DRV8323: SHx Pin AMR absolute maximum rating limited range

Hi Laxmi,

I will need to consult with my team about this. I will aim to get you a response by this Friday at the latest.

Thank you,

Joshua

LAXMI AKSHAYA THELA said:

For example SHx pin AMR with 24 V battery is going to only be 24+7=31V which in my view is a limited range if very high currents and switching node oscillation are involved.  Am I missing something? Is there any remedy for this? I have seen other IC’s for the same application in the market with an absolute max rating of the pin-up to 80V.

Hi Laxmi,

With VM=24v then the Source of the high FET should be SHx=24v when it's turned on, so I think 7v on top of the batt is reasonable rating. I'm interested to check out another driver that you mentioned that has SHx=80v rating with VM=24v.

Brian

7 volt above battery voltage is ok almost always except when the stall current gets to 300 Amps. Then I see 30 V on that node (SHx) which is 5 volt more than limit (18+7=25V=max limit). I looked at 6edl7141 gate driver  and the max abs limit for SHx is 70 volt (not 80 volt sorry for typo earlier)

LAXMI AKSHAYA THELA said:

I looked at 6edl7141 gate driver  and the max abs limit for SHx is 70 volt

It has good and bad merit: 70v rating is good for any VM voltage, but when VM=70 and this driver has zero overhead for SHx as it's rated at 70v also, but this is not your concern. So for your application at VM=24v, the 6EDL7141 is a better product.

Stall current of 300A or braking current? I don't see why during motor stalled that the phase voltage SHx is higher than VM more than 7v as motor has no BEFM. During braking the BEFM added with the inductive current can cause higher voltage at SHx. What mode of braking you choose? Short high side or short lower side, or high Z?

Brian

When we start the motor with locked rotor (stall) the BEMF is zero that means whole 24 volt will drop across motor impedance which is very low. I am also using very low impedance battery which makes the current  even higher.

LAXMI AKSHAYA THELA said:

When we start the motor with locked rotor (stall) the BEMF is zero that means whole 24 volt will drop across motor impedance

LAXMI AKSHAYA THELA said:

7 volt above battery voltage is ok almost always except when the stall current gets to 300 Amps. Then I see 30 V on that node (SHx)

You said at stalled motor there was 24v across motor phases, but the next statement says it has 30v at SHx which is the phase voltage. I'm confused.

Brian

Hi Laxmi,

Please let us know if you require further assistance on this debug. 

If not, please help mark thread as resolved. Thanks 

Best Regards, 
Andrew 

Sorry for confusion. The SHx steady state value is indeed 24 V. By 30 volt I meant only for short time during transient  (MOSFET ringing)

Hello,

As you said earlier that you would confirm with your team and let me know.

I am waiting for your response.

Hi Laxmi,

I apologize for the delay in getting back to you.

During the time that SHx is 30V what is the measured voltage of VM? VM could be increasing along with SHx.

High currents at the phase node will typically go though the external MOSFET to VM and not damage the internals of the DRV8323.

Switching node oscillation can be mitigated by using an RC snubber circuit. For more information on RC snubber circuits see the following E2E FAQ.

Best Regards,

Joshua

LAXMI AKSHAYA THELA said:

Sorry for confusion. The SHx steady state value is indeed 24 V. By 30 volt I meant only for short time during transient  (MOSFET ringing)

Can you post the waveform of the 30v ringing? Wonder if not enough bulk cap or high frequency bypass cap near the FETs.

Brian