Pwm ON and Off States

I was really confused when reading about the theory of H bridges on line. To rotate the motor at lower slower speeds the document says we use pwm signals. That point is understandable for me, but he says in pwm off states several combinations are possible. For example H1 can be ON. This is where it confused a lot for me, till this time i was assuming that pwm off means all switches are in off state. Please tell me the difference between system pwm off and each switch pwm off. Are they different or both same. Please help.

First of all, without a drawing it's a little difficult to explain. Regardless, you don't have PWM on each switch. Usually only the bottom switches receive the PWM signal.

Hi,

Imagine: when PWM_OFF means that all switches are OFF...
Then what does PWM_ON mean?
* both switches ON --> this causes short circuit
* low side ON --> what is the high side good for when it never is ON?
* high side ON --> what is the low side good for when it never is ON?
Three possible modes --> none makes sense

The conclusion is that the first statement (all switches are OFF) is wrong.

Klaus

If you are using a half bridge, both switches can be off during the dead time (that just ensures that they have some time to turn off); this is important because turning on and off does take some time.

Both switches on is like suicide because the power supply line is shorted to ground; that should never happen in real life.

Hence there are two possibilities: H1 is on and that means L1 is off (else both on is not allowed) and the other one H1 is off and that means L1 is on (both off is allowed only for a short while).

Hence H1 and L1 can be driven by complementary signals. They are automatically produced by the driver IC

Thank you all for your patience and answering me some very silly and i think even stupid questions. But here is the link i am referring to
https://www.modularcircuits.com/blog/articles/h-bridge-secrets/h-bridges-the-basics/
In the Drive modes sub heading
https://obrazki.elektroda.pl/9325336600_1505193211.png
https://obrazki.elektroda.pl/5387600200_1505193301.png
I am only confused when he said in the OFF state some switches closed. I am not sure if you have already answered it. Is the OFF state of the system separate master control apart from controlling the switches? Please help. I am also attaching the pdf document of the same.

Hi,

It is explained very well in the first document of post#5.
So where exectly is your doubt?

Take a sheet of paper and draw a timing signal of how you understood.
Then show exactly with thus diagram where your confusion lies.

With motor drive you usually use different modes of operation:
* forward, slow decay --> pwm mainly used for motor speed control
* forward, fast decay --> pwm mainly used for motor torque control
* reverse, slow decay --> pwm mainly used for motor speed control
* reverse, fast decay --> pwm mainly used for motor torque control
* free running
* breaking --> pwm used for deceleration control

Klaus

Ok. Finally got something going. Please find the initial diagram with motor in one direction trying to add all the modes.
https://obrazki.elektroda.pl/3548822700_1505293221.png
Is it correct?

Hi,

If you tell which one of the above modes you tried to generate...

Klaus

electronicsman said:

Ok. Finally got something going. Please find the initial diagram with motor in one direction trying to add all the modes.
https://obrazki.elektroda.pl/3548822700_1505293221.png
Is it correct?

Click to expand...

That drawing tells us nothing; its four colored lines, as far as I can tell.

I think you need to step back and read something (and there's plenty of stuff to read). You need to get a fundamental understanding of how an H-bridge, and PWM, work.

I am really sorry for that drawing and not explaining clearly. Yeah i am studying a lot to understand H bridge, but i don't understand them. With clarifications from this forum i am making some progress, because by just reading i am going nowhere.
As far as the diagram is concerned my requirement is to run the motor in different modes as klaus and mitra have pointed out (I should say both of them helped me a lot). As of now i do not know how to do that. But what ever i have shown is for one PWM cycle of 50% duty cycle. During the entire PWM cycle i am making Q1 permanently ON. Since Q1 is ON Q2 should be OFF. During the ON time Q4 is ON to make the motor move in forward direction. During the OFF period i am making the Q3 ON to provide the path for the motor current. Please correct me if i am wrong. Yes Still lot more to understand. Request your support.

I think you're making this a lot harder than it has to be. Using the document from post #5:
If Q1 and Q4 are on, the motor runs in one direction. If Q3 and Q2 are on, it runs in the other direction. You would apply your PWM signal to Q2 and Q4. If Q1 and Q2 or Q3 and Q4 are on, you have burned up transistors. If Q1 and Q3 or Q2 and Q4 are on, you will be braking the motor (but there are problems to consider with this (voltage overshoot, for one).

electronicsman said:

Ok. Finally got something going. Please find the initial diagram with motor in one direction trying to add all the modes. ...

Click to expand...

No, not yet. First label the drivers: there are four drivers in a full bridge. A full bridge is made of two half bridges (fair enough)- the load sitting in the horizontal leg of the H.

Label the left side as H1L1 and the right side H2L2. Under steady state, current will flow H1L2 or H2L1. It means H1 and L2 are on OR H2 and L1 are on. H1L1 OR H2L2 means a short to the power supply.

