DRV8320: how to determine the best dead time

Hello Hirata-san,

I will consult with the team and aim to provide response before the end of the week.

Best Regards,

Akshay

Hello Akshay-san,

DRV8320 has smart gate drive feature and monitor the gate-source voltage for DRV8320 to understand the best timing to switch over.

In my understanding this feature is automatic dead time insertion.

In addition to this feature we can add the fixed dead time which has 4 option.

So how to determine the best fixed dead time? Do you have any guideline?

Regards,

Hirata

Hello Hirata-san,

You are correct the device has automatic dead time insertion along with different fixed options for you to select. The specific dead time is focused on end application so it depends on how fast you want your lowside MOSFET to turn on after the highside MOSFET. Since it is use specific, we do not have a fixed guideline to follow.

Best Regards,

Akshay

Akshay-san,

In my understanding everyone want to reduce the dead time. Is this not correct?

The default of DRV8320 is 100ns.

Who need to use longer dead time such as 400ns?

Also who can use shorter dead time = 50ns?

Regards,

Hirata

Hello Hirata-san,

I will consult the team and aim to provide feedback before the end of the week.

Best Regards,

Akshay

Hello Hirata-san,

Thank you for your patience.

50 ns – if your system is robust and you can ensure no shoot through will occur based on the input to the pre-driver. But short time can lead to shoot through typically, that is why the default is a 100ns. If your application isn’t for a BLDC motor and has the MOSFETS working separately then shoot through isn’t a problem so 50ns should be fine

400ns – A larger dead time will reduce the chance of a shoot through condition. But high deadtime can reduce motor voltage and may not be ideal if you want high motor voltage. But it depends on your specific application. For customers who want to reduce chance of shoot through and do not need high motor voltage requirement then we suggest longer dead time.

I could ask my team for more information if you can provide some detail on the application of the device.

Best Regards,

Akshay

Akshay-san,

I would like to know the handshaking and dead time relation.  

Regarding below, handshaking process adjust the gate drive time already. Why tdead is needed further? You mentioned shoot through but handshaking process already checked the proper timing of the switching in my understanding.

How does the handshaking process determine the correct time to switch?

For example...

"VGS monitor block" monitors the gate voltage and then tdrive terminates if the VGS becomes 1V.

Remaining gate charge(=1V) should be removed during tdead.

If this is true I understand.   

In this case the guide line might be the below for example.

"Qg @ VGS=1V" / IHOLD << tdead

I would like to know these information.

Thanks.

Regards,

Hirata

Hello Hirata-san,

Thank you for your patience.

We use an internal voltage of VGS = 2V to handshake and determine when to turn on the MOSFET so that the high side and low side are not on at the same time but also the closed loop dead time timer is implemented to ensure that the MOSFETs are on based on the dead time setting chosen.

You can choose dead time so that any gate charge is removed during the t_DEAD setting. We apply IDRIVE for the duration of T_DRIVE, then we implement a brief I_HOLD which keeps the MOSFET at that state and during this time (t_DEAD) the remaining gate charge is removed.

Please consult the figure below which illustrates the principle described above.

Best Regards,

Akshay

Hello Akshay-san,

Thanks for your answer. Can I confirm? Is this below understanding correct?

**********
Phase1.
When gate driver sinks the gate charge from the gate of the FET to turn off, "VGS monitor block" monitors the VGS voltage and tdrive duration terminates if the VGS becomes 2V. This mean this phase 1 duration can be shorter than the tdrive setting. For example if the VGS voltage becomes 2V 600ns after the start of the turn off process, this phase 1 will terminate at this 600ns timing even if the tdrive setting is 1000ns. During this phase 1, the gate charge is sank by the magnitude of IDRIVE setting.

Phase2
Remaining gate voltage(=2V) should be reduced down to FET threshold voltage during tdead at least. During this time the gate charge is sank by IHOLD.
**********

Also regarding below comment, closed loop dead time timer means tdead? I think tdead does not have any feedback loop so i think it is open loop. I think below comment is true. Is this correct? 

"but also the open loop dead time timer (=tdead) is implemented to ensure that the MOSFETs are OFF based on the dead time setting chosen."

Akshay Rajeev Menon said:

We use an internal voltage of VGS = 2V to handshake and determine when to turn on the MOSFET so that the high side and low side are not on at the same time but also the closed loop dead time timer is implemented to ensure that the MOSFETs are on based on the dead time setting chosen.

Thanks for your support.

Regards,

Hirata

Hi Akshay-san,

Sorry for the late reply. I had some summer vacation.

Thanks so much for your answer and now I am clear.

I appreciate your support!!

Regards,

Hirata

Hi Hirata-san,

Happy to help.

Best Regards,

Akshay