Patent Application
20150311723
Electric vehicles include batteries which must be charged regularly, typically every day. For many consumers, remembering to plug the vehicle into a battery charging system at the end of the day is a major inconvenience. For others, there is apprehension in handling a 240V AC (alternating current) power supply, particularly in wet conditions. Inductive charging overcomes many of the issues of prior plug-in charging systems because there is no need to physically handle the plug every day to charge the vehicle batteries. Inductive charging provides hands-free automatic charging when the vehicle is parked adjacent to a charging pad.
The subject inductive power transfer system generates direct current (DC) voltage used to power a device such as a battery charger on an electric vehicle to charge the vehicle batteries. The system includes a transformer including a stationary primary coil and a secondary coil mounted on the vehicle. When the vehicle is parked adjacent to the primary coil, inductive charging occurs. A primary circuit is connected between an AC power supply and the stationary primary coil. The primary circuit includes a rectifier which converts AC voltage to DC voltage and a bridge inverter that creates a pulse width modulated square wave voltage to drive the primary coil. The rectifier and inverter are connected in parallel with the primary coil. In an alternate embodiment, a power factor correction (PFC) circuit can be provided in the primary circuit at the output of the rectifier to provide the DC voltage.
According to a preferred embodiment, a reactor is connected in series between the output of the bridge inverter and the primary coil. In addition, the bridge inverter is an H bridge formed of transistors. A link capacitor is also connected in parallel between the rectifier and the H bridge to filter the rectified DC voltage.
The secondary circuit includes a secondary coil inductively coupled with the primary coil to receive the square wave voltage from the primary circuit. A rectifier is connected in series with the secondary coil to convert the AC voltage to a DC voltage which is used by the battery charger to charge the vehicle batteries. The secondary circuit also includes a link capacitor connected in series with the secondary circuit rectifier.
Other objects and advantages of the invention will become apparent from a study of the following specification when viewed in the light of the accompanying drawing, in which:
The control panel 2 is connected with an AC voltage source 14. The control panel includes a primary circuit which is connected with the stationary primary coil. More particularly, the primary circuit includes a rectifier 16 connected with the AC voltage source and an inverter 18 connected in parallel with the rectifier. The rectifier is formed from a capacitor bank or a plurality of diodes 20 connected in a known manner. The inverter includes a bridge of transistors 22 such as metal oxide semiconductor field effect transistors (MOSFETs) or insulated gate bipolar transistors (IGBTs). The transistors are preferably connected to form an H bridge inverter as shown. A large link capacitor 24 is connected in parallel with and between the rectifier and the inverter.
A reactor 28 in the form of an inductor is connected in series with the output of the inverter. The reactor limits the current output of the inverter so that the capacitor 24 is not a short circuit on the output of the inverter. The reactance of the reactor comprises an imaginary part of the coupling impedance, i.e. the impedance at the output of the inverter. This can be referred to as the reactive or imaginary part of the equivalent series impedance. By selecting the inductance of the reactor, the insertion reactance of the system can be controlled.
In one embodiment, the inductance of the reactor is chosen to be equal to the inductance of the stationary primary coil 10. The insertion reactance is then minimized at the resonant frequency of the system, i.e. the primary 10 and secondary 12 coils of the system transformer. This is true independent of the coupling coefficient between the primary and secondary coils, defined as
k=LM/√(Lp*Ls)
where LM is the mutual inductance;
Lp is primary inductance; and
Ls is secondary inductance.
The benefit of minimizing the reactive impedance is that the output voltage of the secondary is independent of the load applied. This creates a stiff source of voltage to the vehicle charger. Stiff voltage is defined as a voltage which is only dependent on the input voltage and the coupling ratio, and independent of the load value.
Accordingly,
Vout=Vin*k
where Vout is the output voltage to the vehicle charger; and
Vin is the voltage output from the inverter.
This equation is valid where the primary and secondary coils have substantially the same inductance. If the coils are not substantially the same inductance, then
Vout=Vin*k*C
where C is a constant which is dependent on the self-inductance values of the primary and secondary coils.
C also depends on the construction details of the coils. C is independent of load.
Under these conditions, the vehicle coil 12 can be significantly misaligned relative to the stationary primary coil 10 (wide variation of the value of k), while the output voltage to the vehicle charger remains stable with respect to changes of the output load and the system is driven at a fixed frequency.
In another embodiment, the inductance of the reactor is chosen to be different from, i.e. above or below, the inductance of the primary coil. The insertion reactance is then minimized at a frequency which is dependent on the value of k. The stiff voltage output will be at a frequency which may be the resonant frequency of the system or another drive frequency.
The reactor balances the differential mode currents in the charging system to reduce radiated emissions and losses in the system. In a preferred embodiment, the reactor comprises a dual winding over a gapped iron core to balance common and differential mode currents on both sides of the charging system and to control the electromagnetic field for controlling radiated emissions. In alternate embodiments, air, ferrite, amorphous material, or nano-crystalline cores may be used for the reactor, with single or dual windings.
A secondary circuit is arranged within the vehicle adapter 6 and includes a capacitor 30 and rectifier 32 connected in series with the secondary winding 12 and a link capacitor 34 connected in parallel with the rectifier. Like the rectifier in the primary circuit, the secondary circuit rectifier may be formed from a capacitor bank or a plurality of diodes 36. The secondary circuit rectifier converts the high frequency AC output from the secondary coil 12 to a DC output which is delivered to the vehicle charger. The high frequency AC output from the coil 12 is shown in
In an alternate embodiment shown in
In operation, AC power is provided to the control panel and is rectified by the rectifier 16 of the primary circuit. The link capacitor 24 filters the rectified AC into DC. The DC output from the filtering capacitor is delivered to an inverter that creates a pulse width modulated high frequency square wave voltage to drive the parking pad. The high frequency AC is magnetically coupled from the parking pad coil to the vehicle adapter coil where it is rectified back into DC by the secondary circuit rectifier 32 and fed to the battery charger on the vehicle. The reactor 28 at the output of the inverter provides load regulation of the system secondary output voltage. A dual wound reactor balances differential mode currents on both sides of the system. An iron core reactor controls the stray magnetic field to improve radiated emissions.
While the preferred forms and embodiments of the invention have been illustrated and described, it will be apparent to those of ordinary skill in the art that various changes and modifications may be made without deviating from the inventive concepts set forth above.
This application claims the benefit of U.S. provisional patent application No. 61/972,728 filed Mar. 31, 2014.
| Number | Date | Country | |
|---|---|---|---|
| 61972728 | Mar 2014 | US |
