The relentless pursuit of efficiency in sectors comparable to AI, cloud computing, and autonomous driving is making a warmth disaster. As the subsequent technology of processors demand extra energy in smaller areas, the switched-mode energy provide (SMPS) is being pushed to its thermal restrict. SMPS’s built-in circuit (IC) packages have historically used a big thermal pad on the underside aspect of the package deal, referred to as a die connect paddle (DAP), to dissipate the vast majority of the warmth by way of the printed circuit board (PCB). However as energy density will increase, counting on just one aspect of the package deal to dissipate warmth shortly turns into a severe constraint.

A thermally enhanced package deal is a kind of IC package deal designed to dissipate warmth from each the highest and backside surfaces. On this article, we’ll discover the usual thermal metrics of IC packages, together with the composition, top-side cooling strategies, and thermal advantages of a thermally enhanced package deal.

Thermal metrics of IC packages

As a way to perceive what a thermally-enhanced package deal is and why it’s helpful, it’s essential to first perceive the terminology for describing the thermal efficiency of an IC package deal. Three foundational metrics of thermal resistance are the junction-to-ambient thermal resistance (R_θJA), the junction-to-case (high) thermal resistance (R_θJC(high)), and the junction-to-board thermal resistance (R_θJB).

Thermal resistance measures the opposition to the stream of warmth in a medium. In IC packages, thermal resistance is often measured in Celsius rise per watt dissipated (°C/W), or how a lot the temperature rises when the IC dissipates a specific amount of energy.

R_θJA measures the thermal resistance between the junction (J) (the silicon die itself), and the ambient air (A) across the IC. R_θJC(high) measures the thermal resistance particularly between (J) and the highest (t) of the case (C) or package deal mildew. R_θJB measures the thermal resistance particularly between (J) and the PCB on which the package deal is mounted.

R_θJA considerably is determined by its subcomponents—each R_θJC(high) and R_θJB. The decrease the R_θJA, the higher, as a result of it clearly signifies that there will likely be a decrease temperature rise per unit of energy dissipated. Energy IC designers spend a variety of time and sources to provide you with new methods to decrease R_θJA. A thermally enhanced package deal is one such method.

Thermally enhanced package deal composition

A thermally enhanced package deal is a quad flat no-lead (QFN) package deal that has each a bottom-side DAP and a top-side cutout of the molding to instantly expose the again of the silicon die to the atmosphere. Determine 1 exhibits the grey bottom of the die for the Texas Devices (TI) LM61495T-Q1 buck converter.

Determine 1 The LM61495T-Q1 buck converter in a thermally enhanced package deal. Supply: Texas Devices

Exposing the die on the highest aspect of the package deal does two issues: it lowers the R_θJC(high) in comparison with an IC package deal that fully molds over the die, and allows a direct connection between the die and an exterior warmth sink, which might considerably cut back R_θJA.

R_θJC(high) in a thermally enhanced package deal

R_θJC(high) permits warmth to flee extra successfully from the highest of the gadget. Usually, warmth escapes by way of the package deal mildew after which to the air, however in a thermally enhanced package deal, it escapes on to the air. This helps cut back the gadget temperature and reduces the danger of thermal shutdown and long-term warmth stress points. The thermally enhanced package deal additionally has a decrease R_θJA, which makes it potential for a converter to deal with extra present and function in hotter environments.

Determine 2 exhibits a collection of IC junction temperature measurements taken throughout output present for each the LM61495T-Q1 within the thermally enhanced package deal and TI’s LM61495-Q1 buck converter in the usual QFN package deal beneath two widespread working circumstances.

Determine 2 Output present vs. junction temperature for the LM61495-Q1 and LM61495T-Q1 with no warmth sink. Supply: Texas Devices

Clearly, even with no warmth sink connected, the thermally enhanced package deal runs barely cooler, just because extra warmth is dissipating out of the highest of the package deal and into the air. The R_θJA for a thermally enhanced package deal is barely decrease, demonstrating with certainty that, even when solely marginally, this package deal kind will present higher thermals in comparison with the usual QFN with top-side molding, even with none further thermal administration methods. Desk 1 lists the official thermal metrics present in each gadgets’ information sheets.

Desk 1 Evaluating information sheet-derived thermal metrics for the LM61495-Q1 and LM61495T-Q1. Supply: Texas Devices

High-side cooling vs QFN

Combining its near-zero R_θJC(High) high aspect with an efficient warmth sink considerably reduces the R_θJA of an IC in a thermally enhanced package deal. There are three important enhancements when in comparison with the identical IC in a normal QFN package deal beneath in any other case related working circumstances:

• Increased switching-frequency operation.
• Increased output-current functionality.
• Operation at increased ambient temperatures.

For any SMPS beneath a given enter voltage (V_IN), output voltage (V_OUT) situation and supplying a given output present, the utmost switching frequency will likely be thermally restricted. Inside each switching interval, there are switching losses and conduction losses that dissipate as warmth. Switching extra continuously dissipates extra energy within the IC, resulting in an elevated IC junction temperature. This may be irritating for engineers as a result of switching at increased frequencies allows using a smaller buck inductor, and due to this fact a smaller total resolution measurement and decrease price.

