RF Energy Harvesting for mobile phone charging

Manee Shankar

Other Parts Discussed in Thread: BQ25570, BQ25505

Hi TI,

We have a RF to DC conversion circuit which provides a voltage output ranging from (2V to 10V). The RF signal is available at all times and hence providing a constant DC output.

We are looking to charge a mobile phone with this DC output. Although the voltage from the RF to DC circuit seems sufficient, the current is very low (micro amps).

Appreciate if you could suggest any IC or solutions that TI has which can help with the mobile charging. I came across few ICs but not sure which one suits the best. Below are the specifications that may help:

Frequency of RF signal: 70Mhz

Power level of RF signal: -5dBm to 5dBm

DC voltage output: 2V to 10V

Current output: 1uA

Appreciate your feedback in this.

Thank you very much.

Regards,

Manee

over 6 years ago

0 Jeff F over 6 years ago

TI__Guru**** 168436 points

Manee,

Ideally you need a buck-boost based charger with very low quiescent current (nA range) that can operate down to 2V. The closest fit is the boost only based BQ25505 and BQ25570 families which can only boost to 5.5V from a voltage lower than 5.0V.

0 Manee Shankar over 6 years ago in reply to Jeff F

Prodigy 160 points

Hi Jeff,

Thank you for your reply. Im looking at the BQ25570. It has a buck converter output of 1.8V and a battery charging output as well. Which of this can i use to charge a phone? How much current is expected from this outputs? How do I increase the 1.8V output to achieve a higher voltage?

0 Jeff F over 6 years ago in reply to Manee Shankar

TI__Guru**** 168436 points

Manee,

The battery is intended to be charged from the BAT output, as that output has undervoltage protection. However, if your battery has an internal protector, which includes undervoltage protection, it doesn't matter which output that the battery is charged from. Some people set the boost charger output at VSTOR to 4.5V or slightly higher and then use the buck converter output, set to 4.2V, to charge their battery. The buck converter provides a soft start function which may be helpful when the input source cannot provides high power or when the battery is discharged. You can reprogram the buck converter output voltage using the VOUT_SET resistors. The maximum output from the buck is 93mA but only while the VSTOR voltage is at least 300mV above the output. You can use the excel spreadsheet below to size the resistors.

www.ti.com/.../sluc484

Please read datasheet section 10.1 before change the EVM resistors.

0 Manee Shankar over 6 years ago in reply to Jeff F

Prodigy 160 points

Hi Jeff,

Thank you for your reply. Noted will look into this further and will try to charge the phone on the VBAT and as well as the VOUT and see which works best. However, the current 93mA seems quite low. Is there any way I can derive a higher current out from this IC to get a better charging rate? Or do you have any suggestion of other ICs that may help with the mobile charging based on our situation?

I shall get the BQ25570 EVM and try it out as it seems to be the closest fit to our case. Appreciate your feedback.

Thank you.

Regards,

Manee

0 Manee Shankar over 6 years ago in reply to Jeff F

Prodigy 160 points

Hi Jeff,

On the BQ25570 EVM, to reprogram the buck converter output voltage, I should change VOUT_SET resistors right? Meaning I should de-solder the existing resistors on the EVM and replace it?

0 Jeff F over 6 years ago in reply to Manee Shankar

TI__Guru**** 168436 points

Yes

0 Manee Shankar over 6 years ago in reply to Jeff F

Prodigy 160 points

Jeff,

The VSTOR and VBAT voltages are not rising and the input measured at VIN remains at 0.33V although my input is 3.4V. Any idea?

It was working fine before this

0 Jeff F over 6 years ago in reply to Manee Shankar

TI__Guru**** 168436 points

Manshee,

If the output at the panel is 3.4V but the boost charger input sees 0.33V, then you are collapsing the panel. How much current are you pulling from the solar panel? What is its VOC and ISC? Are you clamping the up to 10V panel to no more than 5V output with a zener clamp?

0 Manee Shankar over 6 years ago in reply to Jeff F

Prodigy 160 points

Jeff,

Im using a RF to DC board as my input. N the output from this board is 3.4V. I also tried to give 1V as input using a DC power supply. VIN measured to be at 1V. However VSTOR and VBAT are still 0V.
Any ideas or suggestions?

Appreciate your feedback.

