USB Cable voltage drop

Author:     Published in:2022/5/19 14:08:08    Hits:

Line resistance plays a decisive role in the pressure drop.

Why does pressure drop have a significant effect on charging efficiency?

Before we start talking about charging efficiency, let's call a key word "voltage drop." Pressure drop is the voltage difference that points to wire two ends, the input end of wire for example was connected to the power supply of a 5V, but the voltage that detects 4.8V only in output end, the pressure drop of this wire is 0.2V. So how does the pressure drop come about? Although we actually used in the cable material is a good electrical conductor, but ultimately not superconductors, inside there is resistance, so that when we put the charger with usb data cable and mobile phones, is equivalent to in series with a resistor in a circuit, and after the formation of charging circuit, there will be an electric current passes through in the data line, the presence of resistance and current, Both ends of wire can produce voltage naturally, and the value of this voltage is pressure drop value.

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So why is pressure drop a key word for charging efficiency? That is because in the charging process, the input voltage of the terminal equipment is processed by the data line "voltage drop". For example, when the output voltage of the charger is 5V and the current of the charging loop is 2A, the data line with the voltage drop of 0.2V means that the input voltage of the terminal equipment is 4.8V and the total input power is 9.6W. Using a data line with a voltage drop of up to 0.4V means that the input power of the terminal device is only 2A*4.6V=9.2W, and the wire incurs an additional 0.4W loss. The lower the input power is, the slower the charging speed is. This is the main reason that the pressure drop of wire can affect the charging efficiency.

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And the above is often only a theoretical calculation, in fact, many terminal devices are required to have a minimum charging voltage, for example, a device supports 5V5% charging voltage, that is, 4.75V to 5.25V, when you use a voltage drop of 0.4V in 2A current wire, it is likely that because the charging input voltage is only 4.6V, The charging current has to be reduced to reduce the voltage drop, or even directly stop charging, which can be restored after replacing a reduced voltage data line. Theoretically, when A data line voltage drop reaches 0.4V under 2A current, the voltage drop becomes 0.25V, then the current through must drop to 1.25A, at this time the input power of the terminal equipment is only equivalent to 4.75V*1.25A5.94W, compared with the original theoretical 5V*2A=10W, there is A significant drop.

How is the pressure drop caused and the line resistance plays a decisive role in the pressure drop

Since the "voltage drop" has an obvious influence on the charging efficiency of the data line, which attribute of the data line has an obvious influence on the "voltage drop"? Actually the computation formula that drops according to pressure "voltage = current * resistance" this point we can be informed, the resistance of wire can have apparent effect to pressure drop. Wire resistance is we often say "line resistance", according to the "resistance = resistivity * length/cross-sectional area" calculation formula of learn that when the cable material under the condition of same, that is, the resistivity, the resistance of the wire is proportional to the length, and the cross-sectional area is inversely proportional to, so want to reduce the resistance of the cable, shorten the length, increase the cross-sectional area is the most direct way.

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This is why some of the longer data lines tend to be thicker, because it needs to increase the cross-sectional area to compensate for the line resistance caused by the length, but this approach tends to greatly increase the cost of the wire, so these longer and thicker data lines tend to sell more expensive. However, there are also some products that increase the length without changing the cross-sectional area of the wire. The longer the length of such data line, the more obvious the pressure drop will be. Of course, we are not saying that such data line can not be used, but the charging efficiency of such wire will indeed be lower, and then directly and simply say that: Resistance is also the thickness of the line, the thicker the wire, the smaller the AWG number of the conductor, the greater the current, the thinner the wire, the greater the AWG number of the conductor, the greater the current. Because: the line is thicker, current flows faster, resistance is smaller, resemble conduit one kind, conduit is finer, current is smaller conduit is thicker, of current is bigger, above popular truth.