Underneath the identical working circumstances, utilizing the thermally enhanced package deal and a warmth sink, the warmth dissipated in every switching interval is now extra simply channeled out of the IC, resulting in a decrease junction temperature and enabling a better switching frequency with out hitting the IC’s junction temperature restrict. Simply don’t exceed the utmost switching frequency advice of the gadget as outlined within the information sheet.

The advantages of utilizing a smaller inductor are particularly pronounced in higher-current multiphase designs that require an inductor for each part. Determine 3 exhibits a simplified four-phase design able to supplying 24 A at 3.3 V_OUT at 2.2 MHz utilizing the TI LM64AA2-Q1 step-down converter. If the design have been to overheat and the switching frequency needed to be diminished to 400 kHz, you would need to substitute all 4 inductors with bigger inductors (by way of each inductance and measurement), inflating the general resolution price and measurement considerably.

Determine 3 Simplified schematic of a single-output, four-phase step-down converter design utilizing the LM644A2-Q1 step-down converter within the thermally enhanced package deal. Supply: Texas Devices

Conversely, for any SMPS beneath a given V_IN, V_OUT situation, and working at a particular switching frequency, the utmost output present will likely be thermally restricted. When discussing the present restrict of an IC, it’s essential to make clear that for all high-side FET built-in SMPSs, there’s a information sheet-specified high-side present restrict that bounds the potential output present.

Upon reaching the current-limit setpoint, the high-side FET turns off, and the IC could enter a hiccup interval to scale back the working temperature till the overcurrent situation goes away. However even earlier than reaching the present restrict, it is extremely potential for an IC to overheat from a excessive output-current requirement. That is very true, once more, at increased frequencies. So long as you don’t exceed the high-side present restrict, utilizing an IC within the thermally enhanced package deal with a warmth sink can lengthen the utmost potential output present to a stage at which the usual QFN IC alone would overheat.

There may be one other fixed to make the thermally enhanced package deal versus the usual QFN package deal comparability legitimate, and that’s the ambient temperature (T_A). T_A is a major issue when contemplating how a lot energy an SMPS can ship earlier than it begins to overheat.

For instance, a buck converter could possibly simply do a 12V_IN-to-5V_OUT conversion and assist a steady 6 A of present whereas switching at 2.2 MHz when the T_A is 25°C, however not at 105°C. So, there’s but a 3rd method to take a look at the profit {that a} thermally enhanced package deal can present. Assuming the V_IN, V_OUT, output present, and most switching frequency are fixed, a thermally enhanced package deal used with a warmth sink can allow an SMPS to function at a meaningfully increased T_A in comparison with a normal QFN package deal with no warmth sink.

Determine 4 makes use of a present derating curve to reveal each the upper output present functionality and operation at a better T_A. In an experiment utilizing the LM61495-Q1 and LM61495T-Q1 buck converters, we measured the output present towards the T_A in a normal QFN package deal and not using a warmth sink and in a thermally enhanced package deal QFN linked to an off-the-shelf 45 x 45 x 15 mm stand-alone fin-type warmth sink. Apart from the package deal and the warmth sink, all different circumstances are fixed: working circumstances, PCB, and measurement instrumentation.

Determine 4 Output present vs. ambient temperature of the LM61495-Q1 with no warmth sink and the LM61495T-Q1 with an off-the-shelf 45 x 45 x 15 mm stand-alone fin-type warmth sink. Supply: Texas Devices

When T_A reaches about 83˚C, the usual QFN package deal hits its thermal shutdown threshold, and the output present begins to break down. As T_A will increase additional, the gadget cycles into and out of thermal shutdown, and the utmost achievable output present that the gadget can ship is essentially diminished till T_A reaches a gentle 125˚C. At this level, the converter could not be capable to maintain even 5 A with out overheating.

Examine this to the thermally enhanced package deal QFN linked to a warmth sink. The primary occasion of thermal shutdown now doesn’t happen till about 117˚C. That’s a rise in T_A earlier than hitting a thermal shutdown of 34˚C, or 40%. The LM61495-Q1 is a 10-A buck converter, that means that its really useful most output present is 10 A. However on this case, with a thermally enhanced package deal and efficient warmth sinking, a steady 11 A output was clearly achievable as much as 117˚C – in different phrases, a ten% enhance in most steady output present even at a excessive T_A.

Strategies of top-side cooling

Determine 5, Determine 6, and Determine 7 present among the commonest strategies of top-side cooling. Stand-alone warmth sinks are easy and available in many alternative varieties, supplies, and sizes, however are generally impractical in small-form-factor designs.

Determine 5 Stand-alone fin-type warmth sink, these are easy and available however generally impractical in small kind issue designs. Supply: Texas Devices

Chilly plates are very efficient in dissipating warmth however are extra advanced and costlier to implement (Determine 6).

