, and, assuming a square pattern we see that the length should be at least 22.5 mm for a total area of 506 mm 2. This is slightly less than 1 square inch of copper. In Appendix A, we show detailed results that were obtained from one set of experiments involving the CS5203A, a 3.0 Amp Linear regulator in a D 2 PAK package. It shows how many factors effect thermal performance in a design. Among these factors to consider are: Size of the PCB, percentage of copper on the PCB, thickness of copper on the PCB, orientation of the board, proximity of other PCB s and the regulator location on the board. In this case all the boards were mounted vertically, giving a slightly better air flow than in a case where the board is situated in the bottom of a box with little airflow to the back side of the board. The boards were larger than that typically used in thermal experiments which would also tend to yield better results. On the other hand we only used one and two sided boards, while most PCB s today are multi layer boards. Appendix A PC Board Heatsink Measurements To evaluate the effectiveness of the PCB as a heatsink we used the CS5203A linear regulator in a D 2 PAK mounted on a test PCB s of FR 4 material measuring 139.7 mm (5.5 in) by 83.82 mm (3.3 in) with 1.0 oz., and 2.0 oz., copper as shown in Figure 6. All measurements were made in free air with the boards standing vertically. Since the regulator is soldered directly to the tab we have assumed R CS 0 C W for all measurements. Figure 6. Test PCB Layout 6
R JA (degrees C/Watt) 55.00 50.00 45.00 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Pd = 2.5 W Single Sided 1.0 oz. Copper Pd = 1.7 W Pd = 1.7 W w/ heatsink Pd = 2.5 W w/ heatsink 12.7 mm 19.1 mm 25.4 mm 38.1 mm Pad Length Figure 7. Rθ JA for D 2 PAK vs. Pad Length for 1.0 oz. Copper PCB. R JA (degrees C/Watt) 50.00 45.00 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Pd = 2.5 W Single Sided 2.0 oz. Copper Pd = 1.7 W Pd = 1.7 W w/ heatsink Pd = 2.5 W w/ heatsink 12.7 mm 19.1 mm 25.4 mm 38.1 mm Pad Length Figure 8. R JA for D 2 PAK vs. Pad Length for 2.0 oz. Copper PCB. The results are shown in Figures 7 and 8. The pads are square in all cases so the length refers to the length of one side. In addition the graphs illustrate the difficulty that arises when we use a single figure for Rθ JA. As the power dissipation is increased from 1.7 W to 2.5 W, Rθ JA decreases by approximately 4.0%. This is due to the increased heat transfer from the package. As the power dissipation increases the junction and case temperatures increase and the natural convection currents increase. This increase in convection currents near the surface of the package produces a more effective heat transfer path. Several heatsink manufacturers now offer heatsinks for D 2 PAK. As can be seen from Figure 9, they sit over the package without making physical contact. The heat is conducted through the solder pad to the heatsink and travels by convection to the ambient air. These heatsinks offer some increased thermal capability as can be see from the graphs in Figures 7 and 8, but their benefit declines with increasing copper area. Semiconductor Components Industries, LLC, 1999 November, 2000 Rev. 0 7 Publication Order Number:
ÑÑÑÑÑÑÑÑ Ñ Ñ Ñ Ñ Ñ Ñ Figure 9. Typical D 2 PAK Heatsink Designs Using a double sided board with thermal vias (plated through holes) will also help thermal performance. Figure 10 and 11 shows the thermal performance of double sided FR 4, 1.0 oz. and 2.0 oz. copper respectively. Heatsink Location The placement of the copper area on the board is almost as critical as the size of the copper. In most board level power supply applications, the power components are situated in one corner of the board either because the power supply is the last section to get designed or because it makes it easier to get the supply voltage to the board. The edge of the board is not the best location for the regulator in terms of power dissipation since the heat can spread in only two directions as opposed to four directions if it is situated in the center of the board. To evaluate this we used the CS5203A linear regulator in a D 2 PAK mounted on test PCB measuring 139.7 mm (5.5in) by 83.82 mm (3.3 in) with 1.0 oz. and 2.0 oz. copper as shown in Figure 10. The boards each had a full copper backplane but there was no direct connection between the front pads and the backplane. The regulators were soldered on the boards and operated in identical conditions. All measurements were made in free air with the boards standing vertically. The case and ambient temperatures were measured and Rθ JA calculated in each case. Since the regulator is soldered directly to the tab we have assumed for all measurements. R CS 0 C W R JA (degrees C/Watt) 50.00 45.00 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Double Sided 1.0 oz. Copper Pd = 1.8 W Pd = 2.6 W Pd = 3.5 W 12.7 mm 19.1 mm 25.4 mm 38.1 mm Pad Length Figure 10. Rθ JA for D 2 PAK vs. Pad Length for Double sided 1 oz. Copper PCB. 8
R JA (degrees C/Watt) 50.00 45.00 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Double Sided 2.0 oz. Copper Pd = 1.8 W Pd = 2.6 W Pd = 3.5 W 12.7 mm 19.1 mm 25.4 mm 38.1 mm Pad Length Figure 11. Rθ JA for D 2 Pak vs. Pad Length for Double sided 2 oz. Copper PCB. U1 TP1 TP2 TP3 U1 U1 TP1 TP2 TP3 TP1 TP2 TP3 LEFT CENTER RIGHT Figure 12. Test PCB Layout Semiconductor Components Industries, LLC, 1999 November, 2000 Rev. 0 9 Publication Order Number:
R JA (degrees C/Watt) 38.00 37.00 36.00 35.00 34.00 33.00 32.00 31.00 30.00 29.00 Pd = 2.5 W Power/Ground Plane 1.0 oz. Copper Pd = 1.7 W 28.00 Left Right Center Pad Location (see board diagrams) Figure 13. Rθ JA vs. Pad Location for 1.0 oz. Copper PCB with Full Backplane 36.00 Power/Ground Plane 2.0 oz. Copper R JA (degrees C/Watt) 34.00 32.00 30.00 28.00 26.00 Pd = 2.5 W Pd = 1.7 W 24.00 22.00 Left Right Center Pad Locations (see board diagrams) Figure 14. R JA vs. Pad Location for 2.0 oz. Copper PCB with Full Backplane The case temperatures for the regulator in the center of the board was lower in all cases as shown in Figure 13 and 14. These boards had very good thermal characteristics overall, but keep in mind that they had a full copper backplane that helped greatly. Summary Always design for worst case conditions, use the maximum input voltage, maximum load conditions, maximum ambient temperature, and minimum output voltage. If the chosen package will not meet these requirements first consider other package options. If there is no suitable package available, heatsinking will be necessary to keep the junction temperature within specification. The heatsink can be implemented by bolting to a heat rail, adding a heatsink from one of the heatsink manufacturer, or adding copper foil to the PCB. The thermal shutdown circuitry in most regulators is there to prevent catastrophic failure only, do not make it an essential part of your design. Heatsink Manufacturers AAVID Thermal Technologies One Kool Path P.O. Box 400 Laconia, NH 03247 0400 Phone 603 528 3400 Fax. 603 528 1478 Thermalloy Inc. P.O. Box 810839 Dallas, TX 75381 0839 Phone 1 888 432 8746 Fax. 214 241 4656 Wakefield Engineering Inc. 60 Audubon Road Wakefield, MA 01880 Phone 617 245 5900 Fax. 617 246 0874 10
Notes Semiconductor Components Industries, LLC, 1999 November, 2000 Rev. 0 11 Publication Order Number:
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