Several H bridges on one PCB

Discussion in 'General Electronics Chat' started by secretstep, Aug 19, 2008.

  1. secretstep

    Thread Starter Member

    Jun 27, 2008
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    I want to supply 3V motors with maximum 300mA (that's the stall current).
    That is, the power = 0.9W.

    The H-bridge can supply 0.4A (average) and dissipates 0.95W when mounted on 50mm*50mm*1.6mm PCB board with 30% of copper area.

    The power supply in the lab can draw maximum 3A, so I'm wondering how it will work on 15 MOTORS when the motors are to connected to each H-bridges on one PCB of 50mm*50mm area and 1.6mm thickness.

    1. What does it mean by 30% of copper area on PCB?
    2. Do the PCB over heat if there are several H-bridges (each dissipating 0.9W) on one PCB with 50mm*50mm*1.6mm?
    3. Since there's nothing such as 'identical', would some H-bridges have more current flowing through it than other H-bridges, making some motors to run around 0.4A and burning them up?

    thanks to all of you who can help me with the questions above.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    That's 14.25W to be dissipated by a 2"x2" board. There is not enough surface area to dissipate that much heat. Besides, you would be very hard-pressed to find an H-bridge IC small enough to fit 15 of them in that size of an area, along with routing all of their signals in and out.

    It would help to have a datasheet for the H-bridge that you are considering.
    This is where the datasheet would come in handy. But a good guess would be that the manufacturer of the IC wants to ensure that there is sufficient copper left around the IC to draw heat away.
    With that kind of power in such a small area, yes.
    It depends on the H-bridge driver, and how you have it set up. You can use a sense resistor between the source of the low side and ground to measure the current. 300mA across 1 Ohm = 0.3 Volts. If the voltage rises above 0.3V, turn the lower half of the H-bridge off momentarily, and then turn it back on until you again measure 0.3V.
     
  3. secretstep

    Thread Starter Member

    Jun 27, 2008
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    Thank you for very kind reply.
    The H-bridge I want to use is TA8409SG.
    here, I attach its data sheet.

    The size is around 6mm*6mm.

    Can you assume how many of these can be mounted on 50mm*50mm*1.6mm PCB without overheating (and why if possible)?

    Do you think using a heatsink would help mounting more of them on a single PCB?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    The TA8409S/SG is the SIP model, which is (roughly) 22.7mm x3.4mm.
    The TA8409F/FG is the SMT version, which is (roughly) 6mm x6mm.
    The SMT version can handle about 70% of the power of the SIP model with no heatsink.

    I would definitely suggest using a heatsink. With a board that small, you might use a heatsink with a fan that was designed for a PC with a Pentium processor. Use heat sink compound, and run the fan at a moderate speed.
     
  5. secretstep

    Thread Starter Member

    Jun 27, 2008
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    ah, I was misunderstanding.
    I thought the SMT version is the SIP model.

    I was looking into the other H-bridges, since the TA8409SG can have only 0.75W of power.
    The ideal one I found, had four outputs, and in its data sheet, it says that it can dissipate 1.5W. Does it mean that it can dissipate 1.5W for each motor or for overall circuit?

    and whatever the case it is, we need a heatsink compound with a fan since it is dissipating such a high power for a small board, right?

    I'll attach the data sheet just in case.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    You would've discovered that the moment you opened up your order. ;)

    Total per package. That's at 25°C, or room temperature. If the temp of the IC goes up, that's out the window.

    Yep.

    Good move. Life would be so much easier if there was just documentation on it.
     
  7. secretstep

    Thread Starter Member

    Jun 27, 2008
    17
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    I am having a serious issue about this H-bridge.

    The prof from mechanical department is insisting that there will be small power dissipation in H-bridges using the formula,
    power in = power dissipate on H-bridge + power to run motor.

    So if we have 3V and 300mA of input, and the motor runs at 3V and 300mA, then we have no power dissipation on H-bridge? (which leads to no need of heat sinks or whatever on PCB)

    Then does it mean that even if H-bridge max. power dissipation is 0.5W, if it is 4 channel H-bridge, then we can run four motors that consumes 1W each?

    I thought the input power is going through the H-bridge, so H-bridge handles that much power to transfer the power out to the motor.

    Please let me know what you think.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    There will be some power dissipation in the H-bridge. How much depends upon the resistance of the H-bridge vs the resistance of the motor; or actually what the power due to the voltage drop across the H-bridge will be.

    The Rds(on)=1.2 Ohms @ 25°C; I'm assuming that's per side - so 2.4 Ohms total.

    So, 300mW across 2.4 Ohms = 848.5mV. If your total voltage drop amounts to that (or about 424mV/side) you'll be dissipating 300mW in your driver. I don't know what kind of load you're planning on putting on your motors. Fast start is a heavy load. Keeping them loaded at or near stall is a very heavy load.

    However, if you will have no load on the motors, and will not accelerate them rapidly, they will have a light load.

    Note that you must use diodes on the bridge to protect it from the motors' reverse EMF.
     
  9. scubasteve_911

    Senior Member

    Dec 27, 2007
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    This is unacceptable. Advertise elsewhere.
     
    Last edited by a moderator: Nov 24, 2008
  10. markm

    Member

    Nov 11, 2008
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    You also need to consider duty cycle. A robot arm might have most of it's motors running at the same time for an extended period, but in many other applications, you'll occasionally turn on just one or two of a large set of motors, so your peak power is much lower, and you also have cool-down time in between the peaks. An example of the latter usage that I'm pretty familiar with is ATMs (automated teller machines), which have 6 or more motors for shuffling bills, envelopes, and other pieces of paper around, but only turn on one path at a time and also spend most of their time just waiting for the customer. You can stuff those motor drivers as closely together as board routing allows - but even so, it took a lot more than 4 square inches of space. (This was with fairly hefty motors that required discrete power MOSFET's.)

    The OP seemed to be looking at something more like robotics (3A P/S for fifteen 0.3A motors = running ten at a time?). He'll need extra real estate, and heat sinks and a fan might be necessary.
     
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