Keep it cool

Several of my recent posts have mentioned the very negative impact of heat on power consumption. This is the first of a two part series of posts on thermal management for low power devices. This information is mostly taken from my “Low Power Design” PDF e-book.

As semiconductor geometries have shrunk, in recent years leakage current has become a significant component of the overall power consumed by ICs. As parts heat up, their leakage current typically increases. It is not uncommon for parts to consume twice as much current at their highest rate temperature than at their lowest. For example, the AD8226 op-amp is rated for -40°C to 125°C. The quiescent current ranges from 325uA at -40°C to 425uA at 25°C to 600uA at 125°C. This is nearly a 100% increase across the temperature span and nearly a 50% increase from “room temperature” to the maximum temperature. You should conduct your current measurements at the temperature your product will normally operate at if not at the temperature extremes too.

 Controlling Heat

Even if you don’t have the luxury of airflow in your product, there are a number of things you can do to keep the heat under control and reduce the impact of heat on your power consumption:

  • If your product is vertically mounted, place as much circuitry as you can below the main heat generating parts.
  • If you have the option for voltage regulators and other heat generating parts, select packages with bottom side ground or thermal pads. Dissipating heat into the circuit board can help localize the heat buildup and maintain a lower air temperature within the enclosure.
  • For products that operate in high temperatures and have more than a few ICs, select part packages based on what you want to do thermally for a part. For parts that drive large loads or use high clock speeds it can be beneficial to select a package with a low thermal resistance to help get the heat away from the die. On the other hand, if you must place other parts near heat sources on your board you can choose a package with a higher thermal resistance for those other parts to reduce the heat transferred from the PCB to the die. Most surface mount ICs have several options for package styles that can have a wide range of thermal resistances. For instance, Texas Instruments offers the 74LV74 in 6 different packages with thermal impedances ranging from 47°C/W to 127°C/W.
  • Without forced air flow in your product, the junction-to-ambient thermal resistance spec probably isn’t relevant for selecting parts. You need to pay attention to the junction-to-case thermal resistance specs. Some manufacturers are specifying a junction-to-board thermal resistance which is even better. When comparing the junction-to-case thermal resistance of different packages you must understand where on the case this spec applies. Newer parts and particularly those in SMT packages intended for power applications will use the bottom of the case for this spec while older parts and non-power packages will likely use the top of the case since the assumption is air flow is used to dissipate heat and not the circuit board.
  • To fully realize the heat dissipating potential of low thermal coefficient packages with large tabs or bottom side thermal pads, you have to place several thermal vias in the pad to tie this pad to the internal ground plane or a large copper pour on the back of the PCB. You also need to pay special attention to the datasheet on parts with an exposed bottom side pad. These exposed pads are often connected to ground internally but not always and on some parts the pad may be the only ground connection for the part. On other parts, the pad may not be electrically connected and can be safely grounded or it may be the negative voltage supply on dual supply analog parts (which may or may not be ground in your design). The datasheets usually contain details on the size of the copper area and other PCB layout requirements to achieve the specified thermal resistances. The diagram below shows a typical arrangement for a DPAK and SMT DIP packages with a thermal copper pour and vias to connect to an internal ground plane or back-side copper pour. Most of the IC manufacturers that have packages with bottom side thermal pads provide app notes or even on-line calculators to help determine the minimum size of the copper area and number of thermal vias you need for a given package.

Footprints with Cu pour

  • With thermal vias, more isn’t always better since they can also disrupt the spread of heat across a copper pour or ground plane. The ground plane connections for vias are usually made with four thermal “spokes” and not a direct connection, some of these spokes may be missing if the vis are packed too close together. Some assembly houses may complain about thermal vias in device pads robbing solder from the pad. If so, reducing the via size or covering the via on the opposite side of the board with solder mask can help minimize the amount of solder that may seep into the via holes.
  • When using the PCB for heat sinking, keep in mind that FR4 and other laminates that PCBs are commonly made from are very poor thermal conductors. You are really spreading the heat through the copper in/on the PCB instead of transferring the heat into the PCB. It is best to use at least 2 oz copper for the outer layers and 1oz copper for inner layers. The heavier copper isn’t significantly more expensive than the “standard” 1 oz and ½ oz copper used for most PCBs but does provide significantly better heat transfer across the ground plane and copper pours.
  •  If you have traces on your board that carry more than a few amps, the traces themselves can be a significant source of heat if you aren’t careful. There are a number of good on-line PCB trace width calculators you can use to help prevent this. The maximum allowed temperature increase for the trace is an input for most of these calculators. If a trace normally carries high current you should set this to 5°C or 10°C. If the high currents are of a fairly low frequency and short duration, you can usually go as high as 25°C max temperature rise (but first make certain your product isn’t subject to any specifications that restrict the max temperature rise of traces). The overall trace resistance is also a function of trace length so you can reduce this resistance and resulting generated heat by shortening the trace length. Vias can have significantly higher resistance than copper traces causing them to generate additional heat so above a few amps you should use multiple vias instead of a single via.
  • To some degree the leakage current of an IC is influenced by the die size and circuit density on the die. For typical embedded devices the micro will usually have the largest die size and highest circuit density of the ICs in the device and therefore the highest leakage current. Simply increasing the space between heat generating parts and the micro can drastically reduce the amount of heat the micro is exposed to thereby decreasing its leakage current.
  • If possible, heat sink your heat generating parts to the enclosure. You may be able to do this directly but also indirectly be placing your heat generating parts near mounting holes on the board so the mounting hardware can help carry the heat out of the circuit board.
  • Pay the premium for more efficient voltage regulators. The power that an inefficient regulator wastes is generally turned into heat which can increase the power consumed. You aren’t likely to get into a thermal runaway condition but to some degree this increased power consumption generates more heat which increases power consumption which generates more heat …. you get the point.

That wraps up this part on thermal management. As you can tell while much of thermal management is in the circuit board layout, the extent that heat can be managed in the layout and the impact heat will have on the power consumption of your design are highly dependent on device and part package selection.

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