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Designing interoperable battery chargers

July 02, 2019

anshulgulati-July 02, 2019

Traditionally, all mobile phone chargers followed the USB Battery Charging rev 1.2 specification which allows phones to be charged at 5V via the standard micro-B connector. Since then, numerous companies have improved charging times by creating different standards. Unfortunately, they have done so without consideration of compatibility across standards. Now there are various charging standards in the market including USB Power Delivery 3.0, Quick Charge 4.0, and Adaptive Fast Charging, etc., each evolves at its own pace and creating its own unique features. For example, Power Delivery 3.0 includes Programmable Power Supply (PPS), a feature not supported by Power Delivery 2.0. Similarly, Qualcomm Quick Charger 4.0 has superseded Quick Charge 2.0 and 3.0.

With the need for more frequent charging due to the increasing usage of smart phones throughout the day, the average individual owns at least one extra charger besides the in-box phone charger. It is important to understand the impact of compatibility between a phone and the extra charger; for example, if one charges a Samsung phone with an iPhone charger, it will take longer to charge compared to charging with an in-box Samsung Adaptive Fast Charging (AFC) charger and vice a versa.

Given the great variety of charging standards and smart phones, it is advantageous for OEMs to design chargers based on a programmable solution that can quickly adapt to different standards and their associated changes without significant redesign.

The Interoperability Challenge

The various charging standards, and proprietary solutions have created a mess for consumers. Chargers that work with one device may not work with another, leading to frustration and buying more incompatible chargers for each device. Below is a summary of the main products available today.

USB Battery Charging rev. 1.2

The USB BC 1.2 standard defines three primary types of charging ports:

Standard Downstream Port (SDP): This is the traditional USB 2.0 data port widely available in most desktop and laptop computers. An SDP can supply a maximum current of 500 mA at 5V.

Charging Downstream Port (CDP): This is a downstream port that complies with the USB 2.0 definition of a host or hub. Besides supporting data, a CDP can supply up to 1.5 A at 5V.

Dedicated Charging Port (DCP): This is a downstream port that provides current up to 1.5 A at 5V through a USB connector, but lacks the ability to enumerate as a USB 2.0 data port. It is identified by a short between the D+ and D- signals.

With the above charging options, power is limited to 7.5 W, limiting the speed at which a smart phone can be charged. To overcome this challenge, companies such as Qualcomm, Samsung and Apple, each invented proprietary charging protocols to help reducing charging time for their own smart phones.

Quick Charge 2.0, 3.0 and 4.0

The Quick Charge (QC) standard was developed at Qualcomm. While QC 2.0 supports only fixed voltages such as 5V, 9V, 12V, and 20V, QC 3.0 allows the output voltage to be adjusted in steps of 200mV from 3.3V to 20V. A device can request the desired voltage by setting the terminations on D+ and D-.

Table 1 shows the different voltage modes for QC 2.0/3.0.

D+ D- VBUS Output
0.6V High-Z 5V
3.3V 0.6V 9V
0.6V 0.6V 12V
3.3V 3.3V 20V
0.6V 3.3V Continuous mode

Table 1: QC 2.0/3.0 Voltage modes

In QC 3.0 continuous mode of operation, the increment and decrement commands are signaled as a series of one or more pulses on D+ and D- to adjust VBUS up or down by 200mV for each pulse. These increment or decrement requests are sent as rising edge pulses on the D+ line or falling edge pulses on D- line respectively.

QC 4.0 is the latest standard developed by Qualcomm. It has been defined to comply with the USB Type-C and USB PD 3.0 specification. It also implements programmable power supply (PPS) and supports VBUS from 3.3V to 21V in steps of 20 mV. Commands are sent over the CC lines, a new signal introduced in the USB Type-C standard, instead of the D+ and D- lines used in QC 2.0 and 3.0.

Apple Charging

Developed by Apple exclusively for iPhones and iPads, this standard implements three sets of terminations – Brick ID 1 A, Brick ID 2.1 A, and Brick ID 2.4 A. VBUS is static at 5V and current variations are based on the termination set by the charger.

D+ D- Brick 1D
2 V 2.7 V 1 A
2.7 V 2 V 2.1 A
2.7 V 2.7 V 2.4 A

Table 2: Apple Brick ID Terminations

Adaptive Fast Charging(AFC)

Samsung’s proprietary charging standard that follows the USB BC 1.2 DCP detection mechanism for initial handshake with the device. An AFC interface constitutes a physical layer (PHY) to facilitate bidirectional communication over the D- line. The protocol has one byte assigned per profile to communicate voltage and current values that the charger supports. For example, if the charger supports three profiles (e.g. 5V, 9V and 12V), it uses three bytes to communicate voltage and current values to the device. The device is then expected to make three consecutive requests to the charger for the new voltage/current.

USB is supposed to make life easier by being a universal connector. With different charging technologies, however, USB has becomes segmented. By designing battery chargers to be interoperable, USB can return to its roots. In addition, such chargers will give OEMs more flexibility since a single hardware architecture can meet the requirements of the various charging standards.

Continue reading on page two, USB Power Delivery >>

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