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Posted on 01 August 2019

Achieving Optimum Battery Charging by Power Management

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We see the results in user-friendly form consumer electronics

The increasing number and level of sophistication of consumer devices over the last decade has had a great impact on system operation. Many new designs require the system to be fully operational during the charging process and also when the battery is deeply discharged or removed.

By George Paparrizos, Summit Microelectronics

 

Furthermore, the integration of hard drives and other power hungry components on battery- powered devices adds more challenges in terms of power availability and delivery. Last but not least, new industry standards, such as Universal Serial Bus Charging and Universal Serial Bus On-The-Go, introduce new power requirements in already complex system designs. Addressing such power management requirements becomes a great challenge for engineers, especially given the continuous effort to meet small industrial designs and aggressive market prices. Today’s battery charging IC solutions are beginning to perform power management functions that enable these new sophisticated designs while maintaining small board space and ensuring system reliability.

Power dissipation and thermal design

In many of today’s consumer electronics, industrial design is frequently the driving power behind system development, with a focus on slimmer, light and user-friendly form factors. This introduces great challenges, given that an increasing number of applications require higher average power budgets and batteries with larger capacity. In these cases, traditional linear charging solutions become obsolete because of their inefficient operation and consequently high levels of heat dissipation. Even in cases in which thermal foldback prevents extreme temperature conditions (charge current is limited when junction temperature crosses a specific threshold), two negative side effects remain: a) charge current is reduced resulting in longer charging times, and b) average temperature levels during charging remain high.

High ambient temperature conditions in a compact design introduce many negative effects. From a reliability standpoint, a rise in average ambient temperature shortens the lifetime of the components; a 10 degrees Celsius rise results in a 50% reduction in component life (Arrhenius equation). From a system design standpoint, dissipating high levels of power means that a more conservative design approach needs to be taken, often resulting to larger board space and cost. From a user standpoint, rising temperature translates to a negative consumer experience in the form of longer charging times and “hot spots”.

Per the calculations below, a switch-mode charger, like the SMB137, can result in significantly lower power dissipation, easier power and thermal design, while achieving a compact industrial design. For smart phones and other devices utilizing higher capacity batteries, the charge current level can be 1A or 1.2A, further highlighting the benefits of switch-mode power delivery for battery charging.

Assumptions:
Input (adapter) voltage: VIN = 5V (±5%)
Battery voltage: VBATT = 3.7V
Fast-charge current: IBATT = 800mA (1C charge rate for 800mAh battery pack)

Calculations:
Linear charger power dissipation:
PD = (VIN – VBATT) x IBATT = (5.25V – 3.7V) x 0.8A = 1.24W
Switch-mode charger power dissipation:
PD = (VBATT x IBATT) x (1/η - 1) = (3.7V x 0.8A) x (1/0.9 – 1) = 0.33W

Thermal comparison between linear and switch-mode charging solution

Board space

One of the reasons switch-mode charging solutions were not widely adopted in portable equipment designs up until recently was the fact that they occupied a significant footprint. Modern switch-mode battery charger Ics are offered in packages that are equal or smaller than their “linear” counterparts (example: chip-scale packages). Furthermore, new power architectures allow highfrequency operation that enable the use of tiny, chip inductors. The integration of additional power management functions - as will be discussed below at greater detail - on some of these solutions also eliminates the need of additional components. These space savings compare favorably against linear charging implementations that introduce “hidden” board space penalties, primarily in the form of layout tricks to address sensitive thermal designs.

CurrentPath™

With many of the newer phones absorbing multi-media functions (video, music, etc.), the need for system operation when the power adapter is connected, but the battery is empty or removed, has become a necessity. In traditional charging implementations a removal (or deep discharging) of the battery constitutes a false operation and results in system shutdown. The SMB137 provides Current- Path, which allows power to be provided to the system via a different path than the one used for battery charging. In this case, the power delivered from the wall adapter or other power source (USB hub, etc.) to the battery is limited by the charger configuration, however the system is directly powered by the input power source. This allows the system to instantaneously turn on from the power supplied by the external power source, even when the battery is deeply discharged. Furthermore, this charging configuration reduces the charge and discharge cycles on the battery, thereby extending its operating life.

Splitting system from battery power also allows for a less conservative design, since peak system power can always be supplemented by the battery, when input power is not sufficient. This enables the external adapter to be designed for a lower current (power) rating, significantly reducing its cost and complexity. This operation is demonstrated in Figure 2 when the system needs more current than available, charge current is reduced to prevent a collapse of the system voltage. In cases in which the total current is still not adequate for powering the system, the battery will start discharging via a “noloss” power path and supplement the system power.

Battery is supplementing the power required by the system

OTG power path

The proliferation of handheld consumer devices has led to a number of industry initiatives that address the need for user-friendly operation and inter-compatibility. One of these standards is the Universal Serial Bus (USB) On-The-Go (OTG) one, which allows handheld devices to be connected (and communicate) to each other without the need for a USB hub (example: PC). This opens the door for a number of new applications, like printing photos directly from a digital camera and songs exchange between a cellular phone and an MP3 player. The power requirements for supporting USB OTG implementation requires the “host” device to provide 5V (+4.4V to +5.25V) and a predetermined limited current to the connected peripheral device. The SMB137 takes advantage of its existing power path and, when not in charging mode, allows the battery to become the input power source, and the USB input to become the output power source. This “reverse” operation eliminates the need for an additional boost converter IC (for 5V VBUS) and corresponding inductor, thereby saving board space, system cost and manufacturing complexity. Furthermore, the SMB137 incorporates functionality to ensure that the peripheral device does not draw more current than necessary, thereby eliminating the risk of fast battery drain of the host device.

OTG power path for powering peripheral OTG device

Low Battery Recovery™ path

The SMB137 provides an additional power path that allows the steering of battery power to the system output when USB input power source is present. This additional “reverse” boost operation does not require any external components and allows the SMB137 to function like a 5V step-up regulator for the system. Such an implementation is useful in cases in which a higher current is required for the system to wake-up and initiate some actions. If for example the USB power source is temporarily capable of 100mA only (USB1 mode – prior to enumeration), essential system components running the software necessary for powering up the system might not be able to power-up. In this case the system is “trapped” and system turn-on time might be considerably long. The supplemental power from the battery via the low-battery recovery path addresses this design challenge and enables system “booting” even during a low-battery situation.

Summary

The increasing complexity of modern consumer electronics, coupled with the need for small form factors, has introduced new challenges in power and batter management designs. Charging control circuits are now required to perform many more functions than just charging; they need to ensure that power is available whenever system needs it. New battery charging solutions, like the SMB137, integrate a number of unique power management functions that allow better consumer experience while meeting the strictest standards for system and battery safety.

SMB137 system application demonstrates design flexibility via a high number of power paths

 

References:

1) http://en.wikipedia.org/wiki/Arrhenius_equation
2) USB-IF, On-The-Go Supplement to the USB 2.0 Specification revision 1.3.
3) USB-IF, Battery Charging Specification revision 1.0.
4) http://www.summitmicro.com/prod_select/summary/SMB137/SMB137.htm

 

 

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