Figure 1: Efficiency vs Output current for Wireless Power Receivers
Smartphones with integrated wireless charging capability have been on the market for several years now, with features and technology advancing swiftly with each new release.
While wireless power has many benefits (convenience, no connector, waterproof), users still want wireless chargers to charge at the same rate as when plugged into a cable. Today, this typically means charging at 5W, but pretty soon expectations will be charging times equivalent to those achieved from the “Fast charge” feature now being added to new high-end phones. In certain cases, you can achieve four hours of battery life in just 10 minutes. The new medium power (15W) specification which is about to be approved by the WPC will further support this capability
Several technical challenges are associated with designing even a true 5W wireless power system. Phones are getting thinner and the real estate within the phone is very important; the size and power dissipation of every component must be as low as possible. It is critical that the phone does not get hot – not only can it impact reliability, but it should not feel warm to the user.
A wireless power-charging solution is effectively an extra power stage within a phone, a new circuit to dissipate heat. The most popular wireless-power devices on the market today use a low-dropout regulator (LDO) instead of a switcher to feed the battery-charger circuit. It is higher efficiency and does not need a bulky inductor. To reduce the power dissipation of the LDO, it is critical to keep its on-resistance as low as possible and to minimize the current passing through it. The way to minimize the current at any given output power is to have a higher output voltage, and ideally to make this output voltage programmable. A programmable output voltage allows the circuit to be optimized by taking into account the input voltage range of the phones PMIC and the paramaters of the receiver coil.
TI’s bq51021 family of wireless chargers, including the bq51221 dual-mode (Wireless Power Consortium/Power Matters Alliance) compliant receiver, have been designed to enable true 5W charging. They feature the industry’s lowest Rdson and have an output voltage programmable from 5V to 8V. The bq51025 has an output voltage range up to 10V and the device can support 10W of power. The graph in figure 1 shows the efficiency of TI’s device vs load current and compares it to another product on the market.
As power levels continue to increase, thermal designs will become more challenging. This extra power will enable wireless charging times similar to the latest Fast charge features that now use only a wire.