BQ24171: ACFET and RBFET

Hideki Uehara

Prodigy 150 points

Part Number: BQ24171
Other Parts Discussed in Thread: CSD17571Q2, CSD17313Q2,

Hi
I have two questions.

1.Please tell me how to select ACFET and RBFET.

2.Also, please tell us the reason for selecting the ACFET and BRFET used in the evaluation board.

Please answer.

over 3 years ago

0 Hideki Uehara over 3 years ago

Prodigy 150 points

I have an additional question.
Please tell us about peripheral passive elements.

Please answer.

0 Ricardo over 3 years ago in reply to Hideki Uehara

TI__Genius 10385 points

Hello,

For peripheral passive elements:

https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/196/5140.BQ24133-BQ2417x_5F00_Schematic-Checklist_5F00_V1p0.pdf

For ACFET and RBFET:

The N-channel Power MOSFETs were selected for their low RDSON on and low Qg and Qgd. Any N-channel Power MOSFET with similar RDSON, Qg, and Qgd can be used in place of the CSD17313Q2 such as CSD17571Q2.

I hope this resolves your issue,

Ricardo

0 Hideki Uehara over 3 years ago in reply to Ricardo

Prodigy 150 points

Hi.

Since it is BR24271, the data sheet also includes CGD.

I would like to know the explanation of CGD.
I would also like to know the related equations if possible.

Please tell me.

0 Ricardo over 3 years ago in reply to Hideki Uehara

TI__Genius 10385 points

Hideki-san,

I do not know what BR24271 is, could you please send a link to the datasheet?

C_GD is the capacitance between the gate and drain. This capacitance and the combined total gate capacitance (C_G) must be fully charged for the MOSFET to switch on.

The significance of this is that the charge pump on ACDRV has a limit of 60 uA. By keeping the gate capacitance or gate charge low, you can reduce your turn on time.

Some relevant equations:

Q = i * t and Q = CV

I hope this resolves your issue,

Ricardo

0 Hideki Uehara over 3 years ago in reply to Ricardo

Prodigy 150 points

Im sorry. It was BQ24171 instead of BR24271.
I will ask you a question again.

The BQ24171 datasheet also lists CGD in the "10.2.2.5" "Input ACFET and RB FET" section.

Please tell me the explanation of CGD.
I also want to know the relevant equations if possible

Please tell me.

0 Ricardo over 3 years ago in reply to Hideki Uehara

TI__Genius 10385 points

Hideki-san,

I understand your concern now. The external C_GD and C_GS are to supplement the parasitic capacitances of the MOSFET. These parasitic capacitances limit the turn on time of the MOSFET. To additionally slow the turn on time of the MOSFET you can additional external capacitance.

The turn on time can be calculated as the time it takes to charge the total gate capacitances of the FET as well as the external additional capacitances. You know the gate drive current (60 uA), you will know the turn on voltage of the FET you select, you will know the capacitances of your FET. With all of this considered you can approximate your turn on time.

I hope this resolves your issue,

Ricardo

0 Hideki Uehara over 3 years ago in reply to Ricardo

Prodigy 150 points

Thank you.

I understand that CGD and CGS are for setting the turn-on time of the external MOSFET.

Is the following about how to determine the values of CGD and CGS?

CGS is about 10 times the Cg of MOSFET
CGD is about 10 times the Cgd of MOSFET

Is it correct?

0 Ricardo over 3 years ago in reply to Hideki Uehara

TI__Genius 10385 points

Hideki-san,

That is roughly correct and a good starting point.

I hope this resolves your issue,

Ricardo

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BQ24171: ACFET and RBFET