Designing interoperable battery chargers

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 Similarly, Qualcomm Quick Charger 4.0 has superseded Quick Charge 2.0 and 3.0.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.

Enter USB Type-C and USB Power Delivery

To overcome the challenge of incompatible charging standards, most manufacturers are now adopting the USB Type-C connector that supports the new USB Power Delivery standard, aiming to unify the charger world to a single standard. This is a reversible connector which can be plugged in either orientation (facing up or facing down). This connector resolves the most annoying orientation issue with the USB Type-A and USB Type-B connectors. It is a small connector with dimensions of only 8.4 mm (0.33 in) by 2.6 mm (0.10 in), allowing it to be nicely fitted in ultra-thin notebooks, tablets and mobile phones.

By standardizing the power connector for all the portable devices and accessories on USB Type-C; the need of carrying different chargers and cables is minimized. One charger with the USB Type-C cable can charge one’s mobile phone, laptop, tablet, and other portable device, eliminating confusion and incompatibility forever.

USB Power Delivery

USB Power Delivery (PD) is one the new USB standards for power delivery over the USB Type-C connector. It delivers up to 100 Watts and a voltage/current as high as 20 V/5 A over VBUS. USB PD 2.0 supports fixed voltage profiles ranging from 5 V to 20 V. Next-generation PD 3.0 supports these as well as programmable power supplies in the range from 3.3 V to 21 V in 20 mV steps.

Two Communication Channel (CC) lines – CC1 and CC2 – are used to negotiate the power delivery contract between the charger (power provider) and a device (power consumer). The provider communicates a Source Capabilities[1] message to the consumer. In response, the consumer returns a Request[2] message based on its power requirements. The provider gets into an explicit contract after accepting the request and then sets the desired voltage; this process is termed “power delivery negotiation”. The power delivery negotiation uses PD Messages (as specified in the USB-PD specification).

To illustrates the incompatibility among different charging standards and the need for a universal standard, a series of tests were conducted using several popular smart phones, including Google Pixel 2 XL, Apple iPhone 7, and Samsung Galaxy S8 phones and their associated fast chargers.

The graphs below show the battery percentage vs charging time for all three phones.

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Figure 1: Google Pixel 2 XL Charging time

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Figure 2: iPhone Battery Charging time

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Figure 3: Samsung Galaxy S8 Charging time

The test results reveal some interesting facts and expected outcome:

  • These chargers do not support all charging standards.

  • To reduce charging time for particular phone, a charger supporting the compatible charging standard is required.

While standardizing all mobile phones and chargers to the universal USB PD standard is the future, there is still a huge incompatible installed base that will continue to exist for years to come. This is where a programmable charging solution can be deployed to deal with both the new and old standards. With a programmable solution, developers can implement different charging standards without modifying the hardware and without compromising battery charge-time for consumers. The programmability also helps to keep the solution updated in the field if necessary to keep pace with the changes of all supported charging standards.

The most important components of a battery charger system are:

  1. Power Converter

  2. USB PD Controller and

  3. USB Type-C receptacle

Designing a Universal Mobile Phone Charger

A mobile phone charger becomes programmable if the USB PD controller in the charger is programmable. Such USB PD controller contains a processor and Flash to execute and store firmware and hardware components like Timers, ADC, GPIOs and Communication blocks that can be configured by firmware. Figure 4 shows a block diagram of a generic programmable implementation.

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Figure 4: Programmable Battery Charger Block diagram

An example of a programmable USB charging solution is Cypress Semiconductor’s EZ-PD CCG3PA controller. The CCG3PA is Cypress’ third-generation USB Power Delivery solution and it is a highly integrated controller for all USB-C charger and power bank applications. The CCG3PA has an integrated 32-bit Arm® Cortex®-M0 processor, 64KB flash, 8KB RAM, a built-in USB Type-C transceiver, 2 ADCs, 2 programmable communication interfaces, 4 Timer modules, integrated feedback control circuitry for voltage (VBUS) regulation, and other hardware blocks as shown in Figure 5.

Chargers designed using the programmable CCG3PA controller can be updated/customized easily to support the latest USB PD 3.0 and a long list of legacy charging standards, thus reducing the time to market for charger manufacturers and eliminating confusion for consumers. In this way, a single charger is all a user needs to charge any of his or her mobiles, all at the shortest possible charging time. At the same time, OEMs will benefit from the flexibility of being able to use a single hardware architecture to address the various charging standards. This will simplify design, reduce inventory size, eliminate costs, and improve profitability.

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Figure 5: CCG3PA Power Adapter Block diagram

Footnotes

[1] A Source Port reports its capabilities in a series of 32-bit Power Data Objects as part of a SourceCapabilities Message. Power Data Objects are used to convey a Source Port’s capabilities to provide power.

[2] Message is sent by a Sink to request power, during the request phase of a power negotiation. The Request Data Object is returned by the Sink making a request for power.


Anshul Gulati is currently working on customer/reference designs on USB Type-C and USB PD applications like Car Chargers and Power Banks. She has over 16 years of experience in Embedded System designs and has worked on applications ranging from USB, BLE, HBLEDs to Solar Charger Controllers. She has a degree in Electrical and Electronics Engineering.

1 thought on “Designing interoperable battery chargers

  1. “Ok to be honest, out of all the phones and devices that I've used through the years, I get that we need to keep changing cables and plugs to make sure that everything is kept efficient and to increase the level of charge and power transmitted when somethi

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