Bc1.2 PD protocol

One 、BC1.2

BC1.2 (Battery Charging v1.2) yes USB-IF Subordinates BC(Battery Charging) Agreement made by the group , It is mainly used to regulate the demand of battery charging , The agreement was originally based on USB2.0 Protocol to implement .

BC1.2 Charging port

USB2.0 The protocol specifies that the peripheral is from USB The maximum extraction current of the charger is 500mA,500mA The current limit of can not meet the growing demand for fast charging . therefore ,BC1.2 The charging port identification mechanism is introduced , It mainly includes the following USB Port type ：

1. Standard downlink port （SDP）

SDP Port support USB agreement , Maximum current 500mA, It can be said that SDP It's just plain USB Interface

2. Dedicated charging port （DCP）

DCP Data protocol is not supported , Support fast charging , It can provide large current ,DCP It is mainly used for wall charging and other special chargers

3. Charging downlink port （CDP）

CDP It supports both data protocol and fast charging

Two 、BC1.2 Protocol identification process

1、VBUS Detect Vbus testing

PD（portable device, Portable devices ） There is a test in VBUS Whether the circuit is effective , The circuit has a reference value , Above this value is considered to be VBUS It worked , The reference value is not fixed, usually in 0.8V~4V Between

2、Data Contact Detect Data connection detection

This stage is not necessary , because USB Ports may or may not support data protocols . If this phase times out 900ms We haven't detected D+ or ID PIN The connection of , It is required to start Primary Detection.

3、Primary Detection First test

This stage is mainly used to judge whether the port is a charging port or a data port ：

First of all, will PD+ Pull up to 0.6V, And then test PD- The voltage of , If it is less than the specified reference voltage, the port is a data port SDP; If it is greater than the reference voltage, it is the charging port CDP perhaps DCP.

4、Secondary Detection Second test

This stage is used to confirm whether the charging port can support the data protocol , That is to distinguish CDP and DCP：

First of all, will PD- Pull up to 0.6V, And then test PD+ The voltage of , If it is less than the specified reference voltage, the port is CDP; If it is greater than the specified reference voltage, the port is DCP.

The picture below is CDP Port identification process , After the first test , Enter the second detection stage , First of all, will PD- pull up , And then determine PD+ The voltage of is less than the reference voltage 0.4V, Description port is CDP port .

3、 ... and 、 Private fast charging protocol

because BC1.2 Not a mandatory agreement , Many manufacturers are based on BC1.2 Developed its own private fast charging protocol . For example, Qualcomm's QC2.0/QC3.0, United Development Section PE（Pump Express）/PE+. Because charging power is related to voltage and current P=UI, Increasing the voltage or current can increase the charging power , When the battery capacity is certain , The higher the power, the faster the charging speed . High pass QC2.0/QC3.0 And the United Development section PE The technical principle of the fast charging scheme is the same , Both of them increase the charging power by increasing the charging voltage .

The following figure shows Qualcomm High Voltage DCP Protocol identification process , It can be seen that the first test and the second test are in conformity with BC1.2 The provisions of the , Initial VBUS Voltage is 5V, In identifying DCP After the port VBUS Pull up to 9V.QC2.0 Support 5V、9V、12V Third gear voltage ,QC3.0 On this basis, the voltage range is subdivided to 200mV For the first gear , At the same time, the voltage range is widened to 3.6V~20V.

Four 、 Line loss compensation

from U=IR Know that the greater the charging current ,USB The greater the voltage drop on the charging line . Especially on the on-board equipment , With USB Increase of line or charging current , The voltage reaching the mobile terminal may not reach 5V, Such problems can be solved by line loss compensation .

To deal with such problems , With line loss compensation USB Charging power supply is becoming popular . The so-called line loss compensation is to monitor the change of charging current in real time , Then adjust the output voltage in real time to offset or supplement the voltage drop on the wire . If the charging current is monitored to increase , Increase the charging voltage immediately , On the contrary, reduce the output voltage .

The principle of line loss compensation is shown in the figure , First, in the USB A sampling resistor is connected in series in the power supply line Rsense,Rsense The resistance of must be small enough （ Such as 0.01Ω）, So we need an amplifying circuit to amplify Rsense The pressure drop across the gets Vsense, then Vsense adopt R3 To change FB The voltage of the pin reaches “ cheating ” Purpose of switching power supply . The effect of line loss compensation is shown in the figure 1.9 schematic . Of course , The limitations of such compensation measures are also obvious , The line resistance of the charging line must be known and fixed , Otherwise, it may cause over adjustment or under adjustment .