What Size Heatsink Do I Need?

Today there are a large array of high power modules in the range of 300 to 1,000-watts, both DC-DC converters as well as AC-DC power modules, which are commonly referred to as “bricks.” Even though these devices feature high conversion efficiencies in the area of 85 to 90% (or higher), some power is lost in the form of heat that must be dealt with in order to maximize the lifespan of the end product. For example, a 500-watt power module with 90% conversion efficiency would generate over 55-watts of wasted heat within the module that must be removed to maximize its reliability.


The concentrated power density (watts per cubic inch) within these power modules make them a challenge to cool in real world applications. Most high power bricks are packaged in thermally conductive plastic or epoxy cases with integral metal baseplates. The high power components within the bricks (i.e., semiconductors, inductors, transformers, etc.) are thermally coupled to these baseplates, which in turn can be attached to external heatsinks or liquid-cooled cold plates in order to keep the baseplate at or below its maximum operating temperature (typically 85 to 100°C).

Half Brick – DC-DC Converter

Full Brick – AC-DC Power Module

The maximum baseplate temperature is primarily determined by the maximum internal junction temperature of the semiconductors within the power bricks. The term “thermal management” refers to the designer’s challenge of cooling these power bricks by considering the many levels for heat transfers via conduction (direct contact between solids), convection (contact with air or a fluid) and thermal radiation (electromagnetic infrared energy), both internal and external to the power module.

The diagram above shows the series-connected thermal resistances that impede the flow of heat from one level to the next. These impedances need to be considered, beginning with the internal semiconductor’s junction temperature relative to its case, the thermoplastic module case and its metal baseplate, and ending with a mechanically attached heatsink that conducts away the heat from the baseplate to the surrounding ambient air via natural or forced air convection cooling. Heatsinks are designed to cross thermal barriers primarily by substantially increasing the surface area that comes in contact with the ambient air, thereby providing enhanced convection cooling. Because the mating surfaces of the power module’s baseplates and heatsinks are not perfectly flat, some type of thermally conductive interface material is required to fill the tiny voids. This interface material can range from a thin layer of thermal grease to a custom designed silicon pad.

AC-DC Power Module with Heatsink & Other Components

Selecting the proper size and shape of a heatsink and determining if forced air cooling is required are among the tradeoffs the designer needs to consider. This process begins with a detailed review of the power module’s specifications and knowledge of the end product’s heat loads, internal and external operating temperatures, space constraints, and available air flow sources, paths and restrictions.

The next step in this process is to determine the amount of power that will be lost (wasted) within the power module, based on its efficiency. This information for computing this is usually listed on the power module’s datasheet or installation manual, but it can also be determined by actual measurements of the input and output powers. For this example, we will use a typical AC-DC power module with a 48V/10.5A, 504W output rating, and a typical efficiency of 85% with a 120VAC input. By the way, the 85% efficiency rating is very good considering the fact that this module contains full-bridge rectification and active power factor correction AC front-end circuits as well as an integral DC to DC converter. In addition, this module has a maximum operating baseplate temperature, as measured at its center point, of 100°C.

Based on the above information, to compute the internal power dissipated (wasted heat); we can use the following formula:

Pd = (Pout / η) – Pout
Definitions & Calculation Example:
Pd : Internal Power Dissipated (W)

Pout : Output Power (504W)

η : Efficiency (85%)

Pd = (504W / 0.85) - 504W = 88.9W
To calculate the required baseplate to ambient air thermal resistance that would be needed for this application, the following formula would apply:
θba = Tb - Ta / Pd
Definitions & Calculation Example:
θba : Baseplate to Ambient Air Thermal Resistance (°C/W)

Tb : Baseplate Temperature (100°C)

Ta : Ambient Air Temperature (40°C)

Pd : Internal Power Dissipated (88.9W)
θba = 100°C - 40°C / 88.9W = 0.67°C/W

In this example, we would need a heatsink (with or without air flow) that provided a thermal resistance of 0.67°C/W. However, unless the heatsink includes a thermal interface material like thermal grease or a pad in its rating, we need to account for this additional thermal contact resistance (θbs), which can be on the order of 0.1°C/W. Therefore, the required thermal resistance of the heatsink itself, with the thermal interface material included, can be calculated per this formula and example:

θba-bs = θba – θbs

θba – θbs = 0.67°C/W - 0.1°C/W = 0.57°C/W

The next step in this process is to review specifications for potential heatsinks that have a thermal resistance of 0.57°C/W. In this case, the power module has three optional heatsinks to choose from as shown in the chart below.

