VOLTAGE REGULATORS AND POWER CIRCUITS 312 Chapter 6

(unregulated) I 3A fuse

3 + heat

Figure 6.5. Five volt regulator with outboard pass transistor and crowbar.

and the 33 ohm resistor. Its func- fuse somewhere in the power supply, as tion is to short the output if some circuit shown. We will treat overvoltage crowbar fault causes the output voltage to exceed circuits in more detail in Section 6.06. about 6.2 volts (this could happen if one of the resistors in the divider were to open up, for instance, or if some component in the HEAT AND POWER DESIGN 723 were to fail). is an SCR controlled rectifier), a device that is nor- 6.04 Power transistors and heat sinking mally nonconducting but that goes into saturation when the gate-cathode junction As in the preceding circuit, it is often nec- is forward-biased. Once turned on, it will essary to use power transistors or other not turn off again until anode current is re- high-current devices like or power moved externally. In this case, gate current rectifiers that can dissipate many watts. flows when the output exceeds The an inexpensive power tran- voltage plus a diode drop. When that hap- sistor of great popularity, can dissipate as pens, the regulator will go into a much as 1 15 watts if properly mounted. limiting condition, with the output held All power devices are packaged in cases near ground by the SCR. If the failure that that permit contact between a metal sur- produces the abnormally high output also face and an external heat sink. In most disables the current-limiting circuit a cases the metal surface of the device is elec- short in then the trically connected to one terminal for crowbar will sink a very large current. For power transistors the case is always con- this reason it is a good idea to include a nected to the collector). HEAT AND POWER DESIGN 6.04 Power transistors and heat sinking 313

The whole point of heat sinking is to where P is the power being dissipated. keep the transistor junction (or the junc- Let's take an example. The preceding tion of some other device) below some power-supply circuit, with external pass maximum specified operating temperature. transistor, has a maximum transistor dis- For silicon transistors in metal packages sipation of 20 watts for an unregulated the maximum junction temperature is usu- input of +15 volts drop, 2A). Let's ally whereas for transistors in plas- assume that the power supply is to oper- tic packages it is usually 1 Table 6.1 ate at ambient temperatures up to lists some useful power transistors, along not unreasonable for electronic equipment with their thermal properties. Heat sink packaged together in close quarters. And design is then simple: Knowing the max- let's try to keep the junction temperature imum power the device will dissipate in below 1 well below its specified max- a given circuit, you calculate the junction imum of 200°C. The thermal resistance temperature, allowing for the effects of from junction to case is per watt. heat conductivity in the transistor, heat A TO-3 power transistor package mounted sink, etc., and the maximum ambient tem- with an insulating washer and perature in which the circuit is expected conducting compound has a thermal re- to operate. You then choose a heat sink sistance from case to heat sink of about large enough to keep the junction temper- per watt. Finally, a Wakefield model ature well below the maximum specified 641 heat sink (Fig. 6.6) has a thermal resis- by the manufacturer. It is wise to be tance from sink to ambient of about conservative in heat sink design, since per watt. So the total thermal resistance transistor life drops rapidly at operating from junction to ambient is about temperatures near or above maximum. per watt. At 20 watts dissipation the junc- tion will be 84°C above ambient, or 134°C (at maximum ambient temperature) in this Thermal resistance example. The chosen heat sink will be adequate; in fact, a smaller one could be To carry out heat sink calculations, you use used if necessary to save space. thermal resistance, defined as heat rise (in degrees) divided by power transferred. For heat transferred entirely by conduc- tion, the thermal resistance is a constant, Comments on heat sinks independent of temperature, that depends only on the mechanical properties of the Where very high power dissipation (sev- joint. For a succession of thermal joints eral hundred watts, say) is involved, forced in the total thermal resistance is air cooling may be necessary. Large heat the sum of the thermal resistances of the sinks designed to be used with a blower are individual joints. Thus, for a transistor available with thermal resistances (sink to mounted on a heat sink, the total thermal ambient) as small as to per resistance from transistor junction to the watt. outside (ambient) world is the sum of the 2. When the transistor must be insulated thermal resistance from junction to case from the heat sink, as is usually neces- the thermal resistance from case to sary (especially if several transistors are heat sink, and the thermal resistance mounted on the same sink), a thin in- from heat sink to ambient The sulating washer is used between the tran- temperature of the junction is therefore sistor and sink, and insulating bushings are used around the mounting screws. Washers are available in standard VOLTAGE REGULATORS AND POWER CIRCUITS 4 Chapter 6

TABLE 6.1. SELECTED BIPOLAR POWER TRANSISTORS

rnax rnax h I, min typ rnax npn pnp (V) (A) (A) (MHz) (W) Comments

Regular power: = (typ); = (typ) A 60 4 100 0.2 2 80 40 3.1 150 low cost, gen purp B 80 5 50 0.5 2 60 70 1.8 150 TO-3 60 15 50 2 2.5 125 115 1.5 200 metal, indus std B 60 10 50 2 2.5 125 90 1.4 150 plastic, indus std TO-3 80 25 50 10 4 400 200 0.9 200 TO-3 80 50 30 25 2 700 300 0.6 200 for real power jobs TO-3 100 25 50 8 40 200 200 0.9 200 premium audio TO-3 120 50 50 20 30 400 250 0.7 200 premiumaudio

Darlington power: (typ); = (typ) A 60 4 2000 2 - 30 40 3.1 150 B 80 8 2500 4 4 80 75 1.7 150 TO-3 100 12 3500 5 4 100 150 1.2 200 TO-3 100 20 3000 10 4 150 160 1.1 200 high current

