WPT Rx dissipation at low loads

Andrew,

The Rx should not heat up during a time of only a few mA.  Can you give me more details on the configuration?  What IC are you using?  What transmitter are you using?  Charger?  Can you determine what is heating up?  Heat can come from many sources, IC, Coil, capacitors and other items depending on the design.  Heat dissipation from the IC can be minimized by proper layout and the amount of copper available to dissipate the heat.

Regards,

Dick

Hi Dick,

The WPT Tx is bq500211A (BQ500211ARGZ):
* The Tx coil is Wurth 760308111 (A11)

The WPT Rx is bq51013A (BQ51013ARHLT):
* The Rx coil is Vishay IWAS-3827EC-50
* The current is set for 300mA with a limit of 360mA

The battery charger IC is bq24020 (BQ24020DRCR):
* The battery is 500mAh
* The max charge current is set for 244mA (~0.5C)
* The temperature limits are 0'C & 47.5'C

We have used the datasheet application circuits.

The enclosure is a snug fit around the (50x25mm) PCB.
None of the components become unacceptably hot individually.
However the rise in internal ambient means that the max battery charging temperature of 45'C is being exceeded.
We need to minimise the overall dissipation in the coil, caps & WPT Rx IC, etc.
Without a battery we are in a low load situation.
I was not expecting much dissipation under these circumstances.

This is a project blocker.

Many thanks for your help.

Warmest regards,
    Andrew.

Hi Dick,

I should add that we selected the WPT Rx series & parallel resonant caps as per the procedure described in the datasheet.

Many thanks.

Andrew.

Hi Dick,

One more thing - could you please point me in the direction of some typical steady-state Tx & Rx coil waveforms so that we may be sure that our set up is operating correctly & is tuned properly.

Many thanks,

Andrew.

Andrew,

My apologies for the short response.  I hope to get more time on this tomorrow.

Look at the capacitor series resistance.  Are you using the X7R?  Voltage rating?

Can you send me any layout information?  Copper is my main question - for dissipation.  

If possible, measure the RECT pin voltage. That will give a good idea how much is being dissipated in the IC.

Measure the coil frequency as well.

Regards,

Dick

Hi Dick,

We're using X7R, 25V caps in most positions but C0G 50V across pins AC1 and AC2.
This mimicks what's done on the EVM board.

I have also sent a PDF of the layout but I would emphasise that:
* The enclosure is a snug fit around the (50x25mm) PCB.
* None of the components become unacceptably hot INDIVIDUALLY.
* However the rise in INTERNAL AMBIENT means that the max battery charging temperature of 45'C is being exceeded.
* We need to minimise the OVERALL dissipation in the coil, caps & WPT Rx IC, etc.

I have also attached a photo so you can see that the device is:
* both small & compact, &
* completely enclosed by its housing.

Without any load at all on the WPT 5V we measured:
* Coil is 156.3kHz & 16.4Vpp
* Temperature rise over 45 mins is 18'C above ambient (25'C)
* Vrect = 7.2V (7V expected with Iload = 0)
* As far as we can tell the main heat source is the Rx coil

We are using the bq51013A.
Is the bq51013B much different?

Many thanks for your help.

Warmest regards,
    Andrew.

1016.WPT IC layout.pdf

Hi Dick,

I'm sorry to be a nuisance but this is a project blocker.

Any suggestions would be gratefully appreciated.

Many thanks,

Andrew.

Andrew,

I've got a few things for you to try.  One concern is that the battery is absorbing energy and that's where some of the heating is coming from. It looks like your shield is in pretty good shape, but there are a few things we can do to check that out.

1. Measure the input current into the bq500211A without the RX present.
2. Put the RX into the system in its low power state and measure the input current.
3. Physically remove the battery from the system.  Keep it connected to the system, but away from the TX / RX area.  Measure the input current.
4. Electrically remove the battery from the system.  Measure the input current.

These steps may help lead us towards the issue.  Possibly not.

