Constant Power Micro Load

Question

I am trying to make a circuit that slowly discharges a multi cell lithium battery to storage voltage. I already have a comparator circuit that can turn on or off discharging. I want to support 2s to 6s batteries so we are talking about voltages ranging anywhere from 7.6v to 25.2v. I am shooting for 500mw of power dissipation so I can achieve my form factor requirements for the PCB (11mm x 15mm). Ideally, the same circuit can discharge any cell voltage with constant power. So far I have tried/considered:

• PTC Thermistor load. This will essentially run the PTC perpetually in its "tripped" state. I have an NTC thermistor in series to handle the inrush current until the PTC heats up and protects the circuit. This best captures what I want. However, I contacted a support engineer and they say the use case is not specified and could negatively impact the life of the device. https://www.murata.com/en-us/products/thermistor/ptc/prg

• BJT with an NTC thermistor switching a resistive load on or off. I would preferentially avoid this due to extra components necessary.

• I would like to stay away from MCU controlled constant heaters for simplicity sake.

• Buck or Boost converter is not practical for form factor and cost.

Any help will be appreciated.

More context:

• I am creating a battery discharge circuit small enough that can be integrated into a battery connector (like an XT60 adaptor).
• Users may inadvertently connect a higher cell count than selected or thermally insulate the device so I want overtemperature and overcurrent protection.
• Users can choose cell count with a DIP switch so I want a constant power load.
• I also want to decrease the BOM cost of course, so one device that does it all would be wonderful :). So far a PTC thermistor fits the bill but I am wary of leaving it in a tripped state for extended periods.

Edit: One last requirement that I overlooked (I apologize). The device will be "plug in and forget" so it needs to have a very low quiescent current so it can be left on the battery for months or even years without over discharging the battery (which will damage it). The solution I have right now has a leakage current in the microamp range.

• Current Schematic:

Answer 1

The type of PTC you need is a ceramic one, I don't think the polymer "polyfuse" type are suitable. PTC ceramics were commonly used as fairly constant temperature heaters, and probably still are in hair-tongs etc, where they use a PTC thick film resistor printed on a ceramic plate. However these are commonl;y used today for pretty high temperatures, which might be more than you would want inside a soft plastic connector. If you use overcurrent protectors, you will also find that these are high temperature devices - they will have to idle at (say) 120C. If you use an SMD device. this means the soldered joints and pcb have to take that, which they might not like long term. Against that, this is not really a very high value or very long term device so it might not matter. Leaded disc types transfer much less power to the pcb and solder, so would be a more reliable choice. There are PTCs for sensing with s lower slope, and which have a much more constant slope rising from lower temperatures.

Motor protection PTC's have much more that range you are looking for. When you look at the lower sense temps (50-90C e.g. B59100M1070A070) you will see that the don't have the nasty NTC part at the start of the curve.

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FYI, here is a constant power, constant current circuit. This also meets your objective using very cheap components, and without requiring a very specific thermistor.

The constant current emitter follower Q2, has its reference voltage subtracted by the current mirror Q1Q3 in proportion to the supply voltage. The result is a linear dropping current i.e. constant power when the slopes are balanced. R3 is the primary slope adjuster.

^{simulate this circuit – Schematic created using CircuitLab}

We can simplify this using Q4 to replace the zener (Q3 cancels Q2 VBE, so Q4 provides the constant reference voltage). This gives an NTC, but this is what we want. 2 dual-npn and 3 resistors is about as cheap as you can get.

^{simulate this circuit}

Flipping the circuit around we can gate it with a TL431 to control stop voltage, and give near zero off current

^{simulate this circuit}

Answer 2

I'd use a LM239 or any other cheap dual comparator with open collector outputs.

Comparator #1 with a cheap voltage reference/LDO and a resistor divider to set the discharge voltage.

Comparator #2 with a thermistor as temperature sensor makes a thermostat.

Open collector outputs are joined together, creating a logic AND to only draw current when temperature is below a certain value, and voltage is above the threshold.

You'll need a transistor as a switch, and a SMD resistor to dissipate the heat. Make sure to place the temperature sensor next to the resistor and that their ground pads are connected with lots of copper. If you want to use the PCB as a heat sink you'll need a copper pour anyway.