Heat Sinks for Switching Power Supplies
2.3 Heat Sinks for ICs
immediately after machining, the mounting area should be polished with No. 000 steel wool, then rinsed with acetone or alcohol, and coated immediately with ther
mal grease or compound.
Many aluminum heat sinks are black-anodized for appearance, durability, performance, and economy. Anodizing is an electrical and thermal insulator that offers resistance to heat flow. As a result, anodizing should be removed from the mounting area.
Another aluminum finish is irridite (chromate acid dip), which offers low resis
tance because of the thin surface. For best performance, the irridite finish must be cleaned of oils and films that collect when the heat sinks are manufactured and stored.
Some heat sinks are painted after manufacture. Because paint of any kind has a high thermal resistance (compared to metal), paint must be removed from the heat- sink surface where the component is attached.
2.2.4 Thermal Compounds
Thermal compounds (also called joint compounds or silicon greases) are used to fill air voids between the mating surfaces so as to improve contact between the component and heat sink. A typical compound has a resistivity of about 60°C/in./W, compared to about 1200°C/in./W for air. As a result, the thermal resistance of voids, scratches, and imperfections filled with a compound or grease is about one-twenti
eth of the original resistance.
Thermal compounds are a formulation of fine zinc particles in a silicon oil that maintains a greaselike consistency with time and temperature. There are two com
mon methods for applying the compounds. With one technique, the compound is ap
plied in a very thin layer with a spatula or lintless brush, and then the surface is wiped lightly to remove excess material. The other technique involves applying a small amount of pressure to spread the compound. After the mounting is complete, any excess is wiped away using a cloth moistened with acetone or alcohol.
Some recommended thermal compounds follow:
Astrodyne—Conductive Compound 829
Dow Corning—Silicon Heat Sink Compound 340 Emerson & Cuming, Inc.—Eccotherm, TC-4 General Electric—Insulgrease
George Risk Industries—Thermal Transfer Compound XL500 IERC—Thermacote
Wakefield—Thermal Compound Type 1201
The maximum allowable power dissipation (usually specified as ΡΌ or maxi
mum device dissipation on the data sheet) is a function of the maximum storage tem
perature Ts, the maximum ambient temperature TA, and the thermal resistance from the chip to case. The basic relationship is ΡΌ = (Ts - 7A)/thermal resistance.
Not every switching-regulator IC data sheet necessarily lists all of these pa
rameters. It is quite common to list only the maximum power dissipation for a given ambient temperature, and then show a derating factor in terms of maximum power decrease for a given increase in temperature.
For example, one of the micropower switching-regulator ICs (the Raytheon RC4190) discussed in Chapter 4 has a maximum power dissipation (or PD) of 833 mW (in the ceramic DIP package) at an ambient temperature of 50°C, with a derat
ing factor of 8.33 mW/°C for ambient temperatures above 50°C. If the IC is operated at 100°C ambient, the maximum power is as follows: 100°C - 50°C = 50°C; 50 x 8.33 mW = 416.5 mW; 833 mW - 416.5 mW = 416.5 mW. If the ambient is in
creased to 125°C, then the maximum power is: 125°C - 50°C = 75°C; 75 x 8.33 mW = 624.75 mW; 833 mW - 624.75 mW = 208.25 mW.
In the absence of specific data-sheet information, the following typical char
acteristics can be applied to common IC package types used for switching regula
tors. No IC should have a temperature in excess of 200°C, 175°C being a far safer limit. The following are absolute maximum values.
