The concept of wireless power has been developed for a long time. It is until recent years, with the rapid development of microprocessor, wireless power becomes a viable solution. Today, wireless technology is growing at an exponential rate, with everything from phones to consumer electronics being wirelessly connected. Despite the rapid development in the technology, battery life of these devices remains a problem. Wireless power or wireless charging is designed to solve these problems.
Wireless power is also known as inductive charging. It requires two coils, a transmitter coil and a receiver coil. An alternative current passes through the transmitter coil, generating magnetic field. The magnetic field induces a voltage in the receiver coil, which can be used to power external load, such as to power a mobile device or to charge a battery.
In a wireless power/wireless charging system, a transmitter is connected with a power source. The transmitter contains a primary coil that generates a magnetic field. When a receiver, which has a secondary coil, makes contact or is in a close proximity of the transmitter, the transmitter and the receiver are magnetically coupled. Power transfers from the transmitter through coupled inductors, such as an air core transformer. The receiver takes the inputs from the secondary coil, and passes it through a rectifier circuit.
In modern integrated circuit designed for wireless power devices, the amount of the power transferred is controlled by internal control circuits. Control signals are transmitted from the receiver to the transmitter based on detected conditions at the receiver to increase or decrease power. Further, the receiver monitors receiver conditions and triggers internal protection mechanism. A wireless pick up unit in a wireless power receiver that receives input from the secondary coil, rectifies the input and outputs a rectified output (RECTOUT). The RECTOUT drives a load for the wireless power receiver applications.
The Wireless Power Consortium (WPC) is a standard body that develops and licenses a global interoperable standard for wireless charging. WPC has requirements for WPC medium power to have a separate load switch to shut off the wireless power receiver when an over voltage condition or other internal or external fault condition occurs. When a load switch is used, the load switch is connected to the RECTOUT and outputs a switched output RECTOUT_SW. While in theory the shut down circuit uses a simple switch as the load switch, it is complicated to control how to turn on the load switch in all possible operations. For example when a user puts a device onto a wireless charging station, the secondary coil of the power receiver in the device does not always align with the primary coil of the power transmitter in the charging station. Therefore, the actual available power transferred from the secondary coil is unknown because alignment is not determined until the load switch is turned on and the load is applied to the output. The issue is magnified when an integrated circuit for the wireless power pick up unit is designed to be used in different foreseeable applications, which have different load. For example, in a typical application, a RECTOUT capacitor is connected to the RECTOUT after the rectifier and before the load switch. A RECTOUT_SW capacitor is connected to the RECTOUT_SW after the load switch and connected to the load. The capacitor sizes of RECTOUT capacitor and RECTOUT_SW capacitor vary a large range for different applications. The load on the RECTOUT_SW varies as well.
A problem occurs when the load switch is turned on the voltage on RECTOUT drops because of the RECTOUT_SW capacitor and the load on RECTOUT_SW. If the available power from the secondary coil and the RECTOUT capacitor cannot supply the load to charge the RECTOUT_SW capacitor and the current required by RECTOUT_SW, the RECTOUT voltage will drop too low and triggers resets of the power receiver circuit.
FIG. 1 shows prior art circuit diagram of an integrated circuit 1 for a wireless power receiver. Integrated circuit 1 has two input terminals RX1 11, RX2 12, and a ground terminal GND 110. A series capacitor 3 and a parallel capacitor 4 make up the dual resonant circuit with a secondary coil 74. Secondary coil 74 receives power from a power-transmitter coil in a power transmitter unit and passes through the secondary dual resonant circuit, which includes series of parallel capacitances, capacitor 5 and capacitor 6, to be connected to the two input terminals, RX1 11 and RX2 12 of integrated circuit 1. The dual resonant circuit enhances the power transfer efficiency and enables a resonant detection method.
Full bridge rectifier circuit 40, coupled between input terminal RX1 11 and input terminal RX2 12, provides full-wave rectification of the AC waveform received from RX1 11 and RX2 12. The output of rectifier circuit 40 is connected to a rectifier output terminal RECTOUT 15. A RECTOUT capacitor 8 is connected to RECTOUT 15 and a ground.
A bootstrap circuit is used to power rectifier circuit 40. Two external bootstrap capacitors, bootstrap capacitor 5 and bootstrap capacitor 6 are connected to bootstrap terminal HSB1 13 and bootstrap terminal HSB2 14. A low voltage power, e.g. 5-volt, charges the bootstrap capacitors through a bootstrap diode 31 and a bootstrap diode 32, respectively. The bootstrap circuit, therefore, provides power to high side switches of rectifier circuit 40 in normal operation.
Integrated circuit 1 has detection and monitor circuitry that communicates with other circuits of a wireless receiver. Resistor 202 connects to RECTOUT 15 and a RECTOUT monitor terminal RECMO 18. RECMO 18 outputs a proportion of the rectified output to a monitor circuit of the wireless power receiver. Integrated circuit 1 also outputs regulated power. A 5V regulator 21 outputs a regulated 5 volt power to a 5V terminal 26. A 3.3V regulator 22 outputs a regulated 3.3 volt power to a 3.3V terminal 27. Two external capacitors 28 and 29 are connected to terminal 26 and 27, respectively. Integrated circuit 1 monitors these power outputs. A power OK circuit 23 monitors the output of 5V regulator 21 and sends output to a logic gate 201. A power OK circuit 24 monitors the output of 3.3V regulator 22 and sends output to logic gate 201. Logic gate 201 outputs a signal by taking the AND of the inputs. The output of logic gate 201 connects to a reset terminal 25. When the output of logic gate 201 indicates a power output problem, the signal is sent through reset terminal 25. The reset signal causes the reset of the wireless power receiver, which stops the transmission of power from the transmitter. It is, therefore, important to keep the output voltage at the right range to avoid the reset.
FIG. 1 also shows an output control circuit 50. Output control circuit 50 detects different signals and de-asserts an enable signal when one or more predefined conditions are met. Output control circuit 50 has logic gate 51. Logic gate 51 takes different input signals and outputs a control signal. The inputs of logic gate 51 can be various internal conditions. Examples of internal condition detection circuits include, over voltage (OVP) circuit 53, under voltage lockout (UVLO) circuit 54, thermal shutdown (TSD) circuit 55, and current limit and sense (CUR) circuit 56. The input signal can also include external signals, such as an output enable signal from output an enable terminal OUTEN 16. Upon detecting one or more signal that indicates an output shutdown condition, an output shutdown signal is asserted. Integrated circuit 1 enters output shutdown mode. If all the output shutdown conditions are cleared, the output shutdown signal is de-asserted and integrated circuit 1 enters regular mode. Logic gate 51 outputs an enable signal.
An external load switch 7 is coupled between a load switch (LSW) terminal 19 and RECTOUT 15. When load switch 7 is turned on, an end user load 73 is powered by the output of integrated circuit 1 through load 102. A RECOUT_SW capacitor 9 is connected to load switch 7. FIG. 1 shows an NMOS load switch that connects to a gate drive 59 through LSW terminal 19. When output control circuit 50 outputs a de-asserted enable signal, it turns off gate drive 59 and thereby turns off load switch 7. However, how to turn on the load switch is more complicated. For large end user load 73, the RECTOUT voltage can drop dramatically. If the available power from the transmitter coil cannot supply the large load, the RECTOUT voltage drops too low, the internal detection circuit like logic gate 201 sends a reset signal that resets integrated circuit 1.
Methods and structures for improving such wireless power receiver are sought.