Exploring the Differences Between High and Low Voltage Fast Charging Technologies

In the era of smartphones, we have witnessed processors upgrade from single-core and dual-core to quad-core, octa-core, and now even up to ten cores without surprise. The pixel count of phone cameras has also made a leap from hundreds of thousands to millions, and now to tens of millions. The same goes for screen sizes, from 4 inches and 5 inches to now 6 inches and above.

What is fast charging technology? How is fast charging achieved?

However, the only regret is that battery technology for smartphones is still slow in terms of innovation, and the battery life is not sufficient to meet users' daily needs, which is still the biggest pain point for manufacturers. Previously, the common practice among smartphone manufacturers for this shortcoming was to equip large-capacity batteries under the premise of balancing the thickness and weight of the phone, but this was limited. However, with the emergence of fast-charging technology in recent years, it seems to have given the entire industry hope. After a period of exploration, numerous fast-charging solutions for smartphones have emerged.

So, how do smartphones achieve fast charging? How is it implemented, and is it safe? What types of fast charging are there, and what technological innovations can we expect in the future? Today, Appear Inc will answer these questions one by one.

Let's take a look at the physics formula that determines charging efficiency: energy W (which can be viewed as battery capacity) = power P × time T; power P = voltage U × current I. Therefore, it can be seen that under the condition of constant battery capacity, the magnitude of power determines the speed of charging; the larger the power, the shorter the charging time. According to the formula P = U × I, it can be easily concluded that improving charging speed and reducing charging time, can be achieved by the following three methods:

1. Increase current while voltage remains constant;
2. Increase voltage while current remains constant;
3. Increase both voltage and current at the same time to achieve fast charging.

Regarding the relationship between power, current, and voltage, we can make a simple analogy. It's like filling a bathtub with water. Increasing voltage and current is like increasing the amount of water flowing out per unit time and the water flow speed. When one or both parameters are increased, the water-filling efficiency naturally improves, and the bathtub fills up quickly. The speed of filling water (charging to full) will also be significantly improved. Currently, many manufacturers' fast charging solutions rely on increasing voltage (or simultaneously increasing output voltage and current) to achieve fast charging.

Mainstream fast charging technology classification:

The above three schemes are the major prerequisites in terms of technology. After several iterations of technology evolution by manufacturers, we can divide fast charging into two major categories: high-voltage fast charging and low-voltage fast charging. Among them, Chinese manufacturer OPPO's phones use low-voltage fast charging (high current); chip manufacturers Qualcomm's QuickCharge and MediaTek's PumpExpress use high-voltage fast charging schemes; there is also a PD charging standard that uses a scheme where both voltage and current are increased, but it also belongs to high-voltage fast charging, with a maximum current limit of 5A.

Representative of low-voltage fast charging: OPPO VOOC Flash Charge

VOOC Flash Charge technology was first introduced in the first half of 2014 and was installed on the Find7 phone. At that time, reports claimed that VOOC Flash Charge became the biggest factor that users were willing to pay for the Find7.

OPPO's VOOC Flash Charge uses low voltage and high current to quickly charge smartphones. It increases the charging current while maintaining the inherent 5V charging voltage of USB charging. The biggest advantage of this charging method is that it not only improves the charging speed of Android phones (which can be up to 4 times faster than traditional charging speeds. The official claim is that it can reach 75% of 3000mAh in 30 minutes of charging) but also reduces the heat generated by the adapter and the phone during charging, thereby providing the most direct guarantee for both the charging performance and phone safety.

Fast Charging Solutions:

1. Qualcomm Quick Charge 3.0

Qualcomm is the provider of fast charging technology, and currently, in the fast charging technology of smartphones, Qualcomm's Quick Charge 3.0 solution is widely adopted by mobile phone manufacturers and has a relatively wide range of applications. Qualcomm Quick Charge has now been upgraded to version 3.0, which is more efficient in charging than the previous 2.0 version. According to Qualcomm, Quick Charge 3.0 uses the "best voltage intelligent negotiation" algorithm for the first time, which improves conversion efficiency.

Quick Charge 2.0 only provides four levels of charging voltage: 5V, 9V, 12V, and 20V. The upgraded Quick Charge 3.0 provides voltage selection in increments of 200mV, ranging from 3.6V to 20V. At the same time, Quick Charge 3.0 is also compatible with previous versions of chargers, providing more options for OEM manufacturers, which is one of the reasons why Qualcomm Quick Charge is widely used in the fast charging of smartphones.

In terms of time and efficiency, Qualcomm claims that Quick Charge 3.0 can charge a typical smartphone from zero to 80% in about 35 minutes, while a regular mobile terminal usually takes about one and a half hours.

2. MediaTek PumpExpress

MediaTek has also proposed its own fast charging solution, PumpExpress, which is built into the PMIC power management integrated circuit. Its feature allows the charger to determine the initial voltage required for charging based on the current, and the PMIC sends a pulse current instruction to the charger through the USB Vbus, and the charger adjusts the output voltage according to this instruction, gradually increasing the voltage to a maximum of 5V to achieve the maximum charging current.

