**Power/Alternative Energy??**

# MOSFETs balance supercaps with zero power burn

**Keywords:supercapacitor?
MOSFETs?
op amp?
DC?
**

Without the SAB MOSFETs, V_{OUT} voltage continues to rise towards 4.60V. If it does, it will slowly destroy C2 as the V_{C2 }= V_{OUT} 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 V_{OUT} voltage reaches 2.45V. At this V_{OUT} voltage, all the currents are again changed. V_{C1 }=2.15V and V_{C2 }=2.45V. I_{C1 }is now approximately 2.20 uA (see graph in **figure 2**). I_{OUT1 }is now at approximately 0.003 uA. I_{C2 }is at approximately 0.90 uA and I_{OUT2 }is at approximately 1.303 uA. The leakage currents of I_{OUT1 }+ I_{C1} now add up to 2.203 uA while I_{OUT2 }+ I_{C2 }also add up to 2.203 uA. The leakage currents are now in balance, which stabilises V_{OUT} voltage at about 2.45V. V_{C1 }and V_{C2V}ages 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 V_{OUT} voltage will meet with significant increases of I_{OUT2 }thereby limiting further V_{OUT} voltage increase. At this point V_{OUT} 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 I_{OUT1 }+ I_{C1} is now ~2.203 uA instead of the I_{C1} 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.

**Example**

In this example, the extra power is dissipated by I_{OUT1 }which is about 0.003 uA. I_{C1 }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 I_{OUT1}, plus the I_{C1} current would be equal to the leakage current of MOSFET, I_{OUT2 }plus I_{C2}.

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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