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DRV8714H-Q1EVM: Fast decay reverse MOSFET conduction

Radu Drosescu
Prodigy 20 points
Part Number: DRV8714H-Q1EVM
Other Parts Discussed in Thread: CSD18512Q5B

Hello Team,

In Fig.2-1 c of Appnote “Current Recirculation and Decay Modes“ (slva321a) the N MOSFET transistors AL and BH are assumed to conduct in reverse mode (source to drain direction) in so-called fast decay mode. I used DRV8714H-Q1EVM and a DC motor in tests. In the CSD18512Q5B external transistors datasheet the only reference to reverse conduction is Fig.9 (Typical Diode Forward Voltage). No other reference justifies reverse conduction.

My questions are:

  1. can a N MOSFET (enhanced) conduct inverse, from source to drain? If so, why the reverse conduction is not detailed by characteristics like those specific to direct conduction found in the datasheet?
  2. Why power switches capable of bidirectional current flow are made with two MOSFET transistors connected back-to-back (see slva948.pdf from TI) when a single transistor with both types of conduction would solve the problem?
  3. I tested a DC brush motor using the DRV8714H-Q1EVM and the DRV87xx EVM GUI. The fast decay mode (transistors AL and BH) is not programmed in either PH/EN or PWM mode. The decay current can follow one of the lower or upper recirculation paths corresponding to the passive or active freewheeling mode. I have attached the oscillogram for synchronous low side decay (forward). Indeed, the transistor Q8 active in recirculation is operated with the voltage VGL1 of approx. 11V. However, the current measured with a current probe does not change in form or value when GL1 is disabled. This fact demonstrates conduction only by freewheeling diodes. How can this fact be explained?

Thanks!

Regards,

Radu

  • 0
    TI__Genius 13325 points

    Hi Radu,

    Give me 24 hours to look into this and get back to you.

    best,

    Keerthi

  • 0 in reply to Keerthivasan Kumanan
    TI__Genius 13325 points

    Hi Radu, 

    1) Conduction can occur from source to drain or from drain to source in a N channel device as long as the voltage on the gate is greater than the voltage on the source. Just to clarify are you asking why the three isn't a specification for how much current can flow from the source to drain.

    2) Yes, a single N channel can be a BPS:

    The back-to-back configuration as mentioned in the application note gives you symmetrical off-state blocking. I will check with other team members on the other tradeoffs.

    3) What is the time scale on the oscilloscope capture? Do you know the inductance of the motor and the RDSON of the FETs you are using?

    Best,

    Keerthi 

  • 0 in reply to Keerthivasan Kumanan
    Prodigy 20 points
    Helo Keerthi,
    I repeat the previous message sent for which I received an error message "Delivery incomplete" probably due to the too large size of the attached figures
    First of all  excuse my late reply but I was out of the country on an Erasmus program.
    Indeed, there is the possibility of conduction in both directions only when the body and the source are not linked. Remember that the gate of a MOS device is linked to the source, channel, and drain through an insulating layer, while the substrate is linked to the source , channel, and drain through a semiconductor depletion layer. These two types of links will have different behaviors with respect to the voltages across the boundaries. Essentially, the gate will act as a MOS gate terminal, while the substrate will act as an NPN base terminal. When the source and the body are connected this dual conduction is not possible or rather feasible.
    The MOSFET is the TI CSD18512Q5B and the board I used in the tests is the DRV8714.
    The signals from the oscillograms are noted on the electrical diagrams  (attached files) slow decay. In the tests I used injectors and a high pressure injection pump (so only solenoids) connected in H-bridge. Regardless of the programming mode PH/EN or PWM , Forward or Reverse the circuit called Fast Decay in Fig.2-1c of slva321a. pdf is not active, nor can it be programmed. Only the slow decay circuit is used in the process of dispersing the energy from the coil and involves the transistors from the low side or those from the high side as in the two electrical diagramme. Reverse conduction should occur through the blue transistor, but in reality the current only flows through the body diode. By activating or deactivating the gate of this transistor VGL1 the current through the solenoid coil (blue CH2 signal) does not undergo any change which denotes a conduction only through the diode.
    In addition, the source is connected to ground, so it has a potential of 0V. According to what you said, there should be a negative voltage on the drain, that is, on the OUT1 terminal. But the negative voltage does not appear on the oscilloscope in any test.
    Finally, in the data sheet of the transistor mentioned above (https://www.ti.com/product/CSD18532Q5B), the only characteristic of reverse circulation (source to drain) is that of the built-in diode.
    Thanks!
    Best regards,
    Radu
  • 0 in reply to Radu Drosescu
    TI__Genius 13325 points

    Hi Radu,

    Thank you for your response. Give me 24 hours to look into this and respond back to you.

    best,

    Keerthi

  • 0 in reply to Keerthivasan Kumanan
    Prodigy 20 points

    Hello Keerthi,

    TI makes recognized MOSFET transistors so they also have specialists who can answer my questions. This was also my intention to have a competent opinion. Well, more than a month has already passed and I still haven't received a clear and well-reasoned answer. Is there any point in waiting?

    Thanks!

    Radu

  • 0 in reply to Radu Drosescu
    TI__Genius 13325 points

    Hi Radu,

    I apologize for the delayed response. I will recreate the same setup on my end and will update you tomorrow. 

    Best,

    Keerthi

  • +1 in reply to Keerthivasan Kumanan
    TI__Genius 13325 points

    Hi Radu,

    You are correct, after a certain point the diode is the one that is conducting the current. If you look at the equivalent model as follows then you can see that once the current through the channel reaches this value Vf/ Rdson, the parasitic body diode will start conducting and any increase in current after that point will flow through the body diode (the current through the channel will stay fixed:

    Best,

    Keerthi

  • +1 in reply to Keerthivasan Kumanan
    Prodigy 20 points

    Hi Keerthi,

    Thanks for this reply and I consider the problem solved. This case can be closed.
    All the best!

    Radu