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USB-C Power Delivery--What makes a USB-C cable fast charging?

Aug 23, 2022
USB-C Power Delivery--What makes a USB-C cable fast charging?


Before the Type-C interface appeared, the USB cable was only allowed to provide 2.5W of power, while the USB Type-C cable allowed the maximum power to be up to 5V/3A (15W). If the Power Deliver (PD) protocol is adopted, the voltage and current can be increased to 20V/5A (100W), which allows the power supply of large devices through the USB interface, such as charging a laptop with a large battery.

But, What makes a USB-C cable fast charging?

First, Let's review about USB-C interface and USB-C cable

1. Function Definition of PINs of USB Type-C

Type-C is a form of USB interface. It is the only USB connector that does not care about the front side and back side when it is inserted. It supports USB standard charging, data transmission, video transmission, audio transmission, display output and other functions.





Another difference between USB Type-C and older standards is its dual-role capability. Both ends of each USB Type-C cable are mirrored, which means that the two connected devices must communicate with each other to determine whether they should exist as a host or a peripheral. The communication of the roles needs to be carried out separately for data and power, and this work should be carried out after the cable is connected.

The host port used for data communication is called Downstream Facing Port (DFP), and the peripheral port is called Upstream Facing Port (UFP). In terms of power supply, the power supply end is called the source end (Source), and the power consumption end is called the sink end (Sink). Some devices can have both the Dual Roles of Data (DRD) capability on the data and the Dual Roles of Power (DRP) capability on the power supply. The CC wire defines the role of the power supply during the connection between the two devices, communicating via the Type-C "Configuration Channel Pin CC"



2. How is a USB-C to USB-C cable connected?

The wiring diagram of the full-featured USB-C to USB-C GEN 2 Cable is as follows, provided by P-Shine Electronic Tech Ltd.

Status ① Unflipped direct connection



The image above shows the connection when the cable is Unflipped. From the socket on the left to the socket on the right, the RX1 pair is connected to the RX1 pair, the RX2 pair is connected to the RX2 pair; D+ is connected to D+, D- is connected to D-, SBU1 is connected to SBU2, and CC1 is connected to CC1. .

Sometimes the VCONNs at both ends of the cable do not need to be connected (B5 to B5). When the electronic mark (E-mark) chip is installed on the PCB of the USB-C connector, the B5 of the left plug and the B5 of the right plug need be connected to each other

State ② Flipped connection



When the plug and socket on the left remain the same, and the socket on the right also remains the same, but the plug on the right changes from one side to the other (USB-C supports front and back insertion), the USB-C connection flipped

In this case, from the socket on the left to the socket on the right, the RX1 pair is connected to the TX2 pair, the RX2 pair is connected to the TX1 pair, D+ is still connected to D+, D- is still connected to D-, SBU1 Connect to SBU1, SBU2 to SBU2, and CC1 connected to CC2 via the CC wire. Now, high-speed data is transmitted via RX1+/- and TX1+/- on the left to TX2+/- and RX2+/- on the right.

Both the left and right plugs can be flipped. It seems that there are four different connection methods in total, but there are actually only two, direct (flipping both ends at the same time is equivalent to direct) and one-sided flipped.

Therefore, you can see four pairs of high-speed signal pairs in the 3.1 cable of USB-C to USB-C Cable, but only two pairs are working at the same time, when one-sided plug fliped, the other two free signal pairs may replace the original working pairs. Or as the host and peripheral roles for power supply or data transfer change, signal pairs are constantly toggled.

In the USB 3.1 system, the RX/TX data pairs need to be configured for each possible connection state using a multiplexer so that correct communication can be formed. The figure below shows the routing possibilities of data pairs between USB Type-C ports, the orientation of the plug and socket can be known by measuring the status of CC1/CC2 on each terminal, the CC logic controller can then complete the routing configuration of the multiplexer, either in the multiplexer or in the USB chipset.




3. USB-C Power Delivery--What makes a USB-C cable fast charging?

USB PD3.0 is only related to the power supply of the cable, and has nothing to do with the data transmission. Traditional USB-A charging cables can be only two wires, VBUS and Gound. However, a USB-C to USB-C cable that complies with PD 3.0 requires at least three wires, VBUS, Gound and CC (Channel Configuration).

In a USB Type-C cable that does not use a power transfer protocol, the method of power transfer from the source end to the sink end is shown in the figure below



The source end of the USB Type-C cable always contains a MOSFET switch for turning on/off VBUS, it may have the ability to detect VBUS current, its main function is to detect overcurrent conditions, the VBUS discharge circuit in it will start to work when overcurrent occurs. The detection circuits of CC1 and CC2 exist at both the source and sink ends.

