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A recent question about cyclically charging a capacitor reminded me of something I read once. As I remember, it demonstrated that it's impossible to construct a charge pump that is 100% efficient with ideal components, but it is possible to build a 100% efficient boost converter with an inductor if components are ideal.

Does this resonate (no pun intended) with anyone else? Any way to demonstrate or refute the truth of this?

To be clear: we are assuming we have ideal components. I realize no real circuit will be 100% efficient with real components. Diodes may have zero voltage drop. Transistors may be ideal switches that take no energy to change state. Wires may have zero resistance.

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3 Answers 3

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It is all about dualism. With ideal components, you can make an ideal SMPS type voltage converter (= using an inductor to do the work). You can't make an ideal voltage converter using switched (flying) capacitors. That is not the universe being unfair to capacitors: you can make an ideal current converter using switched capacitors, which is not possible using inductors.

I can't do the math out of my head, but the problem with capacitors and a voltage source is like this: take a voltage source with a certain source impedance (= series resistor). Connect a capacitor to it and load it for an infinite time (any finite time will do too). Calculate the amount of energy lost in the series resistor as a function of its resistance. Now mathematically take the limit of that formula twoards zero resistance. You will find that the energy loss will stay the same. Intuitively this is because a smaller resistor causes a higher initial loading current, and hence a higher RI2 loss.

management summary: You can't connect an ideal voltage source to a capacitor, because that would result in an infinite current which is impossible in itself and would cause an infinite magnetic field which would destroy the universe (just kidding, remember this is the management summary). But you can approach this ideal as closely as you like, and the result will still be the same: a fixed amount of energy is lost while charging the capacitor. Hence: sorry boss, no ideal flying capacitor voltage converter.

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Excellent answer. Now that I understand the problem is unavoidable energy losses in charging capacitors, I was able to find the article I remember. –  Phil Frost Jan 14 '13 at 16:41
Actually, you can't get infinite current. Any circuit of nonzero area has nonzero inductance, and this will limit the current even if there's no resistance. But electromagnetic energy will radiate away from the circuit, so you still can't get 100% efficiency (but this applies to inductor-based switching converters, too). –  Dave Tweed Jan 14 '13 at 17:16
I guess Phil would cheat out of that problem by requiring zero-sized components and conductors :) –  Wouter van Ooijen Jan 14 '13 at 19:40
Also consider the common exam question of connecting a discharged capacitor to an equal value charged one, comparing the total stored energy before connection and after equalization time. –  Chris Stratton Jan 14 '13 at 20:12
@DaveTweed - an ideal capacitor is non-inductive. That you cannot build an ideal capacitor is an entirely different topic, irrelevant to how an ideal capacitor would theoretically behave in a circuit. Two connected ideal capacitors will equalize - the equations governing their ideal behavior require it. –  Chris Stratton Jan 15 '13 at 18:16

Well it really depends upon how far we go with "ideal components". If diodes had a forward voltage drop of 0 volts, BJTs had a base threshold of 0 volts a saturation of 0volts and infinite current gain, and FETs have a gate threshold of 0volts and a Rds of 0 ohms then it may very well be likely be possible to realize a 100% efficient change pump.

Even in the case of the boost converter it will not be 100% efficient unless the switch FET and flyback diode are ideal in the sense that I described above. Likewise the inductor has to have a DCR that is equal to 0.

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We are going all the way with ideal components. FETs that are ideal switches and take no energy to change state and diodes with no voltage drop are both fair. –  Phil Frost Jan 14 '13 at 16:16
@PhilFrost - OK Then. I cannot think why a charge pump could not be 100% efficient then...as long as all the wires are zero ohms resistance too. :-) –  Michael Karas Jan 14 '13 at 16:23
The only way to transfer energy between two capacitors or series-connected groups of capacitors is for there to exist a potential difference between the points where they are connected. Any such scenario may be modeled as connecting two capacitors C1 and C2, charged to voltages V1 and V2. The energy before the connection will be (C1·V1·V1 + C2·V2·V2)/2. The voltage after the connection will be (C1·V1+C2·V2)/(C1+C2), and the energy after will be (C1·V1+C2·V2)·(C1·V1+C2·V2)/2(C1+C2). The only time the two energies are equal is if V1=V2, meaning nothing happened. –  supercat Jan 14 '13 at 17:05
There's a way for an inductorless charge pump to be 100% efficient, but only if certain other conditions are met regarding the inputs and outputs. –  supercat Jan 14 '13 at 17:45

An inductor-less charge pump cannot be 100% efficient when powering a constant-voltage load from a constant voltage source. An inductor-less charge pump made with ideal components may be 100% efficient if the source current and voltage waveforms have the proper relationship with the load current and voltage waveforms. It is possible for either the source or the load voltage to be constant DC, but not both (except in the trivial case where both voltages are the same and the charge pump doesn't have to do anything).

Note: a charge-pump which contained an internal current source could be 100% efficient at converting input power from a constant-voltage source to an external constant-voltage load, with any energy that was drawn from the internal current source in one cycle being replaced on the next. On the other hand, such a current source would simply be taking the place of an inductor.

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Can you elaborate on what the "proper relationship" would be? –  Phil Frost Jan 14 '13 at 18:00
There are an infinite number of possible relationships, and I'm not sure there's any particularly nice way to characterize them. On the other hand, I can offer an example though: suppose one has two capacitors in series, one of which is charged to five volts and the other of which is discharged. Across the two capacitors is a 5K resistor (which will draw 1mA). If one connects a 2mA source to the cap which is discharged, it will charge from 0 to 5 volts at the same rate as the other cap discharges. If one then switches the 2mA source to the other cap, one may effectively repeat the process. –  supercat Jan 14 '13 at 18:14
The 2mA source will see the voltage across it rise from 0 volts to 5 volts, then drop essentially instantly to zero, then rise to five, etc. During the time when the input voltage is below 2.5 volts, the circuit will be taking less energy from the source than is going to the load; the difference between input and output energy to that point will match the change in the total energy of the two caps. While the input voltage exceeds 2.5 volts, the input energy will exceed the output energy, with the differential energy replenishing the caps. –  supercat Jan 14 '13 at 18:18
If what one has is not a constant current source, but at an AC voltage source whose voltage waveform matches the waveform that would have been yielded by a constant current source, the circuit behavior will be the same as it would have been with a constant-current source. Note that while this example for simplicity a voltage which went from 0 to 5 volts, it could have used a voltage that swung from -5 to +5; if one added a polarity switch, one could then accommodate a triangle wave rather than a sawtooth. –  supercat Jan 14 '13 at 18:22

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