Charge controller - power diode question

Why not use a schottky diode pair (to220 package) such as MBR2545 for lower forward resistance and since the diodes are in the same package they will share the load better. Also for similar reasons a power mosfet might be a better choice to handle switching high current more efficiently.

There have been ample discussions about the folly of solar power when you have any alternatives. I'll not repeat that here. I like to help people with their hobbies. I've helped engineer more than one solar system for mountain-top use in the boonies.

Your statements are worrisome on many levels...

Depending on what batteries you're using and the tradeoffs you've made for life vs maximum stored energy, your battery voltage ranges from 10V to nearly 15V or so.

To deal with the 20.0A maximum

Sounds logical on the surface, but there's a troll under that bridge. Most diodes have a negative temperature coefficient. If there's any imbalance, one diode takes more current and warms up which makes it hog more current which makes it warm up. Pretty soon, you have one diode in the circuit and the other two just sitting there. From then on, it's a matter of time until the silicon melts and shorts or the plastic melts and the device comes apart open. According to the spec, that diode at 10 amps is gonna be 150 C above ambient. That's too hot, even if you could make three share equally. Get something with a bolt hole and put it on a heat sink. You're better off with one 40A diode. And a 20V diode will likely be less lossy and cheaper than a 100V one. And I don't mean a lower voltage one selected out of the same bucket. I mean a diode designed for minimum forward voltage often has a lower reverse voltage. It doesn't have to be a fast diode.

But there's a more basic question. Many solar panels have built-in diodes. Are you sure yours don't?

That diode has a forward voltage spec of 1V. You've got 15V at the battery and you're wasting another volt or 20 watts.

And it's more complex than that. Look at the family of curves for the panel. They're not a straight line. And the 20A is at noon in the desert. At 3PM when the panel isn't pointed directly at the sun and there's more atmosphere in the way and there's some haze and some bird poop on the panel, you're gonna be on another curve of that graph. That diode might be the difference between your charge current going to zero at 3PM instead of 3:30 PM. Those lost amp-hours add up. Also, if I have a

Shunt controller sounds simple, but again, the devil is in the details. We built one system that used 4 150W resistors and 4 audio power transistors. A comparator switched the transistors on at 13.7V and off at 13.6V. That worked because The panel was 20A and the batteries were 800Amp-Hour. They didn't have any trouble with that current and voltage. And the thing rarely turned on except on the longest days of the year.

If you had a single car battery and a 20A panel, that wouldn't be a good idea.

A later version used an oscillator and a crude 300Hz PWM to switch the transistors. For a less robust battery system, you'd want a fast switcher and much better charge management. And today, you'd be able to afford fast switching transistors at that current.

For starters, take any 300W plus buck converter design that can run on

10-15V. Put 600W of resistors with values that can draw at least 300W from the output of the converter. Control the converter output voltage to give you the shunt load current you require. Since you're likely to use multiple resistors in parallel. It might be easier to use more buck converters of lower current each. Depends on what you can find in the cheapo bin of the electronic surplus store.

We built a MPPT controller and a sun tracker. Worked neat and gained some additional usable power. Problem was that it was deemed unworkable. It's hard to track the sun when the panel is covered with ice and the roads to the site are closed eight months out of the year.

For remote stuff that just has to work, it's often better to add another panel than to try to eek out a few more percent on the one you have. Reliability trumps efficiency every time.

The hardest part of any project is writing the spec. Decide exactly what you expect to happen under any and all conditions of insolation and load current and battery charge level. Map that all out and decide what to implement. Then figger out HOW to implement...repeat the cycle until it looks like what you want. Then start ordering parts.

If you're on the grid and expect to generate power, you're likely to give up a that point and go play a round of golf.

If you're indulging an expensive hobby, you can have great fun with solar power. It's a lot cheaper than golf...and less risky than a mistress.

Thank you very much for your detailed and insightful explanation. I really loved your last paragraph.

The usual method to avoid this is to put individual series resistors in front of each diode, thus, sharing the current more evenly.

Of course, this dissipates some power.

