How does the QC and PD charging work?

How does the QC and PD charging work?

Issues to be considered in charging technology

First: the battery's charge-acceptance

To put it in a more popular way, charging the battery is to fill the battery. Then first consider how much the battery can accept. With the more popular mobile phones currently on the market, not many have a battery capacity exceeding 3000mAH.

According to 4.35V as the highest voltage and 1.5C charging, the maximum possible charging power is about 20W. Of course, this is the extreme situation. In addition to consider the power receiving capability, current receiving capability shall also be involved.

When charging at 1.5C, the charging current of the 3000mAH battery will reach 4.5A. Therefore, the battery contacts and the current transmission structure inside the battery must be optimized.

Second: the power supply capability of the adapter

Regardless of the bearing capacity of the interface, 20W power is a breeze for the adapter. However, the traditional Micro USB interface has a maximum current carrying capacity of 2A and a maximum voltage of 5.25V in the standard specification.

But only 10.5W is far away from meeting the 20W requirement. How to solve this problem? There are two obviously solutions, increase the current, or increase the voltage. If the physical interface is not changed, it is impossible to increase the current. Therefore, increasing the voltage is the only option in the MicroUSB era. This is how the Qualcomm's QC quick charging protocol come.

Therefore, we can see that 1.5A is the current recommended by the QC standard, because 2A is the limit of MicroUSB. The general consensus in the industry is not to use the device to the limit, but to reserve a margin.

In this regard, OPPO took the opposite path of Qualcomm’s. They patched MicroUSB physically and added additional contact pins to transmit large currents.

The maximum charging current was managed to reach 4.5A, while the voltage remained unchanged at 5V. It also achieves a power transmission exceeding 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 requirements of existing mobile phone batteries.

Third: Charging management and heat dissipation capabilities of mobile phones

Charging management and heat dissipation capabilities of mobile phones. Charging management inevitably involves voltage conversion, constant current control and other links, which brings about a drop in charging efficiency and heat dissipation problems.

Therefore, in theory, the best charging design scheme is that the mobile phone does not do charging management, and is completely controlled by an external adapter. At this point, QC is at a disadvantage, because high voltage and low current input will inevitably lead to energy conversion inside the mobile phone to low voltage and high current.

This will bring about a big problem in mobile phone heat dissipation. Therefore, from a technical point of view, the historical limitations of QC have been highlighted. A more serious problem is that it is strictly forbidden to use methods other than USB Power Delivery (USBPD) to adjust the charging voltage in both the TYPE-C interface and the USBPD.

Qualcomm has made great efforts to persuade the USB-IF organization to try to make QC and PD coexist in the TYPE-C interface. However, it is a pity that Qualcomm was ruthlessly rejected. The latest TYPE-C1.2 and USBPD3.0 maintain the description of this feature.

Therefore, both technically and theoretically, QC will face the danger of being eliminated. Of course, Qualcomm itself is very aware of this trend. Therefore, the USBPD negotiation function has been integrated in the latest processor core. 

USB-PD fast charging communication principle

USB-PD communication is the process of modulating the message of the protocol layer into a 24MHZ FSK signal and coupling it to VBUS or obtaining the FSK signal from USB bus voltage (VBUS) to realize the communication between the mobile phone and the charger.

In the USB-PD communication, the 24MHz FSK is coupled to the DC level on the VBUS through the cAC-Coupling coupling capacitor. In order to prevent the 24MHz FSK from affecting the Power Supply or USB Host’s VBUS DC voltage, the loop At the same time, a low-pass filter composed of zIsolation inductors is added to filter out the FSK signal.

The principle of USB PD, taking the mobile phone and charger support USB PD as an example, is explained as follows:

1) The USB-OTG PHY monitors the VBUS voltage. If there is a 5V voltage of VBUS and it detects that the OTG ID pin is a 1K pull-down resistor (not OTG Host mode, the ID resistance of OTG Host mode is less than 1K), it means that the cable supports USB PD;

2) USB-OTG performs normal charger detection according to the BCS V1.2 specification and starts the USB PD device policy manager. The policy manager monitors whether the FSK signal is coupled to the DC level of the VBUS, and decodes the message to obtain a CapabilitiesSource message. Analyze the message according to the USB PD specification to get all the voltage and current list pairs supported by the USB PD charger;

3) The mobile phone selects a voltage and current pair from the CapabilitiesSource message according to the user's configuration, and adds the voltage and current pair to the payload of the Request message, and then the policy manager couples the FSK signal to the VBUS DC level;

4) The charger decodes the FSK signal and sends an Accept message to the mobile phone, while adjusting the DC voltage and current output of the Power Supply;

5) When the mobile phone receives the Accept message, adjust the charging voltage and current of the Charger IC;

6) The mobile phone can dynamically send a Request message during the charging process to request the charger to change the output voltage and current, so as to realize the fast charging process.

QC3.0 fast charge protocol CX7916

CX7918/ CX7916 is a charging interface control chip of USB mobile device. In particular, it adopts Qualcomm Quick Charge 3.0 Type A/Type B specification for adaptive charging of HVDCP. CX7918/ CX7916 can accurately adjust the HVDCP output voltage according to the voltage request sent by the mobile device, thereby saving up to 75% of the charging time.

When the mobile device is inserted into the USB port, CX7918/ CX7916 can automatically recognize its type and make a reasonable response, so that the mobile device can always get the maximum current from the charging port.

CX7918/ CX7916 supports Apple iPad, Apple iPhone, Samsung Galaxy Note, compatible with BC1.2 or YD/T1591 standard devices and almost all modern mobile devices.

CX7918/ CX7916 will automatically detect whether the connected powered device is compatible with QC2.0 or QC3.0 protocol specifications before starting the output voltage adjustment. If it detects that the powered device is not compatible with QC2.0 or QC3.0 protocol, CX7918/ CX7916 will be restricted from adjusting the output voltage, and only output at 5v voltage to ensure that the old USB powered devices can work safely.


Supports Type A and Type B specifications of Quick Charge3.0

Smart recognition of USB charging interface

The difference between PD protocol and QC protocol

USB-Power Delivery (USBPD) is a protocol specification that supports up to 100W of power transmission and data communication in one cable.

USB Type-C is a brand-new USB connector specification that can support USB3.1 (Gen1 and Gen2), DisplayPort and USBPD and a series of new standards.

The USB Type-C port can support up to 5V3A by default. If USBPD is implemented in the USB Type-C port, it can support the 100W power (5V20A) defined in the USBPD specification.

Therefore, having a USB Type-C port does not mean that it supports USBPD.

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