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Thermal power management considerations for portable devices

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CIOL Bureau
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With all of the power demands being made by portable equipment, clearly thermal management is one of the most critical design challenges for these devices. A well thought out thermal management plan can effectively remove heat from a heat source and surrounding areas, as well as increase safety from fire or explosion hazards.

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Power management -- an introduction

With shrinking form factors, increasing functionality and performance, portable power equipment is demanding more power than ever before – making thermal management one of the most critical design challenges today. Thermal management can be composed of system power management, battery power thermal management and traditional thermal design.

 

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Initially, we’ll address system level power management, including voltage scaling and frequency throttling, to optimize average power demand from the portable electronic system. Then, we will discuss thermal regulation and thermal shutdown controls for improved thermal reliability. This is achieved by actively monitoring the temperature of critical components such as CPU and power management integrated circuits (IC), and adjusting the charging current to maintain safe operations.

 

Battery management from a thermal perspective is a major concern since excessive high temperatures accelerate battery degradation, and may cause thermal run-away and explosion in Lithium ion (Li-Ion) batteries. Finally, thermal management also involves system level heat-flow management to effectively remove heat from a heat source, to the printed circuit boards (PCB), and enclosures through thermally-enhanced packages and thermal vias.

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System level power management

Power consumption associated with thermal management has become one of the most important design challenges for portable power devices that operate with a limited energy source such as batteries. With continued demand for miniaturization and increasing computation power, the use of powerful microprocessors continues to grow. These are needed for running sophisticated, intelligent control software in a variety of portable devices including digital camcorders, smart phones, portable media players and portable medical devices. However, there is an inherent conflict in the design goals for portable battery operated devices: longer battery runtime is always preferred. But for intelligent devices, they require powerful processors, which consume even more energy, thus generating thermal design challenges and reducing battery life.

The fundamental tradeoff between power consumption and battery life is a key consideration for system design. Decreasing power consumption helps extend the battery runtime, and contributes to better thermal management for small form factor portable devices. How can we minimize power consumption and improve thermal management, and still meet critical performance requirements? These challenges can be met with dynamic voltage scaling technology.

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Digital complementary metal-oxide-semiconductor (CMOS) circuits are used in the majority of microprocessors, and have both switching and static power consumption. Static power consumption is caused by bias and leakage currents, and is negligible in most designs. The dominant power consumption for CMOS microprocessors is the switching loss. Every transition of a digital circuit charges and discharges the digital circuit’s output capacitance which generates power dissipation. The power consumption is given by:

     Pcpu=C fs V2core

In the above equation, C equals the total load capacitance, fs is the switching frequency, and Vcore is the supply voltage applied to the microprocessor. When a processor is idle due to a light workload, power is wasted. It shows that clock frequency reduction linearly decreases power consumption, and voltage reduction results in a quadratic power reduction. Thus, reduction of Vcore is the most effective means for lowering power consumption.

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Lowering Vcore, however, creates the problem of increased propagation delay of the CMOS transistor. This in turn restricts the clock frequency and slows down data or signal processing speed in a microprocessor. Do we always need high-speed processing? High performance is usually needed only for a small fraction of the time. For the rest of the time, however, a low-performance, low-power processor would suffice. We can achieve low performance by simply lowering the operating frequency of the processor when full speed is not needed.

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Dynamic voltage scaling (DVS) actively adjusts core voltage and clock frequency in response to fluctuations generated by a processor in use. A voltage scheduler can adjust voltage and frequency when it has prior knowledge or predictions of the system workload. DVS is a technique that provides high core voltage when high performance is needed. However, it lowers the core voltage and reduces the CPU clock frequency in microprocessor idle mode. Figure below, using this voltage scaling technique, shows the power savings for a microprocessor .

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Figure 1. Power Saving with DVS and Frequency Throttling

Potentially, DVS can provide a very large net power savings, making it easy for thermal management through this voltage scaling technique.

