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MOSFETs balance supercaps with zero power burn

Posted: 08 Apr 2016 ?? ?Print Version ?Bookmark and Share

Keywords:supercapacitor? MOSFETs? op amp? DC?

Without the SAB MOSFETs, VOUT voltage continues to rise towards 4.60V. If it does, it will slowly destroy C2 as the VC2 = VOUT voltage exceeds 2.7V maximum rated voltage towards 2.9V, causing for example, an open circuit to C2 and rendering the entire supercapacitor series stack to become inoperative in a catastrophic failure event.

At Step 3, the VOUT voltage reaches 2.45V. At this VOUT voltage, all the currents are again changed. VC1 =2.15V and VC2 =2.45V. IC1 is now approximately 2.20 uA (see graph in figure 2). IOUT1 is now at approximately 0.003 uA. IC2 is at approximately 0.90 uA and IOUT2 is at approximately 1.303 uA. The leakage currents of IOUT1 + IC1 now add up to 2.203 uA while IOUT2 + IC2 also add up to 2.203 uA. The leakage currents are now in balance, which stabilises VOUT voltage at about 2.45V. VC1 and VC2Vages are both within the 2.70V maximum rated voltage limits, and without any further changes, will not damage either of the two supercapacitors C1 and C2.

Any attempt to increase VOUT voltage will meet with significant increases of IOUT2 thereby limiting further VOUT voltage increase. At this point VOUT resists any further changes due to minor changes in supercapacitor leakage currents of both C1 and C2. When this equilibrium point is reached, the total leakage current of IOUT1 + IC1 is now ~2.203 uA instead of the IC1 of 2.80 uA without leakage current balancing. This example illustrates that "negative", or below zero power burn is possible when the balancing circuitry utilising SAB MOSFET is deployed.

In this example, the extra power is dissipated by IOUT1 which is about 0.003 uA. IC1 of C1, now at ~2.20 uA is the dominant leakage component internal to the supercapacitor, and it is less than the reference leakage current specified as 2.80 uA at 2.3V cell voltage. Net additional current burn is 0.003 uA, ~0.1% of 2.80 uA, which is approximated to zero power burn. Note that this 0.1% is that of the leakage current specification of the supercapacitor. So if that leakage current is greater or lesser, for different make or models, the extra power burn can be scaled accordingly.

For circuits described above, MOSFETs sense that the voltage wants to go up, so one of them starts leaking current very quickly, without allowing the voltage to go up much. Because it is exponential in nature and the current goes up, it will automatically float to a point where the MOSFET current IOUT1, plus the IC1 current would be equal to the leakage current of MOSFET, IOUT2 plus IC2.

There is a push-pull dynamic relationship. In other words, there are two supercapacitors and two MOSFETS, but only one MOSFET is turned on harder at any given time while the other MOSFET would be turned on a little softer. Since there is no way to know which supercapacitor has higher leakage, placing the MOSFET across both supercapacitors will balance the network automatically. Since the specific leakage of each cell is unknown, the one that has the higher leakage would be automatically balanced by the corresponding MOSFET. When a MOSFET is placed across each supercapacitor, it automatically balances the system, by equalising whichever supercapacitor has the highest leakage current.

To summarise, MOSFETs can:
???Lower additional leakage current to zero levels
???Completely and automatically balance supercapacitors
???Offers low component count and low implementation costs
???Provide simple and yet elegant solution
???Offer scalability to any number of supercapacitors
???Adjusts for changing environmental conditions and leakage currents.

The examples illustrated above explain the zero power dissipation operation of the balancing circuit action. However, there are numerous other possible combinations, where the SAB MOSFET balancing solution, while adding little or no leakage, does allow a lower voltage bias on the leakier supercapacitor. The actual total leakage current, and hence the power dissipation caused by the series-connected supercapacitors can be potentially less than not balancing the circuit at all.

Selecting the right SAB MOSFET requires knowledge of the supercapacitor operating voltage and maximum rated leakage current. This balancing method limits leakage current better than any other method. SAB MOSFETs also actively adjust to different temperature or supercapacitor chemistry changes. A designer can just pick the maximum operating voltage margin and the maximum leakage current for the particular supercapacitor(s) and look up the correct SAB MOSFET part number.

About the author
Robert L. Chao is the founder of Advanced Linear Devices Inc.

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