However, since there are usually quite a long distance between the panels and the batteries (and diodes) at such low voltage as 12 V (instead of 24 V or 48 V), there are still going to be some voltage drop in the cable with cross section A, before to current goes into three 10 A diodes.

Why not use three separate insulated conductors with A/3 cross section from the panels, each connected separately to each diode ? The total power loss in the wiring will be the same, but the separate wiring will now act as small series resistors to distribute the current more evenly.

If you look at actual diode curves, diode TC is negative at low currents and positive at higher currents. That's because the ohmic component of VF dominates at high current, and it has a positive TC. It's generally safe to parallel identical diodes on the same heat sink.

Couldn't you do a series switcher with no inductor and no catch diode? Just connect the solar array to the battery through a mosfet or SSR, on/off, from a comparator with maybe a little timing, like a clocked d-flop in the path. It's simple and power dissipation would be very low.

Acutually, the solar panels I have in front of me have the diodes in _parallel_ with the panel output.

The purpose, AIUI, is to provide a path for the current of shaded panels when wired in series.

In that case, the provided diodes will not prevent current back into the battery.

They prevent current from an illuminated panel from being loaded by a non-illuminated panel. IOW, electrons only get to spit in one direction from all elements. That being INTO the charging circuit input.

I didn't want to complicate the discussion. What you suggest probably works fine. In our case, there was also a wind generator that had some control mechanisms in the head. It didn't like to be disconnected when the wind was blowing hard. And the solar had a crude controller. The two controllers wanted to fight. And the pieces were a hundred miles away and a mile straight up. Was easier to just shunt the thing into a resistor to regulate it.

It can be a minor consideration, but series control is loss when you need the juice most. Shunt control is loss when you've got excess you can't use.

Hmm, well first I know squat about solar panels. But that seems wasteful. Doesn't it cost a bit more than one solar panel 'photovoltage' to overcome the diode drop of the panel in the shade? Or is there a series stack of 'PV's with one diode across the whole lot?

George H.

Ummm- that observation only applies to the small signal stuff you work with. The power Schottky's stay negative TC all the way: see Fig. 3

Yeah- if the battery was the only energy load on the system, but there might be other things like an MPP inverter which will get all confused with that scheme.

That suggestion is insane for a professional project.

The power Schottky's stay negative TC all the way: see Fig. 3

All diodes eventually get to positive TC. Sometimes that point is above the rated max current, sometimes it isn't. But it's still usually safe to parallel identical diodes on the same heat sink. It's really not much different dynamics than one big slab of diode which, in theory, can have one zone hog the current. But check the data sheet and do the math, of course.

other things like an MPP inverter which will get all confused with that scheme.

If it won't work for you, don't do it.

It sounds like a 24vdc PV panel. usually they are wired up in series for

600vdc to run a Grid-tie inverter. The shading Diode sounds plausable and may be in there for other issues;) It's your standard ~0.5v drop power diode. If you partially shade an array the GTI's tend to shutdown altogether, so I'm not sure there is any benefit.

Cheers

The power Schottky's stay negative TC all the way: see Fig. 3

As a rule, if there is a single component rated to handle the job, it is always more economical than using multiple copies of a component that can't handle the job. There may be exceptions.

be other things like an MPP inverter which will get all confused with that scheme.

I know for a fact the MPP in-/con-verter will not like you periodically clamping its input to Vbatt, so you will have to think of something else.

The power Schottky's stay negative TC all the way: see Fig. 3

more economical than using multiple copies of a component that can't handle the job. There may be exceptions.

be other things like an MPP inverter which will get all confused with that scheme.

clamping its input to Vbatt, so you will have to think of something else.

I didn't have in mind using an MPP inverter, but something much simpler to regulate the battery charging.

All I was suggesting was: solar panel, series switch, battery. The switch is the only high-current part, and maybe a series diode if that's needed. The control is a simple, low-power comparator thing.

One could use a single bidirectional SSR or equivalent as the switch, to handle the daytime and nighttime cases. Saves the loss of a series diode.

If nobody likes the idea, don't use it.