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Impedance matching dc dc converter

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engineer1000

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I have a photo type cell that I want to feed into a dc dc converter using a max668 PWM step up controller. The photo cell has an impedance of approx 5R. I know to get my max power transfer i have to match this resistance with the Rin of my dc dc converter.
Anyone got any suggestions??
 

FvM

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It's wrong to say a solar cell has a specific (fixed) "impedance". Instead the IV characteristic and the respective maximum power point changes with irradiation.

To get maximum power out of the solar cell (so called maximum power point tracking), the DC/DC converter power demand has to be adapted to the actual irradiation level.
 

engineer1000

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It's wrong to say a solar cell has a specific (fixed) "impedance". Instead the IV characteristic and the respective maximum power point changes with irradiation.

To get maximum power out of the solar cell (so called maximum power point tracking), the DC/DC converter power demand has to be adapted to the actual irradiation level.


I think I need an isolated dc dc converter otherwise the impedance will be governed by the load resistance. or I could vary the switching frequency using a pic to match the impedance. I don't want to involve firm ware if I can help it.
IS there another way of doing it?????
 

FvM

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If a DC/DC converter is isolated or not doesn't change it's impedance properties.

You didn't yet tell anything about the intended load, only some (probably wrong) assumptions about impedances.
 

asdf44

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Look up MPPT. This is a problem many people have solved.

As FvM says, you can't model a solar panel as a fixed impedance.

And it doesn't matter if it's isolated.
 

engineer1000

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I don't really know the load other than a max of 5w.

- - - Updated - - -

its actually a teg . I've looked up MPPT and understand that by alteringthe switching frequency you can match the imedance. I am trying to find a way to do this without using a pic or firmware.
 

SunnySkyguy

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A PV is essentially a lossy current source that has a zener like open circuit voltage, Voc that declines slightly with lower solar input.

The maximum power point is approximately 80+/-5% of Voc when the VI product has a shallow peak in power about this centre.

You can hunt for this peak using MPPT regulated current sinking SMPS or approximate it using a CC current sink that regulates the input current by regulating the voltage around this mean. A separate tiny lambertian photo sensor can estimate the solar power input to determine what Voc should be, or you can pulse the main PV with no load and measure it.



Beware that a series choke is necessary to maintain a constant current sink to match the source and large di/dt from choke inductance may cause large voltage transients or large dv/dt may cause large current transients from array capacitance. I believe a Kalman PI filter model provides the means to optimize these and also Solar transients when using PWM control.

PI, meaning Proportional and Integral feedback in the control system to give a fast tracking solution.

Triangular dither and hunt methods are very slow. (> 10s)
 
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