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# Help to understand how the mosfet works in this circuit

#### JoseGD

##### Newbie
Hello,
I'm trying to reverse engineer an electronic board that controls a brush motor.
I came up with this schematic after working with the board for a while.
The U2 mosfet controls the pump (motor) using typical Source Follower.
But the U1 mosfet, I don't know what function it has in this circuit, I don't understand its operation in this circuit.
Could any of you help me understand this circuit.
Thank you so much

Hi,

too much unknown.

What is VCC?
I don´t think "MCU" really goes directly to an MCU pin. Please confirm.
What exact type is D1?
D2 is drawn as a Schottky ... but maybe it is a zener? Please confirm.

What kind of pump is it? DC-Motor, vibration .. anything else?

Klaus

Hi,
Thanks for answering, I clarify the circuit a little more.
- MCU does not really go to the U2 gate, I put it this way to simplify. The U2 gate is connected to a Driver and from the Driver to the MCU. The U2 has a zener between the Gate and the source. But this part of the circuit is easy to understand, what I don't understand is U2.
- Vcc is 12V.
- D1 is MBRB1045G Schottky
- D2 is Zener
- It is an engine of a car's ABS system.

- It is an engine of a car's ABS system.
I still don´t kow whether your "secret pump" is something that needs DC to work or pulses to work.
And I still don´t know how U2 gate is driven.

For me personally speaking: I need such informations to understand (in opposite to guess) how the circuit is meant to work.

Klaus

If the extracted schematic is so far correct, D1 + U1 work as switchable freewheeling diode. U1 on = motor voltage is clamped to -0.3 V, U1 off = motor voltage can swing to negative voltage (fast discharge of stored inductor energy). To allow negative swing, U2 driver most be isolated.
I have no idea why the function is needed, we'd need more application details.

What is that box on the anode of D1?

The schematic is well extracted (I reviewed it many times), but the only thing I can't find is where the U1 Gate is activated.
The diagram is the control of the ABS motor of a car.
On all the plates of these ABS systems there is not always the U1, only in some models.

On all the plates of these ABS systems there is not always the U1, only in some models.
Means what? Only D1 present, cathode to ground or no freewheeling diode at all?
--- Updated ---

What is that box on the anode of D1?
No box, just a symbol with two cathode terminals.

No diode,
I attach a photo.