I am planning a bidirectional motor driver using the AOP605 IC (two complementary P/N channel MOSFETs). I will drive a motor drawing about 300 mA (1.5A when stalled). I will use current sensing to sense the endpoint of the mechanical system (stall of the motor).
The system will be controlled by an Attiny85 (or by an ATMega328 for two channel application).
The first step of this project has been realised using two relays per channel instead of the MOSFETs. In this second step I am planning to replace the relays with MOSFETs.
Simplicity is my No:1 priority. There will be no PWM. The motor will be driven directly until it stalls. So the gate capacitance etc. is not a problem in this setting.
This is the schematic of the H Bridge:


As the datasheet of the AOP605 states, the ON resistance of the P channel MOSFET at a VGS of -4.5V is less than 58 mohm. The ON resistance of the N channel MOSFET at a VGS of 4.5V is less than 43 mohm. This data suggests that these MOSFETs can be used as "logic level MOSFETs" as the working current is relatively low. As the MOSFET side will have a higher supply voltage (about 9 V) the high side P channel MOSFET will have a VGS of more than -4.5V (actually about minus 8 - 8.5 V). So I think the H Bridge should work without any problems.

My question is about the driving circuit. Can I omit the two 2N2222 transistors and drive the gates directly from the microcontroller (maybe using a series resistor)?
In this case I will also omit the two 10k resistors connected to the collectors of the transistors.
Theoretically, 5V for the N channel MOSFET is enough to turn it on with an RDSon of 43 mohm. 0 V from the microcontroller output will lead to a VGS of -9V for the P channel MOSFET and this will turn it ON adequately.

Am I right?

Thank you very much in advance...

PS: Of course there are flyback diodes but I have left them out for clarity.

 

I am planning a bidirectional motor driver using the AOP605 IC (two complementary P/N channel MOSFETs). I will drive a motor drawing about 300 mA (1.5A when stalled). I will use current sensing to sense the endpoint of the mechanical system (stall of the motor).
The system will be controlled by an Attiny85 (or by an ATMega328 for two channel application).
The first step of this project has been realised using two relays per channel instead of the MOSFETs. In this second step I am planning to replace the relays with MOSFETs.
Simplicity is my No:1 priority. There will be no PWM. The motor will be driven directly until it stalls. So the gate capacitance etc. is not a problem in this setting.
This is the schematic of the H Bridge:
View attachment 129270

As the datasheet of the AOP605 states, the ON resistance of the P channel MOSFET at a VGS of -4.5V is less than 58 mohm. The ON resistance of the N channel MOSFET at a VGS of 4.5V is less than 43 mohm. This data suggests that these MOSFETs can be used as "logic level MOSFETs" as the working current is relatively low. As the MOSFET side will have a higher supply voltage (about 9 V) the high side P channel MOSFET will have a VGS of more than -4.5V (actually about minus 8 - 8.5 V). So I think the H Bridge should work without any problems.

My question is about the driving circuit. Can I omit the two 2N2222 transistors and drive the gates directly from the microcontroller (maybe using a series resistor)?
In this case I will also omit the two 10k resistors connected to the collectors of the transistors.
Theoretically, 5V for the N channel MOSFET is enough to turn it on with an RDSon of 43 mohm. 0 V from the microcontroller output will lead to a VGS of -9V for the P channel MOSFET and this will turn it ON adequately.

Am I right?

Thank you very much in advance...

PS: Of course there are flyback diodes but I have left them out for clarity.

When you try to drive it directly the PFET gate will need to go to 9 volts to turn it off. I don't think the micro does this.
There doesn't seem to be an off state. Does it just go back and forth continuously?

 

When you try to drive it directly the PFET gate will need to go to 9 volts to turn it off. I don't think the micro does this.
There doesn't seem to be an off state. Does it just go back and forth continuously?

You are right. The P MOSFET would not turn off. I think there are two ways. I can use 5V instead of 9V as motor supply (change the motor too) or I can use a 2N2222 as gate driver. Thank you very much.

 

Now I have another question. I am designing a PCB for this circuit. With three motor channels and an Atmega328.
The question is, would there be any problems if I connect the emitter of the 2N2222 to the source of the respective N MOSFET instead of connecting it directly to ground?

 

The first circuit is better.

1) there will be heavy shoot-through current when they switch, a larger gate resistor makes this nasty event last longer. Look at the curve of Rds ON when the switching is half way... I highly recommend using 4 IO pins to drive the bridge.

2) if the voltage on R3, the sense resistor spikes, it could cause the transistor to turn off, especially if you are driving it with 3.3V logic.

Motor current sensing never works well as a limit switch.
The motor can jam easily because of the stored rotational inertia- this energy loads up the mechanics harder than you can unload going the other way.

 

Thank you for the reply. For me, the first circuit is the better and correct one but routing the PCB traces is much easier with the second circuit.
You are right. I must use the correct way.
About current sensing...
You may be right in general but in RC aircraft (retractable landing gears) this type of circuit is very popular. I am using a couple of them in my RC aircraft but they use relays instead of MOSFETS. They work fine. Now this project is an impoverd version for my friends who find relays unreliable and want to have solid state circuitry.