The Y axis of this chart shows the thermal resistance between the heatsink and the air (°C/W) and the X axis shows the required airflow velocity for the three heatsinks. In this example, we need to find 0.57°C/W along the Y axis and then move to right along the X axis to where it intersects a heatsink curve. In this example, 0.57°C/W intersects with the HAF-15T heatsink curve at about the 1 m/s airflow velocity point. Therefore, for this application we would select the model HAF-15T heatsink and would have to provide forced air cooling with an air velocity of 1 m/s. To translate m/s (meters/second) into LFM (linear feet/second), use this general conversion factor: 1 m/s = 200 LFM. In this example, since 1 m/s = 200 LFM of forced-air velocity, the fan required for this application must provide 200 LFM.
Based on the above, we have now determined the requirements for cooling this power module with a heatsink, thermal compound and forced air flow. If we wanted to be more conservative and improve the MTBF of the module, we would recalculate the required heatsink with the assumption that we wanted to keep the baseplate temperature at 85°C.

A word or caution should to be injected here. After going through the thermal calculations and selecting a heatsink, air flow, etc., the next step is to confirm the “paper-design” by running actual tests on a sample unit. The tricky part is to get access to the center point of the power module’s baseplate so you can measure the temperature at that point while the module is operating under load. One way to do this is to drill a hole in the center of the heatsink so the leads from a thermocouple can be mounted on the module’s baseplate and routed to your temperature measurement device.
In summary, we have shown how to determine the correct heatsink for power module applications. As the efficiencies of these devices improve the need for cooling will reduce, but the designer should always be aware of the heating effects from not only from the power module, but also from nearby devices. Therefore, it’s always best to run actual thermal tests with thermocouples attached to the power module and inside the end product to insure the design will be as reliable as possible.

Power Supply "Remote Sense" Mistakes and Remedies

Most medium to high power AC-DC power supplies and some DC-DC converters include "Remote Sense" connection points (+ and - Sense) that are used to tightly regulate the supply's output voltage at the load. Since the output cables that connect a power supply's output to its load have some resistance, as current flow increases, so will the voltage drop across the cables (I x R = Voltage Drop). Moreover, since it's best to regulate the voltage directly at the load, the use of the two Remote Sense wires connected from the supply to the load will compensate for these unwanted voltage drops. Refer to Fig. 1 which shows the typical connections when the Remote Sense function is used.

Fig. 1: Power Supply with Twisted "Remote Sense" Wires Connected to the Load

Typical "Remote Sense" Problems & Remedies

1. Most remote sensing circuits are capable of compensating for from 0.25V to 0.75V of voltage-drops across the output cables. However, to be sure, always check your power supply's instruction manual to determine its maximum remote sense compensating range. If the voltage drop across the output cables exceeds the compensating range of the remote sense circuits, the voltage at the load will no longer be regulated. This problem can be remedied by either reducing the length of the output cables or increasing the size (heavier wire gauge) of the output cable's to reduce the excessive voltage drop. Voltage drops across the output cables should be minimized since this is a source of wasted power. For example, with just a 0.5V cable drop with a 100A load, the lost power amounts to 50W in each cable or 100W total.
2. The remote sense function automatically increases the output voltage at the output terminals of the supply to compensate for any unwanted voltage drop in the output cables with heavy load currents. Likewise, the remote sense function decreases the output voltage of the supply when the required load current is reduced. In some applications, the power supply's output needs to be adjusted by the user to voltage higher than its nominal (e.g. 5V nominal, adjusted to 5.5V). Always adjust the power supply's output while measuring the voltage at the load. In addition, care should be taken to assure that under full load that the remote sense function does not push the Vout to a higher voltage that could possibly trip the OVP set-point and shutdown the supply. Therefore, always read the power supply's instruction manual to be aware of the supply's adjustment range and OVP set-point.
3. The remote sense leads carry very little current so light gauge wires can be used. However, steps should be taken to ensure that the remote sense wires do not pick up radiated noise by either twisting the + and - Sense wires together and/or by shielding the wires from the noise (refer to Fig 1). It is best to use different colored sense wires (e.g., black and red) so that after they are twisted it is easy to determine which wire is the + and – Sense.
4. Refer to Fig. 2 below for a simplified schematic of a power supply's remote sense circuits. It is important to observe the correct polarities, i.e., the +Sense wire should connect at the load near the +Vload connection and the –Sense wire should connect at the load near to the – Vload connection. If by mistake the remote sense wires are crossed-connected (+Sense to –Vload and – Sense to +Vload) current will flow in the Sense lines and burn out the internal Rsense resistors, causing a malfunction of the supply. Typically, these internal Rsense resistors are around 10 to 100 Ohms with a maximum rating of 0.5W.

