PD Fast charge protocol in USB charger

IV. PD protocol in the battery charging application

1. First consider the acceptance capacity of the battery.

Charging is like feeding the battery, so you need to know how the battery feels. At present, the battery capacity of smart electronic devices on the market is basically 3700~4600mAH. Assuming a maximum voltage of 4.35V and 1.5C charging, the maximum possible charging power is 20W, but this is only the limit case. In addition to the power reception capability, the current reception capability also needs to be considered. For a 3000mA battery, at 1.5C charging, the charging current will reach 4.5A, so the battery contacts and the current transmission structure inside the cell need to be optimized.

2. The second need to consider the power supply capacity of the adapter.

Without considering the capacity of the interface, providing 20W power is a breeze for the adapter. However, the traditional Micro USB interface in the standard specification of the maximum current carrying capacity is 2A, the highest voltage is 5.25V, can only provide 10.5W power, can not meet the requirements of 20W. So how do you solve this problem?

There are obviously two solutions, one is to increase the current, the other is to increase the voltage. However, if the physical interface is not changed, increasing the current is not possible, so increasing the voltage is the only option in the Micro USB era, which is the origin of Qualcomm QC fast charging method. Therefore, it can be seen that the QC standard recommends a current of 1.5A, because 2A is the limit of Micro USB. The general consensus in the industry is not to use the device to the limit value, but to reserve the margin.

In this respect, OXXO has taken the opposite path from Qualcomm. They have physically patched Micro USB, adding extra contact pins specifically designed to transmit high currents. The maximum charging current reached 4.5A, but the voltage remained unchanged at 5V, also achieving a power transfer of more than 20W. The emergence of the Type-C interface makes this problem no longer exist, because the TYPE-C port supports up to 5A input current, which can fully meet the fast charging needs of existing mobile phone batteries.

3. In addition, the charging management and heat dissipation capacity of the mobile phone also need to be considered.

Charge management must involve voltage conversion, constant current control and other links, which will inevitably bring about the reduction of charging efficiency and heat dissipation problems. Therefore, in theory, the best charging design scheme is that the internal charge management of the mobile phone is not done, and the external adapter is completely controlled. At this point, QC is at a loss, because high power and low current input will inevitably lead to energy conversion inside the mobile phone, into low voltage and high current, which will bring about problems in the heat dissipation of the mobile phone. Therefore, from a technical point of view, the historical limitations of QC have been highlighted. The more serious problem is that the TYPE-C interface and USB-PD are strictly prohibited from using ways other than USB-PD to adjust the charging voltage. Although Qualcomm has made great efforts to try to allow QC and PD to exist simultaneously in the TYPE-C interface, it is a pity that it was ruthlessly rejected, and the latest TYPE-C1.2 and USB-PD3.0 maintain the description of this feature. Therefore, QC will face the danger of being eliminated both technically and theoretically. Of course, Qualcomm itself is already aware of this trend. Therefore, in the latest processor core, the negotiation function of USB-PD is integrated.