MOSFETs balance supercapacitors with zero power burn

March 28, 2016 //By Robert L. Chao
Adding ALD ALD910024 SAB MOSFETs would bend C2 leakage current curve to follow IC2 + IOUT2 curve.
Robert L. Chao, Founder, Advanced Linear Devices, Inc. explains how MOSFETs deployed in circuits that balance voltage and leakage current in supercapacitor series-connected stacks of two or more, can be configured to burn zero power.

The use of supercapacitor series stacks are growing for many systems used in backup power storage or battery life extension. One of the most critical circuit design goals for such systems is minimizing the steady state DC power dissipation. For that reason, MOSFETs deployed in circuits that balance voltage and leakage current in supercapacitor series-connected stacks of two or more, can be configured to burn zero power.

To grasp the concept of zero power burn in balancing the individual supercapacitor cells, I will describe a circuit with current burn of 0.003 micro amperes (uA), or ~0.1% of 2.80 uA. While this is not absolute zero, the amount of energy used is so minimal that it is virtually zero. Using MOSFETs to balance voltage in supercapacitor cells stacked in a series contrasts to balancing voltage using op amp, which burns quiescent current.

Supercapacitors are becoming increasingly useful in high-voltage applications as energy storage devices. When an application requires more voltage than a single 2.7-volt cell can provide, supercapacitors are stacked in series of two or more. An essential part of ensuring long operational life for these stacks is to balance each cell to prevent leakage current from causing damage to other cells through over-voltage. For those seeking more information on how this works, I invite you to read our previous submitted article, “ MOSFET-based current balancing cuts power use in supercapacitor stacks

There are actually three possible scenarios of zero power burn of balancing circuitry. First, the power dissipated can be near zero, meaning that it is substantially less than a reference power dissipation level.

In this case, l use the highest leakage current within a group of a given make and model of supercapacitor as a benchmark reference power dissipation. The highest leakage current of one of the supercapacitors used is the actual minimum leakage current possible for the entire supercapacitor stack, not including any power used by any

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