0 Manee Shankar over 6 years ago in reply to Manee Shankar

Prodigy 160 points

Jeff,

Last thing i did before this happened is i tried charging a 3.7V LiOn batter on VBAT. Voltage measured at Vbat was 3.7V. VIN was 0.33V. Once battery removed, Vbat & Vstor remains at 0V.

Appreciate your feedback. Thanks

0 Manee Shankar over 6 years ago in reply to Manee Shankar

Prodigy 160 points

Jeff,

Any suggestions? Im stuck here

0 Jeff F over 6 years ago in reply to Manee Shankar

TI__Guru**** 168436 points

Manee,

If you apply a power supply voltage at VIN_DC < VBAT_OV setting <5V and VSTOR does not eventually rise to VBAT_OV, then the IC might be damaged. If you use a ohmmeter to measure resistor from VIN_DC to ground, VSTOR to ground and VBAT to ground, what resistance do you get? What is the external resistive load on VSTOR?

0 Manee Shankar over 6 years ago in reply to Jeff F

Prodigy 160 points

Jeff,

What is the default setting of VBAT OV?

Will get u the resistance values shortly

0 Manee Shankar over 6 years ago in reply to Jeff F

Prodigy 160 points

Hi Jeff,

With an input voltage of 4.2V from a power supply, VSTOR still does not rise up. It stays at 0V at both VSTOR and VBAT.

Resistance on VIN DC: 22ohm

Resistance on VSTOR: 0.4ohm

Resistance on VBAT: 632ohm

Confirm that the IC is damaged? Any idea which component might be faulty? All i did was connect a partially charged LiOn battery to VBAT and this happened. Any idea what went wrong?

0 Manee Shankar over 6 years ago in reply to Manee Shankar

Prodigy 160 points

Hi Jeff,

With an input voltage of 4.2V from a power supply, VSTOR still does not rise up. It stays at 0V at both VSTOR and VBAT.

Resistance on VIN DC: 22ohm

Resistance on VSTOR: 0.4ohm

Resistance on VBAT: 632ohm

Confirm that the IC is damaged? Any idea which component might be faulty? All i did was connect a partially charged LiOn battery to VBAT and this happened. Any idea what went wrong?

Appreciate your feedback.

Regards,
Manee

0 Jeff F over 6 years ago in reply to Manee Shankar

TI__Guru**** 168436 points

Manee,

Were you able to confirm that the charger was damaged?

0 Jeff F over 6 years ago in reply to Jeff F

TI__Guru**** 168436 points

Manee,

I haven't heard from you in a while. Are you still having issues?

0 Manee Shankar over 6 years ago in reply to Jeff F

Prodigy 160 points

Hi Jeff,

Happy new year.

Yes the charger is confirmed to be faulty and we just purchased a new unit of the BQ25570 EVM and started testing.

We found that the current on VSTOR or VOUT is low to charge our load. Any ways to increase the current output at the load to allow a faster charging rate?

Waiting for your feedback.

Thank you.

Regards,

Manee

0 Jeff F over 6 years ago in reply to Manee Shankar

TI__Guru**** 168436 points

Manee,

Unfortunately, there is no way to increase the output current capability of the boost or buck converter. You could parallel the VSTOR output of 2 BQ25570's. Is that a possibility?

0 Manee Shankar over 6 years ago in reply to Jeff F

Prodigy 160 points

Hi Jeff,

Noted. As of now we only have one unit of the BQ25570. How much current can we expect if we parallel another BQ25570?

Are there are other ICs or chips with TI that can boost the current output which can be integrated with my design?

0 Jeff F over 6 years ago in reply to Manee Shankar

TI__Guru**** 168436 points

Manee,

Running with a low impedance source (not a solar panel), with 200mA peak output current limit in DCM mode, the BQ25570's maximum input current is about 100mA. So using a power balance of efficiency = pout/pin = (VBAT_OV*IOUT)/(VIN_DC*100mA), you can estimate maximum IOUT for each BQ25570. You can use an efficiency number closest to your application from the datasheet efficiency curves. It is difficult to estimate maximum current when powering the BQ25570 from low impedance source, like a solar panel, due to the on/off switching operation caused by MPP tracking.

I am assuming that you need the BQ25570 because the input source impedance is high and is below the desired output voltage. If so then you need an boosting IC with MPP capability. The BQ255xx family is the only boost converter charger with integrated MPP. Its peak input current is fixed at 200mA per switching pulse.

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RF Energy Harvesting for mobile phone charging