BQ25751: buck-boost switching frequency confirmation and circuit function confirmation

Hello Sun,

sun qiyun said:

My circuit is designed 30V power input, 26.4V float voltage for 24V lead acid battery charging, max charging current is 1A. Switching frequency is 300KHz.

When I  adjust the input voltage as 28V, the frequency is 150KHz.

Can you provide a scope capture when this behavior occurs?

sun qiyun said:

About the UV/OV function, from the datasheet, I find if the input voltage is over voltage or under voltage, the chip will close the ACFET by the pin ACDRV. 

From the functional block diagram, I find if the ACFET is short, the OV/UV function is also active, and the chip will close the buck-boost controller, right?

Yes, This is correct.

sun qiyun said:

For the inductor selection, datasheet recommends inductor is 2.2~15uH, DCR is 1.75~60mohm. Is it necessary to select inductor per this? Or I can select inductor according to buck circuit caculation?

Yes, You will need to select the inductor based on these parameters.

Best Regards,

Christian

Hi， Moyer

Christian Moyer said:

Can you provide a scope capture when this behavior occurs?

Please see the attached test picture.

C1 is SW1 waveform, C2 is input power, C3 is charging current (in my design, the max charging current is 1A).

When the input power is 28V, the SW1 frequency is about 150KHz.

Christian Moyer said:

sun qiyun said:

About the UV/OV function, from the datasheet, I find if the input voltage is over voltage or under voltage, the chip will close the ACFET by the pin ACDRV. 

From the functional block diagram, I find if the ACFET is short, the OV/UV function is also active, and the chip will close the buck-boost controller, right?

Yes, This is correct.

From the block diagram, I only find the OVP signal Vsys_rev_ov, no UV signal, does it show when the ACFET is short, the controller is only be closed when the power is over voltage, but  not be closed in under voltage, right? And I have not find the value Vsys_rev_ov, could you please tell me if ACFET is short, how the controller is closed when OV/UV happens?

Thanks!

Hello Sun,

sun qiyun said:

When the input power is 28V, the SW1 frequency is about 150KHz.

What is the battery voltage when this behavior occurs? Do you see any difference at different battery voltages? Can you also measure SW2 when this behavior occurs?

sun qiyun said:

From the block diagram, I only find the OVP signal Vsys_rev_ov, no UV signal, does it show when the ACFET is short, the controller is only be closed when the power is over voltage, but  not be closed in under voltage, right? And I have not find the value Vsys_rev_ov, could you please tell me if ACFET is short, how the controller is closed when OV/UV happens?

Yes, If ACFET is shorted. The converter will shut off in both overvoltage conditions and under voltage conditions. Im not sure why this is not in block diagram, but I confirmed it in the lab.

Best Regards,

Christian

Hi, Moyer

Christian Moyer said:

What is the battery voltage when this behavior occurs? Do you see any difference at different battery voltages? Can you also measure SW2 when this behavior occurs?

For 28V power input, please find below waveform of SW2, LODRV2 and HIDRV2. The controller gate output is abnormal. But it can also support 1A current charging.

For 30V power input, it is normal. Please help explain.

Christian Moyer said:

Yes, If ACFET is shorted. The converter will shut off in both overvoltage conditions and under voltage conditions. Im not sure why this is not in block diagram, but I confirmed it in the lab.

If I do not use ACFET in my design, the inner controller OV/UV protection threshold is also set by the ACOV/ACOV, right?

Waiting for your reply, thanks!

Hello Sun,

sun qiyun said:

For 28V power input, please find below waveform of SW2, LODRV2 and HIDRV2. The controller gate output is abnormal. But it can also support 1A current charging.

I just want to confirm that this behavior occurs at every battery voltage? For example: VIN=28V,   VBAT=26,VBAT=20,VBAT=15?

sun qiyun said:

If I do not use ACFET in my design, the inner controller OV/UV protection threshold is also set by the ACOV/ACOV, right?

Yes, If you don't have ACFETs, the device will still shut off converter in OV/UV conditions.