Fig. 2: Simplified Schematic of Remote Sense Circuit with External Output & Sense Wires

5. We have seen applications where the user has installed a switch or fuse in series with one or both output wires. This can cause a serious problem if the remote sense lines remain connected to the load, because if the output cable switch or fuse opens, current will flow in the sense lines and cause the internal Rsense resistors to burn up. System debugging can cause similar problems, for example, where the power and sense cables are located on separate connectors and if by error, only the power cable connector is disconnected. 
6. There are applications where the user may not want to use the remote sense feature. In these cases, the remote sense lines should not be left open for optimum load regulation; instead, a local sense configuration must be used. Referring to Fig. 3, to use a local sense set up the + and -Sense lines should be connected to either their corresponding local sense (LS) terminals, which are provided on many power supplies, or connected to the corresponding +Vout and –Vout terminals. Most power supplies are shipped from the factory with these "Local Sense" jumpers installed on the power supply (see photos below).

Fig. 3: Schematic of Power Supply with "Local Sense" Jumpers Installed

Photo of Power Supply with Local Sense Wires Connected (see Red & Black jumper wires)

Photo of PSU with Sense Screw Terminals Connected to Output Screw Terminals with Metal Jumpers

In summary, the "Remote Sense" feature automatically compensates for unwanted output cable drops, which vary as the output current increases and decreases. This feature is advantageous to the user, but is subject to mistakes that should be avoided to insure the proper operation of the power supply and the end-product.

What does SELV mean for Power Supplies?

SELV stands for Safety Extra Low Voltage. Some AC-DC power supply installation manuals contain warnings concerning SELV. For example, there may be a warning about connecting two outputs in series because the resulting higher voltage may exceed the defined SELV safe level, which is less than or equal to 60VDC. In addition, there may be warnings about protecting the output terminals and other accessible conductors in the power supply with covers to prevent them from being touched by operating personnel or accidently shorted by a dropped tool, etc.

UL 60950-1 states that a SELV circuit is a “secondary circuit which is so designed and protected that under normal and single fault conditions, its voltages do not exceed a safe value.” A “secondary circuit” has no direct connection to the primary power (AC mains) and derives its power via a transformer, converter or equivalent isolation device.

Most switchmode low voltage AC-DC power supplies with outputs up to 48VDC meet the SELV requirements. With a 48V output the OVP setting can be up to 120% of nominal, which would allow the output to reach 57.6V before the power supply shuts down; this would still conform to the maximum 60VDC for SELV power.

In addition, an SELV output is achieved through electrical isolation with double or reinforced insulation between the primary and secondary side of the transformers. Moreover, to meet SELV specifications, the voltage between any two accessible parts/conductors or between a single accessible part/conductor and earth must not exceed a safe value, which is defined as 42.4 VAC peak or 60VDC for no longer than 200 ms during normal operation. Under a single fault condition, these limits are allowed to go higher to 71VAC peak or 120VDC for no longer than 20 ms.

Don’t be surprised if you find other electrical specs that define SELV differently. The above definitions/descriptions refer to SELV as defined by UL 60950-1 and other associated specs regarding low voltage power supplies.

Power Supply Rise and Fall Output Characteristics

A common question asked by our customers is “what are the power supply’s rise and fall output characteristics?” Our usual answer “it depends” sometimes raises an eyebrow. Let’s have a look at why.
As you know, there are two ways a power supply can be turned-on and off. One method is to apply or remove the AC input power via a switch or circuit breaker to the supply. Another method is use the Remote On/Off control of the supply, if the unit has this feature. Let’s examine both methods.

The curves below (Fig. 1) show the typical delay between when the AC input power (Vin) is applied and the power supply’s output reaches its rated voltage of 12V under full load conditions. As can be seen (in this example) with a low AC input of 85VAC the output turn-on delay is slightly more than with higher input voltages. However, in this example, the typical turn-on delay is about 250ms and the output rise time is about 25ms.



Conditions:

Vin	: 85VAC (A)
	: 115VAC (B)
	: 230VAC (C)
	: 264VAC (D)
lout	: 100%
Ta	: 25°C

Fig. 1

The next curves (Fig. 2) show the same power supply’s output (with 100% load) when the AC input is removed. The time delay from when the AC power is removed (or lost) is referred to as the Hold-Up time spec. For this supply, the specified minimum hold-up time is 16ms. As shown below, the measured hold-up time is about 30ms, which meets the spec. The output fall time is approximately 10ms, when measured from 90% to 10%. Note that the energy is pulled from the power supply very quickly due to the heavy load on its output.

Fig. 2

By comparison, without a load (zero load), the curves below (Fig. 3) show that the Hold-Up time of the power supply increases substantially to about 1.8 seconds, and the output drops to zero in about 6 seconds!

Fig. 3

Below is a graph (Fig. 4) for this power supply that shows how minimum Hold-Up time varies as the load changes between 10% and 100% (maximum load).