A: plastic pwr (TO-126). large plastic pwr (TO-127). at

transistor-shape cutouts made from mica, relatively low power dissipation (a watt or insulated aluminum, or beryllia two) this often suffices, avoiding the nui- Used with heat-conducting grease, these sance of mounting the transistor remotely add from per watt (beryllia) to on a heat sink with its leads brought back about per watt. to the circuit. An example is shown in An attractive alternative to the classic Figure 6.6. In addition, there are vari- mica-washer-plus-grease is provided by ous small heat sinks intended for use with greaseless silicone-based insulators that are the plastic power packages (many regula- loaded with a dispersion of thermally tors, as well as power transistors, come conductive compound, usually boron ni- in this package) that mount right on a tride or aluminum oxide. They're clean printed-circuit board underneath the pack- and dry, and easy to use; you don't get age. These are very handy in situations of white slimy stuff all over your hands, your a few watts dissipation; a typical unit is electronic device, and your clothes. You illustrated in Figure 6.6. save lots of time. They have thermal resis- 4. Sometimes it may be convenient to tances of about per watt, com- mount power transistors directly to the parable to values with the messy method. chassis or case of the instrument. In such Bergquist calls its product "Sil-Pad," cases it is wise to use conservative design Chomerics calls its SPC (keep it cool), especially since a hot case calls it "Koolex," and Thermalloy calls will subject the other circuit components its "Thermasil." We've been using these to high temperatures and shorten compo- insulators, and we like them. nent life. 3. Small heat sinks are available that sim- 5. If a transistor is mounted to a heat ply clip over the small transistor packages sink without insulating hardware, the heat (like the standard TO-5). In situations of sink must be insulated from the chassis. HEAT AND POWER DESIGN 6.04 Power transistors and heat sinking 315

style part number thermal resistance -

Figure 6.6. Power transistor heat sinks. I, IERC; Thermalloy; Wakefield.

The use of insulating washers externally on the rear wall of the box), it is field model 103) is recommended (unless, a good idea to use an insulating cover over of course, the transistor case happens to the transistor Thermalloy be at ground). When the transistor is insu- to prevent someone from accidentally lated from the sink, the heat sink may be coming in contact with it, or shorting it attached directly to the chassis. But if the to ground. transistor is accessible from outside the in- 6. The thermal resistance from heat sink strument if the heat sink is mounted to ambient is usually specified for the sink VOLTAGE REGULATORS AND POWER CIRCUITS 316 Chapter 6

mounted with the fins vertical and with across the transistors when the current is unobstructed flow of air. If the sink is nearly zero (zero output voltage). With mounted differently, or if the air flow is a short-circuit load, on the other hand, obstructed, the efficiency will be reduced you have maximum load current at the (higher thermal resistance); usually it is worst possible time, namely, with full sup- best to mount it on the rear of the instru- ply voltage across the transistor. This re- ment with fins vertical. sults in much higher transistor dissipation than normal. EXERCISE 6.2 The brute-force solution to this problem A with a thermal resistance from junc- is to use massive heat sinks and transis- tion to case of per watt, is fitted with an tors of higher power rating (and safe oper- TXBF slip-on heat sink of the type shown ating area, see Section 6.07) than neces- in Figure 6.6. The maximum permissible junc- sary. Even so, it isn't a good idea to tion temperature is How much power can you dissipate with this combination at have large currents flowing into the pow- ambient temperature? How much must the dis- ered circuit under fault conditions, since sipation be decreased per degree rise in ambi- other components in the circuit may then ent temperature? be damaged. The best solution is to use current limiting, a circuit tech- nique that reduces the output current un- 6.05 current limiting der short-circuit or overload conditions. For a regulator with simple current lim- Figure 6.7 shows the basic configuration, iting, transistor dissipation is maximum again illustrated with a 723 with external when the output is shorted to ground pass transistor. (either accidentally or through some The divider at the base of the circuit malfunction), and it usually ex- limiting transistor provides the ceeds the maximum value of dissipation foldback. At +15 volts output (the that would otherwise occur under normal normal value) the circuit will limit at about load conditions. For instance, the pass 2 amps, since base is then at transistor in the preceding volt 2 amp volts while its emitter is at +15 regulator circuit will dissipate 30 watts about at the elevated temperatures with the output shorted (+ input, cur- at which regulator chips are normally rent limit at whereas the worst-case run). But the short-circuit current is dissipation under normal load condi- less; with the output shorted to ground, tions is 20 watts drop at 2A). The the output current is about 0.5 amp, situation is even worse in circuits in holding dissipation down to less which the voltage normally dropped by than in the full-load case. This is highly the pass transistor is a smaller fraction desirable, since excessive heat sinking of the output voltage. For instance, is not now required, and the thermal in a +15 volt 2 amp regulated supply design need only satisfy the full-load with +25 volt unregulated input, the requirements. The choice of the three transistor dissipation rises from 20 watts resistors in the current-limiting circuit sets (full load) to 50 watts (short circuit). the short-circuit current, for a given You get into a similar problem with full-load current limit. Warning: Use push-pull power amplifiers. Under normal care in choosing the short-circuit current, conditions you have maximum load cur- since it is possible to be overzealous and rent when the voltage across the transistors design a supply that will not "start up" is minimum (near the extremes ,of output into a normal load. The short-circuit swing), and you have maximum voltage current should not be too small; as a