Best Regards,

Christian

Hi, Moyer

Christian Moyer said:

I just want to confirm that this behavior occurs at every battery voltage? For example: VIN=28V,   VBAT=26,VBAT=20,VBAT=15?

For 24V/4Ah battery (2 12V batteries in series), when the battery is 24V, 26V, if the input power is 28V, the frequency is also 150KHz.

I also design the circuit for 12V/7Ah battery charging, when the input power is 28V, the frequency is 300KHz, it is normal.

I have a question, is there minimum voltage drop requirement?

Hi, Moyer

Christian Moyer said:

No, There is no minimum voltage drop requirement. This looks normal,  the device is operating in buck-boost mode, so the 300kHz, is split between SW1 and SW2.

Could you please help explain in which state, the controller will be in buck-boost mode?

In my understanding, the VBATREG is 26.4V, the input charging power is 28V, it is also bigger than the VBAT, so I think it is always in buck mode. 

For below SW2, HIDRV and LODRV waveform, is it normal?

Thanks!

Hello Sun,

sun qiyun said:

In my understanding, the VBATREG is 26.4V, the input charging power is 28V, it is also bigger than the VBAT, so I think it is always in buck mode. 

Buck-Boost mode will occur around ±2V from the battery voltage. Buck mode will occur when the input voltage is about 2V above the output voltage. Boost mode will occur when the input voltage is 2V below the output.

The IC will seamlessly transition from buck to boost mode.

sun qiyun said:

For below SW2, HIDRV and LODRV waveform, is it normal?

Do you have the boost side Fets on the boards? Previously you mentioned you were removing these fets.

Best Regards,

Christian.

Hi, Moyer

Christian Moyer said:

Buck-Boost mode will occur around ±2V from the battery voltage. Buck mode will occur when the input voltage is about 2V above the output voltage. Boost mode will occur when the input voltage is 2V below the output.

The IC will seamlessly transition from buck to boost mode.

ok, got it.

Christian Moyer said:

Do you have the boost side Fets on the boards? Previously you mentioned you were removing these fets.

I keep all the four FETS on buck side and boost side like below schematic showing in current test. And you suggested these FETs are used to detect the  inductor DRC, so I will keep them. Could you please help check this? Thanks!

Hello sun,

The SW2 waveform does not look normal to me, it should be switching the same as SW2.

I see you are using a very high inductor, the max we recommend is 15uH. Do you see any changes, if you switch out the indcutor?

Best Regards,

Christian.

Hi, Moyer

Christian Moyer said:

The SW2 waveform does not look normal to me, it should be switching the same as SW2.

You mean SW2 should be same as SW1, right?

I change the inductor as 10uH, the waveform as below.

With load:

Without load:

Only in light load mode, SW2 is same as SW1. But the waveform is bad. 

Could you please help check it?

And I have another question is the datasheet showing buck-boost the minimum input capacitor and output capacitor is 160uF. Could you please tell me why? If I use the capacitor less than 160uF,what will happen?

Thanks!

Hi, Moyer

Thanks for your quick reply. I will further test.

I also have serial questions, could you please help give some suggestion?

1. I want delete the back to back ACFETS, as the 30V power is only used for battery charging, will it cause anything?

2. I meet a strange phenomenon. I have tested several boards, and find some boards cannot operate normally. These boards can output normally without load, but once connected with battery, it cannot output and the output voltage is battery voltage. We try to heat the buck high and low MOSFET, it can operate normally. But when the temperature is reducing, the battery cannot operate again. I also replace buck high MOSFET with a new MOSFET. It is ok. The changeable MOSFET is not broken by multimeter test.

I guess the MOSFET shall have quality question. But I cannot sure completely. 

Could you please help confirm is there any requirement for the buck MOSFET selection? In my design, I choose Infineon: BSC340N08NS3G.

3. For the buck-boost output voltage, what is the output accuracy? For example, in my design, the VBATREG is 26.4V, it shall be 26.4±？

Hope I can receive your reply soon, thanks.