Currently, there are two technical specifications for PumpExpress, with output power below 10W classified as PumpExpress and output power above 15W classified as PumpExpress Plus. The fast charging method of MediaTek has cooperated with power chip factories such as Tongjia and Dialog to develop exclusive power management ICs. This solution is commonly used in the low-end smartphone product markets in domestic and emerging countries.

3. USB 3.1 PD

USB Type-C, which has been quite popular in the mobile phone industry recently, has not only made changes in data interfaces and transfer rates but has also improved its power supply capabilities (USB Power Delivery Specification, referred to as USB PD). Type-C currently supports a maximum power supply of 20V/5A. With more smartphones equipped with USB Type-C (USB 3.1) interfaces coming to the market, the market outlook is also very promising.

What are the differences between high/low voltage fast charging technologies, and what are their drawbacks?

Since both can achieve the effect of quickly charging a mobile phone, what are the main differences between high and low-voltage charging methods? Before explaining the differences between these two technologies, let's first understand the process of traditional 5V/1A slow charging.

In China, the voltage used for civil purposes is 220V. During traditional charging, the input end is converted into a charging current output of 5V/1A after passing through the charger. In this process, there is a voltage drop conversion, and not all energy can be converted to 100%, and the energy lost will turn into heat. Specifically, when we use a charger to charge, we can feel that the charger generates varying degrees of heat.

The converted 5V/1A current suitable for charging will then pass through the voltage reduction circuit inside the phone, which converts it into a voltage of 4.2V or 4.4V suitable for charging the battery. This process also involves voltage drop conversion, which means that the phone will heat up during charging.

Now let's look at the differences between high and low-voltage fast charging.

In high-voltage fast charging (including QuickCharge, PumpExpress, and USB 3.1PD), the conversion is from 220V to 20V/12V/9V and then to 4.2V/4.4V. This means that this method will allow the charging end to have a higher voltage input to the phone's voltage reduction circuit. The voltage reduction circuit in the phone will have to withstand more pressure to perform the voltage reduction conversion, and of course, more energy will be lost during this conversion process, which directly results in more obvious heating of the phone during charging.

In contrast, OPPO's VOOC flash charging, which is a low-voltage fast charging method, directly reduces the voltage from 220V to 4.2/4.4V. All voltage reduction processes during the phone's charging are completed in the charger with an intelligent MCU chip, which can directly convert the 220V current into a 4.2/4.4V current that can be used to charge the lithium battery. Since the phone's voltage reduction circuit is not used during charging, temperature rise during charging is well controlled, and there is no obvious heating.

Good temperature control will bring an obvious advantage, which is the overall fast charging experience, or flash charging performance, which is very superior. Here's an example: when using low-voltage flash charging to charge a phone, you can play games while charging, regardless of the size of the game. This will not affect the flash charging process. In contrast, with high-voltage fast charging, there may be two heat sources: one is the voltage reduction circuit working and generating heat, and the other is the phone's CPU generating heat. As a result, fast charging can only last for a while, and to keep the temperature within a safe range, the phone can only be switched back to normal charging.

Normal interface charging cables

Of course, each scheme has its own advantages and disadvantages. For low-voltage charging, currently, OPPO holds a large number of patents, and the technology is limited to OPPO's own products, which naturally has compatibility limitations for consumers. Specifically, OPPO phones have their own customized chargers, 8-pin batteries, and 7-pin interface charging cables. If you change to a normal battery or use an Android data cable from another phone, then fast charging will not work.

How will high/low voltage fast charging develop in the future?

Since it is fast charging, the future development direction is naturally to be faster than fast. The biggest bottleneck for the future development of low-voltage fast charging schemes lies in the battery. The output power of the charger can be made very large, but how to make the battery withstand such a large current input is what OPPO needs to consider next. At the MWC Barcelona exhibition earlier this year, OPPO showed off a brand new SuperVOOC flash charging that uses low-voltage pulse algorithms, which can fully charge a 2500mAh battery in less than 15 minutes. This can be said to be a new breakthrough in fast-charging technology.

For high-voltage fast charging schemes, in addition to solving the battery problem, it is necessary to solve the heating problem caused by fast charging. Assuming that the output power of the charger is increased to 70W under the high-voltage fast charging scheme, and the conversion loss at the voltage reduction circuit of the phone is calculated based on a loss of 10%, the heat generated will reach 7W. This is a very exaggerated number, and the difficulty of solving it is not small.

In summary:

There is no doubt that fast charging is currently the best way to effectively solve the battery life problem of smartphones, and it is also the main battlefield for mobile phone manufacturers and solution providers to compete in the next few years. If more and more users start choosing smartphones with fast charging capabilities, then what we need to choose next are fast-charging phones that charge faster, have more stable performance, and have better compatibility. This is also the development direction of the entire industry.

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Noted: The above information is shared by the Technology Department of APPEAR INC. For more industry and product exchanges, as well as customized needs for batteries and battery packs, please contact bruce@appearhome.com.