What technical progress enabled high-efficiency charge pumps?

Question

I found a lot of information on the internet, most of which told me that charge pumps are only suitable for low-power scenarios and they are not very efficient.

Charging ICs based on the charge pump can provide very large currents and are very efficient. For example, TI's BQ25970 can reach 96.5%@3A.

What happened?

What factors determine the efficiency of the charge pump, and what changes have been made to this kind of charge pump-based charging IC to provide large current and high efficiency at the same time?

Answer 1

The electrical efficiency of charging a capacitor from a constant voltage source through a resistance is given by

$$\eta = \frac{\text{energy transferred to capacitor}}{\text{energy transferred to capcitor + energy dissipated}} =\frac{V_{C(0)}+V_{C(1)}}{2V_{source}}$$

where the capacitor is charged from \$V_{C(0)}\$ to \$V_{C(1)}\$ from a source of voltage \$V_{source}\$.

If a capacitor is charged from 0 V to \$V_{source}\$ through a resistance, then the efficiency is 50%. However, if the capacitor is charged from say 90% of \$V_{source}\$ to \$V_{source}\$, then the charging efficiency will be 95%. The ratio of energy that is dissipated to the energy that is store in the capacitor is only 1:19. So, when using capacitors as energy storage, and when charging them from a constant voltage source through a resistance, it is generally advisable to keep the voltage on the capacitor near the source voltage, to keep capacitor charging losses low.

The loss due to capacitor charging is only one of the loss factors in a capacitive charge pump. However, the above equation tells us that our converter will be more efficient if we ensure that the voltage ripple across the capacitors is low. The voltage ripple will decrease if we increase the switching frequency, and also if we increase the capacitance. The voltage ripple will increases as we increase the current drawn from the converter.

Although they are not new, MOSFETs often permit higher frequency switching than BJTs, and lower conduction losses. Similarly, MOSFETs may be used as active rectifiers, which reduces rectifier forward conduction losses. MOSFETs have the drawback, however, that at high frequencies, gate current losses are high. Another drawback of MOSFETs in capacitive charge pumps is the external components required for driving gates. A voltage doubler using 2 MOSFETs for primary switching and two MOSFETs as low conduction loss rectifiers requires 4 gate drive circuits. This adds complexity to a design. However, complexity entails neither low power, nor low efficiency.

Answer 2

Don't believe everything you read on the internet. Even with passive switching, well-designed Cockroft-Walton circuits can be very efficient, and have been since Cockroft and Walton used the idea to enable their Nobel Prize winning research.

The main loss mechanisms are capacitor charging and switch voltage drop. As @Math points out, capacitor charging losses are minimized by providing adequate capacitance to keep ripple down. For step-up Cockroft-Walton style circuits, the usual way to keep switch voltage drop loss down is to keep the voltage steps large compared to a diode drop.

At lower voltage, other topologies using MOSFET switches instead of diodes yield better efficiency by eliminating the diode drop. The BQ25970 is not a Cockroft-Walton circuit, it's step-down, and it uses active switching.

Answer 3

Charge pumps can be quite efficient if the capacitors are kept close to full charge. That means a few things.

1. The capacitors should be large.
2. The switching rate should be high.
3. The charge pump should not be used for voltage regulation.

Using a charge pump for voltage regulation is much like using a linear regulator. Any excess voltage is simply wasted.

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Documentation on the BQ25970 in english* seems fragmentary, the data sheet is practically useless. It cites a typical efficiency of 97% but says nothing about what conditions under which that is acheived. The normal graphs and so-on which one would normally expect to find in a datasheet are completely missing. There is a block diagram for "simplified application" but no complete example schematics. Heck even the pin descriptions are missing.

There is however an application note, "How to use the BQ25970 for Flash Charging". Reading this application note and also reading the data sheet for the BQ2589x which is mentioned on the "Simplified block diagram". I conclude that this chip is NOT intended to act as a main charger, but instead as a secondary charger to allow faster charging under specific circumstances.

Specifically it is intended to be used in conjunction with the "programmable power supply" feature of USB power delivery. This allows the efficiency loss inherent in using a charge pump for regulation to be avoided by taking control of the input voltage from the external power supply.

* It seems there may be a proper data sheet in Chinese, but since I don't read Chinese that isn't any help to me.