BQ76952: High side MOSFET switch on

Hey Raymond,

is there a specific MOSFET that gets hot?

Thanks,
Caleb

Hey Raymond,

There seems to be no issue with your schematic. Do you mind measuring the voltages on the FETs gate, source, and drain?

Thanks,
Caleb

Hi Caleb, just to confirm, is it measure the Vgs, Vds?

Hey Raymond, 

Take measurements of the gate, source, and drain, with respect to ground.

Thanks,
Caleb

Hi Caleb, 

I have measured w.r.t ground. 

Gate = 62V

Source & Drain = 56V 

The MOSFET which I use is STW65N80K5

One more thing that we have noticed is that when the load is ON, we are unable to switch on the MOSFET. 

When the load is off, then we can turn on the MOSFET. 

Is there something wrong with my schematic that causes this? 

Hey Raymond,

It looks like your gate voltage is only about 6V higher than the source and drain. This could be why you can't turn the MOSFET on while there is a load.

Can you check the Settings:FET:Chg Pump Control[LVEN]  configuration bit to confirm that the charge pump voltage is set to 11V?

This could be contributing to the heat that you are observing. According to the MOSFET datasheet it's expecting at least 10V between the gate and source & drain. Since you're not at 10V, the Rds(on) will be very high. It also looks like your MOSFET already has a very high Rds(on) (70mOhms) even at the required 10V gate voltage.

Thanks,
Caleb

Hi Caleb, 

Noted. Will check the configuration bit. 

Regarding the issue of 

"when the load is ON, we are unable to switch on the MOSFET. 

When the load is off, then we can turn on the MOSFET. "

We removed one MOSFET and it works. 

Not sure whether is the resistor value too high or the chg pump control value. 

Hey Raymond,

I'll be waiting for you to get back to me about the configuration bit. In the meantime, do you mind specifying which MOSFET you removed that caused the circuit to work?

Thanks,
Caleb

Hi Caleb, 

Configuration Bit = 11V. 

We remeasured the voltage again, it is about 10.6V, maybe the previous measurement got some error. 

What we notice is if we removed Q23 in the above schematic, the MOSFET works perfectly. 

When the MOSFETs are in parallel, we notice that the gate voltage becomes a sawtooth waveform and the voltage can go up to 30V. 

And it is unable to turn on when the load is on first. 

Hi Raymond,

Normally we recommend individual gate resistors or inductors for each FET in parallel (see the Multiple FETs with the BQ769x2 application report). Oscillation can be a problem without these components. I wonder if this could be the issue.

Regards,

Matt