I'm designing a PCB heater capable of heating to 55°C in a 20°C environment.
Given the trace length/width that I decided to use, I used the Saturn PCB Design tool to calculate what amperage I would need to get the desired result. The tool estimated that the trace will have a resistance of 1.5 Ohm, a voltage drop of 8.5 V and 5.7 A will be drawn to reach the wanted temperature, effectively using 48.5 W of power.
All of this means that, considering heating element only, I would (at least) need an 8.5 V PSU capable of delivering 5.7 A. Of course 8.5 V PSUs don't exist, and even if they do I would get something more capable of that to have a safety margin. So let's say I use a 12 V PSU: to achieve the 48.5 W needed it would only have to draw 4 A of current, but the resitance in the circuit would always be 1.5 Ohm, effectively drawing 8 A of current (too much).
To solve this, and to have some control on the temperature, I can add an N-Channel MOSFET and regulate the Gate voltage (with PWM + RC filter) to add the right resistance to the circuit and get the desired power draw, ranging from 0 to 48.5 W.
The MOSFET I selected is BUK9M156-100E, which is suitable for microcontroller projects and can withstand ~100 V and more than 8 A (if thermals are managed correctly).
By looking at the graph below I understand that the Vgs range suitable for my application is between 2 V and 2.8 V, which of course needs to be verified with the actual hardware in hand.
The voltage after the heating element will be 12 - 8.5 = 3.5 V, which by going into the MOSFET will be dissipated into 4^2 * 0.15 Ohm = 2.4 W. This considering a max load situation of 4 A @ 12 V.
The rest of the circuit consists of an ESP32, IRM-60-12 AC/DC brick, AP2112K LDO for the ESP32 and an INA226 to monitor current from a shunt resistor.
Being my first high power project I'd like a sanity check, do these calculations make sense?