MPPT power conversion how do they do it?

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unyalli
unyalli Solar Expert Posts: 121 ✭✭
in Advanced Solar Electric Technical Forum #1
How are the MPPT controllers converting high voltage low amps to low voltage high amps? The higher this ratio the more heat produced. What is producing this heat?
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #2
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    Re: MPPT power conversion how do they do it?

    I think this explains it:
    http://en.wikipedia.org/wiki/Buck_converter

    Pretty simple (not) LOL
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  • unyalli
    unyalli Solar Expert Posts: 121 ✭✭
    #3
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    Re: MPPT power conversion how do they do it?

    I figured a transformer of some type was involved but this is DC not AC. So the heat is caused by resistance (isn't it always) in the inductor and resistor. In a multiphase buck this can be spread out over a larger heat sink.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #4
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    Re: MPPT power conversion how do they do it?

    Resistance and "stray capacitance" (junction capacitance across the FETs--The higher the input voltage, the more losses in j-capacitance--Higher frequency, more j-capacitance losses, higher working voltage for FETs, higher junction resistance).

    Electronics are really interesting--Many of the things we work with are "almost perfect" at what they are designed for--But the mucky stuff (losses, capacitance, inductive coupling, etc.) always comes back to bite when pushing the limits.

    Also--Look at the Buck Converter... There is a diode that actually carries 1/2 of the current (when the buck switch is open, the inductor "pulls current" through the forward biased diode--0.2 to 1.0 or more volt drop there (more heat/losses).

    You then look at synchronous rectification--Replace that diode with another FET (lower on resistance/voltage drop, more efficient) and put some electronics around the FET to make sure it is only on at the "right time"...

    This stuff can be a very interesting study--But it is also a pain to design (the devil is in the details).

    Multi-phase converter--Put two or more buck converters in parallel... This reduces the "bumps" on the output voltage/current if the converters are "staggered" in time (less output filtering needed, cleaner output voltage). Also, you can turn off one or more converters when the power usage is low (save energy--single switching losses for minimum loads).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
    #5
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    Re: MPPT power conversion how do they do it?

    My recent reason why all this equipment gets hot is that we have to design around defective parts.

    Resistance, switching losses from switching time, diode charge recovery time, stray capacitance,
    leakage inductance, thermal resistance... It just doesn't stop. It's all a compromise.

    Yes, it gets worse when the input voltage goes up.

    Yeah... Bad parts. We just try to get the best bad parts that are affordable.
    Good parts aren't available at any price, yet.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #6
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    Re: MPPT power conversion how do they do it?

    The buck circuit is the easy part. Try variable loading of an unknown solar array, to find the peak power point, and then not muck it up when a cloud comes by.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • frank.thomson
    frank.thomson Registered Users Posts: 15 ✭
    #7
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    Re: MPPT power conversion how do they do it?

    Yes, the low-voltage input MPPTs are regular buck-converters. But I don't think these high-voltage input MPPTs like the morningstar's or xantrex's 600V input devices can't be a simple buck converters, thats a step-down ration of 10. Long time back in early years, I took a course with Ray Ridley, an awesome guys teaching people basics of electronic switchers. If I was to guess, there would be a circuitry to chop the high-voltage DC into pulsating square-wave type high-voltage AC and convert them to a low-voltage AC with a transfomer and than a rectifier makes it DC. Without a transformer a dead-component could bring the high-input voltage to the output side and zap the guy wiring it. And I would be scared to know if they are not galvanically isolated with a transformer.

    Reason for losses is simple, there are two types: conduction loss, those due to current flowing through cables, inductors, FETs, transformers, etc. and switching loss, those due to the FETs turning on/off at high-speed to chop the voltage.

    Not sure why there are only few who make these 600V ones, may be too complicated or just expensive.

    -Frank

    mike95490 wrote: »
    The buck circuit is the easy part. Try variable loading of an unknown solar array, to find the peak power point, and then not muck it up when a cloud comes by.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: MPPT power conversion how do they do it?

    Gee, you guys, why are you confusing people with science?
    It all works by magic. Hence the term Magic Power Point Tracker! :D
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  • unyalli
    unyalli Solar Expert Posts: 121 ✭✭
    #9
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    Re: MPPT power conversion how do they do it?

    This has been very interesting, thanks to all for the input.

    -Jeff
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #10
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    Re: MPPT power conversion how do they do it?
    Cariboocoot wrote: »
    Gee, you guys, why are you confusing people with science?
    It all works by magic. Hence the term Magic Power Point Tracker! :D

    And Coot - - don't forget the "Magic Smoke" when things go wrong! :D
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