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DRV8705S-Q1EVM: DRV8705S

Part Number: DRV8705S-Q1EVM

Hello,

We would need your technical support on a TI product: The gate driver: DRV8705S-Q1.

We encountered a problem of failure of the DRV8705S following a diagnostics validation test ("Test: Short Circuit of SH1 to GND").

*The driver is used to manage two external N-Mosfets (High Side: HS & Low Side: LS): (RDSON=3.3 mΩ) in split HS and LS configuration (with a shunt current resistor of 3mΩ) to control a load of 2Ω resistance.

**Vbattery=13.5V

Ie: In our case, we don’t implement the freewheeling diode in parallel with Load (R=2Ω , L=10uH).

*Diagnosis is performed in “ON STATE” and in “OFF STATE”.

In “ON STATE”: We set VDS_LVL=0.06V

*In “OFF STATE”, we do three consecutive cycles of diagnostics. Each one lasts 4 ms, as follows:

1- Pull-Down_SH1=1           Pull-up_SH1=0

Pull-Down_SH2=1             Pull-up_SH2=0

 

2- Pull-Down_SH1=0           Pull-up_SH1=1

Pull-Down_SH2=0             Pull-up_SH2=1

 

 

3- Pull-Down_SH1=0           Pull-up_SH1=1

Pull-Down_SH2=1             Pull-up_SH2=0

* We launch a cycle with a PWM=100% (Frequency=4Hz) and we apply a short circuit to ground on the signal relied to Pin 21: SH1 of the gate driver.

-Test result:

The gate driver DRV8705S is no more operational following the application of the short circuit.

Investigation of the failure:

*After checking the 2 N-MOSFETS high side and low side and the gate driver, we found a very low impedance between the following pins:

-SH2-SL2: 79Ω

-SH2-GND:45Ω

-SL2-GND:43Ω

->Interpretation: The internal circuit linked to the measurement of the VDS voltage of the LOW SIDE MOSFET of the 2nd half-bridge is faulty.

 

Questions:

1/-Would you have an explanation for the test result?

2/-Why is it the circuit linked to the Low Side Mosfet that was destroyed and not the one linked to the HIGH-SIDE Mosfet (It's "strange")?

 

Thank you for answering our questions as soon as possible.

Best regards.

  • Hello,

    It is likely that the DRV8705S-Q1 was damaged by an over-voltage event when the protection turns on and HIZs the external FETs.  High-current can push supply voltage up.  Can you please scope the supply voltage during this shorting event assuming it is reproducible?

    Regards,

    Ryan

  • Why is it the circuit linked to the Low Side Mosfet that was destroyed and not the one linked to the HIGH-SIDE Mosfet (It's "strange")?

    Can you confirm if the 2 FETs are damaged or not? I would assume the high FET is damaged but not the lower FET. What do you mean the "linked circuit" to the FETs? Did you measure the resistance from GH1 to GND and GL2 to GND? 

    Regards,

    Brian

  • Hello,

    Both mosfets high side and low side are not damages.Only the gate drive wich fails.

    We have a low impedance between SH2 ans SL2 pins.

    I will tale measurement for GH1 to GND and GL2 to GND.

  • What would you think about the inductance load effect(with applying this test of short circuit) as we are not implementing a freewheeling diode?

    Would you think that a misconfiguartion or registers(with applying this test of short circuit) could lead to this problem ?

    I hope that is reproductible,that i could scope the supply voltage.

  • I don't understand how high current can push supply voltage up?

  • What would you think about the inductance load effect(with applying this test of short circuit) as we are not implementing a freewheeling diode?

    Since you only used a 2 Ohms resistor for the load and no inductance load, then there should be no freewheeling current that need a diode across the load when the upper FET turned off (assuming the driver detect high VDS and turned off the HS FET). 

    Do you have long wires from the power supply to the PVDD pin? As the ABS spec for PVDD is 40v, and if the long wire inductance is large enough to cause voltage spike on top of 13.5v VBATT when the HS FET turned off, the inductor voltage reverses polarity when the HS FET turned off, causing + voltage spike at the PVDD pin.

    Brian

  • Hey Mahfoudh,

    * We launch a cycle with a PWM=100% (Frequency=4Hz) and we apply a short circuit to ground on the signal relied to Pin 21: SH1 of the gate driver.

    According to the functional safety document this should result in HS1 MOSFET being non-operational and the device reporting a VDS fault.  As a class B effect, this should be "No device damage, but loss of functionality".  

    Did you set the VDS_MODE register to the Disabled Mode or Warning Report Only Mode by chance?  I doubt it, but figured I should ask. 

    Is your load a 2Ω resistor for testing, or is it a motor with 2Ω phase resistance? 

    2/-Why is it the circuit linked to the Low Side Mosfet that was destroyed and not the one linked to the HIGH-SIDE Mosfet (It's "strange")?

    I wonder if when the short to ground condition occurs and it makes the High Side MOSFET non-operational, if that causes all the current/voltage to go into the Low Side FET and somehow this results in an overvoltage or reverse voltage/current event on the FET that it can't handle.  Not sure currently, and our more experienced team members are away right now.  

    Regards,

    Jacob

  • Hello Jacob,

    I would think you for your responses.

    Our load is a heating mat with a resitance of 2 ohms and an inductance of 10uH.

    I will check with software team about registers configuration especially the vds reporting fault mode.

    Best regards.

  • Hi Mahfoudh,

    Do you use DRV8705S EVM board or your own design?

    If you use your own board, can you tell if you placed the whole H-bridge ie. 4 Mosfets on board or only 2 Mosfets connected to GH1 and GL2?

    Regards,

    Grzegorz

  • Hello,

    We use our own design in split high side and low side configuration.

    We don't place the whole H-bridge, only 2 N-Mosfet connected to GH1 and GL2.

    Regards,

  • Hi Mahfoudh,

    If there are only two Mosfets then I think the circuit looks a bit like a circuit for avalanche energy test of Mosfets.

    10uH inductance is not a lot but time constant here is L/R = 5us and inductor energy at 6A is 1/2*L*I^2= 180uJ, not enough to destroy power Mosfet with avalanche breakdown but probably enough to destroy signal inputs of DRV8705.

    I think that when LS Mosfet turns off during load current decay (like during OCP fault after SH1-GND short or during normal work) there will be high positive voltage impulse on SH2 that will lead to LS Mosfet avalanche or destruction of DRV8705 SH2 pin. Probably similar situation will be when HS Mosfet turns off causing negative voltage impulse on SH1.

    I would add two small (1-2A) Schottky diodes (replacing two body diodes of 2 missing H-bridge Mosfets), first between SH2 and VBAT (anode to SH2), the second between SH1 and GND (anode to GND). Single diode across load like on picture above may work as well. I would place diodes possibly close to Mosfets and DRV8705 on PCB.

    Regards,

    Grzegorz