Boost converter

Discussion in 'General Electronics Chat' started by jackorocko, Jan 23, 2012.

  1. jackorocko

    jackorocko VIP Member

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    Is it impossible to think that I could use a 12V 50A max car battery charger transformer and make a 45V boost converter to run some steppers with a current draw of 2A per motor. total of 6A.

    I want something simple and I already have this transformer, so even though it is not that efficient of a design. It fits my budget for this redneck project I am brainstorming.

    Something like this, at least from that calculator on that page, it seems very plausible, as long as I can get a 555 or similar device to produce the PWM input.
    http://www.ladyada.net/library/diyboostcalc.html
     
    jackorocko, Jan 23, 2012
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  2. jackorocko

    OLIVE2222 VIP Member

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    Hi jackorocko, for almost the same price the MC34063 can be better at the task.
    He offer a regulation and a current limiting. Just need an external FET, as with the 555 anyway.

    Olivier
     
    OLIVE2222, Jan 23, 2012
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  3. jackorocko

    jackorocko VIP Member

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    1.5A output is not nearly enough output current. I need at the very min 6A
     
    jackorocko, Jan 23, 2012
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  4. jackorocko

    BobK VIP Member

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    That is why Oliver said you need an external (MOS)FET.

    Bob
     
    BobK, Jan 23, 2012
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  5. jackorocko

    jackorocko VIP Member

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    I think I get it now. You are saying to add in a MOSFET, the gate to the switch collector of the output transistor of the 34063 and then the source to ground and the drain to the inductor where the switch collector was previously attached?

    I will whip a schematic up sometime and see if I am on track.
     
    jackorocko, Jan 23, 2012
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  6. jackorocko

    jackorocko VIP Member

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    Ok, so I went over the datasheet for the mc34063. They have a schematic in there for an external NPN transistor. I assume I can just use this design and replace it with a MOSFET.

    I went through the formula table and figured out all my values. This is what I got, did I manage to do it right? If the below information is correct then that would mean I need a MOSFET that can handle 45V @ 42.25A D-S? I am also thinking I should find one with a low gate capacitance to make sure I don't slow down my switching freq. Any other suggestions?

    Ton/Toff = 2.25uS
    Ton + Toff = 10uS (max freq)
    Toff = 3.07uS
    Ton = 6.93uS
    Ct = 277pF
    Ipk = 42.25A
    Rsc = 0.007Ohms
    Lmin = 2.2uH
    Co = 1600uF
     
    jackorocko, Jan 24, 2012
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  7. jackorocko

    BobK VIP Member

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    Actually, I don't think that chip is good for using an external MOSFET. A high power MOSFET needs a push-pull driver so that it can both charge and discharge the gate capacitance quickly. You might want ot look for other boost converter controllers that are designed for an external MOSFET switch.

    Bob
     
    BobK, Jan 24, 2012
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  8. jackorocko

    jackorocko VIP Member

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    Thanks bob, I took a quick look on digikey. I found something that has a gate driver, but the specs look to low for what I need. Maybe I am interpreting it wrong?

    I think this one or the max773 will work for me. I am going through the datasheet now. The max773 datasheet has a schematic for a 100v@100mA output shunt regulated design. I am trying to read through the design procedure to see if it will work. I was confused when it said in the datasheet output power to load 24W(max1771) and 15W (max773), but I guess my load in this case is the MOSFET gate? How does one go about figuring out that?
    http://search.digikey.com/us/en/products/MAX1771CPA+/MAX1771CPA+-ND/1130157
    This one looks more promising, but it has an open collector transistor as the output. I don't think this is what they consider a gate driver, is it? They show in the schematic a MOSFET attached to the output though.
    http://search.digikey.com/us/en/products/TL5001AIP/296-10203-5-ND/380448
     
    Last edited: Jan 24, 2012
    jackorocko, Jan 24, 2012
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  9. jackorocko

    jackorocko VIP Member

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    This is what I come up with. But I have some serious doubts to whether this will work.

    Vcc = 14 - 16V transformer is 12VAC @ 50A

    1. Datasheet says to set Rsense by the graphs, well none of the graphs are even close to the range I will be using. I used the 1R resistor in the schematic on PG 12 FIG 3e.

    2. Input and output capacitors. Again I just took a guess at these. No idea how to figure the values out and the datasheet is no help.

    3. Diode? According to the datasheet I might be better off to look for a high speed silicon diode instead of a shottky. Any suggestions?

    4. MOSFET I chose was, Vds 60V @ 30A No idea, just took another wild guess.
    http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00003405.pdf

    5. The inductor I picked with a ILim of 30A.

    6. How do I know what my output current will be?

    We can start with the above, I might have more questions.
     

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    Last edited: Jan 25, 2012
    jackorocko, Jan 25, 2012
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  10. jackorocko

    OLIVE2222 VIP Member

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    Cheap FET driver are available from microchip
    http://search.digikey.com/us/en/products/TC1427CPA/TC1427CPA-ND/267372 to be used in combination with the MC34063.

    Here under a practical design,with technical highlights, based on a UC3843, same generation as the MC34063, i.e. not optimized for FET driving.


    http://www.labkit.ru/userfiles/file...shm/95-wat_Laptop_PSU_12V_in_-_19V_5A_out.pdf


    Many others components (or components combination's) can be used as you already find out. More recent components use higher frequencies (in the Mhz range) allowing less bulky inductors and caps.
    Unfortunately they tend to be squeezed in harder to handle SMD packages.

    regarding your design the MAX773 is scheduled to be discontinued (not an issue at hobby level however) also the 1 ohms shunt seems quite high they probably need to be in the tenth of mOhms range with 6Amps

    Olivier
     
    OLIVE2222, Jan 25, 2012
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  11. jackorocko

    jackorocko VIP Member

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    You trying to tell me that the UC3843 is for FET driving just not optimized driving? The datasheet says

    sry, just a little confused on what exactly is an optimized driver. I guess that FET driver chip you linked to could be used in between any converter IC and a FET to give me the needed drive current?
     
    jackorocko, Jan 25, 2012
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  12. jackorocko

    OLIVE2222 VIP Member

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    You right jackoroko the UC3843 with the totem pole output stage is well adapted to drive MOSFET compared to the MC34063 open collector output stage. An "optimized" driver must be able to fastly drive the gate despite it's represent a capacitive load.

    Olivier
     
    OLIVE2222, Jan 25, 2012
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