Determine 6 Chilly plate-type warmth sink, these are very efficient in dissipating warmth however are extra advanced and costlier to implement. Supply: Texas Devices

Utilizing the metallic housing containing the facility provide and the encircling electronics as a warmth sink is compact, efficient, and comparatively cheap if the housing already exists. As proven in Determine 7, that is completed by making a pillar or dimple that connects the IC to the housing to allow environment friendly warmth switch. For energy provides powering processors, it’s doubtless that this methodology is already serving to dissipate warmth on the processor. Including an extra dimple or pillar that now offers heat-sink entry to the facility provide is commonly a easy change, making it a highly regarded methodology, particularly for processor energy.

Determine 7 Contact-with-housing warmth sink the place a pillar or dimple connects the IC to the housing to allow environment friendly warmth switch. Supply: Texas Devices

There are lots of methods to implement warmth sinking, however that doesn’t imply that they’re all equally efficient. The scale, materials, and type of the warmth sink matter. The kind and quantity of thermal interface materials used between the IC and the warmth sink matter, as does its placement. It is very important optimize all of those components for the design at hand.

Evaluating warmth sinks

Determine 8 exhibits one other present derating curve. It compares two various kinds of warmth sinks, every mounted on the LM61495T-Q1. For reference, the determine consists of the efficiency of the usual QFN package deal with no warmth sink.

Determine 8 Output present versus the ambient temperature of the LM61495-Q1 with no warmth sink, the LM61495T-Q1 with an off-the-shelf 45 x 45 x 15 mm stand-alone fin-type warmth sink, and with an aluminum plate warmth sink. Supply: Texas Devices

For a visualization of those warmth sinks, see Determine 9 and Determine 10, which present a top-down view of the PCB and a transparent view of how the warmth sinks are mounted to the IC and PCB. The warmth sink proven in Determine 9 is a commercially obtainable, off-the-shelf product. To reiterate, it’s a 45 mm by 45 mm aluminum alloy warmth sink with a base that’s 3mm thick and pin-type fins that stretch the floor space and permit omnidirectional airflow.

Determine 9 The LM61495T-Q1 analysis board with the off-the-shelf 45 x 45 x 15 mm stand-alone fin-type warmth sink. Supply: Texas Devices

Determine 10 exhibits a customized warmth sink that’s basically only a 50 mm by 50 mm aluminum plate with a 2 mm thickness and a small pillar that instantly touches the IC. This warmth sink was designed to imitate the contact-with-housing methodology, as it is extremely related in measurement and materials to the varieties of housing seen in actual purposes.

Determine 10 The LM61495T-Q1 analysis board with a customized aluminum plate warmth sink to imitate the contact-with-housing methodology. Supply: Texas Devices

Underneath the identical circumstances, the stand-alone warmth sink offers a serious profit in comparison with the usual QFN package deal with no warmth sink. The usual QFN package deal hits thermal shutdown round 67°C T_A. For the stand-alone heat-sink setup, thermal shutdown isn’t triggered till the T_A reaches about 111°C, which is a serious enchancment. Nevertheless, the aluminum plate heat-sink setup doesn’t hit thermal shutdown in any respect. With the aluminum plate setup, the converter continues to be capable of provide a steady 10-A present on the highest T_A examined (125˚C), demonstrating each the significance of selecting the proper warmth sink for the system necessities in addition to the recognition of the contact-with-housing methodology.

Addressing fashionable thermal challenges

Energy provide designers more and more cope with thermal challenges as fashionable purposes demand extra energy and smaller kind components in hotter areas. Normal QFN packaging has lengthy relied on dissipating the vast majority of generated warmth by way of the underside aspect of the package deal to the PCB. A thermally enhanced package deal QFN makes use of each the highest and backside sides of the package deal to enhance warmth stream out of the IC, basically paralleling the thermal impedance paths and lowering the efficient thermal impedance.

Combining a thermally enhanced package deal with efficient warmth sinking ends in important thermal advantages and allows higher-power-density designs. As a result of these advantages are derived from lowering the efficient R_θJA, designers can understand only one or all of those advantages in various levels. Enhance the utmost switching frequency and cut back resolution measurement and value. Enabling a better most output present for increased energy conversion. Allow operation at a better T_A.

Jonathan Riley is a Senior Product Advertising and marketing Engineer for Texas Devices’ Switching Regulators group. He holds a BS in Electrical Engineering from the College of California Santa Cruz. At TI, Jonathan works within the crossroads of promoting and engineering to make sure TI’s Switching Regulator product line continues to evolve forward of the market and allow clients to energy the applied sciences of tomorrow.

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• Energy Suggestions #101: Use a thermal digital camera to evaluate temperatures in automotive environments
• IC packages and thermal design
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• QFN? QFP? QFWHAT?

Further sources

• Learn the TI utility observe, “Semiconductor and IC Bundle Thermal Metrics.”
• Take a look at these TI utility stories:
  • “AN-2020 Thermal Design by Perception, Not Hindsight.”
  • “AN-1520 A Information to Board Structure for Greatest Thermal Resistance for Uncovered Packages.”
• See the TI utility transient, “PowerPAD Made Straightforward.”
• Watch the TI video useful resource, “Enhance thermal efficiency utilizing thermally enhanced packaging (TEP).”

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