Mosfet selection for Buck converter

Hello to all.
I would like to design a Buck converter to control speed of dc motor,350w the max speed is:1500 rpm.
The input voltage of Buck is 70v and my frequency switch is 5 KHZ.
I have chosen IRF3710. Is it okay or not? Any advice for MOSFET selection.
Very thank you.

5khz? Are you sure you didn't forget a zero or two? Have you considered what this does to the physical size and cost of your magnetics and capacitors?

Why not use a modern switch mode IC controller with speeds in the hundreds of kilohertz? There are very valid reasons why switching speeds have risen to those levels.

Because I don't have so much knowledge about design a high frequency switching power, so I thing 5 KHZ is okay specially with my load is a motor by high Inductance.

avr_fred:
5khz? Are you sure you didn’t forget a zero or two?

4kHz, 8kHz are standard PWM rates for big motors, its sounds very reasonable. Remember big motors
have lots of winding inductance, switching at high rates isn’t needed for smooth current waveforms.
Unnecessarily PWM frequencies means larger iron losses so are generally avoided.

"Buck Converter" usually means the transistor is on the high side - between the power supply and the load. It also usually means a power supply with much higher switching frequency.

You want to control the low side - between the load and ground. Putting the transistor on the low side means an N-Channel FET like the IRF3710 can be driven directly by the Arduino. If you put the transistor on the high side you need a level shifter and you either a voltage supplie higher than the motor voltage or a P-channel FET.

The IRF3710, is adequate, but could be damaged or overheat depending on the specifics of your circuit and motor. You probably need a flyback diode across the motor. Depending on the switching characteristics you probably want a heatsink on the FET and diode.

If you don't know much about electronics I recommend using a pre-made motor driver board to start with. Pololu, Sparkfun and others offer drivers that can control 5A, but make sure it's rated for more than 70V.

This is the circuit of my Buck. If it has any problem, please tell to me.

Very thank you.

Firstly you've drawn the circuit right to left which is very hard to follow.

You are high-side switching using a low-side gate driver - can't work.

You are using a saturable inductor and have no fast over-current protection - it will blow the top
of the MOSFET if the inductor saturates and your capacitors discharge through the device at
massive current levels.

Normally you use the motor itself as the inductor, you wouldn't bother to buy an extra one
of lower specification!

For a big motor you need the same over-current protection circuitry due to stalling the motor,
rather than saturation (which is unlikely in a motor as there is so much more iron you'd need
to saturate).

You are high-side switching using a low-side gate driver - can't work

Why it can not work?
Thx.

so,What can I do.If I change the low side ground,Do everything will be OK?
Thx.

No, you need a different gate driver and possibly a different MOSFET (depending on whether you use a p-channel fet with a driver designed for those, or an N-channel fet like the 3710 and a gate driver capable of generating the voltage you need to make it switch.

What kind of switching power do you suggest?
I mean, I need to a converter which could have a variable voltage between 0 to 70 in output and the maximum voltage of my input is about 70 volts, What kind of SMPS can work and is easy to build.
Very thank you.

I think this is an Arduino forum.
So where does the Arduino fit in.

I think designing a 70volt/350watt buck converter is not a task for a beginner.
Study electronics, and try again in 5years time.
Leo..

leoncorleone:
What kind of switching power do you suggest?
I mean, I need to a converter which could have a variable voltage between 0 to 70 in output and the maximum voltage of my input is about 70 volts, What kind of SMPS can work and is easy to build.
Very thank you.

Do you need a motor speed controller or a power supply? They are done differently, a motor controller
uses PWM into the motor and uses the motor's own inductance to smooth the current.

A power supply has to have its own inductor(s) - you wouldn't bother using variable voltage supply to
speed control a motor since a motor driver is a simpler circuit.

Normally a motor controller is bidirectional, power supplies are not.

Do you need a motor speed controller or a power supply?  They are done differently, a motor controller
uses PWM into the motor and uses the motor's own inductance to smooth the current.

Yes, Actually I need a motor controller, So can I use Buck without its output C and L?
Thx.

You need a motor driver. Usually that means an H-bridge, and yes you need plenty of capacitance on
the DC rail for that, but not on the output (other than perhaps 10nF to 100nF ceramic directly across the
motor terminals to suppress RF noise/EMF).

If you dont need bidirectional control a half-H-bridge or a single switching MOSFET may be enough.

But say more about what you want to achieve and the precise details of your motor and supply....

Partial motor/driver cross-post list.

Nuf said.

Does somebody know how to switch this Mosfet by arduino when Drain is about 70 volts and Source isn't connected to Ground?
Thx.

The motor will always be running, independent of gate voltage, because the body diode is conducting.

I see that the picture has been edited (grrr).
Now, fet is "off" if gate is the same as motor(+), and "on" if gate is ~10volt higher than motor(+).
Not easy to drive.
Try the fet between motor(-) and ground.
Then the drive circuit can also be grounded.
Leo..

Very thank you,every one.
Now I know what to do.
When you wanna switch the Mosfet like Buck circuit that is not connected to Ground(High Side Switch) you should use P Channel Mosfet or boot-strap Circuit. P Channel Mosfets usually have Rds(2 to 3 times larger than N Channel), so the best way is using boot-strap circuit.
There are many ICs that have boot strap circuit like UCC27200, and ICs that can switch High Side are like IR2111,IR2153,IR2104,IR2102
The below link was so helpful to me
https://engineering.purdue.edu/Courses/ECE433/exp5_5th~6thweek_.pdf
At the end thanks from Arduino.

An N-channel fet with bootstrap circuit (high-side switching) is mainly used in H-bridges.
Or in circuits where you can't disconnect the load from ground.
AFAIK (from previous posts) you only (or mainly) need the motor to turn one way.
Why complicate the circuit. Grounded source is a lot easier.
Leo..