The role of the CC (Channel Configuration) wire is to configure the power supply for two connected devices. Initially, there is no power supply on the VBUS of the USB Type-C interface. The system needs to define the role of the device during the cable connection. The device with the voltage of the CC line on the socket pulled up will be defined as the power supplier (source), while the device with the voltage pulled down will be defined as the power consumer (sink).



The figure above shows how to determine the roles of power supply and consumption, cable orientation, and current supply capability. The CC1 and CC2 at the source end are pulled high by the resistor Rp, and the monitored CC1/CC2 is always at a high voltage when nothing is connected. Once the sink is connected, the voltage of CC1 or CC2 is pulled down by the resistor Rd . Since there is only one CC wire in the cable, the source can tell which side of the CC is pulled low. The voltage of CC1/CC2 at the sink is also monitored, once a CC wire is found to be pulled up, the change in its voltage level will let the sink know the current supply capability of the source. The pull-up resistor Rp in the circuit can also be replaced with a current source, which is easy to implement in an integrated circuit and can be immune to V+ supply voltage errors.

The defined value of the pull-down resistor Rd at the sink is 5.1KΩ, so the voltage of the CC wire is determined by the value of the pull-up resistor Rp at the source (or the current value of the current source). There are 3 levels of bus current that have been defined. The lowest CC wire voltage (about 0.41V) corresponds to the default USB power specification (500mA for USB 2.0 or 900mA for USB 3.0), and the higher CC wire voltage (about 0.92V) ) corresponds to a current capability of 1.5A. If the CC wire voltage is about 1.68V, the corresponding Maximum current supply capability is 3A. Relevant data can refer to the following figure



The figure below demonstrates a measurement case in which the power supply side (Source) is connected to the power consumption side (Sink), using a normal USB-C to USB-C cable.

Initially, both CC1 and CC2 on the source socket are pulled up to a high voltage by the resistor Rp, and both CC1 and CC2 on the sink are pulled down to a low voltage by the pull-down resistor Rd.

After the cable is connected, CC1 or CC2 is pulled up to a higher voltage depending on the insertion direction of the cable. The cable in this case is not in a flipped state, the CC1 at the source end and CC1 at the sink end are connected, after the voltage on CC1 is affected by Rp and Rd, a new value appears, this voltage will be measured by the sink and thus know what the current supply capability of the source is.

In this case, the voltage of CC1 after connecting is about 1.65V, which means that the source can supply a maximum current of 3A.

After the CC wire connection is established, the 5V voltage on VBUS will be turned on.

In systems without a power delivery protocol, the current supply capability on the bus is determined by Rp/Rd, but the source only supplies 5V



After adopting the Power Delivery (PD) protocol, the bus voltage of the USB Type-C system can be increased to a maximum of 20V, communication between the source and sink regarding bus voltage and current is accomplished by transmitting serial BMC codes on the CC wire

The system frame diagram of the USB Type-C system including the PD protocol from the source side to the sink side is shown in the figure below



As shown in the figure above, the source side contains a voltage converter, which is controlled by the source side PD controller. The voltage converter can be a Buck, Boost, Buck-Boost or flyback converter depending on the input voltage conditions and the highest bus voltage requirements. PD communication through the CC wire is also under the control of the PD controller. The USB PD system also needs a switch to switch the Vconn power to a CC wire.

When the connection of the cable is established, the SOP communication of the PD protocol starts through the CC wire to select the specification of power transmission, the sink will ask the power configuration parameters (voltage and current data of the bus) that the source can provide. Since the power demand of the sink end is often related to the device connected to the sink (such as a charger), the embedded system controller of the sink end needs to communicate with the PD controller of the source end to determine the corresponding specifications.

The figure below demonstrates an example of a sinking PD controller requesting a higher bus voltage.



The communication between the sink and the source on the CC wire looks like the following steps:

1. The sink side applies to obtain the capability data of the source side.

2. The source provides its capability data information.

3. The sink selects the appropriate power configuration parameters from the capability data information provided by the source and sends a corresponding request.

4. The source accepts the request and modifies the bus voltage to the corresponding parameter. During bus voltage changes, the current consumption of the sink is kept as small as possible. The process of raising the bus voltage at the source end is carried out according to the defined voltage raising speed.

