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Power Dissipation: My Arch-Nemesis

When I started my career, I was working with the 4000 series CMOS devices. Unless they were running at very high speeds, they drew micro-amps, so any voltage regulation was handled by a simple linear regulator with no thought for heat sinking. Back in 1976, I was unaware of switch-mode power supplies, if they had been developed, and the linear regulator was all the rage. I seem to remember the LM317 being selected of the product of the year by Electronics magazine. (Anyone remember that?) I was lulled into a false sense of security.

A couple of years later, I moved on to NMOS micros like the 8085 and 8748. These drew tens of milliamps (or more), but 100 mA isn't much, is it? And yet the voltage regulators I was using were overheating. How could that be? Well, of course, I was using a high input voltage, and 10 V across the regulator at 100 mA is 1 W — well outside the absolute maximum of 750 mW for a TO92, and in reality pretty stressful for a TO-220 package without heat sinking.

Over the years, I have developed a rule of thumb: If I place my thumb on a component, and I can't keep my finger there, it is too hot. My thumb seems to be calibrated to about 55°C. Now, I know that cost is the imperative for some engineers, and they need to design right up to the limit, taking into account the maximum ambient temperature combined with the temperature gradient across the heat sink and package arriving at the maximum junction temperature. I work in a different world. We produce very small volumes, and the prototype is the final unit. It has to work the first time, so I give myself plenty of margin for error.

Most of my designs are still simple, with few ICs (one is often a microcontroller) in a limited space, so I still go for linear regulator. The newer complete switch-mode regulators (like the TI LMZ14201) can be very helpful, if a little pricy. But a mentor of mine once said, “When it comes to power dissipation, there is no magic. Bigger is better.” The micro may be speced at a particular current, but this current is not the amount drawn when an emulator is plugged in or there is some malfunction on the circuit. After several instances of overheating, I started to upsize. For instance, I use a DDPAK, rather than a SOT23, for a regulator package.

Often I have designed two linear regulators in series or a series zener before the regulator in order to distribute the heat and reduce the individual component power dissipation. Also, if I am driving LEDs or relays, I will power them directly from the incoming power (and switch them through open collector transistors or something similar) to save the additional current through the regulator. Even a relatively small temperature rise can affect your measurements in a case like a current shunt, and I have used two or more in parallel to limit this effect, though this introduces a conundrum if you are looking for a Thevenin connection.

Wait. There's more that I would like to point out. Make sure you read your components specification completely. Let's look at the PR02 resistor. This is supposed to be a 2W resistor, but look at page 10 of the data sheet. The hotspot temperature at the 1.6W power level leads to a temperature rise of around 120°C. The resistor itself may handle that temperature, but it will heat up everything around it. I have seen PCBs suffering from heat damage as a result of hot resistors, and it may even get hot enough to melt the solder connecting it to the board. The operating temperature will certainly fail my thumb test.

You may think that this resistor is unique. It isn't. Take a look at the Heat Rise Chart on this data sheet, which indicates that a 5W resistor (from this particular manufacturer) will go up more than 100°C at only 3 W.

Something else to note from the PR02 data sheet (on the same page) is that the length of the leads and the height above the PCB affect the power dissipation, as well. This is not restricted to resistors. Here is the data for an MR751 diode. Pay attention to Figure 3.

Another thing you should be aware of is derating. When the ambient temperature increases, the part's ability to dissipate its own heat drops off to zero. All the data sheets I've cited so far include a derating curve.

Now, I don't design switch-mode power supplies (SMPS), but I know enough that heat dissipation is a problem there, as well. In particular, I am told you need to pay attention to switching the MOSFETs with a sharp edge, or the transistor will be on for a period of time, and the conduction of current in the linear zone will generate heat. The coils, of course, must also be correctly rated — a topic that is way beyond my knowledge frontier.

You would think that capacitors wouldn't get hot, since they carry only reactive power. That's the theory, anyway. But capacitors, like everything (and everyone) else, are imperfect. There is a property of capacitors called equivalent series resistance (ESR), and the heating by any AC current is the same I2R as any resistor, so the capacitor can get warm, as well. This phenomenon is not restricted to a SMPS. Where safety is not an issue, there is a technique where a mains AC input is rectified and regulated to a low DC voltage with a device like the Supertex/Microchip SR10 regulator. A large portion of the voltage is dropped across a capacitor, and the ESR must factor into that.

Heat dissipation concerns are not restricted to discrete components. Integrated circuits will generate heat through the sheer number of transistors that are switching and through the current being sunk or sourced at their I/O pins. In today's world of miniaturization and surface mount technology, power dissipation has become even more of an issue. The use of copper thermal pads on the underside of the IC package is becoming very common, necessitating the development of heat spreading techniques on the PCB, including multiple vias for large surface areas on the top and the bottom of the board.

My advice to you: Whenever you introduce a component into a design, consider its power dissipation first. Unfortunately, it seems to be a case of “Do as I say, not as I do.” Even though 38 years of experience should have taught me that, it always comes back to bite me. Only last week, I made an error with 200 V across a high-voltage linear regulator, the LR8. The current through the voltage biasing resistors was a measly 4 mA with no other load, and 800 mW is just too much for a TO92.

Do you have burn callouses on your fingertips, as well?

15 comments on “Power Dissipation: My Arch-Nemesis

  1. David Ashton
    September 4, 2014

    Good practical advice as always Aubrey.  I also use the thumb test.  I recently had to fix a 5v regulator built into a lighter plug – the original switched mode reg had given up the ghost.  It was for a GPS that draws in excess of 500 mA.  I used a 7805 for simplicity – with a bit of a heatsink – but it is dissipating in excess of 500mA x 7V = 3.5 W and it gets very warm.  I think I will have to put a switcher back in there.  NS (now TI of course) simple switchers should do the job, and if I get a PCB done I can use it in the future I am sure…..

