BQ25750: MPPT charging architecture problem

Hello Allen,

Allen Yang said:

Does this mean that when using MPPT, there should be no system load on VSYS?

Yes, we recommend that there should be no system load when using MPPT.

Allen Yang said:

If the system must be connected at the VSYS, are there any guidelines to improve MPPT accuracy and avoid unstable behavior during the sweep?

I recommend making sure that the BQ25750 and the system load together won't brown out the power source. You can help make sure of this by setting the ACUV or VAC_DPM voltage of the BQ25750 higher.

Best Regards,
Ethan Galloway

Ethan Galloway What will be happened to bq25750 charger circuit if we enable the MPPT w/ VSYS loading ??

Ethan Galloway We did run a field testing w/ power supply by a 100W solar panel in the bright sun condition in California and VSYS loading w/ a device (< 10W) connected, and the battery ran into a protection mode w/o power (no charge/discharge) when MPPT enabled when 3min. full sweep timer was up.

1. As you mentioned, it should be work fine w/o power shutdown by connecting to a low impedance power source

2. Do we need to modify the bq25750 charer circuit for MPPT function w/ constant VSYS loading ?

Hello Matt,

Thanks for working on this.

Do you know why the battery went into protection mode here? Did the register logs of the battery throw any flags?

MATT CHANG said:

Do we need to modify the bq25750 charer circuit for MPPT function w/ constant VSYS loading ?

You shouldn't need to modify the circuit to support a constant VSYS loading. What type of load was applied to VSYS?

Best Regards,
Ethan Galloway

Ethan Galloway My understanding is when MPPT enable and start a full panel sweep for the solar panel input, the device (bq25750) will enter HiZ mode temporaryly to measure the VOC (open circuit voltage) of soalr panel. We might lose the power source on VSYS at that moment, and the VSYS drained below ACUV to cause the full system power off shut down.

Questions:

1. What the bq25750 did when it enter HiZ mode before a full panel sweep ??

2. How to prevent a power drop to trigger the ACUV protection for the situation ??

3. Any recommended bq25750 application circuit for the stable MPPT process w/ Solar input + VSYS loading ??

Hello Matt,

Thanks for being patient with this. We are swamped at the moment. Let me look into this and I'll get back to later this week.

Best Regards,
Ethan Galloway

Hello Matt,

Once again, thanks for being patient with this.

Yes, the BQ25750 turns-off the ACFETs when the BQ25750 starts the full panel sweep. This can cause VSYS to brown-out. Can you connect the system load in parallel with the battery to improve this behavior?

Once again, I'll look into this and get back to you.

Best Regards,
Ethan Galloway

Ethan Galloway Can you help to provide a reference circuit design how to connect the system load in parallel for the MPPT issue ??

 Ethan Galloway What's the side-effect if we remove the ACFET (Optinal) on the bq25750 charger circuit ?? ACUV/ACOV protection ?? MPPT full sweep ?? 

Hello Matt,

MATT CHANG said:

Can you help to provide a reference circuit design how to connect the system load in parallel for the MPPT issue ??

You can follow the BQ25756 application example for solar panels in the BQ25756 datasheet.

MATT CHANG said:

What's the side-effect if we remove the ACFET (Optinal) on the bq25750 charger circuit ?? ACUV/ACOV protection ?? MPPT full sweep ?? 

This is a pretty good idea. There's only 1 side effect I can think of the moment. When the BATFETs are ON, there will be nothing to block battery current from flowing through the BATFETs and into the input. You can add a diode here to block this reverse current. Or, you just remove one of the ACFETs. Then, system can still get power from the solar panel's body diode and the reverse battery current will be blocked.

Let me know if either of these suggestions will help your system.

Best Regards,
Ethan Galloway

Ethan Galloway As your suggestion, we just need to add a 0R on the left side MOSFET of ACFET to keep the battery charging path always on, and still wondering what's the downside for it ??
- Will we still have the ACUV/ACOV protection ?
- What will be changed when running MPPT full sweep scan ?

As mentioned before, VIN from 100W solar panel and VSYS connected to a device w/ 10W loading:

- Are we able to get the max. power point from Solar input w/ 10W loading at VSYS ?

- Will the VSYS be power down when start to running MTTP full sweep (bq25750 enter HiZ mode by disable ACFET) ??

Hello Matt,

MATT CHANG said:

As your suggestion, we just need to add a 0R on the left side MOSFET of ACFET to keep the battery charging path always on

Yes, this is correct.

MATT CHANG said:

Will we still have the ACUV/ACOV protection ?

ACUV protection will still work. However, ACOV protection will not work. I recommend shorting the ACOV pin to ground. If the charger detects the ACOV condition, it will turn-off the ACFETs and turn-on the BATFETs. However, voltage will still be able to conduct through the body diode. This might allow current to flow from VAC directly into the battery.

MATT CHANG said:

- What will be changed when running MPPT full sweep scan ?

When running the full sweep, the BQ25750 IC will act as normal. However, when the BQ25750 goes into HIZ mode and turns-off the ACFETs, current will still be able to flow through the ACFET and power the system load.

MATT CHANG said:

- Are we able to get the max. power point from Solar input w/ 10W loading at VSYS ?

This will depend on the load at VSYS and the power available from the solar panel. The BQ25750 will find it's max power point from its battery charge current independently of the system load.

MATT CHANG said:

- Will the VSYS be power down when start to running MTTP full sweep (bq25750 enter HiZ mode by disable ACFET) ??

VSYS should stay powered from the current flowing through the body diode of the ACFETs.

Best Regards,
Ethan Galloway

Ethan Galloway Thanks for the feedback and it does help a lot. With the recommeded single ACFET architecture for solar input power passing through the body diode of ACFET, it seems that could help to resolve the VSYS shut down issue when MPPT full sweep started w/ HiZ condition of bq25750.

• One more question to be clarified is you recommended to connect ACOV to PGND, so we have to re-calculate the ACUV resistors dividor w/o RAC3, right ?
• A following question is not sure if we will hit the ACUV protection when MPPT enabled ... to be checked ??

Hello Matt,

MATT CHANG said:

One more question to be clarified is you recommended to connect ACOV to PGND, so we have to re-calculate the ACUV resistors dividor w/o RAC3, right ?

Yes, that is correct.

MATT CHANG said:

A following question is not sure if we will hit the ACUV protection when MPPT enabled ... to be checked ??

The solar panel voltage should fall to the either the ACUV_DPM voltage or the VAC_DPM voltage, whichever setting is greater. We have more information on the ACUV / ACOV pins here.

If the system load fluctuates, the MPPT voltage could fall and cause the BQ25750 to enter into ACUV protection, though.

Best Regards,
Ethan Galloway

Ethan Galloway Aother option, it seems to add a schottky diode on the ACFET can have us a always on AC charging power path, and also keep the ACUV/ACOV working, right ? Will that be a better solution than bypass the left side of ACFET ?? 

Hello Matt,

Adding a schottky diode here will bypass the left-side ACFET, but the ACOV function still won't work as designed. The left-side ACFET will always be bypassed by the schottky diode. Can you go into more detail on why the system needs the ACOV function?

Adding the schottky diode here will decrease the voltage drop from the solar panel to the system load when the ACFET is OFF, though.

Also, just for reference, we have an FAQ on the power path switchover that goes into more detail on when the BATFETs / ACFETs are ON / OFF.

Best Regards,
Ethan Galloway