bq24600 will not charge at more than 3A when configured for 6s

Bruce Pearman

Other Parts Discussed in Thread: BQ24600, CSD18533KCS, CSD19534KCS

I have been using the bq24600 for several years in designs with 4s LiPos and 5A with no problems. Now I am trying to use it with a 6s LiPo and a charge current of 5A. If I configure it for 3A it works fine, but as soon as I change the value to 5A it shuts down with no current. I have tried changing the inductor from 3.3 uH to 2.2uH as the datasheet for the EVM suggests for higher current with no change in behavior.

I am using it on a board design that has been very successful at lower voltage and current. I have plenty of heat sinking; the MOSFETs run at 45 deg C at 3A. I am using different MOSFETs than I have used in the past but they are TI 60V, 118A NexFETs (CSD18533KCS).

Any ideas would be welcome.

Thanks, Bruce

over 8 years ago

0 Steven Bangdiwala over 8 years ago

TI__Expert 8690 points

Hi Bruce,

The FETs you are using have high Qg (3x more than what we use in the EVM: SiR426DP) it will slow down the switching time and may provide problems. Change FETs to lower Qg and test again. Can you send me a schematic?

Thanks,
Steven

0 Bruce Pearman over 8 years ago in reply to Steven Bangdiwala

Intellectual 280 points

Steve,

I thought that might be a problem. I'll try to find devices that meet all of the criteria and have lower Qg. I'll update this post after I get the other parts.

Thanks, Bruce

0 Bruce Pearman over 8 years ago in reply to Bruce Pearman

Intellectual 280 points

Hi Steven, I have replaced the MOSFETs with TI CSD19534KCS devices. They have a much lower Qd and higher Vds. I still experience the same behavior. I can get the circuit to work fine if I keep the current under 3A. As soon as I change Viset even a slight amount the charge current suddenly drops and about 50% of the time the IC fails before I can remove power. I'm going to make some measurements and try to pinpoint the failure mode. I would attach a schematic but I don't see how to do that. Any thoughts would be appreciated. Bruce

0 Bruce Pearman over 8 years ago in reply to Bruce Pearman

Intellectual 280 points

Steven, can you suggest a MOSFET better suited for this application in a TO-220 package?

0 Steven Bangdiwala over 8 years ago in reply to Bruce Pearman

TI__Expert 8690 points

Hi Bruce,

You can send the schematic as an attachment in the html reply format. Just click on the "Insert File" button on the toolbar.

Can you send a scope shot of the PH node at 3A right before failure? I want to see if there is anything abnormal.

Edit: Also please provide HIDRV, LODRV scope shot as well.

Thanks,

Steven

0 Bruce Pearman over 8 years ago in reply to Steven Bangdiwala

Intellectual 280 points

Steven,

My Tek 'scope is in calibration--I should get it back today or tomorrow. In the interim I am using a 40+ year old Tek analog scope that doesn't take pictures. I have looked at PH, HIDRV and LODRV and found considerable ringing. I'm trying to track the source down.

Bruce

0 Bruce Pearman over 8 years ago in reply to Bruce Pearman

Intellectual 280 points

0 Bruce Pearman over 8 years ago in reply to Bruce Pearman

Intellectual 280 points

0 Steven Bangdiwala over 8 years ago in reply to Bruce Pearman

TI__Expert 8690 points

Bruce,

You will need to reduce the high power loop in order to diminish the noise. You want C41, C42, Q7, Q8 in a very closed loop, and make sure the GND loop does not pass through any small signal circuitry, like ISET, VFB, SRP-SRN. If possible, you want to reduce loop for C47 and C48 as well, as these will reduce the inductor current ripple and have some AC components. You also want C43 and C44 as close to the IC as possible, as these are used as charge reservoir to drive the FETs. If you still have ringing, the first thing you should try is to add a RC snubber at the PH node.

Thanks,

Steven

0 Bruce Pearman over 8 years ago in reply to Steven Bangdiwala

Intellectual 280 points

Just to put a little closure on this problem. I took more measurements and found considerable parasitic oscillation in the high-power loop. This is what I believe was heating up the FETs and blowing out the bq24600 IC because the P-P excursions of the ringing was exceeding the ICs maximum voltages. I have designed an RC snubber for the high-side MOSFET which attenuated the ringing from > 0.3 Vpp to < 1mV. The modified circuit (not good enough yet) will charge at 4.5A (target 5A+) and the hottest objects on the PCB (MOSFETs and bq24600) do not exceed 45° C.

I am n the process of redesigning the PCB to incorporate all of the changes I have learned from this experience.

Thanks for your help Steven.

Bruce.

0 Steven Bangdiwala over 8 years ago in reply to Bruce Pearman

TI__Expert 8690 points

Bruce,

I'm glad to have been of some assistance.

Thanks,
Steven

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bq24600 will not charge at more than 3A when configured for 6s