Burst Power Application

Discussion in 'The Projects Forum' started by FardinBiper, Aug 21, 2012.

  1. FardinBiper

    Thread Starter New Member

    Aug 21, 2012

    I am working on a project that needs to meet these specs:
    - Supply is an automotive lead acid battery, voltage range 11 - 14 V
    - Need to drive 8 resistor based components (0.9 Ohm) simultaneously. When these resistors are put in parallel, they require minimum 112A for 100ms duration. This current pulse can repeat at 1Hz frequency.
    - MOSFET has already been chosen: SQM120N02-1m3L manufacturer is Vishay.
    - We are using 4 of these MOSFETs in parallel so that each can drive 2 resistor component which need approximately 30 A. (112/4)
    - a driver is needed to drive these 4 MOSFETs simultaneously.
    - If we were to draw this current from the battery it would be really bad for the battery, so other energy storage options have been looked at such as Supercaps. The problem here is we have limitation in space. The maximum we have is a cylindrical space D 19mm * H 34mm.
    - Ideally we like to have a supercap that fits in that space, rated at 20V and 0.25 Farads capacitance.

    Overall, what I am asking for help is a suitable capacitor and a driver for MOSFETS.

    Thank you,
  2. DMahalko

    AAC Fanatic!

    Oct 5, 2008
  3. cork_ie

    Active Member

    Oct 8, 2011
    Is the load purely resistive i.e. bulbs, or inductive ie solenoids, or a mix ?

    Is the resistance Ohmic?

    0.9Ω @12V = 13.33A x 8= 106.67A

    @10% duty cycle ≈ 128 Watt/second ,

    At that kind of heat your resistance is hardly likely to remain steady unless you have some kind of serious heatsink.
  4. FardinBiper

    Thread Starter New Member

    Aug 21, 2012
    Hi and thanks for the replies,

    The resistance is purely ohmic, and it can handle that much power. So there is no need for a heat sink.
    Your calculations are correct, however each of the 8 resistance based components have 14 A running through them at 12 VDC, which means 168 W for each of them. In total they will consume 1344 W.
    The point of this question is to design a compact driver to supply this energy to these 8 component.

  5. cork_ie

    Active Member

    Oct 8, 2011
    Circuit design is not really my thing but here is where I would start.

    Basically there are two distinct areas in your circuit.

    1) Continuous charging of the capacitors from the battery,
    2) Firing of the main Mosfets while simultaneously cutting (or reducing) the current from the battery to the capacitor

    I would examine these 2 options. I am sure that others know a lot more than I do and probably find a better way.

    1a) If the capacitor is charged via a mosfet with a gate bias so it is normally switched on. This will charge the capacitor.

    2a) Use a standard 555 timer circuit in astable mode. You will need to do the calcs for the frequency and duty cycle you require. You can use the output signal to drive both the gate signal of your main mosfets and a small transistor to pull the gate of charging mosfet to ground - shutting it off during the discharge cycle.

    The main problem I see is the charging mosfet is going to be about 15A rating and needs to be fitted and heatsinked in your very limited space.