Hey everyone. I'm back again..

I've been studying the use of buck/boost converters for MPPT. Here's what I have learned so far:

1) - Buck converters step down voltage, boost converters step up voltage.
2) - For a given set of conditions, there is a certain voltage Vmpp, at which a solar cell outputs the maximum power that it can.
3) - This point i.e Vmpp is always lower than Voc (the open-circuit voltage), as it should be because the power virtually drops to zero at Voc because there is no current.

Now if 1) is true, then the appropriate Dc-dc converter to use is the buck converter because the voltage (and thus power) adjustment that has to be made for operation at Pmpp is always behind the Voc point i.e, we must go down from Voc and seek Vmpp (this is just a consequence of solar V-I characteristics). The voltage range of a buck converter is 0 to Voc, and that is exactly what we need.
If instead, we choose a boost converter, then that makes no sense, because its voltage range is from Voc to infinity, and a solar cell operating at Voc or beyond outputs zero power.

The point of writing all this is merely to ask this: Why is the boost converter an option at all, when deciding upon the dc-dc converter topology to use in an MPPT system? If our aim is to maximize power, which can only be achieved behind Voc, then where does the boost converter come in, considering that it outputs voltages at or above Voc?

I doubt I have expressed my question very well, but I hope you guys will see what I mean.

Thanks!

 

The boost converter would be needed if the desired load voltage is higher than Vmpp for the panel...

You aren't always going to charge a 12V battery with a 20Voc panel

 

To avoid having to use a more complex buck/boost design you would design the panels and battery such that the panel voltage is always above (or below) the battery voltage for any combination of battery charge and panel output MPPT conditions.

 

Either a boost or a buck converter will work depending on how much voltage is needed, more or less (literally).

One thing I realized a page back in this post is to have an MPPT convrter you need a switcher with a continuous input current. A basic buck converter is not acceptable; while very efficient using what current it does draw it wastes 100% of the available current it does not use.

There does exist buck schemes that do draw a continuous input current.

 

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One thing I realized a page back in this post is to have an MPPT convrter you need a switcher with a continuous input current. A basic buck converter is not acceptable; while very efficient using what current it does draw it wastes 100% of the available current it does not use.

That's why all switching converters have a large capacitor at their inputs--to smooth out the current pulses of the switching function so that the source sees the average current, not the peak current. No current is wasted.

 

Such a cap may help for EMI but does little to turn series of current pulses into a relatively constant current.

 

The boost converter would be needed if the desired load voltage is higher than Vmpp for the panel...

You aren't always going to charge a 12V battery with a 20Voc panel

Right. But if we want to be operating the panel at any point other than the Vmpp, we can forget about maximizing the power, right?

Thanks guys!

 

Right. But if we want to be operating the panel at any point other than the Vmpp, we can forget about maximizing the power, right?

Thanks guys!

Charging a 12V battery with a 20Voc panel is one of the places where a MMPT converter makes so little difference as to not be worth the trouble. Just connect the damn panel to the battery with no intervening crap, and you will get to a fully-charged battery in only minutes longer than a MPPT controller could do it.

 

Charging a 12V battery with a 20Voc panel is one of the places where a MMPT converter makes so little difference as to not be worth the trouble. Just connect the damn panel to the battery with no intervening crap, and you will get to a fully-charged battery in only minutes longer than a MPPT controller could do it.

I agree. MPPT performance gain is really over sold. I have seen claims of 20 to 30 percent improvement where 8 - 10 percent is really what you get. The case of charging a 12v battery with a 20v panel is a great example of keeping it simple. Sure, you could throw some spiffy electronics on it and gain 10% or so, but is it worth the cost? There certainly are cases where adding a switching converter/battery charger with MPPT is needed, but not always.

 

Such a cap may help for EMI but does little to turn series of current pulses into a relatively constant current.

Respectfully, I think it does help, and in fact, I rely on it for the power converter on my solar bike. The converter draws pulses that can peak at 40 amps, but the panel is only capable of about 8 amps (and I've never seen that in actual sunlight). Having 2000 uF of low ESR electrolytic plus 30 uF of ceramic capacitors right at the switcher input allows me to draw a relatively steady current from the panel.

note: slight change in values since my reply #36 above, as the design evolves experimentally

 

I would like to talk about some solar battery charger applications, from simplest to most complex, their uses, and pros and cons.

- Portable 20Voc panel connected to a 12V battery - This is the simplest and cheapest way to charge a battery. This is a good match between panel and battery voltage. The down side is that there is no regulation and you could over charge the battery.

- Portable 20Voc panel connected to a 12V battery, plus a linear voltage regulator - Adding a regulator allows you to leave the battery connected. This is a good configuration that requires little attention.

