MCU Buck_converter BLDC driver  - Page 1
Forum section: electronics / beginners

author: RamyHaddad
author_rank: Newbie
timestamp: June 04, 2025, 11:07:23 am
content: Hi,I'm building a PID hot air station, I tried Pwm to control the BLDC fan but it wasn't working as expected.After some search I knew that BLDC fan have internally a PWM controller and it should work only with constant DC supply, tested on main and found the range to be 12 v to 24 v.To control it from an MCU I designed Pwm_buck_converter driver with Frequency about 50k hz to 100k hz (schematic attached below).I do have 3 voltage level in the full design, I used 12V to trigger push-pull gates which driver the Nchannel mosfet ( low-side switching ), I have IRFZ44N/IRF540 and IRL630 ( logic level but not sure if I should use it directly to MCU without a driver circuit. )I'm thinking this is little bit overkill as my load will consume less than 1 amp but I got these components on hand.The issue is that when I simulated the circuit, the output voltage is always about 23 V and does not get affected by the PWM. I will try to breadboard the circuit but it is not easy as some components are in SMD but I tried the mosfet driver circuit and it works.Any ideas what could be the issue or what improvements should I add ?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 07, 2025, 12:42:40 am
content: Your voltage divider (probably) does not work as expected, because it is not referenced against the GND level. When the FET is off, then both sides of the motor will go up to the power supply voltage.Putting 24Vdc on a uC adc pin with just a 3k resistor and a low resistance motor armature in series is also likely to damage the pin. Just look at the DC current paths when the FET is on or off.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 07, 2025, 07:41:28 pm
content: Remove C2. This capacitor will make current spikes and act like peak detector.Move coil L1 to the line below like in attached drawing.After removing C2, the buck converter circuit will be correct, but it needs to be tested with a load. Without a load, the voltage will be at its maximum.

author: RamyHaddad
author_rank: Newbie
timestamp: June 15, 2025, 10:25:09 am
content: Thanks for your help,so I should use a basic voltage follower ( op amp ) setup from the voltage divider to the adc pin ?also I can only measure the voltage if the FET it ON to avoid the ground reference issue, will that be enough?

author: RamyHaddad
author_rank: Newbie
timestamp: June 15, 2025, 10:26:27 am
content: Thanks for the suggestions, but what difference will it make to move the inductor ?

author: Circlotron
author_rank: Super Contributor
timestamp: June 15, 2025, 10:53:37 am
content: The different position of the inductor won't make any practical difference, Probably a drawing he already had on hand.I question the need for feedback. Why not set the PWM duty cycle to the voltage for whatever fan speed you want (with the fan connected) and leave it at that?

author: MariuszD
author_rank: Regular Contributor
timestamp: June 15, 2025, 01:53:15 pm
content: Different position of the inductor is important. With inductor on the lower line there is no AC between load and ground, this limits EMI and makes measurement possible.