Figure 2 shows a high-efficiency 2.25 MHz dc/dc converter with DVS that minimizes power consumed by a microprocessor. A one-wire serial interface is used to dynamically program the output voltage. Figure 3 shows the dynamic output voltage transition between two output values, depending on the command from the microprocessor.

 

Figure 2.  DC/DC Converter with Dynamic Voltage Scaling

 

Figure 3.  Dynamic Voltage Scaling Output Voltage

Additionally, linear regulators usually have a lower power conversion efficiency than switching-mode dc/dc converters, where there is a large difference between the input and output voltages. To conserve battery power and improve thermal reliability, high-efficiency switch-mode dc/dc regulators are used to achieve greater than 90 percent efficiency over a wide range of load currents, while enabling pulse frequency modulation at light loads to extend the standby time. Furthermore, in portable devices, a number of power regulators are usually used to power the CPU, I/Os, back-light displays, memories, wireless system and a back-up supply for a real-time clock. Each of these supplies has an on/off control to manage power needs. Depending on the operation mode, some of these supplies are switched off to minimize the power dissipation, thus, improving thermal stress.

 

Battery power thermal management

In battery-operated portable devices, high-power consumption usually occurs when the battery is charging and the system is operating simultaneously. One solution is to add a thermal regulation loop and dynamic power management function.

A cost-effective linear battery charger usually is used to charge a single-cell Li-Ion battery in portable devices. The highest power dissipation occurs during the transition from pre-charge to fast-charge mode, where a highest voltage drop occurs across the pass transistor. In order for the junction temperature of the device to remain below 125oC, the thermal regulation loop is introduced by reducing the battery charging current. Now the devices always will operate in a thermal safe-operation region, and the possibility of overheating is minimized.

Moreover, most of the power management ICs have a thermal shutdown feature that takes over when the silicon junction temperature exceeds 150oC. This provides a second-level thermal protection as well. A negative temperature coefficient (NTC) thermistor often is used to monitor the battery cell temperature for qualifying the battery charging and discharging. The typical temperature qualification range for Li-Ion battery charging is between 0oC and 45oC.

Battery degradation speeds up when the battery temperature is below 0oC while charging because metallic Lithium is deposited on the anode. On the other hand, charging under high cell temperatures may cause serious safety hazards since the active material in the Li-Ion battery reacts with electrolyte. In this situation, the heat generated from the reaction may cause the battery to catch fire or explode.

Furthermore, the power being demanded by the system is quite dynamic and pulsating. The total power required from the system and battery charging may exceed the maximum power available from the adapter. This can result in overheating the adapter and crashing the system power rail. Dynamic power path management monitors the system bus voltage VOUT to detect over-load conditions. Once the system bus voltage falls to reach the pre-set threshold, the battery charge control system attempts to regulate the system bus voltage by reducing the battery charging current. Therefore, the total current demand from the system and battery charger reaches the maximum current available from the adapter to avoid adapter overheating. Figure 4 shows a dynamic power path management battery charger with a thermal regulation loop.

 

Figure 4.  Dynamic Power Path Management Battery Charger with Thermal Regulation

 

Taking effective measures for thermal management as described above also benefits Li-Ion battery safety. Excessively high temperatures are disastrous for Li-Ion batteries and are a specific safety concern with them. High temperatures can accelerate cell degradation in general. When the cell temperature rises to about 150oC or above, the highly active material in Li-Ion battery self-heats and positively contributes to the elevated temperature. This results in a thermal runaway, followed by a fire hazard and explosion.

However, temperatures also can increase rapidly if the battery is overcharged at high current or if shorted. During overcharge of a Li-ion battery, active metallic lithium is deposited on anode. This material can react explosively with a variety of materials, including electrolyte and cathode material used in the cell. Cathode material, such as LiCoO2, starts reacting with electrolyte when the temperature exceeds its thermal run-away threshold of about 175oC.

