Inductive Charging Circuit Design Help

[ryanrocket]

@Lucien Nunes

Ok so I just conducted some testing to find the output of the receiver module to be 5 volts when the 2 coils are practically touching. Of course, the output of the receiver coil directly affects the voltage it will be able to provide. For the sake of this, lets just assume it's at an optimal distance and is outputting 5V.

As for the drone battery; the battery is labeled to charge at that specification, however after a buddy and I used a bench PSU to test this, we found that the drone can actually charge at anywhere within the range of 3 volts @ ~3 amps to 12 volts @ ~1.5 amps, where the required amperage is inversely related (as voltage decreased, amperage needs to increase).

Though I am not exactly sure what the PCB on the receiver module is doing (due to both my lack of knowledge and the lack of documentation), that's where I am getting my 5V DC from. I need to figure out what to put in between that receiver circuit and the drone's battery circuit to prevent it from the aforementioned effects.

 

[Lucien Nunes]

the receiver coil is able to pick up 5V at 2 amperes when in an optimal range.

the drone can actually charge at anywhere within the range of 3 volts @ ~3 amps to 12 volts @ ~1.5 amps, where the required amperage is inversely related (as voltage decreased, amperage needs to increase).

What is probably happening now is that the two are interacting like a relaxation oscillator. Receiver outputs 5V; drone tries to take >2A; voltage decreases; drone takes more current (this is positive feedback at work); voltage collapses to a point where the drone stops charging; voltage recovers; cycle repeats. Watching the voltage on a scope would show the behaviour clearly. The underlying cause is that the inductive coupler modules and coils are not powerful enough for the load of the drone charger.

Your two inital bullet points were actually pretty well on the money, but I would add a third:

• Reduce the drone's current demand at 5V so that it's within the receiver's ability to supply.
• step down the 5V with minimal loss so that there is enough increase in current to satisfy the drone's demand at a lower voltage.
• Get an inductive coupler system that is powerful enough for the job.

Let's suppose the last one is not possible for now. For my own purposes I would begin by looking at the first option, by reverse-engineering the drone's charging circuit. Depending on the design, it might be possible to increase the battery current sensing resistor or alter the associated feedback network to make it charge at a lower rate. This would mean the charger IC gets a false impression of what is going on, which can impact on the termination of the charge. If the current sense resistor is shared by the discharge monitoring circuit, then it must not be changed or it will also throttle the drone power. I can't honestly recommend going this route if you don't have a good knowledge of battery charging circuits and methodology.

That leaves us option 2: step down the receiver output to a voltage at which the drone input power is lower. The receiver is offering us a maximum of 5 x 2 = 10W. The drone wants anywhere between 3 x 3 = 9W and 12 x 1.5 = 18W. So there is a possibility of a precarious stable zone at 3V if we can do the step-down at an efficiency of 9 / 10 = 90%, which is just about possible with an off-the-shelf buck converter.

I have to go now but you might like to have a look for the most efficient stepdown converter or switching regulator with 3.3V output that will work on 5V input. Obviously a linear regulator won't work as that reduces the voltage without increasing the current. It's a moot point whether even a switching regulator is going to work. It's so very close to there being not enough power at all, and then you'll have three active circuits interacting and possibly fighting each other. I really can't predict what will happen although technically it could work.

You might just like to try a trick first, of putting say 470μF in parallel with the receiver output. If there is another metastable state where the load lines of the converter output and the drone input cross, it might settle there if the time constant is drastically changed. I accept no responsibility for the possible outcome of the receiver blowing up (if it goes into more violent oscillation.)

 

[Lucien Nunes]

I'll add that you might also get some joy from adding a small amount of series resistance. Because the drone input current increases with decreasing voltage, it exhibits negative dynamic resistance. It's not a true negative resistance and is probably far from linear, and we know it's not a constant power input, so there might be spots where the negative resistance is not very high. If you add enough real resistance in series to make the net resistance positive, you might find a stable operating point. A few hundred milliohms might be worth a try.

It's a total kludge and probably won't work, but apart from modifying the drone charger or using a larger inductive coupling setup, any answer is likely to be a kludge.

 

[ryanrocket]

I'll add that you might also get some joy from adding a small amount of series resistance. Because the drone input current increases with decreasing voltage, it exhibits negative dynamic resistance. It's not a true negative resistance and is probably far from linear, and we know it's not a constant power input, so there might be spots where the negative resistance is not very high. If you add enough real resistance in series to make the net resistance positive, you might find a stable operating point. A few hundred milliohms might be worth a try.

It's a total kludge and probably won't work, but apart from modifying the drone charger or using a larger inductive coupling setup, any answer is likely to be a kludge.

Alright. After some in-depth testing, we got the drone to charge using the buck converter, capacitor, and a new micro USB cable. It used 3.7v at like 2 amps. Thanks for the guidance and help! Now to print the PCB's.