Connect with us

Big DC to DC converter toroid?

Discussion in 'Electronic Design' started by BobG, Jan 31, 2007.

Scroll to continue with content
  1. BobG

    BobG Guest

    If I wanted to build a 15KHz 15KW DC to DC converter to hump 48 or 96V
    at a couple hundred amps up two or three times to run a motor, could
    someone tell me about how big the cross section of a ferrite toroid
    would have to be? Gauss? All I know so far is magnetics get smaller
    with increasing freq, and core cross section has to do with power....
     
  2. Terry Given

    Terry Given Guest

    a couple of things:

    - you have several different bits of magnetics. A transformer, and
    (it'll be buck derived) some form of choke (which will be on the HV side
    no doubt).

    - the transformer cross-section is chosen to keep the peak flux density
    below saturation, then low enough to keep losses down. At 15kHz, you
    will be saturation limited, so peak flux density can go up to 250mT
    (2500G) for ferrite, no worries.

    - the relevant equation is:

    Vin*Ton = Np*Bpp*Ae

    Np = no of turns on primary

    Vin = primary voltage (V)

    Ton = on time (s)

    Ae = core cross-sectional area (m^2)

    Bpp = peak-to-peak flux density (T)


    you can vandalise the units if you wish, but MKS is easiest.

    In theory you can let Bpp = 2*Bpeak = 500mT. BUT unless you have peak
    current-mode control, you can run into problems when you first start up,
    as Binitial = 0T. Once its running, B either starts at -Bpeak then ramps
    up to +Bpeak, or vice-versa, hence Bpp = 2*Bpeak


    Assuming you have PCMC and can let Bpp = 500mT, you can then calculate
    Np*Ae:

    Np*Ae = 48V*0.5/(15kHz*0.5T) = 3.2E-3 for 48V.

    For one turn, this requires Ae = 3200mm^2. thats a big toroid.

    for 10T, Ae = 320mm^2. you can get toroids this big, easily.


    The real problem for the transformer is the winding resistance. 15kW/48V
    = 313A, so 1mOhm will dissipate 98W or so.

    As you increase frequency Np*Ae goes down, so for the same core fewer
    turns are required. If you fill up the winding volume (or at least use a
    constant amount) then the resistance is proportional to the square of
    the number of turns. Fewer turns = more copper *and* shorter, so
    resistance drops. So going to say 30kHz will give you 5T on a 320mm^2
    core, and the resistance will be 4x less than at 15kHz.

    (If you have multiple layers, this doesnt work, proximity effect buggers
    everything up).


    BUT as you increase F, core loss goes up. At some point core loss gets
    high enough that you have to start decreasing Bpeak. You still win, but
    by less (and less and less as f keeps getting higher).

    with modern ferrites you can be saturation-limited up to around 100kHz
    or so.


    FWIW I have a 2500W dc-dc toroid I designed sitting on my desk that runs
    from 16V - 32Vdc, and the overall transformer is 50mm OD x 25mm high. it
    runs cool, too.


    Cheers
    Terry
     
  3. You might consider doing this at a lower frequency with a steel core
    toroid. I made a prototype for this type application from a nominal 500 VA
    toroid that is only about 3" dia and 2" thick. It should work up to about 2
    kHz (30x), with an output power of 15 kVA. It had about 0.2 V/turn at 60
    Hz, so it should be 6 V/t at 2 KHz. You will probably want 360 VDC (for 240
    VAC motor), or 720 VDC for a 480 VAC motor. The hard part will be getting
    10 to 20 turns of wire rated at 150 to 300 amps through the hole. The
    secondary will be about 50-100 turns of #11-#14 for 20-40 amps.

    Paul
     
  4. Joerg

    Joerg Guest

    A regular 50/60Hz core at 2kHz? Must have been a high quality core. Did
    it get hot?
     
  5. I did not build the prototype for full power, and I would probably not be
    able to test it if I did. I would need 350 amps at 48 volts for 15 kW
    output. However, I did an LTSpice simulation, which produces 17 kW at 706
    VDC with 95% efficiency. Of course, this depends on many factors, and I
    have modeled a nearly ideal output transformer. A well-constructed tape
    wound toroid may be efficient enough to work at 2 kHz. It will also work at
    1 kHz, with a core twice as large. I think it is worth a try. My ASCII file
    follows:

