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Story

I am designing a boost converter for a battery powered board. I am planning to use single Ni-MH battery. The needed output voltage is 3V3. The device will be in deep sleep mode (drawing few microamps from the 3V3 output) most of the time. Therefore light load efficiency is one of the key points for me. I have found TPS61021A from TI. However it doesn't implement any protection for batteries.

One of the alternatives is TPS61200. It provides at least UVLO protection but has worse efficiency than TPS61021A at light load.

I haven't designed a power converter circuit containing Ni-MH batteries yet. I think that it is better to include some protections for battery. This won't be a commercial product, it is a personal project. Cost isn't a big issue. I want to learn how to handle Ni-MH battery correctly during discharge.

Operating ambient temperature of the circuit is between -20C and +50C.

As far as I searched, I couldn't find a complete power management IC for Ni-MH batteries, especially for single cell case. Moreover, most of the literature are for lithium batteries. Since the nominal voltage of Ni-MH battery (~1.2V) is lower than lithium ones ( >3.5V), designing a proper protection circuit is more challenging.

So, I have thought 3 protection topcics so far. Please free to add if you think more is necessary.

Undervoltage lockout protection

I want to switch off the converter when the cell voltage is below 0.9V or 1V. Since TPS61200 doesn't have UVLO feature, I have to implement UVLO externally. One option is disabling the converter IC using EN pin. To do this, I may use a voltage reference IC and a comperator or a single transistor basically. However it should also have hysteresis to prevent oscillation.

Overcurrent protection

I am planning to use a resettable PTC fuse to avoid dead shorts actually. Since the battery will only be connected to converter IC, short is possible in case of IC fault. I want to include a loose protection and I think PTC fuse is just fine.

Overtemperature protection

I don't know whether it is meaningful for Ni-MH battery or not. It may be unnecessary since there will be a overcurrent protection. A thermistor based circuit to disable the converter IC via EN pin may be a good option, I think.

For these protections, I couldn't find ready-to-use ICs or schematics. I will be happy if you can direct me for single cell Ni-MH protection.

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  • \$\begingroup\$ Which NiMH cell do you intend to use (size, capacity, type etc.), and what operating time are you expecting? \$\endgroup\$ – Bruce Abbott Aug 21 '16 at 19:56
  • \$\begingroup\$ I am planning to use a standard AA cell battery, like this one: gpbatteries.com/int_en/2700aa which has 2700 mAh capacity. I haven't estimated the operating time yet, just trying to design an efficient circuit. \$\endgroup\$ – Alper Aug 21 '16 at 20:05
  • \$\begingroup\$ There aren't a ton of ICs for standalone protection of NiMH cells because they're substantially less volatile than Lithium cells. A PTC is one good way to go for good passive protection. If you stay in the nominal operation range specified for your cells explicit overtemp protection isn't needed if your circuit does not need to handle charging of the cells. If your circuit is charging the cells you'll definitely want temperature protection. For undervoltage lockout you could build a simple comparator circuit with the desired hysteresis to switch a FET, or your converter's enable line. \$\endgroup\$ – alphasierra Sep 8 '18 at 1:39

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