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DC Charge controller

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IamSmooth

Wed Jan 16 2008, 03:12AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I plan to build a DC charge controller to protect an inverter from overvoltage. Let's say that the inverter can't take a voltage over 600v; anything above 600v will destroy the unit. Furthermore, I only want to divert excess voltage away from the unit and not divert everything. This way, I can still keep the inverter running at full power.

I was thinking of using zener diodes parallel to the DC source.

I will have a string of them set, for example, to 550v with enough resistance in series to protect them from the overvoltage. I figure that as the voltage rises above the zener threshold I can use this to turn on a transistor switch to divert excess current to a dump load.

Is this a good plan? If so, what transistor/transistors should I use to handle to power? There can be 500-1000w going through the diverter into the dump load. If this isn't a good idea what is a better one?

I would prefer not to simply activate a mechanical relay as this will divert all power away from the inverter, which is not the goal.

Thanks

Sulaiman

Wed Jan 16 2008, 11:47PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Depending upon the charging/power source, a series-regulator may be better than a shunt regulator in terms of efficiency.
What is the maximum input voltage and output current expected?

IamSmooth

Thu Jan 17 2008, 01:33AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

The voltage will go from 0 to 500v. Current will go from 0 to about 3A.
What do you think would be the best approach, and why? I believe if I remember the regulators correctly, the shunt regulator has the emitter and collector in parallel with the voltage source; the series regulator it is in series. However, I don't know when one would choose one method over the other.

Bjørn

Thu Jan 17 2008, 01:52AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Are we talking about a huge amount of solar cells in series? If so, what about a relay that changes the configuration to two parallel strings that generates half the voltage?

If the inverter is expensive you can make it failsafe by adding a MOV of some sort in parallel that will short the input if it goes too high in case your ordinary protector fails.

Electroholic

Thu Jan 17 2008, 03:56AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

if you are using solarcells, and you wnat to get the most out of them, you need an MPPT, maximum power point tracker. Basically a buckboost regulator, programmed to track the maximum power point on the VI curve of the solarcells, and with a constant voltage output.

IamSmooth

Thu Jan 17 2008, 10:45PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I am talking about a wind generator, and not solar cells. Solar cells have a narrow voltage range depending on the string configuration. Wind generators have voltages that depend on the wind. No wind means zero voltage; moderate winds mean high voltages; strong winds can produce voltages that will blow the inverter. For this reason, I need a means to limit the high end voltage, while still maintaining enough power through the inverter so all the wattage is not waisted.

Basically, anytime the voltage is between 0 and 500v, I want to collect the power; any power due to voltages above 500v I want to divert away from the inverter.

IamSmooth

Fri Jan 18 2008, 04:42PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I need help with someone to go over explain how to choose the right values, and to verify if my design is resonable:

For the purpose of an example, I picked some values: Vinput is 300v/5A, Vout not to exceed about 200-210v, and a power transistor that can handle 50w. I am using a 200v zener diode with a 5w power rating. I calculated that with 100v above 200v, I can only let 1/40A go through the zener, giving me a 4k resistor with a 2.5w rating.

I don't know how to properly calculate the value of R2. I arbitrarily picked 10k to let some current get to the base when the voltage goes above 200v, but I don't know if a value of 50k, 5k, or 100k makes a difference.

Finally, I need help with Q1. What parameters determine the 50w the transistor can tolerate? Do I multiply 300v by the current through the transistor, or do I use 200v times the current? Also, what should the Rload value be so that the output voltage is 200v? How do I calculate the right value or range of values? If my maximum input current is 5A does this mean that Rload can not exceed 40 ohms? Finally, how do I determine if I need more transistors in parallel?

All help is appreciated. jk

Steve Ward

Fri Jan 18 2008, 08:16PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

From the fact that you are unwilling to simply disconnect the generator (or short it) when the voltage gets too high, sounds as if you expect to often exceed the capability of this thing. I would suggest you move to 900V or 1200V silicon instead of any sort of voltage clamp method. Then you might still want a protection scheme, but it would be "all or nothing" and really there only for protection, and not to keep the thing running under those conditions.

Also, designing a converter (or inverter if you need AC output) to operate on such a huge voltage input range is going to be a very compromising design.

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