IGBT or MosFet heater board

Discussion in 'The Projects Forum' started by RobVerm, Apr 7, 2014.

  1. RobVerm

    Thread Starter New Member

    Apr 7, 2014
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    Admitting That my electronics knowledge is basic I really need some help from your community. Here is what I am trying to do.

    What we are doing:
    I need a heater coil fired. It is a pre-heater for an RC Helicopter engine block that we use for aerial reconnaissance during Alaskan wilderness surveys. Extreme cold requires us to preheat the block. We currently us resistance wire wrapped into a low ohm coil 0.05 ohms. We use the battery on the RC to fire the coils manually for about 10 seconds. With a fresh battery we are running @ 4.2 volts so we are drawing ~85 amps. We would like to radio control the heater so we can land the Chopper for extended times and take video without consuming fuel. but we are unable to preheat the block by remote.

    What we have:
    What I have Tried is to connect a manual micro switch to a servo and that works fine to fire a SSR but the SSR we have is bulky and to handle the amps we need also very costly.

    What we need.
    I have been trying to build a 555 timer circuit with a STGW40NC60V IGBT from mouser to fire the coil. Activating it with power to the gate portion of the circuit provided by the micro switch. and I just can't get it to work. Anyone willing to drop a schematic to point me in the right direction would be great. Here is our application and requirements.

    Battery power is provided from an RC Li-Po battery Volt range from 3-4.2 volts it is 4.2 @ takeoff and drops as low as 3 volts after extended flight.

    Coil Resistance is 0.05ohm fired off 4.2 volts so on paper the amp draw is ~84 amps.

    Activation is by on board mechanical servo switch rates @ 500ma.

    we went with the STGW40NC60V for availability and price and rewarped the coil to .1 ohm but want to co back to the out of the box 0.05ohm coil.

    The Ultimate
    I have been told if we use a 556 dual timer we can use 2 IGBT firing 50% cycles alternating and spread the load out. This would be ideal.

    Any help would be great.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Does the onboard battery really have enough reserve capacity to provide this much juice, without risking being stranded?

    FWIW, the internal resistance of your battery is probably limiting your current to far less than the calculated value. You might want to measure the voltage across the coil when it is being fired. I'll bet it's not 4.2V !!
     
  3. richard.cs

    Member

    Mar 3, 2012
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    Electrical heating seems a very poor choice here, like Weyneh I'm suspicious how much power the battery actually delivers, some measurements or some detailed information on the battery would be good.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Generally a MOSFET would be better for this job then an IGBT since an IGBT has a much higher voltage drop when on which will waste battery power (unless you use the engine as a heat-sink for the transistor).

    And, as the others have noted, you load would appear to have way too small a resistance for the battery. You want a high enough load resistance so the battery voltage drop is perhaps less than a volt when the load is connected, otherwise you are dissipating a lot of the power in the battery instead of the load.
     
  5. #12

    Expert

    Nov 30, 2010
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    You could isolate your timers by using a simple 9V battery, or even smaller, a 12 volt the size of a AA battery (part number includes"123"). That removes some of the uncertainty about sufficient gate drive voltage under high loads. Still, any battery can freeze if the ambient temperatures are like what you find in Alaska.

    Attaching your transistor to the engine seems a good way to avoid losing that power in a useless place, but will the engine get hot enough to fry the transistor?
     
  6. wayneh

    Expert

    Sep 9, 2010
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    I have a hunch something like this might give more heat per weight than the coil and battery approach.
     
  7. RobVerm

    Thread Starter New Member

    Apr 7, 2014
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    Thanks for the feedback changing what we have is not viable. It works fine manual 10-15 seconds on the button makes enough heat to pre warm the block oil tank and combustion area.

    The heating system uses Kanthal resistance coils the coils are inserted into our CUSTOM engine blocks. When the 8 coils located around the engine are connected in parallel with resistance is under 0.1Ω sometime as low as 0.05Ω.

    The battery is a 130c li-po high drain. @ 4.2 Volts and even on a low coil build of 0.04 thats only 105 amps. off a 12AH battery that is more than enough to let it park and reheat several times.

    Changing what we have would cost Thousands of $ a new engine block meaning months of field testing.

