MPPT with constant voltage output

Discussion in 'Power Electronics' started by Ande, Sep 8, 2018.

  1. Ande

    Thread Starter Member

    Feb 3, 2017
    I have been studying various MPPT algorithms particularly the P&O and the perturb and observe method. I want to use an MPPT method along with an interleaved boost converter for solar energy conversion. My concern however is that all these methods change the reference voltage of the controller thereby changing the output voltage of the boost. I however need a stable output from the boost, is there any MPPT that can do this? I'm still new at this so please bare with me and my not so smart questions
  2. ebp

    Well-Known Member

    Feb 8, 2018
    The normal method is to regulate the input voltage, and in doing that the output current must be allowed to change.

    The MPP converters I have designed, all but one for battery charging, used three control loops.

    When the power available from the PV array is less than that required to meet the load requirement (which is why you need MPPT in the first place), the loop "regulating" the input voltage controlled the PWM, limiting the duty cycle to keep the array at the MPP voltage. Since the output voltage at any given instant in a battery charger is more or less held constant by the battery itself, this means that the output current must be allowed to vary.

    A second control loop limited the maximum current delivered to the battery to protect both the battery and the circuitry. If the array were small relative to the battery's ability to accept current, this would not be required. If a controller that does cycle-by-cycle current limiting is used, that function may be entirely adequate for protecting everything, especially with a boost converter from a PV array.

    A third control loop precisely regulated the output voltage, provided there was sufficient input power from the array to allow it.

    With a deeply discharged battery, there could a situation where the power required at the current limit would be lower than that available from the array at MPP with the illumination available. The array would then operate at a voltage above the MPP point and the current limit loop would dominate. As the battery voltage rose, the amount of power allowable into the battery would also rise (same current limit and higher output voltage). During this time the input voltage regulating circuit would typically take over, preventing the array from operating below MPP. The output current would fall to whatever level was necessary. If clouds passed, the maximum power from the array would drop dramatically, but the MPP voltage would stay about the same and the output current would fall dramatically. Eventually the battery would reach nearly full charge and require less power than available from the array. At this point, the output voltage control loop would take over and precisely regulate to maintain the battery at "float" voltage (lead acid batteries, though I used the same general scheme for lithium ion batteries that should not be kept at float voltage for long periods).

    Various methods can be used to control the duty cycle. I typically used the controller IC's internal error amplifier at unity gain and used an external reference and error amplifiers. Depending on the controller, the internal reference is directly connected to the internal error amp, so you must work around that.

    Interleaved converters can be a good way to go these days, in no small part because it can allow you to use off-the-shelf inductors. It reduces the ripple current in capacitors, which is very helpful for longevity. With surface mount designs it can make thermal management easier.

    MPP tracking with rapidly changing illumination or dynamic loads without a battery can pose a difficult challenge because large, fast changes in duty cycle are required.
    Kjeldgaard, Ande and digsys like this.