    Paul

    ========================= 48V-720V-IRF1405.asc =========================

    Version 4
    SHEET 1 880 680
    WIRE -656 96 -976 96
    WIRE 272 128 224 128
    WIRE 320 128 272 128
    WIRE 496 128 384 128
    WIRE 560 128 496 128
    WIRE 592 128 560 128
    WIRE -112 144 -368 144
    WIRE 48 144 -112 144
    WIRE 96 144 48 144
    WIRE -864 176 -1104 176
    WIRE -736 176 -864 176
    WIRE -976 192 -976 96
    WIRE -864 192 -864 176
    WIRE 48 192 48 144
    WIRE 224 192 224 128
    WIRE 320 224 304 224
    WIRE 496 224 496 128
    WIRE 496 224 384 224
    WIRE -736 240 -736 176
    WIRE 96 240 96 224
    WIRE 96 240 -736 240
    WIRE 496 240 496 224
    WIRE 592 240 592 128
    WIRE 96 256 96 240
    WIRE -32 272 -32 192
    WIRE 48 272 32 272
    WIRE 304 272 304 224
    WIRE 304 272 224 272
    WIRE -864 288 -864 272
    WIRE -784 288 -864 288
    WIRE -112 288 -784 288
    WIRE -64 288 -112 288
    WIRE -864 304 -864 288
    WIRE -656 336 -656 96
    WIRE -560 336 -656 336
    WIRE -464 336 -560 336
    WIRE -64 336 -64 288
    WIRE 32 336 0 336
    WIRE 48 336 48 272
    WIRE 48 336 32 336
    WIRE 96 336 48 336
    WIRE 272 336 272 128
    WIRE 320 336 272 336
    WIRE 496 336 496 304
    WIRE 496 336 384 336
    WIRE -976 352 -976 272
    WIRE -928 352 -976 352
    WIRE -368 352 -368 144
    WIRE -224 352 -368 352
    WIRE -112 352 -112 288
    WIRE -1104 368 -1104 176
    WIRE -560 368 -560 336
    WIRE -464 368 -464 336
    WIRE 0 368 0 336
    WIRE 160 368 0 368
    WIRE -656 384 -656 336
    WIRE -368 384 -368 352
    WIRE -224 384 -224 352
    WIRE 0 384 0 368
    WIRE 160 384 160 368
    WIRE -976 400 -976 352
    WIRE -784 400 -784 288
    WIRE 304 432 304 272
    WIRE 320 432 304 432
    WIRE 496 432 496 336
    WIRE 496 432 384 432
    WIRE 544 432 496 432
    WIRE 592 432 592 320
    WIRE 592 432 544 432
    WIRE -560 464 -560 448
    WIRE -464 464 -464 432
    WIRE -464 464 -560 464
    WIRE -432 464 -464 464
    WIRE -416 464 -432 464
    WIRE -112 464 -112 416
    WIRE -64 464 -64 416
    WIRE -64 464 -112 464
    WIRE -48 464 -64 464
    WIRE -432 496 -432 464
    WIRE -272 496 -272 464
    WIRE -272 496 -432 496
    WIRE -112 496 -112 464
    WIRE 112 496 112 464
    WIRE 112 496 -112 496
    WIRE 544 496 544 432
    WIRE -1104 528 -1104 448
    WIRE -976 528 -976 480
    WIRE -976 528 -1104 528
    WIRE -864 528 -864 400
    WIRE -864 528 -976 528
    WIRE -784 528 -784 464
    WIRE -784 528 -864 528
    WIRE -656 528 -656 464
    WIRE -656 528 -784 528
    WIRE -368 528 -368 480
    WIRE -368 528 -656 528
    WIRE -224 528 -224 480
    WIRE -224 528 -368 528
    WIRE -80 528 -224 528
    WIRE 0 528 0 480
    WIRE 0 528 -80 528
    WIRE 160 528 160 480
    WIRE 160 528 0 528
    WIRE -80 608 -80 528
    FLAG -80 608 0
    FLAG 544 496 0
    FLAG 560 128 Vout
    FLAG -112 144 Vd1
    FLAG 32 336 Vd2
    SYMBOL ind2 80 128 R0
    SYMATTR InstName L1
    SYMATTR Value 1m
    SYMATTR Type ind
    SYMATTR SpiceLine Ipk=100 Rser=5u
    SYMBOL ind2 80 240 R0
    SYMATTR InstName L2
    SYMATTR Value 1m
    SYMATTR Type ind
    SYMATTR SpiceLine Ipk=100 Rser=5u
    SYMBOL ind2 240 176 M0
    SYMATTR InstName L3
    SYMATTR Value 250m
    SYMATTR Type ind
    SYMATTR SpiceLine Ipk=10 Rser=100u
    SYMBOL nmos -416 384 R0
    SYMATTR InstName M1
    SYMATTR Value IRF1405
    SYMBOL nmos -48 384 R0
    SYMATTR InstName M2
    SYMATTR Value IRF1405
    SYMBOL voltage -1104 352 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 24 132 Left 0
    SYMATTR SpiceLine Rser=5m
    SYMATTR InstName V1
    SYMATTR Value 48
    SYMBOL diode 320 144 R270
    WINDOW 0 32 32 VTop 0
    WINDOW 3 0 32 VBottom 0
    SYMATTR InstName D1
    SYMATTR Value MUR460
    SYMBOL diode 320 240 R270
    WINDOW 0 32 32 VTop 0
    WINDOW 3 0 32 VBottom 0
    SYMATTR InstName D2
    SYMATTR Value MUR460
    SYMBOL diode 384 352 M270
    WINDOW 0 32 32 VTop 0
    WINDOW 3 0 32 VBottom 0
    SYMATTR InstName D3
    SYMATTR Value MUR460
    SYMBOL diode 384 448 M270
    WINDOW 0 32 32 VTop 0
    WINDOW 3 0 32 VBottom 0
    SYMATTR InstName D4
    SYMATTR Value MUR460
    SYMBOL polcap 480 240 R0
    WINDOW 3 24 64 Left 0
    SYMATTR Value 560µ
    SYMATTR InstName C1
    SYMATTR Description Capacitor
    SYMATTR Type cap
    SYMATTR SpiceLine V=1000 Irms=2.9 Rser=0.18 MTBF=10000 Lser=0 ppPkg=1
    SYMBOL res 576 224 R0
    SYMATTR InstName R1
    SYMATTR Value 30
    SYMBOL voltage -656 368 R0
    WINDOW 3 -135 215 Left 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR Value PULSE(0 10 10u 40n 40n 250u 500u 2000)
    SYMATTR InstName V2
    SYMBOL npn -928 304 R0
    SYMATTR InstName Q1
    SYMATTR Value 2N5550
    SYMBOL res -880 176 R0
    SYMATTR InstName R2
    SYMATTR Value 50
    SYMBOL res -992 176 R0
    SYMATTR InstName R3
    SYMATTR Value 100
    SYMBOL res -992 384 R0
    SYMATTR InstName R4
    SYMATTR Value 1k
    SYMBOL res 64 176 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R5
    SYMATTR Value 100
    SYMBOL cap 32 256 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName C2
    SYMATTR Value 220n
    SYMBOL res -576 352 R0
    SYMATTR InstName R6
    SYMATTR Value 60
    SYMBOL res -80 320 R0
    WINDOW 0 36 30 Left 0
    WINDOW 3 36 61 Left 0
    SYMATTR InstName R7
    SYMATTR Value 60
    SYMBOL schottky -448 432 R180
    WINDOW 0 24 72 Left 0
    WINDOW 3 -16 11 VRight 0
    SYMATTR InstName D5
    SYMATTR Value 1N5818
    SYMATTR Description Diode
    SYMATTR Type diode
    SYMBOL schottky -96 416 R180
    WINDOW 0 24 72 Left 0
    WINDOW 3 69 18 VRight 0
    SYMATTR InstName D6
    SYMATTR Value 1N5818
    SYMATTR Description Diode
    SYMATTR Type diode
    SYMBOL zener -768 464 R180
    WINDOW 0 24 72 Left 0
    WINDOW 3 24 0 Left 0
    SYMATTR InstName D7
    SYMATTR Value BZX84C10L
    SYMATTR Description Diode
    SYMATTR Type diode
    SYMBOL nmos -272 384 R0
    SYMATTR InstName M3
    SYMATTR Value IRF1405
    SYMBOL nmos 112 384 R0
    SYMATTR InstName M4
    SYMATTR Value IRF1405
    TEXT 72 384 Left 0 !K1 L1 L2 L3 1
    TEXT -528 632 Left 0 !.tran 1s startup
     