    Adding an IGBT or Mosfet setup to fire the thing for 10 seconds would cost <$100.

    The Remote switch is already on the RC Choppers its just a simple 1amp relay that was used for a flood light that we do not use and have removed.

    The coils are warped in silica and placed into machined holes in the block. again re engineering cost a lot a bread board a 555 or 2 and a couple MosFets = cheap and it will expand our operational time by conserving fuel meaning we get more data.

    I tryed a simple 555 timing setup with the stated chip and cant get it to work on my own as the schematic I have was not intended for that chip. Just want a solution to making it fire please. No trying to burn some battery to pieces this is a proven good route for us in the manual capacity.
     
  8. RobVerm

    Thread Starter New Member

    Apr 7, 2014
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    Seriously? a $70,000 long range RC Chopper with hand warmers for block heater? How do you suggest we activate these after it is sitting in the middle of a clearing for 12 hours? The coils work with the battery package.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Haha, I had no idea what we were talking about. I was picturing a kid's toy. :eek:

    My point was just that you need energy to heat a block, and chemicals can store more energy than a battery. More BTU per unit of weight or volume.

    Let's get back to switching your giant load. You want to have a circuit hold the coils on for ~10 seconds following the RC signal. This would normally call for a 555 monostable or "one shot" circuit. You say your 555 circuit didn't work. Can you share the schematic? What went wrong?

    If it was me, I'd just parallel a few cheap MOSFETs. (I use IRF540N for generic stuff, and it would take at least 4 of these.) But we should be able to get it working with your IGBT if you already have that.

    Is your highest onboard voltage just the battery 4.2V? That introduces some issues.
     
  10. RobVerm

    Thread Starter New Member

    Apr 7, 2014
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    I am open to whatever will work. if stacking 4 Mosfets is a better option I am game for that. The 555 timer schematic I used was something pulled off the internet.

    The other 2 batteries are the same voltage one is for the radio camera and servos the other is for the starter.

    This is the exact one here.

    Instead of 12+ we had it connected to the 4.2 volts

    Instead of the BD139 we used a STGW40NC60V.

    I omitted the part that has the reversing relay and instead of a motor we have our coils.

    For Vm and the +12vDC both tied to the +of the battery.

    Ground was to the - of the battery

    To the best of my knowledge of electronics the N4001s are dropping voltage to something usable by the 555 so there were omitted.

    I assume that is what this problem was. I know enough to build what is in front of me but changing it to suit what our idea was is a tad past me.

    I also suspect that the Caps may have been wrong for the IGBT we used.

    We are open to using a 2s 8.4v (freshly charged) battery and adjusting the cycle time of the mosfet to ~50% and get the same wattage. that should I assume increase usable battery time since it is 50% cycle?
    Adjusting the R1 should do that for us.

    FYI this was a funded research and Gov. Grant was canceled in budget cuts we are working out here working on our own money now. Fuel cost seams nominal to some but @ $40 a day per chopper (5 of them) shutting down would allow us to take longer videos and save tons of fuel.

    Also the 10 sec timer isn't needed the on time of the relay can be programmed into the remote.







    image url to 555 timer we used
    http://www.next.gr/uploads/3/DC%2BMotor%2BController%2BCircuit%2Bwith%2BNE555.png
     
    Last edited: Apr 7, 2014
  11. wayneh

    Expert

    Sep 9, 2010
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    Aha, so all you need at this point is something to switch the heater on and off?
     
  12. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Sounds like using a few parallel MOSFETS is the way to go, but with a simple boost converter to generate a high enough gate drive voltage (~12v) to ensure that the FETs are driven heavily into conduction, even as the battery voltage sags. If you have another dependable source of 12 V, you could use that to drive the gates too?

    FET's with really low drain-to-source resistance typically need a healthy gate drive voltage to get there.
     
  13. RobVerm

    Thread Starter New Member

    Apr 7, 2014
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    As previously noted we thought it was going to be as easy as a 555 and an IGBT. now we seam to be leaning to stacking FETs

    sounds good but as I said in the OP I can take a schematic and put something together with my basic understanding of components.

    What I am having trouble with is changing what I have available from google to work in my application.

    If someone could toss up a schematic of say.

