MOSFET package issue

Discussion in 'General Electronics Chat' started by chimera, Oct 21, 2011.

  1. chimera

    Thread Starter Member

    Oct 21, 2010
    122
    2
    Hello! I am in the process of developing a stand alone charger for Li battery.

    To keep things simple, I have attached two images. The first picture 'MOSFET' is of FDC638P P-type mosfet.
    The second pciture 'IC' is of a chip connected to the mosfet (circuit configuration shown)

    I have never worked with a MOSFET with 6 pins. Normally, there are only three pins; so henceforth the following post:

    In IC picture, the chip is connected the GATE of the mosfet and the Souce of the in conenced to the power suppy. The circuit is carried on through the DRAIN of the mosfet.

    If you observe the attached image 'MOSFET', there are 6 pins: 4 Drain, 1 Gate and 1 source. The Drain pins are tied together internally.

    So in order for me to develop the circuit in the 'IC' image, do I need to need connect any three DRAIN pins together to one remaining DRAIN pin and then use 'summed up' DRAIN in the circuit as needed?

    Or have I though too much into it and just simply connect one of the DRAIN pins as shown in the IC circuit, since all of the DRAIN pins are connected internally and it should be okay :confused:

    Thanks for looking and sharing your thoughts on the matter!!!
     
  2. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    The extra pins are for cooling. They want to be connected to something so the heat can get out.
     
  3. chimera

    Thread Starter Member

    Oct 21, 2010
    122
    2
    Thanks for the reply. So what you are trying to get at is that I can make do with 1 DRAIN pin. The other three can be left unconnected?

    I will be trying to push 2A through the MOSFET. Should I not connect three DRAIN pins to the remaining 1 DRAIN pin and have the current pass through the other three pins as well?

    Thts just my speculation...
     
  4. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    If the current can get in one pin, it can get out one pin. However, the other 3 pins cost money, so they must have a purpose, like letting the heat out. Connect them if you can.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    The data sheet for the device clearly makes reference to cooling by the drain pins. Normally these would all be soldered to a fairly big copper land. Simply connecting a wire or a thin PCB trace to one pin may encourage overheating.

    http://www.fairchildsemi.com/ds/FD/FDC638P.pdf
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Multiple source or drain pins should be connected together via a wide trace/copper area to reduce trace parasitics as well as to accommodate larger current requirements.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If all of the drain pins are connected together and attached to the pads shown in the datasheet, with 2A current flow and Vgs=-4.5v:
    Rds=39m min (156mW), 12.168°C rise on 1"^2 pad; 24.336°C rise on minimum pad
    Rds=48m max (192mW), 14.976°C rise on 1"^2 pad; 29.952°C rise on minimum pad

    That's with the MOSFET fully turned ON. However, you have not mentioned anything about the rest of your circuit. I have a suspicion that you may be planning on using the MOSFET as a linear regulator. If you attempt that, your MOSFET will be fried to a crisp.

    You also don't mention the type of Lithium battery that you are planning on charging. Plain lithium batteries are primary batteries and cannot be recharged; if you attempt to do so, you risk that the batteries will burst or explode and possibly catch fire.

    Li-Po and Li-Ion (chemically very similar; just differences in contruction) need well-engineered controls/monitoring when charging them, or fire/explosion could result.
    the Battery University website has information on charging Lithium-based batteries:
    http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
     
  8. chimera

    Thread Starter Member

    Oct 21, 2010
    122
    2
    The lithium battery is a secondary battery i.e. its rechargeable. I am developing a typical circuit configuration for charging a single cell battery Li-Ion battery as mentioned in the data sheet for stand alone battery charging IC.

    The mosfets only purpose is to switch a load current of 2A on and off. Thats why I am trying to solve the issue I am having regarding the connection of DRAIN pins in the MOSFET.

    Well, if you look at the diagram, then you can see that the DRAIN pins are connected internally. I was wondering if I need to connect them Externally.


    That makes sense. I made the part in Eagle and have made the exact land pattern as mentioned in the datasheet.



    I might have missed that bit in the datasheet. I will give it one more thorough read. The land pattern I have created/chosen for this mosfet pins, as I replied to #12, is the same as I suggested in the data sheet. I was just thinking about making a copper pour around the mosfet and connecting all the DRAIN pin to that. In that way the heat can dissipate easily. Before I go ahead and do that..

    My only question: Should I connect all the DRAIN pins together externally as well? They are already tied together internally in the MOSFET. And in case I do not need to tie them externally, will one DRAIN pin be enough to deliver 2A out of it.


    Thanks for the help everyone. Hopefully I can get something out of this thread and use it my design.
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    YES connect them externally.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    One drain pin would be enough to carry the current, but the current isn't the problem - it's the dissipation of heat.

    You need to connect ALL of the drain pins to the copper pour; otherwise the thermal rise will be higher than if you did.
     
  11. chimera

    Thread Starter Member

    Oct 21, 2010
    122
    2
    Great responses!! Im going to do it and post back with a screen shot of the copper pour
     
  12. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    I can't imagine why you would even consider designing an elaborate method of laying out copper traces to externally connect to all the drain pins and not have them connected to each other.
     
Loading...