BQ24630: BQ24630 is sinking too much current even when not charging

It is the IC VCC pin that we are concerned about. Our battery charge current is ok and steady at 1A.

We are measuring the IC VCC current using the 10 Ohm resistor (R6 on the attached diagram).

Our IC is getting hotter than before and that is the problem.

Thanks.

Yes, we did.

Also, I don't think insufficient VIAs would double the amount of current that is entering the chip.

As I mentioned, this board design worked beautifully when we assembled and tested the first sample.

We only are having problems with the second test board (same PCB, same batch).

We had to make some BOM changes due to lack of the original parts, but we selected all substitutes carefully.

One such substitution was the D13. Original was ZLLS350TA and the new one is NSR05T40XV2T5G. Can this be a problem?

Thanks,

Alex

Dear Alex,

Can you please send the both the original BOM and secondary BOM so I can compare all of the differences?

Also, following Section 9.2.2.6 the best output stability occurs when the LC resonant frequency is between 10 kHz to 15 kHz. Your output frequency is 7.6 kHz.

Thanks,

Mike Emanuel

Hi Mike,

BOMs are attached.

Please note that our board has four independent charging channels with four BQ24630 chips and the BOMs are for the full board (see photo).

For some reason the specification sheet of our inductor indicates RF of 10KHz, which is almost in good range.

Thanks,

Alex

BOM_Comparison.zip

Hello,

Can you please report the input voltage and battery voltage used to test both of these boards? I need to know the input and output characteristic at 1 A so I can better understand the system.

The LC resonant frequency depends on both the inductor and the output capacitor. Please see Section 9.2.2.6 for the calculation of the frequency.

Thanks,

Mike Emanuel

Hi Mike,

Our input is 24V and the battery is a 4S LiFePO4 at 12.8V with a max charging voltage set at 14.4V.

To simplify things, we eliminated the battery from our test and we are only comparing the two boards in that state.

So, no charging and PG LEDs are the only ones glowing.

In this state the new board is recording a 0.097V drop across the 10 Ohm VCC resistor and the older board is showing 0.052V.

We see this as a problem and we trying to understand where is this extra 5mA current is going.

Thanks,

Alex

Dear Alex,

I will get back to you by the end of business tomorrow.

Thanks,

Mike Emanuel

Hi Mike,

I don't know if this will help but I am attaching two readings taken at the note where the bootstrap diode, resistor and the capacitor meet.

Both readings were taken while the battery was disconnected.

One is from the earlier "good" board (left) and the other one from the new "bad" board (right).

Thanks,

Alex

Hi Mike,

I understand.

Can you please do me a favor and have a look at two pictures below (R6 is the 10 Ohm VCC resistor).

Can you please tell me if this tells you anything.

Thanks,

Alex

Hi Mike,

Yes, I am using a differential probe.

I would be less concerned about all this if the average current through VCC was lower than 10mA when idling and lower than 20mA when charging.

These numbers are way above (double) of what the BQ24630 sheet says they should be!

These numbers are also confirmed by the outer case temperature jump of 8C compared to the chips on the old board.

I can't just wire off all this to some mysterious "active components" and call it a day.

One of the main active components on our board is the BQ24630 itself and we could be dealing with a defective TI chip.

I can't eliminate the TI chip as a cause of the problem unless I have at least something that tells me otherwise.

As of right now this chip is eating too much power compared to what the specifications are saying and I can't find yet were is it all going.

If you can give me some kind of a hint as for why do we have four times the number of spikes on our scope when idling, compared to the old board, then I can get somewhere.

Again, all I need is a hint and I will not hold you responsible for it if it turns out to be a wrong suggestion.

Thanks,

Alex

Dear Alex,

Please switch the ICs between the "good" and "bad" board. Please report back the outcome. This will help to eliminate the IC or the board as the problem.

Thanks,

Mike Emanuel

Hi Mike,

Unfortunately that is not realistic. I can't do it without sending the boards back to the assembler and that is not an option for us at this point.

Are you saying you can't make any guesses at all based on the data I provided?

Thanks,

Alex

Hi Mike,

We removed one of our 10 uF output capacitors which would bring our LC resonant frequency to 10.7 KHz.

Unfortunately this did not change anything.

Regards,

Alex

Hi Mike,

Our boards already have populated R5 and unpopulated R7.

I did remove the D12 but no change in the results.

In the meantime I noticed that those spikes in power consumption coincide very closely to moments when the R10-D13 junction peaks drop to 28V and the LODRV has a refresher spike (see attached, taken at different times).

Is it possible that we simply chose the D13 poorly and the bootstrap capacitor now needs charging 4 times more often?

Is it possible that this is where the extra current is going?

Thanks,

Alex

Dear Alex,

Can you explain what waveforms you presented? What is the first waveform and what is the second waveform?

However, if you think the diode may be the issue, could you desolder the original D13 from the first board and place it on the second board to see the difference?

Thanks,

Mike Emanuel

Please click "Resolved" if this answered your question.

Hi Mike,

As I mentioned above, these are taken at R10-D13 junction (bootstrap) and LODRV pin against the ground.

I did some calculations and I decided to wait with the diode exchange.

I found out that our new high-side FETs have over twice the gate charge (75nC) of the old ones (31nC) and this will have much bigger effect on how often we have to top up the bootstrap capacitor than the diode leakage current.

I think we need to find new FETs if we are to keep the same 100nF bootstrap capacitor.

I think this could be our main problem.

Thanks,

Alex 

Dear Alex,

Please keep me posted!

Thanks,

Mike Emanuel