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LM2104: Designing with LM2104

Sv
Intellectual 320 points
Part Number: LM2104
Other Parts Discussed in Thread: DRV8245-Q1, DRV8873,

Hello Team,
We want to design an H Bridge circuit to drive a DC Motor.
The motor is a 12V DC motor with a maximum current of 2A.
The inrush can be up to 10A but for only few micro seconds.

My confusion is with the back emf of the DC Motor.
Can you please clarify the following doubts I am having.

1). Is this the discharge path of the back emf.
 
That is from the motor to the high side FET and back to the motor through the power supply and the low side FET at the opposite side?.
2). Can I use a bidirectional TVS diode with a breakdown voltage greater than 15V across the Motor leads to clamp the reverse voltage due to back emf?.
3). What will happen/ or how will the motor discharge if the power supply is removed abruptly?.
4). If the power is valid throughout the operation, the MOSFETs Q2 and Q3 will act as the discharge path for the motor and we don't need any control over then since they are just diodes.
If such a simple approach is available, then why should we go with some complex decay modes mentioned in the application note?.
5). The device we are planning is a test system. One of the test will be this motor driving.
So we are planning to place the DC motor in a separate PCB (like a test jig PCB) to which our PCB under test (having H Bridge) will be connected via relays. This will greatly reduce the wiring time for testing.
When the power to the test jig PCB is disturbed, the relay will disconnect while in the middle of the motor operation. So there is no place to discharge the back emf.
Can I use a bidirectional TVS diode with a breakdown voltage greater than 15V across the Motor leads to clamp the reverse voltage due to back emf in this scenario?.
I have observed some scenarios in our dotted PCB like the MOSEFTs in the H Bridge got damaged when the power to the relay is disconnected and reestablished again. What could be the reason for this? 
6). What will happen to a motor if a safe discharge path is not given?


Looking for your reply

  • 0
    TI__Mastermind 21575 points

    Hi SV,

    Is there a particular reason LM2104 was referenced? It is a half-bridge gate driver. For full-bridge options we have several devices in our portfolio for BDC (brushed DC) motor applications. If the application is industrial you can consider the DRV8873 which has both hardware control H and SPI control S options. For automotive we have the DRV8245-Q1 and family of devices with S and H options that would be a good fit do 12V BDC motor. These devices have integrated MOSFETs for the H-bridge.

    1.  Typically in BDC H-bridge drive the BEMF will be dissipated as recirculation current in slow decay. For example the DRV8245-Q1 does a high side recirculation for slow decay as well as brake or motor stop. The path you described will happen if the H-bridge is Hi-Z to stop the motor while it is running at full speed. The BEMF could be higher than the VM motor supply resulting in pumping back to the power supply sometimes increasing it momentarily to a level that'd damage the FETs. I'll show a better image from a related application note, https://www.ti.com/lit/an/slva321a/slva321a.pdf.

     

    The above image describes BDC in one direction. The first image is the BDC drive current path for this direction. During the dead-time (while bridge is reversed) or bridge Hi-Z current path in the second image happens. This is similar to you described. Instead of 2 if you use slow decay or brake to stop the motor the current due to BEMF will flow in the FETs and not reach the power supply. The MOSFETs should be capable of handling this current which would be similar in profile amplitude and time duration of the inrush during startup, especially when the motor is instantly stopped this way. To avoid an over current situation a PWM ramp up can mitigate startup inrush and PWM ramp down can mitigate brake current.

    The modern integrated H-bridges I highlighted in the first paragraph have bridge control logic to support this slow decay brake. I hope this helps. 

    Regards, Murugavel

  • 0 in reply to Murugavel4637
    Intellectual 320 points

    Hello Murugavel,
    Thank you for your reply.
    Sorry for replying late.


    Can you please comment on the following question I am having?.

    1). The device we are planning is a test system. One of the test will be this motor driving.
    So we are planning to place the DC motor in a separate PCB (like a test jig PCB) to which our PCB under test (having H Bridge) will be connected via relays. This will greatly reduce the wiring time for testing.
    When the power to the test jig PCB is disturbed, the relay will disconnect while in the middle of the motor operation. So there is no place to discharge the back emf.
    Can I use a bidirectional TVS diode across the motor with a breakdown voltage greater than 15V across the Motor leads to clamp the reverse voltage due to back emf in this scenario?.

    Looking for your reply

  • 0 in reply to Sv
    TI__Mastermind 21575 points

    Hello SV,

    "Can I use a bidirectional TVS diode across the motor with a breakdown voltage greater than 15V across the Motor leads to clamp the reverse voltage due to back emf in this scenario?.". Yes you can do this. Be sure to size the TVS diode to absorb the current without getting damaged. Typically the peak current will be equal to the stall current of the motor for several milliseconds.

    Regards, Murugavel