Leading the Charge in Mobile Phones

­We rely upon our mobile devices and wearables more and more, so we constantly seek enhanced performance and functionality from them. However, with this comes the need for increased battery capacity, despite recent advances in efficient operation. Convenience is the key, allowing users to charge these high-capacity devices rapidly and efficiently, as well as benefitting from recent innovations such as charging wirelessly and charging one device from another.

With billions of batteries deployed, the majority of which are charged daily, there is a significant impact on energy grids. Therefore, it is critical that charging is as efficient as possible—not only does this reduce the load on the grid, but it also keeps temperatures low and speeds up charging.

However, there are significant challenges associated with delivering the performance that consumers desire in a robust, compact, and cost-effective charging solution. In this article, we will discuss how smartphone charging is evolving and look at some of the technologies that are supporting this.

For years we have been able to transfer data wirelessly through technologies such as Wi-Fi and Bluetooth, among others. However, until recently, portable devices were shackled to a wall outlet via a charging cable to replenish their batteries. Clearly, change was needed and today many (if not most) devices offer the ability to be charged wirelessly, without the need for a cable.

In fact, the market for charging phones wirelessly is now very significant. According to a report by Prophecy Market Insights, the global wireless phone charging market was worth around US$11.9 billion in 2020 and is estimated to reach US$79.0 billion by 2030—a CAGR of over 20%.

One of the primary factors driving this growth is user convenience. With so many device manufacturers using different connectors (micro-USB, USB-C, or proprietary) different cables were needed—and had to be remembered when traveling. With chargers being low-cost items, many people had several for a single device and billions of these chargers ended up in a landfill. Recognizing this challenge, some manufacturers (such as Samsung) no longer ship chargers with their latest devices, assuming the user will have several already.

With wireless charging standards such as Qi (pronounced ‘chee’) being adopted by many manufacturers, there is now greater interoperability and most devices are compatible with the majority of charging pads.

Fast Charger Design

Given the relatively low power levels involved, some may assume that the charger design would be trivial. However, this is not the case as the small volume available drives significant challenges. Small size and low weight are always requirements in mobile device design, often simply so that the design will fit and also for user convenience.

Efficiency is the single most important factor. The higher the efficiency, the lower the demand on the grid from the billions of mobile devices being charged. Additionally, if the charger is efficient then charging can be quicker and with less waste energy being converted to heat. Protection is an important consideration, to ensure that the charging circuit and battery are not damaged, should an overload or fault occur.

One topology that is commonly used for fast charging of mobile devices is the charge pump, which uses a combination of switching devices (MOSFETs) and capacitors to reduce and control the energy used for charging. In fact, switched capacitor-based fast charging architectures are now used in many mid-level smartphones, as well as in the high-end devices where they were originally found.

Halo Microelectronics’ HL7132 is a 30W charge pump-based low voltage 2:1 fast charger that provides CC (Constant Current) and CV (Constant Voltage) regulation via an internal FET for devices that use single-cell Li-ion and Li-polymer batteries. The integrated solution includes a dual-phase switched capacitor converter and reverse-blocking MOSFET (QRB FET) allowing it to achieve 97.11% efficiency with a 4.5V output while delivering a 5A charging current.

The integrated switched capacitor converter and FETs are optimized to operate at 50% duty cycle in charge pump (CP) mode, allowing the output voltage (V_OUT) to be around half of the input voltage (V_IN) and output current to double the input current, reducing input power cable loss and limiting temperature rise in the application. The dual-phase architecture reduces the input capacitance requirements (saving space and cost) as well as reducing the input voltage ripple.

The HL7132 has a full suite of protection including OTP, VIN UVP/OVP, IIN OCP/UCP, VOUT UVP/OVP, VBAT OVP, IBAT OCP, PMID to VOUT OV/UV, C_FLY SCP, VIN SCP, VOUT SCP, QRB RCP, and watchdog timer. The device also features a 10-bit ADC to provide VIN, IIN, VOUT, VBAT, IBAT, VTS, and TDIE information to optimize charging control.

Integrated wireless charging

With wireless charging, the requirements are quite similar to fast charging – efficiency is key to maximizing power transfer and minimizing heat generation. However, EMI is a constant challenge with power design, and with wireless charging the challenge is greater as the main transformer is split between the charger and device, allowing the potential for radiated EMI.

Early incarnations of wireless charging simply allowed a device to be placed on a charging pad to be charged. However, technology has evolved such that one wireless-capable device is able to charge another. For example, a smartphone could be used to charge a wearable device, such as a pair of earbuds.

Essential to this dual operation is a smart power load switch such as Halo Microelectronics’ HL5502 wireless RX TX coil switch.  This technology is embodied in the Samsung Galaxy S22 Ultra, which controls the coil size. Depending on whether charging is phone-to-phone or phone-to-wearable, coil elements can be switched in or out to optimize the coil size for power transfer and minimize any unnecessary radiation. This ensures optimum efficiency and minimizes radiated EMI.

The HL5502 is a high-voltage protection wireless coil switch that protects any system against positive or negative coil pin voltages that do not exceed ±40V. The MOSFETs inside the device exhibit a low typical ON resistance Rds(on) of just 90mΩ. With a pair of internal MOSFETs connected in series that two power pins, COIL and AC, are completely isolated when it is disabled.

Click image to enlarge

Figure 2: HL5502 RX coil switch application diagram

The device employs a floating gate drive architecture to pass DC or AC at frequencies up to 1MHz from either the coil or AC pins. Control is effective through the ENn pin and the EN_POL pin. When the ENn pin is driven low and EN_POL is tied to GND, then the internal power FETs are enabled if either VIN or VCC is higher than its under-voltage lockout (UVLO) threshold.

Halo continues to innovate in this area to improve performance and flexibility for designers. One device that is close to launching is the HL7133 - another low voltage dual phase switched capacitor fast charger. However, this offers 45W capability and four operation modes 2:1 and 1:1 in forward and reverse directions. The new device can achieve 97% efficiency (4V output @ 8A).

In CP (2:1) mode the HL7133 operates in a very similar manner to existing devices. However, the device also offers a forward / reverse 1:1 Bypass mode (BP) that allows VIN to pass through the internal power FETs to the output without any conversion.

In common with other Halo charging devices, the HL7133 provides CC and CV regulation and includes all the necessary protection mechanisms to ensure safe operation. Also built in is a 12-bit ADC to provide accurate system information for optimized charging control.

The HL7133 is packaged in a 70-bump WLCSP package measuring just 4.27mm x 2.97mm.

Summary

The rollout and adoption of 5G and the proliferation of USB Type-C connectors are two key drivers within the mobile communications market. Together they are fuelling the adoption and proliferation of fast charging for all types and categories of mobile devices.  As a result, manufacturers are introducing new products that implement fast charging at power levels from 20W to greater than 50W. 

Supporting these trends, Halo Microelectronics has introduced a suite of innovative devices. The HL7132 achieves the industry's smallest footprint for a 30W fast charger, thereby reducing the solution size by more than 30%. The HL5502 load switch allows coils to be rapidly configured (and re-configured) to suit different sizes of devices such as smartphones and wearables.

With their comprehensive range of fast charging solutions, Halo Microelectronics offers the broadest selection of 1-cell and 2-cell products in size, charging speed, and cost optimization to support end-product goals. 

Halo Microelectronics