High current dc blower fan mosfet control questions

[J.Amaral]

Hello everybody, I have some basic knowledge about electronics, and I found a puzzling circuit, it´s a mosfet and driver for a internal blower fan motor for a Hyundai I30 (it´s used in other KIA-Hyundai cars), and I´ve always believed that power mosfets should always be driven in full on off states, but in this it´s using a analog gate signal, it ranges from 6v to 7v (before the 110r resistor) on speed control and 12v on full speed, the heatsink is quite big but still gets REALLY hot even with the blower being used to cool it, is that an acceptable way to drive a mosfet? It´s like a sound amplifier with BJT´s, I don´t know if they do it like this to avoid the pwm noise or something else. I wanted to do a circuit for the same purpose but I´m kinda disappointed that it´s not a PWM driven gate. It doesn´t have a reverse current diode on the motor, only a TVS diode between source and drain. I really searched on the internet and found nothing about driving this way a mosfet on a "high" current dc motor... I found some "not so professional" schematics where people used a potentiometer to control the gate on mosfet or base on BJT, but for small motors... I didn´t find any circuits for driving high current dc motors with only 1 mosfet, only H bridge stuff... I´ve opened a few of those modules for blower fans (from other manufacturers), and apparently they all work in this way.

Any feedback would be appreciated.
Thanks.

P.S. mosfet datasheet can be found here https://www.infineon.com/dgdl/irfp064npbf.pdf

[Someone]

I´ve opened a few of those modules for blower fans (from other manufacturers), and apparently they all work in this way.

Older cars used a series resistor to perform the same function, bypassed with a relay for "full" speed. Also got flaming hot despite being in the airflow.

The schematic needs some more context around it. There is PWM coming in on MCU50, but is there feedback returning from CN22-10? I'm not immediately seeing any negative feedback to keep the current or voltage across the MOSFET stable.

[J.Amaral]

No feedback to the mcu, only 3 pins from the microcontroller the pwm one and 2 for the dual transistor ic... the cn22 actually connects the mosfet module to the controller, i should have removed it, since I already put the mosfet module components on the schematic my bad! All happens with the op amps the feedback goes to the second op amp channel only, no mcu magic there, only raw pwm and if I understood one of the transistors for full speed. Thanks for the feedback!

P.S. The mosfet, TVS diode, r19, r18 and DZ1 are on the mosfet module, the rest on the controller module

[Stringwinder]

Some years ago I took the fan control module from a SAAB 9-5 apart and
made some reverse engineering of the electronics. A large mosfet is used
with a servo-like circuit to control the fan current from 0 to 35 A in
proportion to a control voltage 0-5 V Dc coming from the climate control
unit. No PWM but the mosfet on the low side of the fan to ground.
PWM of 35 A current sounds like a good way to "pollute" the other electrical
systems that need clean DC (like audio systems). Filtering would need
large and expensive components to handle 35 A. Current is measured with
a low ohm shunt resistor to ground with an op-amp that also controls the mosfet.

[jmelson]

At the bottom left of your schematic it says PWM 250 Hz 5V.  That makes me suspect this is actually a PWM speed control.  Your measurements of the gate voltage are probably averaged by a DVM, but are really pulses of varying width.
Jon

[inse]

Nope, the PWM is converted to DC by R3 and C2 but the schematic is drawn so chaotically that it‘s difficult to understand the functioning.

[J.Amaral]

Sorry my first time drawing an schematic, I was simply tapping the components and puting on the software in any manner!!!  I think Stringwinder is right, they probably do that to avoid power fluctuations, or else they would need a big cap to filter it, also the motor would make noise if not over 20khz, weird that I´ve never seen mosfets bein gated with an analog signal, the circuit is actually straight forward, they read the voltage between the mosfet and the motor, and level the gate voltage using the mcu filtered pwm as reference, and one transistor to apply full on when set to max speed. I was curious to see if that´s a normal way to drive a mosfet, since I´ve always seen mosfets being driven with pwm, but this looks more like a audio amplifier with BJT´s. I did a search on the internet and found nothing about driving big loads this way.

[Stringwinder]

You need to realise that just a capacitor without any series impedance
will not work. Do the math on a 35 Amp LC or RC filter and see what you end
up with.