- Portable high voltage panel connected to a 12V battery, plus a switching voltage regulator - Going to a switcher improves the charging capabilities by 20 to 50 percent, allowing you to charge your battery quicker. (This is not an MPPT. It is just the advantage of a switcher over a linear regulator.)

- High voltage panel(s) connected to a switching battery charger, connected to a battery(s) - This is a common configuration found in RVs that are used for occasional "dry camping". Battery power is used for lighting, small appliances, and starting the generator, (to recharge the batteries and power larger appliances.)

- High voltage panels connected to a switching battery charger with MPPT, connected to a battery array - You will find this configuration in RVs that are used extensively for dry camping and for off grid living. This is where you are living from the solar energy that strikes your panels. If you have a cloudy day, you may not have the power to do what you want to do unless you run a generator. In this situation, a MPPT based charger makes sense. It will cut the amount of gas you use in your generator by 10%.

I hope the above examples add some perspective. You don't need a super efficient system to charge your cell phone or keep the battery for your trolling motor charged. Advanced solar systems make sense when you have limited access to the power grid.

 

Respectfully, I think it does help, and in fact, I rely on it for the power converter on my solar bike. The converter draws pulses that can peak at 40 amps, but the panel is only capable of about 8 amps (and I've never seen that in actual sunlight). Having 2000 uF of low ESR electrolytic plus 30 uF of ceramic capacitors right at the switcher input allows me to draw a relatively steady current from the panel.

note: slight change in values since my reply #36 above, as the design evolves experimentally

One may draw 40 amps from a 2000uf cap if one can live with a 1volt droop every 55 microseconds. Also, after that pulse you must wait at least 220 microseconds for the cap to recharge. That is best case with constant current loading and sourcing the cap, zero ESR and DCR.

I fail to see how this plays into the workings of an MPPT converter that controls the current being drawn to be at the variable point of maximum power.

 

One may draw 40 amps from a 2000uf cap if one can live with a 1volt droop every 55 microseconds. Also, after that pulse you must wait at least 220 microseconds for the cap to recharge. That is best case with constant current loading and sourcing the cap, zero ESR and DCR.

I fail to see how this plays into the workings of an MPPT converter that controls the current being drawn to be at the variable point of maximum power.

Think about what happens between the panel and the capacitor. When your buck converter pulls that 40 amp pulse from the cap, and the voltage droops, it's going to draw more current from the panel, and whatever you do to maximize the average MPPT you'll get lots of pulses of inefficient power conversion in the panel. You need to even out the current demand at the panel.

I hedged it by making a dual buck regulator with each side synchronized 180 degrees out of phase with the other. Depending on how the pulse waveform looks, you may run a triple or quad to even it out even more. And in steady state operation of a buck regulator, your curve may not include that big a pulse.

Most solar systems use symmetrical-drive inverters that keep a fairly even load on the panel, which only requires minor smoothing with an input capacitor.

With the load on the panel being fairly uniform, you can then apply MPPT regulation to the bucks or inverter to optimize that load.

 

One other thing... When you're dealing with buck regulators, think in terms of current primarily, and then figure out what the voltage is doing. Bucks are current-pump devices; they don't care much about the voltages as long as they're high enough to make the thing operate, and, of course, subject to the output voltage-regulation circuitry.

In a solar MPPT application, ignoring for the moment the pulsatile nature of the input current, but at least on a phase where the input current is somewhat uniform: You Perturb the current pump ability (i.e., decrease PWM duty cycle) and Observe (measure) the average input voltage and current (during the active phases, in this case).

If the panel I-V curve is sub-optimal in the direction of requiring less current, you override the buck's voltage-regulation circuit (which is telling you that you need to pull more current by increasing the PWM duty cycle) and lower the PWM duty cycle until you reach MPP (or overshoot, and then correct, part of P&O).

If the panel I-V curve is sub-optimal in the direction requiring more current, you increase PWM slightly and (and then re-check, part of P&O cycle), if and only if (while) the output voltage is below the buck voltage-regulator setpoint.

You see, the PWM adjusts the current you're pumping through the buck, and the output load draws this current and in effect determines the output voltage while your MPPT is limiting current.

If the current demand is low enough on the load, you'll find yourself trying to perturb the MPPT and finding that you're at your voltage regulator setpoint and the panel I-V only wants to increase the current so you have nothing to do. However, if the current demand is high, you'll find yourself adjusting your PWM as the energy falling on the panel changes (or your load changes).

If you're using a battery charger as a load, which is essentially a voltage regulator/current limiter, what you'll find is that when your panel has less energy than the battery charger really wants to do its maximal charge rate at the time, the MPPT will limit the current but the battery charger will further limit the current as the input voltage drops below its minimum operating voltage, so you'll have a chain of current limiters adjusting the charge current on the battery to whatever the panel can maximally apply.