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

Transcription

1 312 Chapter 6 3A fuse (unregulated) I 3 + heat Figure 6.5. Five volt regulator with outboard pass transistor and crowbar. and the 33 ohm resistor. Its function is to short the output if some circuit fault causes the output voltage to exceed 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 723 were to fail). is an SCR controlled rectifier), a device that is normally nonconducting but that goes into saturation when the gate-cathode junction is forward-biased. Once turned on, it will not turn off again until anode current is removed externally. In this case, gate current flows when the output exceeds voltage plus a diode drop. When that happens, the regulator will go into a limiting condition, with the output held near ground by the SCR. If the failure that produces the abnormally high output also disables the current-limiting circuit a short in then the crowbar will sink a very large current. For this reason it is a good idea to include a fuse somewhere in the power supply, as shown. We will treat overvoltage crowbar circuits in more detail in Section HEAT AND POWER DESIGN 6.04 Power transistors and heat sinking As in the preceding circuit, it is often necessary to use power transistors or other high-current devices like or power rectifiers that can dissipate many watts. The an inexpensive power transistor of great popularity, can dissipate as much as 115 watts if properly mounted. All power devices are packaged in cases that permit contact between a metal surface and an external heat sink. In most cases the metal surface of the device is electrically connected to one terminal for power transistors the case is always connected to the collector).

2 HEAT AND POWER DESIGN 6.04 Power transistors and heat sinking 313 The whole point of heat sinking is to keep the transistor junction (or the junction of some other device) below some maximum specified operating temperature. For silicon transistors in metal packages the maximum junction temperature is usually whereas for transistors in plastic packages it is usually 1 Table 6.1 lists some useful power transistors, along with their thermal properties. Heat sink design is then simple: Knowing the maximum power the device will dissipate in a given circuit, you calculate the junction temperature, allowing for the effects of heat conductivity in the transistor, heat sink, etc., and the maximum ambient temperature in which the circuit is expected to operate. You then choose a heat sink large enough to keep the junction temperature well below the maximum specified by the manufacturer. It is wise to be conservative in heat sink design, since transistor life drops rapidly at operating temperatures near or above maximum. Thermal resistance To carry out heat sink calculations, you use thermal resistance, defined as heat rise (in degrees) divided by power transferred. For heat transferred entirely by conduction, the thermal resistance is a constant, independent of temperature, that depends only on the mechanical properties of the joint. For a succession of thermal joints in the total thermal resistance is the sum of the thermal resistances of the individual joints. Thus, for a transistor mounted on a heat sink, the total thermal resistance from transistor junction to the outside (ambient) world is the sum of the thermal resistance from junction to case the thermal resistance from case to heat sink, and the thermal resistance from heat sink to ambient The temperature of the junction is therefore where P is the power being dissipated. Let's take an example. The preceding power-supply circuit, with external pass transistor, has a maximum transistor dissipation of 20 watts for an unregulated input of +15 volts drop, 2A). Let's assume that the power supply is to operate at ambient temperatures up to not unreasonable for electronic equipment packaged together in close quarters. And let's try to keep the junction temperature below 1 well below its specified maximum of 200 C. The thermal resistance from junction to case is per watt. A TO-3 power transistor package mounted with an insulating washer and conducting compound has a thermal resistance from case to heat sink of about per watt. Finally, a Wakefield model 641 heat sink (Fig. 6.6) has a thermal resistance from sink to ambient of about per watt. So the total thermal resistance from junction to ambient is about per watt. At 20 watts dissipation the junction will be 84 C above ambient, or 134 C (at maximum ambient temperature) in this example. The chosen heat sink will be adequate; in fact, a smaller one could be used if necessary to save space. Comments on heat sinks Where very high power dissipation (several hundred watts, say) is involved, forced air cooling may be necessary. Large heat sinks designed to be used with a blower are available with thermal resistances (sink to ambient) as small as to per watt. 2. When the transistor must be insulated from the heat sink, as is usually necessary (especially if several transistors are mounted on the same sink), a thin insulating washer is used between the transistor and sink, and insulating bushings are used around the mounting screws. Washers are available in standard

