Contactless power transmission apparatus with stable bidirectional power transmission

A contactless power transmission apparatus, comprising:

• a primary device;
• and
• a secondary device configured to transmit and receive electric power to and from the primary device, wherein
• the primary device comprises:
• a first transmitter coil configured to transmit and receive electric power to and from the secondary device;
• a first capacitor connected in series to the first transmitter coil;
• and
• a first converter circuit connected to the first transmitter coil through the capacitor to convert direct current power to alternating current power flowing through the first transmitter coil and convert alternating current power flowing through the first transmitter coil to direct current power,
• the secondary device comprises:
• a resonant circuit including a second transmitter coil configured to transmit and receive electric power to and from the primary device and a resonant capacitor connected in parallel to the second transmitter coil;
• a second converter circuit connected to the resonant circuit to convert alternating current power flowing through the resonant circuit to direct current power and convert direct current power to alternating current power flowing through the resonant circuit;
• a coil connected in series to the second transmitter coil between the resonant circuit and the second converter circuit;
• and
• a second capacitor connected in series to the second transmitter coil between the second transmitter coil and the second converter circuit such that one end of the second capacitor is connected directly to the second transmitter coil and another end of the second capacitor is connected directly to the coil, and
• the second capacitor has a capacitance at least ten times larger than a capacitance of the resonant capacitor, the value of the second capacitor selected so as to reduce biased excitation in a first power transmission from the secondary device to the primary device without reducing resonance in the resonant circuit during a second power transmission from the primary device to the secondary device.