bq40z50 DSG Drive

The DSG FET has probably been damaged before the gauge attempts to turn it off. This may be due to thermal overload. Does the gauge report any faults?

There are no faults repoted by the gauge. I will send you some measurements later on. Can you tell me until that point whats the main reason to include the Mosfet V4???

That FET was added to protect against a reverse charger connection. If the charger voltage is reversed, then the FET will force the DSG FET off and prevent reverse current flow.

Now I figured out the problem.

The device is not able to disable the Mosfet in the right way.

attached you can see some measurements. One successful run with 12A and two unsuccesful runs with 15A and 24A as current thresholds.

the current value shown is wrong. you ghave to multiply it with a factor of 5!

I am using the IPT007N06N Mosfet from Infineon with a big area for the drain area and a 1,8Ohm preresistor.

Do you have

Hello again,

I have tested some additional circuits in order to turn off the FET as quick as possible. Unfortunatelly the DSG pin does not support big currents and the datasheet does not give any declaration about the current. How much current can the DSG pi9n support during sink and source mode?

We have successfully used a 1k DSG Gate resistor with a similar device and are currently testing a 1k resistor with the BQ40z50 - with no issues so far (though it's early days)

We were finding that when short-circuiting a pack (>100A), the slow turn-off of the 5.1k resistor was causing the DSG FET to fail about 50% of the time.

I can't say what the rated current of the DSG pin is though.

I am using already a resistor of 270 Ohm. Which Mosfets do you use?

I have not tested the turn off during a short circuit. Right now trying to turn off the Mosfet with a contant current of max. 40 A.

As I can see by measuring the Gate-Source voltage with small currents the voltage drops from 11.5 to 4V and stays at this level for around 50ms and then falls below the threshold voltage of 2,3 - 2,7 V and finally turns off the Fet.

Can you give me some more information about your schematics? So I can figure out the difference.

The maximum drive strength of the charge pump will be close to 10 uA worst case. The output current strength decreases somewhat as the output voltage reaches the full 11V. It tends to be strongest at the first 35%.

As for the turn off, there is a nFET pull down, which will appear as a current source when the nFET is saturated (most of the transition) and as a resistor as the FET linearizes near the bottom of the pull down.

I've not done any probing of the BQ40z50, but with the previous chip (BQ20z95) the gate dropped steadily from on to off. Reducing Rg simply shortened the time taken - there was no hanging at a mid-level or anything.

That said, the DSG gate drive on my current board has just gone - it no longer drives above P+. Not sure what I did, possibly static damage though or maybe heat as I've re-soldered parts of the PCB a few times.

Looking at the datasheet, the internal charge pump shows a series 2k resistor. Would this not limit the current to a few mAs anyway?

My design is as per the BQ40z50 reference except I have put a 1M0 on the gate of Q4 and Rgs is 2M2 rather than 1M0 (our application needs minimal idle/leakage current). We also don't expect to see any reverse polarity charge events, so Q4 is not too crucial.

Hi Thomas
What's the current from saturated pull down nFET?
Also is there data available that shows this current pulse/rate of change?

Best regards

The charge pump can drive about 100uA (worst case) during the about the first 50% of turn on and then it decreases to about 10uA (worst case) as the voltage approaches 11V. The pull-down drive strength is about 1mA.