In addition length and sectional area are equal wire wire resistance is not likely to be the same, what kind of material wire is used is also a very key factor. At present, copper wire is generally used in data lines. Some high-end products may use silver-plated wire or even pure silver wire to reduce line resistance, but some low-end wires will use aluminum. The conductivity of aluminum is not bad, but it is much lower than copper. For data lines with very short length, it may have little influence, such as products with only 10-15cm in length, but for wires with a length of 1 meter, 1.5 meters or even 2 meters or above, the influence of line resistance brought by aluminum can not be ignored.

Using pure copper and pure aluminum, the resistivity of the latter is 1.6 times that of the former, which means that for the same length and cross-sectional area, the pressure drop of the latter is 1.6 times that of the former. We have previously tested the standard charging cable of Apple'S MacBook Pro 16. The resistance of the cable is 0.125 , and the voltage drop is about 0.6V when passing through 4.7A current, which is equivalent to losing 2.82W of energy. If the wire were to change from copper to aluminum, the wire resistance would theoretically become 0.200 , and the pressure drop at 4.7A current would become 0.94V, equivalent to A loss of 4.42W of energy.

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It is worth mentioning that remains in the mainstream of quick charge mode "high voltage low current" is in order to solve the wire line charging efficiency to reduce problems caused by the pressure drop and development, is also A charger 18 w power output, under the environment of 5 V is equivalent to 3.6 A current, in A line on the resistance of 0.1 wire pressure drop is 0.36 V, already exceeded the requirements of plus or minus 5%, The loss of energy is up to 1.3W; For a 9V environment with 2A current, the voltage drop is just 0.2V, equivalent to a 0.4W power loss, which is less than a third of the former. So whether the mainstream pattern of "high voltage low current" or gradually incline to unified PD charge agreement, as the charging power has increased, the range of the voltage boost tend to be bigger than the size of the current increase, both to reduce the power loss on the wire, but also can avoid the wire rod in order to pass the large current and become too thick, not making it easy for users to use.

Of course, data cables are not only used for charging, but also for transmitting data. But compared to the charging circuit, used for the transmission of the data line to the requirement of wire rod can be said to be quite low, it is because the current is very small, by which the more time is only used to express the level height, so when we cut a cable, will see its power lines cables will significantly more coarse than data line cables. Also because of such, to the wire that transmits data, join whether firm can compare wire whether quite thick, resistance quite low want more important. Because of this, most of the third party data lines now emphasize the current passing ability of their products, and the data transmission ability is more expressed by whether USB 3.0/3.1 or Thunder 3 is supported.

Industry experience sharing

General USB cable signal line in accordance with 2.0 test standard design for 28AWG transmission as the mainstream, if the test attenuation, the longest can pass about 4 meters; If the dc voltage drop is 125mV as required by the Association, the length of the 28AWG power line is recommended to be within 1 meter, the length of the 26AWG power line is recommended to be no more than 1.7 meters, the length of the 24AWG power line is recommended to be no more than 2.7 meters, and the length of the 22AWG power line is recommended to be no more than 4.3 meters. It is recommended that the length of a 20AWG power cable not exceed 5 m. According to USB2.0 standard test specification reference; The association recommends the following 5 specifications for the cross-sectional area of the conductor power supply: Picture

How to choose a good data line

General ordinary consumers choose thick data lines. General regular manufacturers are using the way of increasing the cross-sectional area to make up for the line resistance brought by the length, but this approach tends to greatly increase the cost of wire, so these longer and thicker data lines tend to sell more expensive. However, some products do not change the cross-sectional area of the wire while increasing the length. The longer the length of such data line, the more obvious the pressure drop is. Of course, we do not say that such data line can not be used, but the charging efficiency of such wire will indeed be lower. In addition length and sectional area are equal wire wire resistance is not likely to be the same, what kind of material wire is used is also a very key factor. At present, copper wire is generally used in data lines. Some high-end products may use silver-plated wire or even pure silver wire to reduce line resistance, but some low-end wires will use aluminum. The conductivity of aluminum is not bad, but it is much lower than copper. For data lines with very short length, it may have little influence, such as products with only 10-15cm in length, but for wires with a length of 1 meter, 1.5 meters or even 2 meters or above, the influence of line resistance brought by aluminum can not be ignored.




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