need help on overload cutoff circuit for inverter

Discussion in 'The Projects Forum' started by livayahurd, Oct 12, 2009.

  1. livayahurd

    Thread Starter New Member

    Oct 12, 2009
    16
    0
    Hi,

    I made an inverter/charger of 300W, 230v, Battery - 12V.
    Was working fine. My circuity had auto charging with over voltage cutoff for battery.

    Recently i burnt all the four mosfet. I found out that the inverter was over loaded. So i thought of installing an overload protection circuit. I searched the net and tried to study some commercially available inverter circuit boards.

    I don't understand how they work nor did get any information from the net.
    I cant just copy and use, i want to know how it works.

    I am sure that some of you know how it does. Please be willing to share...

    Thanks in adavnce.
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Are these four mosfets in an H bridge? If so, the controller of these mosfets should be arranged so that the instantaneous current in each transistor is monitored. The ON cycle of each mosfet is cut short if the current goes over a limit.

    Some PWM controller chips have this function built in. If you are controlling using a micro-controller you will need to program this in.

    Normally, only one sensing resistor (and a comparator) is needed in the bottom "tail" of the H-bridge. This has been discussed recently in these forums!
     
  3. livayahurd

    Thread Starter New Member

    Oct 12, 2009
    16
    0
    Mate,
    Am not using micro controller. Pulse is provided to the each pair of mosfet from 4047 cmos IC. The transformer center tapped. As you said to monitor the current to make the overload cutoff circuit, thats what i don't know, like i tried but failed.

    I really need a help on that with some circuit diagram just that section alone. Need to understand that before i proceed.

    Anyway thanks for your reply.
     
  4. livayahurd

    Thread Starter New Member

    Oct 12, 2009
    16
    0
    no one here knows how it works? Kindly give consideration. If its the way i have asked for help , then please accept my apology..

    I am stuck with this..
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    When you post a schematic, we can tell you more.

    Greetings,
    Bertus
     
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    As your inverter takes quite a bit of current as 12V, the simple sensing solutions (like a resistor in the source connection of the lower FET) are not ideal, you will lose significant power.

    I'd try just a magnetic circuit breaker in the 12V supply cable, so any sudden overload disconnects the supply, plus a thermal switch on the heatsink to shut down the drive circuit if you get a gradual overheat due to slight overload or lack of cooling etc.

    The only ways I can think of doing it electronically are either a hall effect current transducer on the supply cable, which will give a voltage proportional to the current without introducing any resistance, or use 'sensefets' in the lower legs of the bridge.
    These have an extra source pin that has a calibrated fraction of the current passing through it to allow measurement of higher currents with low power circuitry. I believe then need some extra active elecronics to give useful results, I can't remember the details.

    Edit: I was thinking H-Bridge, if it's a straight two-fet plus centre tapped transformer setup, ignore the reference to 'lower' fets, it's just the power FETs.
     
  7. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    1,758
    98
    Have you tried a fuse?
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    We did that on one of our power supplies for the military. We put a small transformer (single loop) in series with the converter transformer winding on the FET drive side of the H bridge and used the secondary of that little Xformer to create a voltage that would trip a comparator and shutdown the converter for a time interval then retry after it clocked out. The little Xformer put out pulses proportional to FET current and we had circuitry that would react very quickly if it went too high.

    Current mode cotrollers can do this more easily since they sense the peak current on every switch cycle.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Won't work. Switching converters blow really quickly if they are overloaded because the switch FETs have a lot of current through them and voltages across them. The current limiters have to act very fast to save them.
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    That's actually why we used a small current transformer on that military supply to measure FET current and feed the overload protector: it ran off 12V and put out 800W, so FET currents were very high. Sensing FET current with a small transformer was very low power loss.
     
  11. Thav

    Member

    Oct 13, 2009
    82
    0
    You might be able to get away with sensing voltage across the FETs to get a sense of overcurrent. Since a FET's on resistance should be pretty well described by the datasheet, you may be able to pick a point that's well enough above your current limit trip. To do this you will have to coordinate so you don't trip when the FET is blocking.

    You could use any current limit setting to maybe trigger a monostable that suppresses your controller output for a certain amount of time. Or you could have it trigger a flip flop that suppresses it until you reset manually later.
     
  12. livayahurd

    Thread Starter New Member

    Oct 12, 2009
    16
    0
    i have added 2 MOSFETs on each side. (check the pdf file).
     
  13. livayahurd

    Thread Starter New Member

    Oct 12, 2009
    16
    0
    Fuse did no good. The resistance. Drain-source on state resistance is 0.022 ohm. Mosfet - IRFZ44N. I am asking because i want to keep a circuit so that i can understand how it functions as well..
     
  14. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Deeply worried by the way you are driving your MOSFETS. The MOSFET gate basically needs no current (unlike a bipolar transistor) to switch it on. But what it does have is a large capacitance to the source. To make it switch fast, you need to charge and discharge this current fast. Therefore the driver needs to be able to source and sink current. Your 2N3055 only sources current, there is no way for the charge to be removed from the MOSFET gate. So the MOSFETS will be very slow to switch off - if at all! This is probably why you are blowing things up!

    It is best to use an integrated MOSFET driver chip. This has the necessary "totem pole" output transistors built in. These drivers need capacitors across their supplies - to supply this current.

    I have attached a quick sketch that shows this and a possible way to do a fast overload current limit. Notice that the current is monitored using a series resistor. This will be a non-inductive type, probably a fraction of an ohm. If the current goes over the limit the MOSFET is instantly switched of by reseting a flip-flop. This is set again by the driver at the start of the next ON cycle. If no over-current occurs, the driver resets the flip-flop at the end of the ON cycle.
     
  15. livayahurd

    Thread Starter New Member

    Oct 12, 2009
    16
    0
    thats working.. thanks a lot..
    thanks for resolving that issue with mosfet driver too.. :)

    Before I stop i have one more doubt, before attempting to build inverter on my own.. i used assembled inverter kit.. that board had some circuit with a preset that stops the inverter in overload conditions.. It also uses an opamp.. Its a local manufacturer. According to him the preset (10K circled in green in pdf file) near the LM339 does the work. But i dont see any sensing resistor (pardon my ignorance) or any circuit that senses the current through the mosfet and feeds that information to the micro controller or the opamp. I would like to know how it works. So that i can try it own my own.
     
  16. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Can't see if this has any current limiting. It might use the resistance of a PC board track or a wire as a crude resistor.

    Your circuit diagram, incidentally, is a example of a badly drawn circuit. It is more of a layout diagram. Circuit diagrams should not show components in their actual places but arranged to show how the circuit actually works. Don't show pictures of chips. Multiple devices in an I.C. like the LM339 should have their separate comparators drawn separately.

    I know you didn't draw this - just one of my favourite rants!

    The only way to get to grips with this is probably to redraw this circuit. It might become clearer then.
     
  17. livayahurd

    Thread Starter New Member

    Oct 12, 2009
    16
    0
    Sorry mate, sorry for that trouble

    Well it was scanned from that book which the manufacturer gives away with his inverter kit.

    Do you have any alternative method for overload sensing.
     
  18. Thav

    Member

    Oct 13, 2009
    82
    0
    This schematic is basically what I was referring to earlier about suppressing the PWM output. I would still recommend a monostable or something to suppress the output for longer than one switching cycle, otherwise you will go and hit current limit every time and could end up building a MOSFET oven.
     
Loading...