Ceramic DIP Package
Maximum junction temperature—+175°C Thermal resistance 0JC—45°C/W
Thermal resistance 0JA— 150°C/W
Storage temperature range 65 to +150°C Operating temperature range—0 to +70°C Plastic DIP Package
Maximum junction temperature—+125°C Thermal resistance 0JA— 160°C
TO-3 Package
Operating junction-temperature range 55 to 150°C Thermal resistance 9JC— 1.5°C/W
Thermal resistance 0JA—35°C/W
Storage temperature range 65 to 150°C TO-220 Package
Operating junction-temperature range 40 to +125°C Thermal resistance 0JC—2.0°C/W
Thermal resistance 9JA—55°C/W
Storage temperature range 40 to +125°C
2.3.I Working w/f/i Switching-Regulator ICs
Many present-day switching-regulator ICs require low power (typically less than 1 W) and can be operated without heat sinks. However, most switching ICs used in higher power applications do require heat sinks (either an external heat sink or by direct contact with a metal surface). Power ICs generally use some form of metal package (TO-3, etc.).
The data sheets for these power ICs usually list sufficient information to select the proper heat sink. In addition, the data sheets or other literature often recommend how to mount the power ICs. Always follow the IC manufacturer's recommenda
tions. In the absence of such data, and to familiarize the reader with the terms, the following sections summarize considerations for power ICs used in switching regu
lators.
2.3.2 Maximum Power Dissipation
From a simplified-design standpoint, an IC regulator is a complete, pre
designed, functioning circuit that cannot be altered in regard to power dissipation.
That is, if the power-supply voltages, input signals, output loads, and ambient tem
perature are at the recommended levels, the power dissipation will be well within the capabilities of the IC.
With the possible exception of the data required to select heat sinks, the sup
ply designer need only follow the data-sheet recommendations. There are worked- out design examples for switching supplies, using off-the-shelf ICs, in Chapter 5.
These include calculations for operating with heat sinks and/or for operating the IC at a lower power-dissipation capacity because of increased temperature (derating factors).
2.3.3 Thermal Resistance for Switching-Regulator ICs
ICs designed for switching regulator applications usually have some form of thermal resistance specified to indicate the power-dissipation capability. This can be 9JC or GJA, or both, in addition to maximum device dissipation, or PD. As in the case of discrete transistors, regulator IC characteristics can change with ambient-temper
ature changes and with changes produced by variations in power dissipation (such as changes in load current passing through the IC). As discussed in the following paragraphs, most switching-regulator ICs have circuits to offset the effect of temper
ature.
In most applications, thermal resistance is measured from the semiconductor chip to the case. On those regulator ICs where the cases are bolted directly to the mounting surface with a threaded bolt or stud (similar to that shown in Fig. 2-2), the thermal resistance is measured from the chip to the mounting stud or flange.
2.3.4 Thermal Runaway in Regulator ICs
Regulator ICs are subject to the problems of thermal runaway (as discussed for transistors in Section 2.1.5). However, unlike discrete transistors, most ICs have built-in circuits to prevent thermal runaway. A typical arrangement is to place a diode in the reverse-biased circuit for one or more of the transistors in the IC. The diode is fabricated on the same chip as the transistors and thus has the same temper
ature characteristics.
This thermal-protection circuit is arranged so that the reverse bias is increased (by a decrease in diode resistance) when there is an increase in temperature. When temperature increases because of an increase in internal transistor current (or vice versa), the diode resistance changes and increases the reverse bias. This offsets the initial change in current caused by temperature changes.
2.3.5 Thermal and Excess-Current Shutdown
Most switching-regulator ICs (such as those described in Chapter 5) have a built-in thermal-shutdown circuit. If the IC temperature reaches a given value (typi
cally 175°C but possibly 150°C), the circuit turns the IC off. Most regulator ICs also have a current-limit provision for turnoff in case of excessive current flow through the IC. In some regulators, the current-limit is an automatic function within the IC.
In other regulators, the current-limit turnoff is controlled by components external to the IC.
2.3.6 IC Heat-Sink Calculations
The calculations for IC heat sinks and power dissipation are essentially the same as for transistors described in Section 2.1 ; they are not repeated here. There are worked-out design examples in Chapter 5.
Where there are a large number of ICs (as well as transistors and rectifiers) operated in a confined area, heat sinks may not do the job. The designer may want to consider the use of forced air (such as from a fan or blower) to keep the ambient air within an acceptable tolerance. Forced air is often used in commercial power sup
plies where large operating currents are involved.