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Maintaining supply voltage on load

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manycalavera

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I have designed a circuit to keep the heating element at a constant temperature. The digital part of the circuit is being feed from a 5V LDO which is powered by two 18650 batteries in series and the heating element is directly connected to the output batteries. Also heating element wire(approximately 2ohm) is controlled with an N channel low side mosfet by the microcontroller.

What i noticed is that the LED and the seven segment display(connected to 5V output of LDO) seems to flicker when controller toggles the mosfet because of the voltage drop on the supply. Is there anything i can do to isolate these rails so that current draw for the load does not affect the 5V components?

The highest supply voltage for the system is 8.4V and currently the load roughly uses 2A. The problem is becomes more significant when the supply voltage gets lower. I don't think that any inrush current situation occurs because voltage continues to stay below 5V at the output of LDO when the load is on after few seconds. Also i tried with both batteries(which i am sure that can supply a lot more than 2A) and a lab bench power supply with no luck. I read that bulk capacitors can fix instant voltage drops but can they prevent constant voltage drop? If so where should i place these bulk capacitors?

Here is my schematic and pcb:
IoWzH.png

7bj1w.png
 

better to have a pwm buck converter drive the heater from the batteries such that there are no sudden changes in the battery voltage

OR: drive the fet with a 470k resistor such that it turns off and on much more slowly - which will also reduce the glitch effect on the batt volts ...
 

Is there anything i can do to isolate these rails so that current draw for the load does not affect the 5V components?
...
I read that bulk capacitors can fix instant voltage drops but can they prevent constant voltage drop? If so where should i place these bulk capacitors?

For an easy thing to try, install a resistor to isolate the sensitive device from the noisy supply. Add a reservoir capacitor as shown below.

noise from supply isolated via interpose resis-n-capa filter.png

This won't make up indefinitely for sagging supply voltage. You may wind up needing to boost voltage to the control device (as well as regulate voltage). Such as a boost converter or voltage multiplier.
 

Hi,

There is no GND plane? Is it?
(Maybe you used copper pour --> this is not compareable to a GND plane)

If not: good luck.
Especially with your switching power application.
Did you test:
* functional reliability
* EMI compliance
* EMC compliance

Klaus
 

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