The difference between readings from step 1 and 2 shows us the power being sent by the TX.  If you know IOUT, you can calculate the power being used by the RX.  If the delta in readings is significantly higher then we can assume the excess power is being turned into heat.  Any metals in the magnetic field could be absorbing some of the magnetic energy.

The main difference between the bq51013A and and the bq51013B is that the B is WPC v1.1 compliant meaning it has the foreign object detection (FOD) enabled.  That actually is what the concern may be here.  FOD looks at the difference between the power consumed by the RX and the power sent by the TX.  If the delta is too large, the TX will shut down.  At the risk of throwing out too many part numbers, the bq51003 is similar to the bq51013B but is aimed at lower power circuits (< 2.5W).

Another quick calculation shows the RX coil power is not very high even under high load conditions.

You're using the RHL package and that has very good thermal dissipation so even if the silicon was warm (and I don't think it is) there would not be much heating as a result.

It's tough to tell from the layout how much copper you have on your board.  Anything you can share on that?

I'll keep an eye on this post over the weekend.

Dick

Andrew,

One more thought.  If you have an bq51013A EVM you should measure that on the transmitter as well to see if there are differences between your board in standby and the EVM.

Regards,

Dick

Andrew,

In one of your earlier posts, you state:

* None of the components become unacceptably hot INDIVIDUALLY.

Have you done any thermal imaging?  We have seen some components raise the temperature without realizing what it was.  The thermal camera image helps us identify those components.  You can see from this image that although there is widespread heat, there are some very hot spots.

Regards,

Dick

Hi Dick,

Many thanks for that.

We will measure the WPT Tx current:
* with different WPT Rx loads, &
* with the WPT Rx coil in position & remote from the board / battery.

It's pertinant to mention that we have sometimes had false FOD reporting when putting units with full batteries on the charger.

We retained as much copper as possible with ground & power flood fills on all 4 layers.
Copper is only absent under the antennae (which are groundplaneless).

The WPT Rx coil ferrite is just large enough for the battery but does not shield the extremities of the PCB.
Presumably the board copper planes could also aborb directly?

As I understand it:
* The PCB is locked to the enclosure.
* The WPT Rx coil is stuck to the battery.
* There battery is slighlty smaller than the enclosure so its position may vary a little from unit to unit.

Many thanks for your help.

Warmest regards,
    Andrew.

Hi Dick,

We played with the Tx & Rx EVM boards before we started but they've been returned now.
As I recall, on the Rx EVM board, the Rx coil was a good distance from the PCB?

Unfortunately we don't have thermal imaging yet (it's on the WIBNI list).

Even if we charged the battery at 1C (ie 500mA) that would only be 2.5W
Would the bq51003 help with our current predicament?

Initial results indicate that having the battery & board in the field does significantly increase the Tx drain.
We will attempt to establish whether it's the battery, the board or both.

What are our options for a solution please?
* Presumably maximising the Rx ferrite area would help (although we can't go too close to the antennae).
* Would reducing the size of the Tx coil help at all?

Many thanks for your help.

Warmest regards,
    Andrew.

Hi Dick,

One other question please.

Is the Rx tuning (series & parallel resonant capacitor selection) just for the comms or does it impact the power transfer as well?

Many thanks,
Andrew.

Hi Dick,

These were useful pointers.  We measured the current into the transmitter and noticed that the current significantly increase when putting the battery behind the receiver coil.

We placed some ferrite material behind the coil shield, and this reduced the current taken by the transmitter.

The next task is to measure the temperature of the receiver with this ferrite material in place and see if it makes a difference.

Andrew,

A lot of good question in several of the posts.  I'll try to cover them all here - in no particular order.

1) Tuning the series resonant capacitors is directly tied to the power.  The transmitter is tuned to 100 kHz as well.  The 100 kHz tuning allows the systems to operate on the same side of the peak resonance.  That makes the frequency directly tied to the power level.  Lower frequency is higher power.  If the systems were not tuned properly, there is a possibility that one of the systems (TX or RX) would be on the "wrong" side of the peak.  This could mean that when the RX requested more power, the frequency change would result in less power.  There's a good application note that covers this in much better detail.  http://www.ti.com/lit/an/slyt479/slyt479.pdf  Several other good application notes are at www.ti.com/wirelesspower under the Technical Documents tab.

