Solar charging issue with MC34673IC

[rahulksgift]

Hi All ,

I have built a solar charger to charge a lithium iron battery
Solar panel I used 5W/8.5 Vpm.
Li-on charger is MC34673.set to 1.A constant current charge .
The issue I'm facing is ,
Solar panel VOC is more than 10 V when there is 100% SUN.But charger IC input over voltage protection limit is 6.8V .When i connect solar panel
with Charger IC , IC it get disabled internally.and there is no charging .

Is it good to connect 6.8V Zener across Panel ?
Or Any other method to regulate solar panel voltage with out much power loss ?

regards,
Aneesh

 

[chuckey]

Your maths don't seem to add up if the panel is rated at 5W/8.5 Vpm., Then at 1 A output, the panel should produce 5V ( 5V X 1A = 5 W). the data sheet I looked gives the max input for the MC34673 as 28 V. I suspect that the chip is getting too hot and closing down. Is it on a heat sink?
Frank

 

[tpetar]

Its very bad idea to put zener across panel! You can read some zener tutorial how zener works, and how is used in design, and when.

 

[rahulksgift]

Your maths don't seem to add up if the panel is rated at 5W/8.5 Vpm., Then at 1 A output, the panel should produce 5V ( 5V X 1A = 5 W). the data sheet I looked gives the max input for the MC34673 as 28 V. I suspect that the chip is getting too hot and closing down. Is it on a heat sink?
Frank

Hi Thanks for the reply, 28V is the maximum input voltage we can apply without damaging the IC. But datasheet says at 6.8V it will be disable internally ." 28V maximum voltage for the power input with 6.8V over-voltage protection threshold" fifth point of feature in datasheet thanks ..

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Its very bad idea to put zener across panel! You can read some zener tutorial how zener works, and how is used in design, and when.

Okay...So can you suggest me any other alternative idea ?

 

[chuckey]

The best (and most expensive) method is to use a buck converter, but if you build a sophisticated one you don't need your regulator chip!. One crude but easy way is to put a Zener in series with the regulator to drop 12 - 6.8 = 5.2 V @ 1A. This will also reduce the input to the regulator at lower solar cell output, so at Vcell = 8.5V, Vin to reg = 8.5 - 5.2 = 3.3 V. A better way would be to use a pre-regulator, but at low solar cell output, there is still a problem of switching the series pass transistor on hard enough, to reduce the voltage drop across it. I would try a PNP transistor in the positive line, so the base can be turned on by the current flowing from the negative line, with the output taken from its collector.
An idea that intrigues me is to use a very crude PWM system, i.e. the switching the solar cell voltage on and off and putting the result though a LC filter to recover the mean DC voltage. So it goes like this, a PNP pass transistor (see above) driven from an astable, where one base bias resistor is taken to a stabilized source (zener diode +resistor). As the input voltage changes, the mark/space ratio will change driving the pass transistor. Run it as fast as you can (100KHZ?) to minimise the size of the L. Who knows it may just work!
Frank

 

[tpetar]

Okay...So can you suggest me any other alternative idea ?

A solar-powered buck/boost battery charger
**broken link removed**