3 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 low cost, gen purp B TO metal, indus std B plastic, indus std TO TO for real power jobs TO premium audio TO premiumaudio Darlington power: (typ); = (typ) A B TO TO high current A: plastic pwr (TO-126). large plastic pwr (TO-127). at transistor-shape cutouts made from mica, insulated aluminum, or beryllia Used with heat-conducting grease, these add from per watt (beryllia) to about per watt. An attractive alternative to the classic mica-washer-plus-grease is provided by greaseless silicone-based insulators that are loaded with a dispersion of thermally conductive compound, usually boron nitride or aluminum oxide. They're clean and dry, and easy to use; you don't get white slimy stuff all over your hands, your electronic device, and your clothes. You save lots of time. They have thermal resistances of about per watt, comparable to values with the messy method. Bergquist calls its product "Sil-Pad," Chomerics calls its SPC calls it "Koolex," and Thermalloy calls its "Thermasil." We've been using these insulators, and we like them. 3. Small heat sinks are available that simply clip over the small transistor packages (like the standard TO-5). In situations of relatively low power dissipation (a watt or two) this often suffices, avoiding the nuisance of mounting the transistor remotely on a heat sink with its leads brought back to the circuit. An example is shown in Figure 6.6. In addition, there are various small heat sinks intended for use with the plastic power packages (many regulators, as well as power transistors, come in this package) that mount right on a printed-circuit board underneath the package. These are very handy in situations of a few watts dissipation; a typical unit is illustrated in Figure Sometimes it may be convenient to mount power transistors directly to the chassis or case of the instrument. In such cases it is wise to use conservative design (keep it cool), especially since a hot case will subject the other circuit components to high temperatures and shorten component life. 5. If a transistor is mounted to a heat sink without insulating hardware, the heat sink must be insulated from the chassis.

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

5 316 Chapter 6 mounted with the fins vertical and with unobstructed flow of air. If the sink is mounted differently, or if the air flow is obstructed, the efficiency will be reduced (higher thermal resistance); usually it is best to mount it on the rear of the instrument with fins vertical. EXERCISE 6.2 A with a thermal resistance from junction to case of per watt, is fitted with an TXBF slip-on heat sink of the type shown in Figure 6.6. The maximum permissible junction temperature is How much power can you dissipate with this combination at ambient temperature? How much must the dissipation be decreased per degree rise in ambient temperature? 6.05 current limiting For a regulator with simple current limiting, transistor dissipation is maximum when the output is shorted to ground (either accidentally or through some circuit malfunction), and it usually exceeds the maximum value of dissipation that would otherwise occur under normal load conditions. For instance, the pass transistor in the preceding volt 2 amp regulator circuit will dissipate 30 watts with the output shorted (+ input, current limit at whereas the worst-case dissipation under normal load conditions is 20 watts drop at 2A). The situation is even worse in circuits in which the voltage normally dropped by the pass transistor is a smaller fraction of the output voltage. For instance, in a +15 volt 2 amp regulated supply with +25 volt unregulated input, the transistor dissipation rises from 20 watts (full load) to 50 watts (short circuit). You get into a similar problem with push-pull power amplifiers. Under normal conditions you have maximum load current when the voltage across the transistors is minimum (near the extremes,of output swing), and you have maximum voltage across the transistors when the current is nearly zero (zero output voltage). With a short-circuit load, on the other hand, you have maximum load current at the worst possible time, namely, with full supply voltage across the transistor. This results in much higher transistor dissipation than normal. The brute-force solution to this problem is to use massive heat sinks and transistors of higher power rating (and safe operating area, see Section 6.07) than neces- sary. Even so, it isn't a good idea to have large currents flowing into the powered circuit under fault conditions, since other components in the circuit may then be damaged. The best solution is to use current limiting, a circuit technique that reduces the output current under short-circuit or overload conditions. Figure 6.7 shows the basic configuration, again illustrated with a 723 with external pass transistor. The divider at the base of the limiting transistor provides the foldback. At +15 volts output (the normal value) the circuit will limit at about 2 amps, since base is then at volts while its emitter is at +15 about at the elevated temperatures at which regulator chips are normally run). But the short-circuit current is less; with the output shorted to ground, the output current is about 0.5 amp, holding dissipation down to less than in the full-load case. This is highly desirable, since excessive heat sinking is not now required, and the thermal design need only satisfy the full-load requirements. The choice of the three resistors in the current-limiting circuit sets the short-circuit current, for a given full-load current limit. Warning: Use care in choosing the short-circuit current, since it is possible to be overzealous and design a supply that will not "start up" into a normal load. The short-circuit current should not be too small; as a