Inductive charger

Hi there Roger,

It seems that you are trying to re-invent the wheel :-) SMPS chargers (3-5W) are readily available on the market for just a few dollars. However if the task is related to 'having fun' this is something else. However if you don't have expierence in switch mode power supplies this task is not easy. Remember also that you are dealing with a device which has a safety function. (a user must be able to touch the output voltage safely). You as designer are responisible for this safety function. Given all this, your circuit should be a flyback converter. Getting a suitable (and safe) transformer is difficult, other components are easy to get.

A switching frequency of 60kHz is not uncommon. Your power switch is a MOSFET and the controller is a small 8pin device from a supplier such as ST, ON, etc. If you look at the datasheet of such a controller (UC3842B is old but a generic part) you will find a typical application. You can downscale it for your needs.

If you have specific questions i might be able to help you.

Best regards and lot's of luck

Enrico Migchels

Power Conversion Design Engineer

Heliox. B.V.

Best, The Netherlands

www.heliox.nl

These can be quite simple and use 60 Hz - Just think of it as a transformer with really bad coupling or high leakage inductance. It may not be the most efficient, but for the kind of application it does not really matter much. My toothbrush uses just this simple of an approach. A 60 Hz, poorly coupled transformer to charge the batteries in 24 hours.

Also see,

http://en.wikipedia.org/wiki/Inductive_charging

HTH - Steve H.

A coil in the charging base (always plugged in and on) couples to a mating coil in the hand unit to form a step down transformer. The transistor, Q1, is used as an oscillator at about 60 kHz which results in much more efficient energy transfer via the air core coupling than if the system were run at 60 Hz. The amplitude of the oscillations varies with the full wave rectifier 120 Hz unfiltered DC power but the frequency is relatively constant.

     E1           CR2          R1                                E3
  AC o----+----+--|>|-----+---/\/\---+----+----------------+-------+  Coupling
          |   ~|  CR1     |+   1K    |    |                |        ) Coil
        +-+-+  +--|<|--+  |          |    / R2             |        ) 200T
    RU1 |MOV|     CR3  |  |      C1 _|_   \ 390K           |        ) #30
        +-+-+  +--|>|--|--+   .01uF ---   /          CR5   |     E4 ) 1-1/2"
       E2 |    |  CR4  |       250V  |    \ MPSA +---|<|---|----+--+   
  AC o----+----+--|<|--+             |    |   44 |         |    |
              ~        |-     R3     |    | Q1 |/ C    C3 _|_  _|_ C2
                       +-----/\/\----+----+----|     .1uF ---  --- .0033uF
      CR1-CR4: 1N4005 1N4005  |     15K               |\ E  250V  |    |  250V
                       |                R4       |         |    |
                       +---------------/\/\------+---------+----+
                                        1K

For the toothbrush, a 4 position switch selects between Off, Low, Medium, and High (S1B) and another set of contacts (S1A) also is activated by the same slide mechanism. The motor is a medium size permanent magnet type with carbon brushes.

                                       S1B
                              S1A  +--o->o
                 D1           _|_  |       R1,15,2W
             +---|>|---+------o o--+   L o---/\/\---+
    Coupling |         |                   R2,10,2W |
       Coil  +        _|_ BT1          M o---/\/\---+
       120T (          _  2.4V                      |
        #30 (         ___ .5A-h        H o----------+
     13/16"  +         _                            |
             |         |        +-------+           |
             +---------+--------| Motor |-----------+
                                +-------+

Hi Roger,

Be carefull building circuits with free running oscillators, these devices are known to overheat in low load condictions. This is also the case when the inductance of the primary coil drops (receiver removed). The switching frequency rises to such high values that the switching losses can overheat the switching devices. I think a better approach is using a dedicated quasi resonant flyback controller (such as On semiconducter NCP1207). This controller limits the maximum switching frequency and has additional benefits. Additional to your primary winding should be a second winding which acts as a demagnetization detection and supply voltage. This winding is also in the transmitter part. In your case (as a feedback circuit to monitor the output voltage is missing), this winding tells you what voltage is on the secondary winding (ratio aux /sec winding). An even bigger advantage is that these devices have a high voltage current source (to startup from the mains supply voltage) and switch off  when the voltage on the VCC pin reaches a certain value. In your case the voltage on the VCC pin can only reach a steady value if the receiver is attached and therefore you have a detection circuit (receiver attached or not). There are still a few difficulties. Due to the bad coupling betrween primary and secondary winding the voltage on the output is not exactly similar to the ratio between aux/sec. Also the drop in inductance (receiver gone) is tricky, however the maximum current trough the winding is limited by the controller (there is a primary peak current detection on the controller). Look at the datasheet, there are also application examples.

Best regards,

Enrico Migchels

Power Conversion Design Engineer

Heliox B.V.

Best - The Netherlands

www.heliox.nl

Hi there Roger,

Luckely for you your situation isn't that bad. The output power for a flyback converter is given by the following formula:

Pout=0.5 x Ip^2 x Lp x fs

Ip = primary peakcurrent set by sense-resistor in source of FET, Ip= 1V/Rsense, 1V is the level given by the controller datasheet).

Lp is the primary inductace of your flyback transformer,

fs = switching frequency.

For the input power (pushed into the core) you should calculate with a factor Pout/0.85 (85% efficiency assumed).

For the following example: Ip = 3A and Fs, min = 40kHz, Lp = 500uH, your output power is 90W.

Note that increasing the peak current increases the output power fast, but take care that the ferrite core can not be exited more than 300mT, otherwise you will saturate the ferrite.

Best regards,

Enrico Migchels

Power Supply Design Engineer

Best - The Netherlands

www.heliox.nl