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MOSFET Deisgn

I just designed (if it works out) a super efficeint Beam Solar engine that works at low voltages. Before I post it I want to ask a few questions to see if I need to revise my design. 1) MOSFETs work on voltage, no current, right? 2) Do MOSFETs have a voltage drop, like biploar transistors have a voltage drop of about 0.6 volts? EDIT: I got a new design that I want you guys to evaluate that doesn't use mosfets. It's incredibly simple and I want to know what you guys think. How it Works: The solar cell energizes the transformer coil while turning on the transistor. The transistor shorts the circuit and then the transistor turns off, then it repeats creating pulsed dc that drives the stepup transformer. I chose this design because there's very little components and there's no voltage drop across the transistor to the transformer. Theoretically this could run on as low as 0.7 volts, which is good for BEAM robots.

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i dont understand your topic.
westfw9 years ago
1) Yes; voltage, not current.
2) If you're talking about a minimum gate voltage to turn the transistor on, then the relevant datasheet value is "VGS(thresh)", which is usually about 4.5V for regular mosfets, and maybe 2V for "logic level" mosfets. (the consequence is that mosfets tend to like relatively high voltages, even though nearly zero current is involved.)
guyfrom7up (author)  westfw9 years ago
For 2) I mean like whenever you put a voltage across it, does the voltage get lowered? like if you put 1 volt through a bipolar transistor, the other side you get .4 volts, does that happen with mosfets (assuming that the transistor is on)?
If I understand your question correctly, there is no "voltage-drop" per se. It is similar to asking what the DC voltage drop across a capacitor: It is (transiently) infinite (or, more correctly equal to the supply voltage) initially, then drops to zero (dependent on the RC time) when fully charged. The gate current is thus, initially very high (just as it is when beginning to charge a capacitor) but drops to nano Amps when in the fully on state. This gate current is actually just a leakage across the insulated junction. The question I have is: have you tried this circuit as written? Generally, MOSFETs turn on at (if I remember correctly) about 4 volts (ie above your supply voltage and do not fully turn-on until 10 volts or more (just like the current dependence of a bipolar: it isn't just on-off). Your circuit depends on creating an oscillator within the solar cell-MOSFET-transformer reactance, and on this oscillator switching your source-drain current. As written, when the gate goes positive, you short the solar cell across the source:drain of the MOSFET, resulting in heat in your MOSFET, not current through your transformer primary (as an inductive reactance recall, it opposes rapid changes in current.
One last comment: Your CONCEPT is excellent:: an efficient charge pump in the voltage/current domain of a solar cell is excellent. Charge pumps become most efficient at high frequencies (at low frequencies you need a big-ole, ie heavy, transformer core; at high frequencies, transformers become much more effecient without requiring a heavy iron core). Charge pumps can have efficiencies of 85-95% at high frequencies, good enough for what you need. The problem then would be to design them to work at the proposed voltage and current. I am looking forward to your latest work on this topic.
The 0.6V drop that bipolar transistors are famous for happens between the base (control signal) and emitter, NOT on the switched current (which goes from collector to emitter.) The Vce voltage drop is typically much lower than 0.6V on bipolar transistors (usually referred to as Vce(sa), and frequently down around 0.2V or so.) In bipolar transistors, the voltage is quantum-physics related, and stays more-or-less constant regardless of current. The similar path on FETs (including MOSFETs) is purely resistive, so the voltage drop DOES vary with current (and is very small for small currents.) This is the commonly quoted Rds(on) parameter. The lower the better, and down around a milli-ohm for good power MOSFETs. (homework assignment: if a power MOSFET has Rds(on) of 10mohms, at what current does it break-even with a bipolar transistor having Vce(sat) of 0.2V ?)
guyfrom7up (author)  westfw9 years ago
thanks!
westfw9 years ago
(now commenting on the schematic...) hmm. So it's more-or-less a "Joule-thief" circuit powered by the solar cell. You'll certainly need a resistor in the base connection for the transistor... The Joule thief isn't noted for its efficiency, but this could be an interesting way of getting higher voltages from relatively high-current low-voltage solar cells. (I think the JT requires that the power source deliver relatively substantial currents, at low voltages. In theory, that's a good match for some modern solar cells...)
guyfrom7up (author)  westfw9 years ago
hmmmm, well 2mA isn't that much current...
Does it work, or is it still all on paper?
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