MOSFET PWM Bridge: few questions on drivers and circuit

[meereck]

Hello,
I would like to control a DC motor with MOSFET transistors and PWM.
Basically, the motor will run in one direction only, and I need to control speed and braking.
Do I need a full H-bridge?

The power supply will be fixed at 18V DC. The motors has to have its one pin connected to ground so I believe I will need high-side switching, right?
It would be good to have the highest voltage possible on the motor at full throttle (i.e. 100% PWM duty.)

I have searched the internet and I am rather confused about P-channel and N-channel MOSFETs. Plenty of circuit found on the web contain N-channel as a high-side switch, and the manufacturers also sell much more of N-channel types than P types.
But N-channel type needs higher Vg voltage than Vs so how can somebody use it as a high-side driver, it would not work, would it?

Can anyone please tell me what the circuit should look like for controlling speed and brake in one direction?
My idea is to use one MOSFET for speed and one for controlling brake. Each mosfet would have separate PWM input.
Is it viable?
Can I use driver IR2110?

Cheers
Meereck

 

[meereck]

I finally understand that IR2110 contains a bootstrap capacitor so that it can operate a N-MOSFET transistor as a high-side switch.

 

[FvM]

Yes, you can use a half bridge for motor operation and brake. The brake can also operate regenerative, if required.

You should consider, that the bootstrap operated high-side driver has a duty cycle limitation below 100% due to the
drivers opeation principle.

 

[meereck]

Hi, thank you for the response.
I am pretty sure I know you from CCS forum

as for duty cycle limitation: Would you have a clue what the maximum duty cycle for bootstrap circuit is?
Since I really want to get the maximum power from the motor, it might be better to use P-channel MOSFET at the high side even if the mosfet transistor would have bigger resistance (Ron).

Can you also please confirm me the following operations of the circuit in the attachment (of course, I will use better transistors such as IRF1405):
1) power:
low-side transistor is off
I use PWM on the high-side transistor to control motor force

2) active braking:
high-side transistor is off
low-side PWM controls braking force

3) recuperation
both transistors are off, energy is fed back to the power supply through diode D1.

I hope I got the idea of the half-bridge circuit right.
Thank you,
Meereck

 

[FvM]

I have e.g. used a limit value of 96% duty cycle for 100 kHz PWM.

I basically agree with your explanation, except for case 3 recuperation. It would only feed back energy, if the motor e.m.f. is higher than 24 V, respectively the motor is running considerably faster than in active operation - rather unlikely. Regenerative operation usually implies boost operation of the PWM by active control of at least the low side transistor.

More generally, you can perform PWM operation by switching only one transistor, but it implies diode operation of the other branch. Active operation of both branches always brings higher efficiency.

 

[FMradio]

using a relay contactor across the motor is not a bad option to STOP its motion

 

[meereck]

hello guys, thanks for reactions.
I am confused about half-bridge operation now.
I have taken a look at PIC ECCP module and half-bridge duty cycle is described as

1) T high-side switch is on
2) dead-band delay
3 (1-T) low-side switch is on
4) dead-band delay
that would mean that motor is braking in every cycle.

In my previous post, I thought that I shall feed PWM signal into high-side switch when rotation of the motor is desired (and the low-side switch is off all the time).
Or I feed PWM to the low-side switch so that I control braking force (high-side switch is off all the time).

Thank you for clarifying.
Meereck

 

[FvM]

that would mean that motor is braking in every cycle

No. PWM implies a voltage conversion utilizing the motor inductance.

 

[meereck]

that would mean that motor is braking in every cycle

No. PWM implies a voltage conversion utilizing the motor inductance.

I am afraid I am rather puzzled.
Do you mean the voltage conversion for opening the high-side N-mosfet?
Can you please extend the explanation.

 

[Tahmid]

Hi,
For controlling speed in one direction, a half bridge will do, furthermore, even one MOSFET(either N-channel or P-channel) will work.
If you use one MOSFET, the N-channel will work as low-side or if you choose P-channel, it will work as high-side.
Tahmid.

 

[meereck]

right,
my question is about the half-bridge control signals in general:
which one of the following is correct for operating a half-bridge when the motor has to be moving?:
1)
PWM comprises just the high-side switch:
low-side switch is off all the time
There is a PWM signal at the high-side switch, which is used to control the speed of the motor

or
2)
PWM duty comprises both switches:
if the motor speed shall be 50%, there are the following sequence on both switches:
a) high-side switch is on, low-side is off
b) high-side is off, low-side is on

Thank you,
Meereck

 

[FvM]

Both modes are possible, but mode 2 is achieving higher efficiency.

 

[meereck]

Both modes are possible, but mode 2 is achieving higher efficiency.

thanks for the reply.
just one follow-up for mode 2 so that I finally understand the topic.
pwm signal on the switches are complementary to each other, right?

When braking, PWM duty cycle is, say 5%. This means that the upper switch is on for 5% of pwm period, and the lower switch is on for 95% of time.
But if the emf voltage of the motor is not higher than the power supply's voltage, the motor will be fed from the power supply (during those 5% of time) which causes the motor to rotate even though it is supposed to brake.
Am I still overlooking something or this is ok?

Thanks again,
M.