2) The plot here shows an example of how the frequency changes across loads.  This changes based on many factors so your frequency response will be different.  Does this cover the request for "steady-state TX & RX coil waveforms"?  It's not exactly the request, but it may answer the tuning question.

3) Your charge current is ~500mA and you ask is switching to the bq51003 would with the heating.  I don't think you'd see much of an impact, if any.  The bq51003 is the 500mA (2.5W) version of the bq51013B, so it has the same FOD calibration (which the bq51013A does not).

4) You noted that having the battery in the system increases the TX power which means it's absorbing some of the energy.  Yes, maximizing the RX ferrite helps.  Also, reducing the TX coil can have an impact as well my minimizing the magnet field.  Based on the previous discussion, it seems that WPC compliance is not required.  Changing from a standard coil makes the TX non-compliant.  But, it can still work for your needs.  For smaller footprint size applications, we have better results with a smaller (non-compliant) TX coil.  Again, tuning the system properly is key.

Let me know if I've missed anything or you need more information.

Regards,

Dick

One more quick question.

When we tested the WPT without anything connected to the output power terminal (unloaded), the transmitter took more current than when it was loaded.  Is this a potential source of heat?

Andrew,

Late response.  To make sure I understand I want to make sure my understanding is correct.

You have the Receiver on the Transmitter with a load and take a measurement (I_TX-RX_Load).

You have the Receiver on the Transmitter without a load and take a measurement (I_TX-RX_Open).

You see:  I_TX-RX_Open > I_TX-RX_Load.  Is that correct?

Can you tell me the values and the RX_Load you're using?

Regards,

Dick

Hi Dick,

Firstly many thanks for all your help - we seem to be on the right track now.

Our actual results were as follows:

WPT Tx 5V Drain:

1 Nothing at all in field
42mA

2 Only WPT Rx Coil in field

2.1 No load on WPT Rx 5V
150mA

2.2
WPT Rx 5V powering battery charger IC without battery connected
115mA

2.3
WPT Rx 5V powering battery charger IC with battery connected
165mA

3 WPT Rx 5V powering battery charger IC without battery connected

3.1 Rx coil & battery in position, board remote
275mA

3.2 Rx coil & board in position, battery remote
220mA - 320mA depending on alignment of coils with comms (still charging)
330mA - 780mA with no comms

3.3 Rx coil, battery & board all in position
290mA

So our reference is 115mA, to which
* positioning the battery only adds 160mA
* positioning the board only adds 105mA
* positioning both the battery & board adds 175mA

This suggests that the WPT Rx coil shielding is insufficient.

Initially we used the remainder of the ferrite sheet sourced for Rx tuning.

Deploying the entire sheet to increase both area & thickness gave a reduction from 290mA to 135mA.

Cutting the ferrite to match the Rx coil ferrite dimensions of 38x27mm also gave a reduction from 290mA to 135mA.

So the ferrite area is adequate - it is simply too thin!

We then removed the wire from several Rx coils leaving just the ferrite.

The WPT Tx 5V Drain drops as the number of ferrite layers is increased:
1 layer  290mA
2 layers 180mA (a 110mA improvement)
3 layers 155mA (a 135mA improvement)
4 layers 135mA (a 155mA improvement)

I am curious as to why the Tx drain was 150mA with no load on the WPT Rx 5V but fell to 115mA when we connected the battery charger IC with no battery connected?

Again, many thanks.

Warmest regards, Andrew.

Andrew,

I took some measurements in the lab this morning and did not confirm the results you saw.

With the bq500211A EVM powered but no RX on the system, the input current was about 12mA.  There was a "ping" every 400ms or so that increased the current briefly.

With the bq500211A EVM powered and a bq51003 EVM (no load) placed on the system, the input current went to about 160mA.

