inductive charger question...about dc to ac to dc conversion

[androidmj]

I'm attempting to create an inductive charger. What are the most efficient ways to convert a dc power supply, into ac, and then after that ac is picked up by the recieving coil, what is the most efficient way to convert the ac back to dc? Right now i am using zener diodes, and am wasting a lot of power.

 

[sreid]

I would square wave chop the DC to AC using a power H-Bridge. The frequency must be high enought and the primary inductance high enough to minimize magnetizing current. On the secondary to get from AC to DC I'd use a full wave diode bridge.

 

[ScottyUK]

Search Google for 'DC-DC Converter'. Numerous companies make them. Do you need to build your own? Commercial products are almost always cheaper than custom or home-grown designs in the long run, unless you are looking at really high volume where a custom design might pay off.



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If we learn from our mistakes,
I'm getting a great education!

 

[androidmj]

I've looked at the DC-DC converters (the isolated ones of course), but they are all for relatively high current situations and their efficiencys drop off considerably when the output current is under 10 amps. The task requires less than an amp of current at around 3-5 volts. Oh yea, the volume will be very high, so I'd like to just design it myself.

 

[ScottyUK]

There are dozens of little DC/DC converters in the 10-15W range that meet the voltage and current ratings you are after. Look up Vicor, Power One, Artesyn, Newport as possible suppliers. Those names just spring to mind quickly - there are numerous others. Hope this helps.



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If we learn from our mistakes,
I'm getting a great education!

 

[sreid]

Androidmj,

Can you describe more fully what you need to design. Originally you asked about an inductive charger but the discussion now seems to be about a DC-DC power supply.

 

[androidmj]

I am talking about an inductive charger. I want the device to work much like the inductive chargers used in electric toothbrushes or waterproof electric shavers. The reason i expressed interest in the dc-dc converters that were mentioned was because after i began looking at them, their designs looked like a good idea because they include a transformer, which i could make into a two part transformer, which comes together when the product is placed on, or very close to the base. Also, there has been alot of research on these designs and much has been done to make them extremely efficient. So thats why i've asked about them.

 

[androidmj]

Also, I can't use those dc-dc converters because they come in single units, I'm pretty sure I'd have to make my own, which is what I wanted to do anyways because every penny is going to count here, but the circuit designs used in them are very helpful.

 

[ScottyUK]

Realisation dawns... it helps if you tell us all the facts, not just a few of them! Your initial post mentioned nothing about one half of the circuit being physically detachable from the other half. That is quite a significant addition to the scope of discussion.



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If we learn from our mistakes,
I'm getting a great education!

 

[Senselessticker]

Perhaps I'm missing something, but if you have a DC source already, why not just pull DC from there and use a voltage regulator. 5 volt regulators rated for less than an 1 amp are relatively inexpensive.

 

[ScottyUK]

Senselessticker,

I think androidmj wants to get power from one piece of equipment to another separate pice of equipment without any galvanic connection between them. This normally uses two tranformer cores, one in each piece of equipment, which are brought into close proximity when power transfer is required.

When I worked for a previous employer we designed a means of transferring significant levels of power between two sealed units. The development unit used two ferrite cores, each carrying a winding, which when brought together completed a magnetic circuit. The magnetic circuit had a sizeable airgap (actually filled with plastic insulation, but in magnetic terms it was an airgap) which made its magnetising inductance large. This was nearly ten years ago so my recollection is sketchy, but the principle details were a half-bridge inverter driving the primary winding at a few kHz, fast rectifier on the secondary side for low losses, smoothing and a conventional DC/DC converter for the load. The magnetising inductance was tuned for resonance with a capacitor to reduce the drive requirements, and the drive circuit swung its frequency either side of nominal to maintain resonance when the magnetising inductance changed as the secondary core was removed or inserted - this was important in our application because we were transferring several kW across the gap and off-resonance currents were high.






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If we learn from our mistakes,
I'm getting a great education!

 

[androidmj]

Scotty, sorry i haven't posted in a while, but yes that's exactly what i am trying to accomplish. I want to charge a product, much like an electric toothbrush charges, but this may have a larger airgap at times. Right now i'm driving it at 125kHz and am not getting enough power out on the secondary coil side. I want 5-6 volts and about 400ma (so about 2.5W), but my coupling coefficient is terrible and I am only getting around 4volts and 100ma, sometimes less. I'm using a zener diode on the secondary circuit, which i know is wasteful, but i'm trying to keep the circuit as small as possible. Were you using a full wave rectifier on the secondary side?

 

[nbucska]

Can U tell me more ? with more data ?
Can you reduce the power requirement?


<nbucska@pcperipherals DOT com> subj: eng-tips

 

[Senselessticker]

androidmj,

Can you describe what kind of load the desired DC output will see? Are you trying to charge a battery?

 

[ScottyUK]

androidmj,

One of your problems is that the leakage reactance is high with this kind of application. At the frequency you are using the coupling will be very poor as most of your driving voltage is dropped across the leakage reactance. You need to draw out the transformer equivalent circuit and examine how the leakage and magnetising reactances interact with your drive circuit. Consider using a much lower frequency - about 10kHz or so.

Yes, we used a full wave rectifier. We were transmitting about 6kW across the air gap with a secondary voltage of about 350V or so, so the power was considerably higher than in your application.



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If we learn from our mistakes,
I'm getting a great education!