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DRV8876: Drives VFD Filament with H-bridge motor driver

KSHoe
Prodigy 80 points
Part Number: DRV8876
Other Parts Discussed in Thread: , DRV8220, DRV8870

I apologize if this is not the right page for this question, but I couldn't find a relevant forum for my topic. 

I am tasked to redesign my current VFD filament driver that has been obsoleted, and I think either the DRV8876/DRV8876N or the new DRV8220 might serve the purpose. But I need advice on a few things.

Before that, let me brief you the required specification of my VFD. My board don't have access to the center tap of the transformer so I can only drive it with bipolar pulses. I think a H-bridge driver is capable of doing this for me. The filament supply will be generated from the on-board 12V supply to create a alternating pulse drive with 22% duty cycle at 10kHz. The waveform of the pulse will be provided by a 3.3V MCU to this driver. IN+ will be a 10kHz D=22% pulse and IN- is the same pulse but with 50us delay.

This should give me roughly 6Vrms to turn on the filament of the VFD. The peak current is around 450mA and the average output current is abt 300mArms.

From here, I have a few thing that need to be addressed and I hope you can help me with this.

1. My VFD requires Ek = 0.8VDC for proper filament bias. I think I could connect a forward bias diode to the PGND of DRV8876 to create this Ek. Can it be done this way?

2. There is a few functionality that I do not need from DRV8876, and I found your new DRV8220 which I think can be used. However, DRV8220 does not have a PGND that I can use to create Ek. Can I connect the the diode to GND pin of the DRV8220 instead?

3. When can I expect DRV8220 to begin mass produce and widely available?

4. Will there be any issue if I were to use a motor driver to drive my filament? If yes please let me know. I would also appreciate if you could introduce some alternative for me.

  • 0
    TI__Mastermind 37683 points

    Hi Kok Soon Hoe,

    Thank you for posting to the forum.

    I am personally not familiar with VFD so this is an interesting application for our motor drivers. If you need bipolar control, then both of these devices will allow for bidirectional control of the current. Your control configuration will also be supported by these two devices. Please see below for the answer to your questions:

    KSHoe said:
    1. My VFD requires Ek = 0.8VDC for proper filament bias. I think I could connect a forward bias diode to the PGND of DRV8876 to create this Ek. Can it be done this way?

    Can you clarify what you mean by Ek=0.8VDC, is this the minimum voltage needed? In general, adding a diode between the PGND (device GND) and the board GND, will create a voltage offset on the PGND. The control inputs (IN1 & IN2) of these devices are with respect to PGND so this offset can create issues. As you can see in the figure below,  a sense resistor, or a diode in your case, will create an offset on the GND pin of the device. So when IN1 or IN2 is LOW, the voltage of the INx pin will be negative with respect to device GND. The DRV8220 has a minimum absolute rating of -0.5V for IN1 and IN2. Adding a 0.8VDC forward vias diode will be outside the operating range of this device.

    If you wish to evaluate the DRV8220, you can get the DRV8220 evaluation board. The evaluation board has a 0Ω placeholder sense resistor between the device GND and board GND. You can remove this resistor and place a diode to evaluate your application with the DRV8220.

    KSHoe said:
    2. There is a few functionality that I do not need from DRV8876, and I found your new DRV8220 which I think can be used. However, DRV8220 does not have a PGND that I can use to create Ek. Can I connect the the diode to GND pin of the DRV8220 instead?

    As you can see in the above image, It is possible to place the diode on the GND pin of the device and the board GND. However, you have to make sure the GND offset is below 0.5V (for the DRV8220) and 0.3V (for the DRV8876).

    KSHoe said:
    3. When can I expect DRV8220 to begin mass produce and widely available?

    This device is in preview at the moment but we expect it to release to market by second half of this year. However, this date could change. Please check the evaluation module if you wish to evaluate the device. Also check the ti website within the next few weeks to check when the device is released.

    KSHoe said:
    4. Will there be any issue if I were to use a motor driver to drive my filament? If yes please let me know. I would also appreciate if you could introduce some alternative for me

    As I said, I am not very familiar with filaments. Please give me a day to do my research and determine if there are any issues using a motor driver for this application. Expect an answer by 3/21 or earlier.

  • 0 in reply to Pablo Armet
    TI__Mastermind 37683 points

    Hi,

    I want to correct my following statement:

    Pablo Armet said:
    Also check the ti website within the next few weeks to check when the device is released.

    I meant to say, check the website within the next few months not weeks.

    I apologize for that.

  • 0 in reply to Pablo Armet
    TI__Mastermind 37683 points

    Hi,

    I was looking at some of our other motor drivers and I think the DRV8876 and DRV8220 might not be suitable for this application because of the GND offset that I described.

    The DRV8870 will be a good alternative device. this device has a dedicated high current GND pin which is typically used to connect a sense resistor for current regulation and sensing. However, you can connect a diode here to create the 0.8VDC offset you need. The only drawback is that current sensing/regulation will not be possible. If you don't require current regulation, then this might work. The DRV8870 ISEN pin has an absolute maximum rating of 1V so it is possible to add a 0.8VDC diode.

    Let me know if you have any questions about this part.

  • 0 in reply to Pablo Armet
    Prodigy 80 points

    Hi,

    Thanks for the recommendation. The DRV8870 does look like a better fit for my work. 

    I would like to order the evaluation module. Who can I talk to via email on other stuff for quotation for the eval module and component pricing?

  • 0 in reply to KSHoe
    TI__Mastermind 37683 points

    Hi Kok Soon Hoe,

    You can send me a private message by hovering over my user name and clicking "send private message". We can continue this conversation privately.