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# BQ25910: question about 3 level convert ，during the work , how did the Cfly charge to VIN/2, who control the Cfly charge

Part Number: BQ25910

HI ,I have a question about  3 level convert.I have read about BQ25910 spec.

Before the 3 level convert start to work ,if D<0.5,the Voltage on Cfly is 0; Who charge it to VIN/2?

I know during the first stage,the VIN charge the Cfly  to VIN/2.

Here is the question,how did the charge work？

why charge to VIN/2?

How to control charge?

who control the charge?

Why charge stop when the  Cfly charged to VIN/2? Why Vsw is VIN/2?

• Hi

Before start switching, bq25910 use an internal current source to charge Cfly to Vin/2.

Charge Cfly to Vin/2 can reduce SW to half, because VSW=Vin-VCFLY. That means switching loses is reduced.

Alen

• During the  3 level convert working ,  Dose the  internal current source which use to charge the Cfly  still working in every cycle（first stage ）？

• Hi User5873469,

The Internal current source just use for pre-condition(before converter switching).

Cfly can be charged during the status of normal working below:

• Oh I konw.Before the Internal current source make sure  Cfly charged to Vin/2.

But during the status of normal working，without the Internal current source to make sure the Vin/2.,how and who make sure the Cfly charge to Vin/2 ?

• Hi Hi User5873469,,

As I mentioned before, bq25910 can adjust the duty cycle of Cfly charge status(state 1) and the duty cycle of Cfly discharge(State 3) to balance Vcfly.

Alen

Please click "This resolved my issue" button if this post answers your question

• HI Alen：

I know the time of state 1 is as much as state 3.。state 1 charge，state 3 discharge .I just understand how it  charge CFly  which voltage is 0 after stage3,than charge to Vin/2 in stage1.

During and after stage 3 the voltage on Cfly is still  Vin/2.

Cfly dischanrge in stage 3,why the voltage still  Vin/2?   Should it be dropped?

From this picture,  Voltage on Cfly didnt change , charge and dicharge, the  Voltage on Cfly is still the same.

WHY?Is the power take from Cfly  too small to change the Vcfly during stage 3?

• Hi user5873469,

You are right.

The charge in/out Cfly during single status/phase is quite small compared to capacity of Cfly. so Cfly is big enough to keep Vcfly almost stable.

Alen

• Thank you!! Thank you very much!!!