Because there are 4 switches and each switch can be either on or off, there will be 2^4=16 possible combinations. I will not go into details here.

As mentioned, H1L1 AND H2L2 represent short of the power supply. That will remove 7 out of 16; we need to look at the possible 9.

One corresponds to the dead time H1, L1, H2 and L2 are all off. Remove that too. We have still 8 to consider.

Two special cases: H1H2 and L1L2; they represent the motor terminals shorted to ground or supply. (We shall see them again!- often motor ends are shorted in an emergency brake)- 6 states to go...

H1L2 and L1H2 are the two standard modes; they just reverse the current through the motor. Useful if you want to reverse the direction.

There are 4 states where only one driver is enabled; H1, L1, H2 or L2.

Now we should discuss about the torque and speed.

- - - Updated - - -

No, I am not yet complete. Please consider the load to be a resistor- simplest example. But that is not the most interesting, we are interested in running a motor and making it run the other way from the software.

Now, when a motor runs, it has a back emf and, except for the small ones, motors act like inductors. This back emf is very close to the applied voltage and the current is not in phase with the external voltage.

These motors (anything more than 50-100W) do not like to be interrupted. If you switch off the current very fast, the voltage increases very high and that can damage many things. Hence after you turn off the current, you should think about the consequences. Once you know how to take care of this, you will understand how to handle the beast- may be a 50W toy or a 100KW locomotive.

Speed Mode forward direction slow decay
H1 ----> permanently ON
L1 ----> permanently OFF
H2 ----> pwm control
L2 ----> pwm control

Speed mode reverse direction slow decay
H1 -----> pwm control
L1 -----> pwm control
H2 ----> permanently ON
L2 -----> permanently OFF

Forward Torque mode fast decay
H1 ---> pwm mode
L1 ----> pwm mode
H2 ----> pwm mode
L2 ----> pwm mode

My thinking is that forward torque mode is same as reverse torque mode. The difference will come based on duty cycle if is above 50% then it is forward torque else it is reverse torque mode. Is it correct?

Braking
H1 ---> permanently ON
L1 ----> permanently Off
H2 ---> permanently ON
L2 ---> permanently OFF

I am bit confused about free running mode. Please advice.

Hi,

Speed Mode forward direction slow decay
H1 ----> permanently ON High sides can´t be permanently ON --> PWM it
L1 ----> permanently OFF --> invert PWM
H2 ----> pwm control --> permanently OFF
L2 ----> pwm control --> permanently ON

Speed mode reverse direction slow decay
H1 -----> pwm control --> permanently OFF
L1 -----> pwm control --> permanently ON
H2 ----> permanently ON --> PWM
L2 -----> permanently OFF --> invert PWM

Forward Torque mode fast decay
H1 ---> pwm mode
L1 ----> pwm mode --> permanently OFF
H2 ----> pwm mode --> permanently OFF
L2 ----> pwm mode --> may be permanently ON (motor acts differently compared to pure inductance)

My thinking is that forward torque mode is same as reverse torque mode. The difference will come based on duty cycle if is above 50% then it is forward torque else it is reverse torque mode. Is it correct?

Braking
H1 ---> permanently ON --> permantly OFF
L1 ----> permanently Off --> should be PWM´d to control break, else expect very high currents
H2 ---> permanently ON --> permantly OFF
L2 ---> permanently OFF --> should be PWM´d to control break, else expect very high currents
The breaking energy is dissipated as heat in the MOSFETs and the motor windings.. and one part is feed back to the power supply.

I am bit confused about free running mode. Please advice.

Click to expand...

Free running: All OFF. The motor runs free without braking.... until rotation energy is zero.

Klaus

When a motor is running, the kinetic energy of the rotor is taken up by the load; the current continuously feeds energy to keep the motor at a constant speed.

When the motor terminals are open but the motor was running (just before that), no current can flow because the circuit is open. However, the motor produces back emf that do not produce any effective work.

Under this condition, the motor is free- the rotor energy continues to be passed on the load and the motor comes to a halt because of friction etc etc. (you can consider the load to be dissipative)

Such a state when H1, L1, H2 and L2 are all off, the deadtime, the motor is in free running mode. Motors used in presses, heavy machines etc., may be disconnected from power (free running mode) for a short time and the whole energy of the rotor may be dumped to the load. The rotor experiences mechanical but no electrical stress in this time.

When the two terminals of the motor are shorted (H1H2 OR L1L2), there will be no voltage but lots of current. Remember that this current will flow in the opposite direction (when the external voltage was present) and will try to stop the rotor (because the current is in opposite direction)- now all the energy of the rotor is getting dumped (also consider the kinetic energy of the load, if applicable) into the coils. For a small toy motor the energy is mostly lost into the diodes but a part will also be lost into the windings. For big motors, this is not a recommended way (the emergency brake; plugging) but you can certainly do PWM to control the rate of braking.

For small motors, you can get away with many simplifications.