Fig. 4

Next, we shall review how the remote on/off control (if used) can affect the power supply’s output rise/fall, and delay times.

Conditions:

Vin	: 115VAC (A)
lout	: 100%
Ta	: 25°C

Fig. 5

In the above curves (Fig. 5), the remote On/Off control is the bottom trace and above that is the power supply’s DC OK signal level. This situation assumes the AC input in always on. As can be seen, from the time the Remote On is activated (goes from high to low logic level); it takes the output about 150ms to reach its full rated voltage. And, the DC OK signal changes state when the output level is about 75% of its rated voltage, which in this case is about 125ms after the Remote On signal is activated.

The next set of curves (Fig. 6) show the reverse situation, where the Remote On/Off signal is used to turn the power supply off.

Fig. 6

There is about 25ms delay after the Remote On/Off changes state to when the Vout reduces to approximately 75% its nominal. In addition, at that time the DC OK signal changes state. The output fall time, from 90% to 10% of nominal, is approximately 15ms.

From the above curves and explanations, I believe the reader can see why our answer at the beginning was "it depends." In these types of measurements, delay times are in many cases more important than rise and fall times. Other considerations include the AC input voltage, the load, operating temperatures, measurement criteria and the design specifications for the power supply.

How to Avoid Being Ripped Off When Buying Power Supplies

When shopping for power supplies, remember these wise words, “If it seems too good to be true, it probably isn’t (true).” Time and again, we hear or read about cases where counterfeit and/or fake power supplies are sold to those who are enticed by ultra low prices, but end up with big headaches instead. Here are some examples: 

• 700W ATX Power Supply with ‘PFC Coil Made from Cement’: http://www.hardwaresecrets.com/blog/Hantol-PSU-Fake-PFC-Made-of-Cement/220

The yellow PFC coil is a fake that is actually made of cement

• Fake iPhone Chargers; Raspberry Pi foundation looks at counterfeit Apple power supplies: http://hackaday.com/2012/10/10/raspberry-pi-foundation-looks-a-counterfeit-apple-power-supplies/
• Exposing some fake electronics with too-good-to-be-true prices: http://hackaday.com/2012/01/04/exposing-some-fake-electronics-with-too-good-to-be-true-prices/
• Low Cost Power Supplies Can Be Expensive: http://power-topics.blogspot.com/search/label/Power%20Supply%20Failure
• Don't buy a cheap replacement power supply!: http://www.youtube.com/watch?v=7nTRIxloDcI
• UL Warns of Power Supplies with Counterfeit UL Listing Mark and UL Energy Verified Mark: http://www.wric.com/story/20226384/ul-warns-of-power-supplies-with-counterfeit-ul-listing-mark-and-ul-energy-verified-mark
• Fake! Counterfeit UL mark found on LED power supply: http://www.edn.com/electronics-blogs/led-zone/4395270/Fake--Counterfeit-UL-mark-found-on-LED-power-supply

 

Because there are bad guys out there who are dishonest, we come back to the question of ‘how to avoid being ripped off when buying power supplies.’ The answer is simple. Buy from reputable and well established companies who have been providing power supplies for at least ten (10) years. For example, TDK-Lambda has been designing and manufacturing reliable power supplies for well over 50 years and offers limited lifetime warranties on some models http://www.us.tdk-lambda.com/lp/ . Avoid buying ‘seemingly’ bargain priced power supplies from eBay or other online discount websites. Fake chargers can kill your expensive cameras, smart phones and other devices.

 

If you work for an electronics manufacturer, remember that the switchmode power supply is the heart of your end-product or system and your company requires power supplies that will provide excellent performance and trouble free operation for many years. The last thing an OEM needs is a lawsuit or the need to recall thousands of products from the field.

 

If in doubt, ask the power supply vendor for the UL listed file number for the supply you are interested in using. For UL Listed and UL Recognized products, the file number is usually printed near the UL mark and typically begins with an E prefix followed by a 6 character number, e.g., E133400. If the file number is not shown, most reputable power supply vendors/manufacturers will provide the end-user with this information. Anyone can go to the UL website to check on a UL file number to make sure the supply is actually listed (approved) and to confirm the name of the manufacturer. The ‘UL Online Certification Directory’ is available at http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/index.html. At this website you can enter the ‘UL File Number’ in the box provided (e.g., E133400) and then click the SEARCH button. If the power supply is in the UL database, information about the manufacturer will be shown. If you then click one of the items shown, the associated model number(s) that conform to the specific UL safety specification(s) will be listed.

 

By following the above recommendations, you’ll become a wise power supply consumer and thereby avoid costly mistakes in your next power supply selection and purchase.