# Maximum power out of thermoelectric generator?

The conventional method to extract maximum amount of energy from ThermoElectric Generator (TEG) is to load it with a resistance equal to the TEG's internal resistance as the maximum power transfer theorem. However, if the load of TEG is not resistive but inductive like boost converter. How would you use maximum power transfer theorem here?

Also how about the case the boost converter operates in DCM mode so the time which NMOS switch ON is much larger than the time for which PMOS switch ON? Is there a special to do maximum power transfer in this case? The TEG sees as its load the voltage across Cin.

If Cin is large enough to reduce the switching effect of the boost converter to a small ripple, then there is simply a steady(ish) current flowing through Rteg to the steady(ish) voltage on Cin. That is resistive.

The boost converter should then contrive to draw (on average) this current out of this voltage on Cin.

The problem is that this circuit does not have access to Vteg, so does not know what Vteg/2 is. There are a number of solutions -

a) hunt around for the voltage on Cin that gives max power output (this is the conventional closed loop MPPT algorithm)

b) assume conditions will change slowly at the TEG, and switch off the boost converter briefly from time to time to let VCin rise to Vteg, to measure it. This loses little power throughput if the switching frequency of the boost converter is >> the Rteg.Cin time constant, to minimise the fraction of time the boost converter spends off

c) assume a value for Rteg, and control the mean current the boost converter takes from Cin to be equal to Vin/Rteg. This will work for both DCM and CCM.

d) you might want to reference the paper from your other post in your question, as maybe controlling the ON time was a clever way to do (c)? I still haven't figured out why they think it should work, but somebody else may have more time to spare.

• Is R teg constant and Known .If it is then [c] will be easy to get working . – Autistic Mar 14 '16 at 21:04
• Yes, Rteg is assumed constant and known here. – anhnha Mar 16 '16 at 10:45
• @Neil_UK: with c, do you mean that the power transfer to inductor when mean inductor current is equal to Vin/Rteg? I also don't see how to implement this idea. – anhnha Mar 16 '16 at 10:47

Power is the rate at which energy is delivered to or taken from a source. The energy taken per cycle by the nmos switch being on is related to how much current is flowing through the inductor when the nmos goes open circuit.

Energy = $\dfrac{L I^2}{2}$

The current of course linearly ramps up to some value during the period of closure of the nmos switch and this represents a small but finite amount of energy. This then becomes power when you consider this is done several thousand times per second so: -

Average power transferred is $\dfrac{fL I^2}{2}$

This means the maximum power transfer theorum holds up if you want it to. Also, if you don't care about the way the energy is removed in bursts you can make the input to the booster look more conventionally "DC" resistive by using a large valued capacitor at the terminals to the converter.

This hasn't really got anything to do with DCM or continuous mode.