  2. Victor Lorenzo
    September 4, 2014

    I remember a colleague from the 90's that had a very displeasant experience with one CD4000 CMOS chip.

    Something seamed to be hot in the board and he started touching the chips. One CD4000 entered latch-up and… well, the metallic pins soldered to the board and some pieces from the encapsulation were the only lasting things from the chip.

    The board was a unique prototype, working at 12Vdc with no fuse or current limiting at all, too bad.

  3. antedeluvian
    September 4, 2014

    David

     I used a 7805 for simplicity – with a bit of a heatsink – but it is dissipating in excess of 500mA x 7V = 3.5 W and it gets very warm.  I think I will have to put a switcher back in there.  NS (now TI of course) simple switchers should do the job, and if I get a PCB done I can use it in the future I am sure…..

    It's a pity that TI discontinued the PT5101N, a SMPS which would plug right into the TO220 socket. If you just need one for replacement, I think I could scare one up- just let me know.

    If you are going to make the PCB anyway you may want to look at the TI LMZ14201 (that i mentioned above).  It is big enough to handle by hand, although the ground pad/heat sink on the underside is more of a problem for hand assembly. It appears to be a simple switcher with the inductor integrated into the package.

     

  4. antedeluvian
    September 4, 2014

    Victor

    One CD4000 entered latch-up and… well, the metallic pins soldered to the board and some pieces from the encapsulation were the only lasting things from the chip

    I used to have a CMOS chip (as a memento) where the silicon got so hot it melted the plastic such that there was this rectangular crater all the way down. It almost looked like a demonstration sample.

     

  5. David Ashton
    September 4, 2014

    @Aubrey – you just reminded me that I have some 78ST212s which I got out of some old plug in disk drive modules years ago when an old server was being chucked out.  Again plug in replacement for TO220 and 2A capability.  Simlar sort of thing to your recommendations, just a bit older and bulkier, but I think they would fit.  I should have thought of them when I first did this.  Thanks for the memory jog!

    PS tried to link to the 78ST212 datasheet but I think that is stopping me posting, you can google it if you need.

  6. goafrit2
    September 5, 2014

    In ASIC especially in digital designs, one area I pay a lot of attention is static power dissipation. I know that my dynamic power is always what anyone will have interest when they know my clock speed and other factors. But static power is one key element that defines a good product. Do not overlook what it means to design with CMOS and ensures that when the device is OFF, no energy is leaking.

  7. goafrit2
    September 5, 2014

    As we move closer to the nanometer CMOS region, power dissipation will become an increasingly big issue for designers. There are many factors in play here. I think power management will remain a great competitive weapon in the industry. Those that can extend the battery lives of products will win the mobile era.

  8. Netcrawl
    September 5, 2014

    @goafrit2 you're right power management will always remain a great competitive force here, the market for power management is growing fast, fresh demands from alternative energy and consumer electronics is driving much of the demand. There's also some great development in this area, new semiconductor technologies such as the CMOS-7HV hwihc can help electronics companies sew up power leakages that will enable the use of smaller more powerful batteries.  

  9. Netcrawl
    September 5, 2014

    @goafrit2, low power design has become a major concern for all IC designers in recent years because of the colling and packaging considerations, design techniques such as the asynchronous design, although more complex than the usual and common synchronous design is more power efficient. Because the power hungry clock circuitry is a major part of a total power consumption. 

  10. fasmicro
    September 6, 2014

    >> he market for power management is growing fast, fresh demands from alternative energy and consumer electronics is driving much of the demand. 

    Mobile is the beast driving this market in the consumer sector. The problem with power management is that most of the top universities do not pay a lot of attention to it in the VLSI programs. 

  11. fasmicro
    September 6, 2014

    >> design techniques such as the asynchronous design, although more complex than the usual and common synchronous design is more power efficient

    Theoretically in asynchronous design, you do not have a lot of power problems. Remember that you have no clock and that means in the dynamic power, you have zero Watts.

  12. goafrit2
    September 10, 2014

    As mobile continues to shape all industrial sectors, power management will remain a growth business. What has surprised me though is how we are not taking advantage of the asynchronous CMOS design paradigm to solve some of these issues.

  13. fasmicro
    October 7, 2014

    >> Because the power hungry clock circuitry is a major part of a total power consumption. 

    In asynchronous design, there is no clock which makes the argument invalid. Theoretically, you can have a power of zero since there is no frequency in asynchronous design!

  14. zeeglen
    December 15, 2014

    in a case like a current shunt, and I have used two or more in parallel to limit this effect, though this introduces a conundrum if you are looking for a Thevenin connection.

    This is an interesting thought. I would think that as long as the 2 shunt resistors are equal in value, including the copper etch that symmetrically carries the current from a split to the resistors, it should be possible to make the voltage sense connection to both shunt resistors using an additional split etch separated from the current carrying etch.

    I well remember a colleague who did not feel he needed to heat sink a linear regulator to the sides of a small metal box that would house his finished design.  I gave him a quick&dirty demonstation by putting a mockup of the circuit into a small cardboard matchbox, the type used for wooden strike-anywhere matches.  Within just a few minutes the regulator overheated and shut down – he then figured out to heatsink the regulator to the real enclosure.

  15. etnapowers
    December 16, 2014

    The overheat protection is really important for each voltage regulator whatever it is linear or switching. The two resistors have to be correctly matched, this imply the need of realizing a layout optimized having the two resistors very close each other.

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