    Parallel MosFets to = 120-140 amp capacity.
    555 PWM adjustable with POT (or whatever else works.
    Available Power is one LiPo battery full charge 4.2v its cutout is 3.0 volts
    Load being switched is 0.04-0.05Ω Heater coil bank.

    If a boost converter would be needed a dual 555 chip would work to drive part of the boost and the PWM yes?

    I have nothing really to offer for payment or compensation other than a note or sticker with your name on the chopper and knowing you are helping local wildlife studies lol.
     
  14. richard.cs

    Member

    Mar 3, 2012
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    Well that sounds rather do-able, probably just the 555 and 2-diodes PWM circuit and since the gate drive needed is almost zero current the gate drive can just be a multiplier off of the PWM clock. When I get a chance I'll throw something together.
     
  15. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Here is a rough idea of what I was thinking:

    It's a boost converter that generates a higher gate voltage with a simple inductive flyback converter, the charge stored on the gate is replenished periodically by pulses from the oscillator to keep the gate fully on.
    The zener diode dissipates the excess energy once the gate is charged.
    When you shut it off, another transistor shunts the charge to ground and holds it solidly there.

    I looked at some MOSFET data sheets and the gate capacitance is MASSIVE for the really high current ones, like 4000 pf each! If this heater needs PWM to reduce the power, you could do it at a slow rate at the input of the circuit, I am sure the thermal time constant of the engine block is more than a few seconds, a 20 Hz PWM would work fine.

    Anybody good at math want to take a whack at component values?
     
    Last edited: Apr 8, 2014
  16. richard.cs

    Member

    Mar 3, 2012
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    Rough schematic attached. 555 with diodes gives low frequency (1 kHz ish) PWM, 10% to 90% or so by twiddling the 100k pot represented by R7 and R8. The same oscillator drives the multiplier for the gate drive. Q1 level shifts, Q2 and Q3 are the gate driver and Q4 with D9 inhibit gate drive until the multiplied supply is ready (~10 ms after switch on) to prevent excessive mosfet heating.

    The IRFH6200 is a surface mount mosfet good for 100A Id if you can keep the case below 100C.
     
  17. RobVerm

    Thread Starter New Member

    Apr 7, 2014
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    I know you said rough but after downloading I cant read some of the component values.

    Is it really that simple? I feel inadequate. So you can just add how ever many MosFets by adding a resistor to the gate. woh.

    As far as the IRFH6200 goes the price tag will kill me...................:D
    and as far as keping it under 212F that should be ok but would the load not be distributed over all 4 making heat distribution somewhat even?

    I think V_Bat is to the battery + where are the coils connecting? prior to V_Bat?

    90% Duty cycle does that mean we loose our top end 4.2 volts to 3.78?
     
    Last edited: Apr 8, 2014
  18. richard.cs

    Member

    Mar 3, 2012
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    I hadn't realised it had come out so small, I've attached a better copy.

    The MOSFET was largely chosen because it's beefy and in the LTSpice library, there are many, many others that will do the job. The MOSFETs look resistive D-S so they naturally current share and each takes a quarter of the total current. The gate restor is only there to prevent HF oscillations of the gate capacitance with the inductance of the driving PCB trace.

    Few of the components are critical, part numbers indicate the model I happened to simulate with but the transistors and schottky diodes can be pretty much any general purpose type, the zener around 12V. The 555 should be a bipolar type not a CMOS one as it has to deliver quite large current peaks into the multiplier.

    The 50 mΩ load is connected between the MOSFET drains and the battery positive.
     
  19. wayneh

    Expert

    Sep 9, 2010
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    It's one of the reasons MOSFETs are loved for this sort of application. You will sometimes see a low ohms resistor (0.1Ω, for instance) in series with the load for each MOSFET, to help ensure that the load balances across the MOSFETs. It's not always required though, because the resistance of the MOSFET will naturally rise with heat. So if one MOSFET is hogging the current, it'll get hot and resist more than the others, shifting the load.
     
  20. RobVerm

    Thread Starter New Member

    Apr 7, 2014
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    So what is R5 doing exactly is that a simulated load where the coils would be? if you want to sujest a popper 555 from mouser.com I am looking @
    Mouser Part #: 595-NE555P or Mouser Part #: 595-NE555D.
     
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