Regards,

Dick

Hi Dick,

Many thanks for that - we seem well on the way to getting this sorted now. :-)

To keep you in the loop, our results were as follows:

WPT Tx 5V Drain:

1 Nothing at all in field
42mA

2 Only WPT Rx Coil in field

2.1 No load on WPT Rx 5V
150mA

2.2
WPT Rx 5V powering battery charger IC without battery connected
115mA

2.3
WPT Rx 5V powering battery charger IC with battery connected
165mA

3 WPT Rx 5V powering battery charger IC without battery connected

3.1 Rx coil & battery in position, board remote
275mA

3.2 Rx coil & board in position, battery remote
220mA - 320mA depending on alignment of coils with comms (still charging)
330mA - 780mA with no comms

3.3 Rx coil, battery & board all in position
290mA

So our reference is 115mA, to which
* positioning just the battery adds 160mA
* positioning just the board adds 105mA
* positioning both the battery & board adds 175mA

This suggests that the WPT Rx coil shielding is insufficient.

Initially we used the remainder of the ferrite sheet sourced for Rx tuning (series & parallel cap selection).

Deploying the entire sheet to increase both area & thickness gave a reduction from 290mA to 135mA.

Cutting the ferrite to match the Rx coil ferrite dimensions of 38x27mm also gave a reduction from 290mA to 135mA.

So the ferrite area is adequate - it is simply too thin!

We then removed the wire from several Rx coils leaving just the ferrite.

The WPT Tx 5V Drain drops as the number of ferrite layers is increased:
1 layer  290mA
2 layers 180mA (a 110mA improvement)
3 layers 155mA (a 135mA improvement)
4 layers 135mA (a 155mA improvement)

Do you have any idea why the Tx drain with no load on the Rx 5V is greater than it is with the charger IC connected but the battery disconnected?

Many thanks for your help.

Warmest regards,
Andrew.

Hi Dick,

I'm sorry, I've effectively posted the same question twice.

Please accept my apologies.

We have identified a ferrite from Wurth designed for this application, which should do the trick.

I have attached the data-sheet, in case you're interested.

Again, many thanks.

Andrew.

0181.354006 0.3mm.pdf

Hi Dick,
One more question please.
With a view to reducing the Tx coil size it has been suggested to me that we might use the Rx coil as the Tx coil as well.
If the ferrite saturates when Rxing it will also saturate when Txing so we will still need extra ferrite.
However the Rx coil is 15 turns, 10.7uH, 183mR whereas the Tx coil is 10 (bifilar) turns, 6.3uH, 20mR.
How much effort is required to modify the Tx cct for a different coil?
Many thanks,
Andrew.

Andrew,

Changing the TX coil makes the system non-Qi compliant.  I expect you knew that.

What is the intended output power of this device?  I'm not sure we've covered that before.  

We have been experimenting with a smaller (non-Qi) coil with very good results.  This is geared towards <5W solutions.  It is a 30mm diameter coil.  I don't know if it is released yet by the vendor, but I'm checking.

The same bq500212A schematic works for both the standard coil and this 30mm coil.

Regards,

Dick

Andrew,

The small TX coil is produced by Wurth and is released as part number 760308101103.  Let me know if you need contact information.

Regards,

Dick

Hi Dick,

The extra ferrite helps - but not enough, I'm afraid.

So we're going to try the new Wurth Tx coil, as well.

I'll let you know how we get on.

Again, many thanks for all your help.

Warmest regards,

Andrew.

Hi Dick,

The smaller Wurth Tx coil has arrived.

We've dropped it onto our (5V, bq500211A, A11) charger but no joy I'm afraid.

I thought it was a drop in replacement - any ideas?

Many thanks,

Andrew.

Hi Dick,

I've been informed that it's now working with the smaller Wurth Tx coil.

I'm not sure what the problem was - probably just 'finger trouble'.

Again, many thanks for all your help.

Warmest regards,

Andrew.

Andrew,

Congratulations!  Well done.  Wireless power solutions are never as easy as first expected.

Thanks for marking as verified.  

If you have more questions, don't hesitate to start a new E2E post.

Regards,

Dick