Ah Qi, here we go again

After seeing Alexis, we noticed that the package of the previous component that we thought would do the job was based on BGA package… so we’re back to the previous step !

Basically the ideal component should :

  • be Qi compliant (low power profile and extended power profile)
  • have a 15 W power output
  • not have a BGA or CSP package which is definitely the hardest part

For a moment I thought I found the perfect match in the TS81000 : Qi compliant, up to 40W power output and QFN package ! Well yes, but actually no.

Alexis brilliantly pointed the fact that this component was only the auxiliary part of the receiver, and the other part which was the TS5111 has a WCSP package. This makes this solution unsuited to our project.

I also came across the BQ51013 which is interesting but can only deliver up to 5W. We think this might be too low for our project but as we still haven’t a precise idea of the power consumption of our device we might come back to it later.

For now we’re going to put aside the wireless power transmission and will stick to a basic USB port, that we will try to hide as much as possible.

To Qi, or not to Qi, that is the question (that was actually)

About Wireless Power Transmission

Because we wanted to create a design centered around aesthetic for our device, we chose to implement wireless charging for our battery.

There are plenty of ways to implement WPT. First of all there is near-field power transmission and far-field power transmission.

For our project, we will focus on a near-field power transmission. In this field, one of the most common technique is inductive coupling. But it is not the only one : capacitive coupling is an interesting technique for our project.

Capacitive coupling VS inductive coupling

Because inductive coupling is based on converting AC current to a magnetic field, there is a high risk that it will cause strong interferences and perturbations with the magnetic fields of the marbles, thus complicating our control over our device.

Regarding this point, capacitive is interesting because relying on a different medium to transfer energy i.e. not a magnetic field. Alas, the major problem with capacitive coupling is that there’s poor documentation on the subject and therefore is hard to implement.

In the end we choose to go for inductive coupling which is very well documented with the advantage of having many organizations working on the subject.
To counter undesirable effects of inductive coupling, we plan to implement electromagnetic shielding.

Qi standard and the Wireless Power Consortium

The Wireless Power Consotium is one the biggest and most important entity working on WPT. They established the Qi standard which is currently in version 1.2.3, with the specification downloadable here.

At first, when looking how we were going to implement Qi compliant WPT we looked for easy implementable solutions like the Würth Elektronik development kit after seeing a similar solution suggested by our comrades working on LitSpin.
Unfortunately implementation of the said kit would be too much space consuming and we would end up with a much thicker device than expected thus possibly ruining the aesthetic aspect.

Because of that and the fact that implementing by ourselves the Qi compliant receiving part of the WPT chain might involve an unknown quantity of unexpected work, we looked for other solutions which weren’t necessarily Qi compliant.

Long story short, the chinese components we could have used as a replacement would mean forgetting interoperability between transmitter and receiver. Therefore after discussing about this with Alexis, we decided that it’d be more safe to stay on the Qi standard as the specifications on the chinese components were vague. Moreover, using the Qi standard we’ll easily be able to order a Qi compliant power transmitter, already packed and beautiful like this one (this one has bad reviews but it’s just an example of the aesthetic we’re looking for).

How to Qi

We will be working on the Mobile Device part of the Wireless Power Charging | Source : http://www.idt.com P9415 datasheet

The research for Qi-compliant Wireless Power Receiver IC ended up being longer than expected because the components category name isn’t consistent throughout websites. In the end, and after going through pages and pages of datasheet we found this device that we need to further discuss with Alexis before order.

Particularly we need to know what’s behind the two arrows between Receiver and Load on the above image.

Going back and forth the H-Bridge

We need to be able to drive our little magnet marbles and flip them according to a magnetic field direction or the opposite one. To do so current must be able to flow both ways through our self-inductances. Moreover we’re going to need current’s intensity of a much higher value than what is able to flow out of or flow in the GPIO.

One way to do so, is to use a H-Bridge as we can see on this site that we’ve already mentionned in an earlier post.

What’s an H-Bridge

Basically an H-Bridge is a electronic circuit built with drivable switches in a way that allows us to flip the voltage on a load and thus allows us to flip current polarity.

H-Bridge circuitry in red | Source : en.wikipedia.org

There are two important configuration for us :

  • the configuration where S1 S4 are closed and S2 S3 are open and
  • the opposite configuration

By switching from one mode to we are able to revert current polarity by changing which voltage source terminal is connected to which load terminal.

FET transistors are usually used as switches to drive the load. Below we can see a typical implementation with NMOS and PMOS.

H-Bridge implementation with MOSFETs | Source : os.mbed.com

How we plan to implement the H-Bridge

There’s two ways to implement H-Bridge circuit. Either we do it ourselves hand picking the transistors and doing the circuitry ourselves or we pick a H-Bridge driver integrated circuit. We choose the later solution as implementing the H-Bridge by hand can be tricky and unnecessarily complicated for our project.

Moreover, H-Bridge drivers are more advantageous as they have protections against over-voltage, under-voltage, overcurrent etc.

How we choose the driver

There’s two ways to implement H-Bridge circuit. Either we do it yourself hand picking the transistors and doing the circuitry yourself or we pick a H-Bridge driver integrated circuit. We choose the later solution as implementing the H-Bridge by hand can be tricky and unnecessarily complicated for our project.

Moreover, H-Bridge drivers are more advantageous as they have protections against over-voltage, under-voltage, overcurrent etc.

How to choose the driver

There are many criterias that we considered to choose the driver.

First of all, one criteria that seems obvious but we didn’t know about before talking with our professor in charge, is that the driver must be designed to be used with inductive loads. This is not always the case as we can find H-Bridge drivers for capacitive loads.

Initially we’d like to have a 32×32 marble matrix and if not possible a 20×20 matrix at least. It is consequently obvious that price is a critical criteria. As each of our marbles needs to be individually drivable, we often need for one component to order as many as we have marbles.

One way to reduce the number of H-Bridge drivers we’ll need, is to look for drivers designed for multiple loads. This is an important criteria as it possibly allows us to reduce the final cost for this component but also because it may also allows us to gain space on our PCB.

Likewise, driver package size is equally important as we’ll need for each marble a set of components (self-inductance, Hall effect sensors, H-Bridge drivers). It’d be too bad to have to space out the marbles because we don’t have enough space under them to fit all components.

Last but not least are the voltage of the power source and the maximum current that can drives through. We estimated that we won’t use currents higher than 1A to generates the magnetic fields so we looked for drivers in this range. As for the power source, we thought that 7V should be more than enough.

Giving all these criterias we looked for a good compromise and we ordered these two drivers for testing :