  6. Joerg

    Joerg Guest

    That one word you used (well-constructed) is exactly the point. With
    regular EI cores it's the same, some are great, almost good enough for a
    decent audio transformer. Then there are others which are, as Archie
    Bunker would have put it, lousay. But the difference won't show much in
    SPICE, it'll show during extended full load testing.

    [...]
     
  7. John

    John Guest

    Core cross section is not all there is to it. The power handling capability
    of a core at a given frequency is a function of its AcAw product where Ac is
    the core area and Aw is the window area. Obviously, the higher the
    frequency, the greater the power handling ability. But, then again, the
    higher the frequency, the greater the core and copper losses.

    You can write the equation for the power handling capability of a core based
    on its core area. You can also write the equation for the power handling
    capability of a core based on its window area. Solving the two equations
    simultaneously gives the power handling capability of the core. In the
    process of writing the equations, you will discover that you need some more
    information. It turns out that you will either need to specify temperature
    rise or regulation. For transformers smaller than a certain size, it is
    regulation that dominates the requirement. For transformers larger than that
    certain size, it is temperature rise that is the dominating requirement. In
    your case, your requirement will be determined by temperature rise of the
    transformer.

    To be more exact, it is the geometry and dimensions of the transformer that
    determines its capability. For a given core, it will have a certain surface
    area and volume when filled with wire. The temperature rise will be a
    function of the surface area divided by the watts per cubic inch of loss.
    There is a section in The Radiotron Designer's Handbook by RCA that
    discusses transformer design and it starts off, as I recall, with a
    temperature rise requirement. I believe that book is now online.

    If you have a starting point (such as Terry Given's post - last paragraph),
    you might be able to ratio his power-handling capacity to your requirement
    if he can give you his AcAw product (don't forget to include the effects of
    frequency). Otherwise, you will need to think deeply for a long time, read
    the book, and write your own equations. Or, you can experiment. If your job
    is to design things, maybe you need to do the math. OTOH, if this is a
    one-shot project, experimentation may be the answer.

    In any case, I wish you luck with your project.

    Cheers,
    John
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-