How to choose the copper wire specification of type C data cable

Since the type C data cable began to run 3a and 5A current, the problem of how to select the copper wire specification to meet the current and power has been frequently asked. Today, we will talk about how to select the copper wire specification of the type C data cable. The most basic electrical parameters of the USB data cable are resistance, voltage drop, voltage, etc, In fact, the most frequently asked questions are, of course, how much current the corresponding data cable can withstand and how large a conductor is cost-effective. Let's talk about it today.


In the data cable of type C data cable, the pin angle related to current transmission is defined as VBUS and GND. Let's review the description of USB type-C pin, and then return to this topic

The above signals can be divided into five categories according to their functions

Class I: power related signals, including

  1. a) VBUS, the bus power of USB cable.
  1. b) Vconn (this signal only exists on the plug) is used to supply power to the plug (it can be inferred that there may be circuits in some plugs).
  1. c) GND, ground

The second type: USB 2.0 data cable, D + / D -, they have only one pair at the plug end, which is consistent with the old USB 2.0 specification. But in order to support the positive and negative random plug. Two groups are defined in the socket end, so that the socket end can map appropriately according to the actual situation. The third type: usb3.1 data cable, TX + / - and Rx + / - for high-speed data transmission. There are two groups of plug and socket end, which are used to support forward and reverse random insertion.

Class 4: signals used for configuration. For the plug, there is only one CC. For the socket, there are two CC1 and CC2.

The fifth category: the signal required by the extended function. The specific use scenario is determined by the corresponding extended function.

For different types of sockets and plugs described in 3.1, these 24 pins and signals may not be all used. For details, please refer to the specification of USB type-c. in addition, you may notice that among the 24 pin signals of USB type-C, the power class (GND / VBUS) and the data class (D + / D - / TX / Rx) are completely symmetrical (for power, no matter how it is inserted, it is the same; For the data cable, a simple route can work. The rest, including CC, SBU and vconn, are used for direction and line type detection


What the hell is VBUS on USB?


We all know about Gen D, but when it comes to VBUS, many people just hear it. According to the USB specification, USB devices can be divided into two types: self powered and bus powered,

The self powered equipment does not use the power of host / hub, but has its own power supply; Bus power supply means that the equipment power supply comes from VBUS.

If it is a bus power supply device, USB specification specifies two kinds of devices according to the current absorbed by the device when it works: low pwer and high power device. Low power device shall not absorb more than 100mA current under any circumstances, and high power device shall not absorb more than 100mA current before it is correctly configured.

If it has been configured, it is not allowed to draw more than 500mA current under any circumstances. If the device enters the suspend state, the low power device shall not absorb more than 500uA current under any circumstances, and the high power device shall not absorb more than 2.5mA current when it has been correctly configured and the remote wake-up function is enabled by the host, otherwise it shall not exceed 500uA.

In a word, the simple point is: VBUS cable is the power cable that host / hub supplies power to USB device.

Now let's go back to the topic of today's discussion. How to choose this power cord and how to choose the appropriate length to meet the current demand depends on the resistance. The larger the resistance is, the smaller the current is, and the smaller the resistance is, the greater the current will be. To put it more directly, the resistance is the thickness of the wire [AWG number of conductor], and the thicker the wire [AWG number of conductor] the greater the current will be, The thinner the wire [the larger the AWG number of conductor], the higher the current; Because: the thicker the wire, the faster the current flow, the smaller the resistance, just like the water pipe, the thinner the water pipe, the smaller the water flow, the thicker the water pipe, the larger the water flow. The above common truth, but when considering the USB Association specification, we will not simply consider a single current as the standard to measure the quality of the wire, but choose the voltage drop, USB type-C specification release 1.2 specification definition: VBUS: voltage drop range is less than 500mv GND: voltage drop range is less than 250Mv finished circuit = 500mv + 250Mv = 750mv.

Why take voltage drop as the standard to measure current instead of single current?

Let's first look at the calculation formula of voltage drop: (output current * conductor resistance) * wire length + contact resistance

Voltage drop, also known as voltage or potential difference, expressed as u, unit volt (V), is a physical quantity describing the power of electric field force moving charge

For example:

The voltage of a power supply is transmitted through a section of line or other parts. A part of the voltage will be consumed and reduced. The reduced part is the voltage drop of this section of line. The difference between the voltage at the starting point and the voltage at the ending point of the power supply is the voltage drop

Take a simple example, for example, if the output voltage of the substation is 220 V and the voltage of your home is 215 V, then the voltage drop of the circuit from the substation to your home is 220 v-215 v = 5 v

The voltage drop should be explained as follows: there is resistance in the wire itself. When the current flows along the wire, a voltage must be applied to overcome the resistance, otherwise the current cannot pass through; On the same wire, the greater the current passing through, the higher the voltage needed to overcome the resistance (v = I * r). For the power supply, the voltage to overcome the resistance causes a "voltage drop" (the voltage sent to the power supply is reduced). The greater the power consumption (I increase), the greater the voltage drop. The larger the wire resistivity, the smaller the cross-sectional area of the wire The longer the wire (r increase), the greater the voltage drop

It can be seen from the above that measuring voltage drop is more reliable, stable and comprehensive than simple current.