5. After the bus voltage reaches the final value, the source will wait for the bus voltage to stabilize and then sending a power ready signal to the sink, at this point, the sink can increase its current consumption. The same communication process occurs when the sink wants the bus voltage to drop, during the bus voltage drop, the source activates a shunt circuit that rapidly reduces the bus voltage through active bus discharge. After reaching the rated value, the source will wait a little longer for the bus voltage to stabilize before sending a power-ready signal to the consumer

This method of communication ensures that any power changes on the bus are within the capabilities of the source and sink, avoiding uncontrollable conditions. When the connection of the Type-C cable is disconnected, the power on the bus is also turned off. Any new connection will definitely do the cable connection detection, and the voltage is always at 5V, so that it can avoid high voltage when the cable is connected from one device to another.

The USB PD communication uses Bi-phase Mark Code (BMC), which is a single-line communication code. The transmission of data 1 requires a switching process between high and low voltages, and the transmission of data 0 It is a fixed high voltage or low voltage. Each data packet contains a 0/1 alternating preamble, a start of packet (SOP), a packet header, information data bytes, a CRC cyclic redundancy code, and an end of packet (End of Packet) code. Packet, EOC), see the figure below:



The figure below shows the waveform of a PD communication that requires a bus voltage increase from dense to expanded. The sequence of the preamble can be seen from the last expanded waveform.



BMC communication data can be decoded with a USB PD decoder, such as Ellisys' EX350 analyzer. With this tool, the data of PD communication can be captured and the meaning of each data packet is displayed, which contains time-related data such as bus voltage value, waveform on the CC wire, etc., see the figure below




4. Power configuration list

The USB PD 3.0 specification defines the following power supply configuration list:



There are 4 separate voltage values that are predefined: 5V, 9V, 15V and 20V. For 5V, 9V and 15V, the maximum current is 3A. In a 20V configuration, if the cable is normal, the maximum allowable output is 20V/3A (60W). If a specially customized cable with Electronic Mark (E-Mark) is used, the corresponding data can be enlarged to 20V/5A (100W). A system that supports the highest voltage and power grade must also support all lower voltage and power grades.


5. Cable with Electronic Mark (E-Mark) and How is an E-Mark chip works?

The USB Type-C specification defines a variety of cables with different specifications. There are no special requirements for a low-speed USB 2.0 cable. But for USB 3.1 cables that support super fast data transmission, or cables with currents exceeding 3A, Electronic Mark must be used. The cable shown in the figure below contains an IC whose function is to identify the characteristics of the cable. This lively cable can also contain IC for signal shaping, all of which require power from the VCONN wier of the cable.



The Vconn in the cable containing the Electronic Mark chip contains a pull-down resistor Ra of 1KΩ, and its value is smaller than the resistor Rd, which is typically 5.1kΩ. When such a cable is inserted, the source end will see the voltage drop of CC1 and CC2. The specific voltage change will tell the host which end is pulled down by the 5.1kΩ resistance of the sink end, and which end is pulled down by the 1KΩ resistance of the cable, so the insertion direction of the cable can be determined. The pull-down effect of Ra also allows the source end to know that VCONN needs a 5V power supply, so it needs to supply power to the CC end to meet the power requirements of the Electronic Mark.



The figure below shows a test case, which the power supply end (source) is connected to the power consumption end (sink) by a cable with an Electronic Mark, and the cable is in a flipped state. It can be seen that when the cable is connected, a CC wire at the source end is pulled to a very low voltage by a 1KΩ resistance from the VCONN end.

The source end will detect this voltage and know that the cable contains an Electronic Mark chip, so it will connect the 5V VCONN to the CC wire to supply power to the internal circuit of the cable.

The PD communication that occurs later will include the communication between the source and the Electronic Mark (called SOP' or SOP"),  and the communication between the source and the sink (called SOP)




6. Dual role of power supply

Some USB Type-C devices can be used as both a source and a sink, and they are called devices that support dual roles (Dual Role for Power, DRP). The CC1 and CC2 terminals of this device are in a state of alternating high and low levels. Before interconnection, once the connection occurs, the CC terminals of both will change, as shown in the figure below.


In this case, the DRP device on the left is selected as the source, and the DRP device on the right is selected as the sink. This situation can also be reversed, unless a DRP device has been set to source first (such as when it is powered by an external power adapter), or set to sink first (such as when it is powered by a battery).

Power role switching can also occur during connection, as long as one of the two DRP devices initiates the role switching request. The following figure shows the process of such a role switching.






 

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