Voltage regulator for heatpad with PWM

Hi, I am fairly new to electronics and would like to ask if I should be concerned about thermal shut down for the voltage regulator.

I am trying to use a Pololu 12V step up regulator (U3V12F12), powered from two series LiPo batteries (~7.4V total), which then enters an Adafruit motor driver. This is then controlled via a PWM and ground signal, to heat a heatpad to certain temperature (a sensor feeds back to the uC, to control the duty cycle).

I have a couple of instances of breaking the MOSFET. I am now using an anti-static mat and bracelet, and was worried it could be due to LC voltage spikes so have added the 33uF capacitor between Vin and GND as recommended.

I need to power the heat-pad for excess of 12 hours. The load draws 0.5A at 12V, but the PWM duty cycle should never be higher than 50%. Furthermore, I may double the size of the heat-pad, so this may draw ~1A in future designs. Given thermal shut down with continuous operation, would I be safer using the Pololu 12V Step-Up/Step-Down Voltage Regulator S18V20F12 model?


Could you clarify what was breaking (i.e. the regulator or the motor driver)?

With an input voltage of 7.4V and a 0.5A load at 12V, the input current would be around 0.9A, so I generally expect the U3V12F12 regulator to be fine (as mentioned on its product page, it can typically handle input currents as high as 1.4A).

However, if you double the load to 1A, you should use a more powerful regulator. The S18V20F12 regulator you mentioned would probably be fine, but I recommend the U3V40F12 regulator instead. It is a smaller form factor, can handle more current (so it should generally run cooler), and is more economical right now since the S18V20F12 is currently rationed due to the global part shortages.


Hi Brandon,

Thank you for getting back to me so quickly. It was the motor driver that would break (no matter what PWM signal, there was no output). Thank you that is very helpful.

Do I need to place capacitors around these regulators due to the LC spikes?

Kind regards

Hi again,

I also have a further question: I am powering my circuit from rechargeable lipo batteries (I am considering putting two in series to help with efficiency of the regulators, so 3.7V*2=7.4V total). For my project, I need a 12V regulator for heat pads, 5V for the uC, and 3.3V for a sensor (I’d rather power it not through the uC). I am trying to work out the most efficient arrangement – is it possible to connect all three regulators to the battery pack, and how would I determine how much current is distributed in theory? Or is it better to step down to 5V from the 12V regulator, and then the 3.3V from the 5V regulator?

Thank you very much.

If your heat pad uses something like a peltier device to create the heat, PWMing that might be particularly stressful on a motor driver since the current doesn’t getting smoothed by any inductance like it would with a motor (similarly to how the heat gets smooths because of thermal inertia). You could try putting a large capacitor at the power input to the motor driver.

Concerns with LC spikes are generally from the power input to the regulator; you likely don’t have to worry about that with a 2S LiPo battery in this kind of setup, but it wouldn’t hurt either.

As far as using multiple different regulators for different parts of your system, it should be fine to power them all directly from the LiPo. There is no concern about power getting distributed unevenly if they aren’t connected to the same load. Cascading the regulators like you mentioned (i.e. stepping up to 12V, then down to 5V, then down to 3V) would work as well. There are trade-offs between each configuration. For example, stepping up to 12V first, then down to 5V would probably allow the 5V regulator to run a little cooler (since it’s stepping down from a higher voltage), but your 12V regulator would need to be more powerful since everything is going through it and the efficiency losses from each regulator in the chain will be stacking up as well.


Thank you that’s very helpful!

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