I'm not sure which exact circuit is suggested by this description, but I doubt, that is able to fulfill the criteria for high efficiency SC voltage conversion. You should particularly consider, that transferring energy to a capacitor without utilizing an inductor burns a part of the energy according to (Vin-Vc)/Vin.It is feasible to chop the supply into pulses, then feed them to the top of a capacitor. The capacitor then powers the load at a reduced voltage.
The load voltage is determined by adjusting the duty cycle.
You're on the right track. Indeed you can't efficiently transfer energy between capacitors unless their voltage is very close together.I'm not sure which exact circuit is suggested by this description, but I doubt, that is able to fulfill the criteria for high efficiency SC voltage conversion. You should particularly consider, that transferring energy to a capacitor without utilizing an inductor burns a part of the energy according to (Vin-Vc)/Vin.
It is a nice solution, but are you sure it is more efficient than linear regulation (assuming low power regulator)?
Let us ignore the power needed by the "Pulse Generator". When the PNP transistor is turned on a current charges the capacitor for a short time in each cycle. But between the higher supply V+ and the capacitor terminal there must be a sort of resistance to limit this charging current. A power will be dissipated by this equivalent limiting resistance. I believe, we can find out (by calculation) that the power efficiency will turn out be the same of a linear regulator (if its own power can be neglected).
Would you explain what a constant gm circuit is?As suggested you can use switched capacitor constant gm circuit. Use this current to take the reference voltage, I am not sure about regulation though.
This is just like using a LDO but switching it on and off at some duty cycle. It's still a LDO, and it still has the same efficiency limitations, since when conducting the transistor still drops a voltage of Vin-Vout. So no this can't work.I follow your reasoning. It adds weight to the view others have expressed, that distributing power from one capacitor to another will not solve the efficiency problem.
My approach started with the idea of switching power to the load. All on. All off. More efficient than the linear method. A slight amount of power is wasted in the transistor/mosfet. I had the idea that this was the only waste in my schematic. It should be no more than is wasted in a conventional switch mode converter.
I admit my method displays a certain amount of "Let's apply a short enough pulse to a large enough capacitor, and see if that gets us in the vicinity of the load voltage."
This post is a follow-up for the sake of exploring possibilities, since we did not expect to find a solution in the linear (voltage drop) method.
Diagram to demonstrate the premise:
Step 2. Add a capacitor to reduce ripple (smooth out pulses). This diagram is like my schematic in post #7 but with the positions of load and capacitor reversed:
I suppose this method would be well-known as a working method if it really worked. So there goes my million dollar idea, alas.
I think it boils down to this: in order to transfer energy and change voltage, you need a component whose stored energy does not change when its voltage is changed. The only thing that satisfies that condition is an inductor. So with the exception of certain switched capacitor circuits (voltage inverters/doublers/etc), you need inductors to make efficient DC/DC converters.What would be excellent is if some offshoot of this concept could work with an LED load. However LED's are not an ohmic resistance, and they instantly **** all spare charge from a capacitor when it's greater than their turn-on threshold. So a resistor still is the easy way (and inefficient way) to limit current through LED's.
However for loads which are an ohmic resistance, I'm surprised if the simple switching method in my schematic can't be used (or else modified) to be more efficient than the linear method.
Okay, I'm fairly certain now that switched capacitors won't help; their theoretical max efficiency is the same as for linear regulators. I don't believe you'll find a solution to your problem without in inductors. And even then, meeting your specs will be difficult (switch mode DC-DC converters generally don't work efficiently at low output voltages and low output power levels like yours). Is it really unacceptable to require an external inductor, or use an external regulator altogether?
You'll however find examples of energy efficient SC converters in literature, partly using several levels of capacitor parallel/series switching to achieve optimal efficiency for different voltage ratios. As a simple example, you can refer to National LM3350, a fractional 3/2 or 2/3 converter with 90% efficiency.
Correct, and circuits like are efficient because the voltage of each capacitor in the circuit is equal and pretty much constant. So with additional capacitors and stages, it's you can get any Vout/Vin ratio with good efficiency. But 3.3V to 1.2V is a pretty inconvenient ratio (11/4), so you'd need a ton of stages and switches. Maybe using a 1/3 divider could work as well.You'll however find examples of energy efficient SC converters in literature, partly using several levels of capacitor parallel/series switching to achieve optimal efficiency for different voltage ratios. As a simple example, you can refer to National LM3350, a fractional 3/2 or 2/3 converter with 90% efficiency.
To me, charging a capacitor with (Q) during a time (t) means the capacitor should be supplied by an average current of (Q/t).
Kerim
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