Although there are several thermal protections in the battery cell such as melting the pores of the separator to permanent shutdown of the battery, it is crucial to monitor the battery temperature. For a typical multi-cell Li-Ion battery pack design, when the battery temperature is above 60oC, it will turn off two back-to-back protection MOSFETs in series with the Li-Ion cells, and will not allow the battery to discharge. When the battery temperature exceeds the second level safety temperature, 70oC for example, it will permanently disable the battery by blowing the thermal fuse in the battery pack.

Multiple thermisters gradually are being used to monitor the temperature of multiple cells, improving thermal monitoring and battery safety. The temperature of the battery cell can be reported through SMbus so that the proper system level protections can be taken as well.  It is strongly recommended to keep the battery away from system heat sources such as the CPU. Not only will this improve the battery life cycle and reduce the self-discharge, but it will also make for a safer system.

Power components and PCB thermal design considerations

Although DVS optimizes power consumption and performance at different processing load conditions, the traditional thermal management design remains a challenging task for system designers. Heat generated from the system must be redistributed and dissipated, as portable devices have very limited space and air flow . Due to the size and space constraints of portable devices, a fan and/or traditional heat sink devices cannot be used to transfer heat generated by the power components.

Today, with thermally enhanced surface mount technology and ICs with power pads, the printed circuit board (PCB) has become an integral part for removing the heat. The thermal impedance provided by an IC manufacturer is tested under certain conditions such as one square inch copper and thermal vias under the thermal pad. For example, a 20-pin QFN package has a thermal impedance of 47oC/W. However, you may get a much higher thermal impedance if there are no thermal vias and/or enough PCBs connecting to the thermal pad. Such common design mistakes include inadequate thermal optimization of the PCB layout, lack of thermal vias, as well as improper placement of components on the PCB and overall enclosure design. All of these mistakes create heat-transfer problems.

PCB design rapidly is becoming a critical design stage in thermal management. Copper is a good thermal conductor when used along its length for a large cross-section. At least one full copper layer is needed to spread the heat. The larger the perimeter of the power pad (circumference), the greater the cross-sectional thermal conduction area, the lower the PCB’s thermal impedance, and the cooler the PCB and IC.

The top PCB layer usually has many external components connected to the IC pins, so there is little available top PCB area that can be connected to the thermal pad to remove the heat from the power components. One approach for removing heat is to use multiple thermal vias. This removes heat generated by the power components to the bottom PCB layer. Heat transfer through a typical 13-mil thermal via. Ten 13-mil thermal vias under the power IC thermal pad have only 8oC/W thermal impedance. Therefore, multiple thermal vias are strongly recommended to enhance the heat extraction from the heat source.

Figure 5 shows the layout of a typical surface mount power device. For example, light emitting diodes (LEDs) are commonly used as a display in portable devices. The high current LED can dissipate over two to three watts. In order for the LED to spread the heat over the PCB, enough copper with a number of thermal vias should be connected to the cathode of the LED. On a multi-layer PCB, if the power ground does not have enough surface area or metal for heat dissipation, there is not enough surface or thermal conductivity for the heat to dissipate. As a result, heat may become trapped, causing thermal-related hazards. To combat this, engineers should practice careful board layout. For instance, enlarge the power ground with more metal, such as two ounces of copper PCB to avoid thermal traps on the PCB.

 

Figure 5.  Typical Layout of a Surface Mount Power Device

 

 About the Author

Jinrong Qian is an Applications Engineering Manager and Distinguished Member of the Technical Staff for the Portable Power Battery Management group at Texas Instruments. He has published more than 40 peer-reviewed power electronics transactions and conference papers, and holds 19 U.S. patents. He earned a Bachelor of Science degree in Electrical Engineering from Zhejiang University in 1985, and a Ph. D. from Virginia Polytechnic Institute and State University in 1997. Jinrong can be reached at ti_jinrongqian@list.ti.com. 

This Article was published earlier in Portable Design (print)

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