Draft Issue 14 of 2017_01_01
WORK IN PROGRESS
This article describes the basic procedure for selecting power transistors: BJTs, MOSFETs and IGBTs for power circuits: audio power amplifiers, inverters, linear and switch-mode power supplies for example. To simplify the explanation, only a power bipolar junction transistor (BJT) will be described, unless otherwise stated, but the information applies equally to the other power devices.
Heat is a form of energy and can move from one location to another by conduction and radiation.
Heat conduction can be likened to the flow of water. Just as water flows from a high pressure to a lower pressure, so heat flows from a high temperature to a lower temperature. And just as a small tube will restrict the flow of water so materials have a thermal resistance and restrict the flow of heat.
In the parlance of thermal calculations the resistance to heat flow is measured in degrees Centigrade, Watt (DCW). Degrees Kelvin Watt (DKW) is also used but they are both the the same thing.
Radiation is the way that the sun's heat travels through space and warms the earth. There can be no conduction through a vacuum (free space) but, just the same as radio waves, heat can be radiated through a vacuum. In fact vacuum tubes (valves) rely heavily on radiation to dissipate heat from their electrodes.
TRANSISTOR POWER DISSIPATION
It is so tempting to think that all you need to do is chose a transistor which has a higher power rating than it will be dissipating in your circuit, but this in not the case. In fact the power rating of a transistor is mainly for marketing and has little practical significance.
The two overriding consideration are the transistor's maximum junction temperature (TJmax) and safe operating area (SOA).
MAXIMUM JUNCTION TEMPERATURE
The TJmax must not be exceeded, even momentarily. Typical maximum junction temperatures are 150 Deg C and 175 Deg C, but the ubiquitous 2N3055 and MJ2955 have an exceptionally high maximum junction temperature of 200 Deg C.
SAFE OPERATING AREA
The other area of concern is the transistor's safe operating area (SOA), which is temperature dependent. The SOA defines the maximum current that a transistor can conduct at the full range of voltages across its C/E. For example a 2N3055 can conduct 15A Amps at 7.5 Volts C/E, but only 3A Amps at 40 Volts C/E.
The SOA is also time dependent, so a transistor is able to handle a higher current for a short duration.
A heat-sink's functions are to,
(1) Conduct the heat away from the transistor mounting area by a thermal conducting material (ally or copper but stick with ally for explanation) with a low thermal resistance. This means thick for a sheet of aluminum.
(2) Once the heat has been conducted away from the transistor mounting area, present a large surface area to the air to efficiently transfer the heat to the air by convection. This is normally done by fins. The best fins are vertical because heated air rises, but there must be a free flow for the air through the fins (open top and bottom)
(3) The heatsink surface area must be in free flowing air as cold as possible. The transfer of heat from a heat sink to air is proportional to the difference between the heatsink surface area, the temperature of the heatsink, and the temperature of the air.
For example, if the heatsink surface is 60 deg C and the air in the area of the heatsink is also 60 deg C there will be no convection and no cooling of the heatsink. 60 deg C is not unusual inside equipments.
If the air was 70 deg C, again not uncommon in an equipment cabinet, the air would actually heat the heatsink up instead of cooling it!
There is stacks of data on the net and elsewhere about heatsinks and also about making heatsinks
Heat-sinks are specified by their thermal resistance in deg/CW, to free air This is a measurement of how much heat the heatsink can dissipate into the air A small heatsink would typically be 10 deg/CW and a large one 0.5 deg/C W
Although there are a few power transistors where the case is electrically insulated, normally the collector of a power transistor is connected to the metal part of the case, so unless the heatsink is going to be live, an electrically insulating, but thermally conductive, washer is required between the transistor case and the heatsink.
Alumina followed by mica have the lowest thermal resistance out of the freely available and low cost group of insulating washers. Mica has the best and most stable dialectic followed closely by alumina.
Aluminum oxide has an even lower thermal resistance but is expensive, while Beryllium oxide has the lowest thermal resistance, but is very expensive and is also toxic, so it is now banned. Diamond has the lowest of all thermal resistance, but obviously is not used for insulating washers.
Take an example:
A transistor has a maximum junction temperature of 170 deg C, as shown on the data sheet. That temperature must not be exceeded or the transistor will be destroyed.
The transistor is dissipating 50W. The maximum air temperature in the vicinity of the heat sink is 50 deg C.
Thus, the temperature difference between the transistor maximum junction temperature and the air is:
170- 50 = 120 deg C
The transistor dissipation is 50W so the maximum thermal resistance of the heatsink is 120/50= 2.4 deg C W, a perfectly achievable figure.
It would not be wise to operate the transistor with a maximum junction temperature. You only do that if you absolutely have no option.
A reasonable max junction temp design aim would be 90% of data sheet Tjmax.
So now, Tjmax_design = 0.9 * Tjmax_data_sheet = 0.9 * 170 = 153 deg C. That is now the design maximum junction temperature.
Doing the heatsink calculations again:
Tjmax_design- Tamb = 153- 50 = 103 deg
Now the heatsink thermal resistance is, 103 deg C/50W = 2.06 deg C W, a lower figure you will notice.
For an actual design, the thermal budget calculations are exactly the same priciple, excpt you would also have to take into account the thermal resistances between the junction and case, and case to heatsink. When you include these thermal resistances the situation becomes much more critical.
A typical thermal resistances from junction to case for a high power TO-3 transistor, is around 0.8 deg C W and, using a mica washer between the case and heatsink, the thermal resistance between the case and heatsink is 1.1 deg C W.
Finally, the total thermal resistance between the junction and the air is, 0.8 + 1.1 + heatsink thermal resistance.
(1) 2N3055/MJ2955 data sheet: http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF
(2) TIP35/TIP36 data sheet: http://www.onsemi.com/pub_link/Collateral/TIP35A-D.PDF