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help with h bridge design using MOSFETS and 555 timer for PWM

jogzzz

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i am a 2nd year electronic engineering student. my task was to design an H bridge circuit to control the speed and direction of a DC motor that is rated at 2A, 5V using MOSFETS and any simple circuit for PWM (to control the speed) on multisim. I chose to use a simple 555 timer PWM circuit and designed the h bridge as in the attached file. both circuits ( the h bridge and the PWM circuit) function correctly on their own. I simulated the H bridge using a function generator and it worked perfectly but when i added in my own PWM I keep getting simulation errors. please help lead me in the right direction. I don't have access to lab technicians or lecturers to help due to the corona virus lockdown and i am desperately trying to figure out what to do to make this circuit work with my very limited knowledge. thanks in advance for any help whatsoever.
 

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BradtheRad

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Your zip file decompressed is a single file ending in .ms14.
The first line contains "MSMCompressedElectronicsWorkbench".
An unfamiliar type.

Can you reply to this same thread and post a schematic? Then you are likely to get more informed replies.
 

dick_freebird

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555 lacks many features necessary for an H-bridge
of any decent voltage / power level. From drive
strength and levels, to shoot-through control.

Is the extra dime for a proper PWM IC too much,
or is it verboten to use a sensible part for the
application?
 

jogzzz

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i am a 2nd year electronic engineering student. my task was to design an H bridge circuit to control the speed and direction of a DC motor that is rated at 2A, 5V using MOSFETS and any simple circuit for PWM (to control the speed) on multisim. I chose to use a simple 555 timer PWM circuit and designed the h bridge as in the attached file. both circuits ( the h bridge and the PWM circuit) function correctly on their own. I simulated the H bridge using a function generator and it worked perfectly but when i added in my own PWM I keep getting simulation errors. please help lead me in the right direction. I don't have access to lab technicians or lecturers to help due to the corona virus lockdown and i am desperately trying to figure out what to do to make this circuit work with my very limited knowledge. thanks in advance for any help whatsoever.
attached are screenshots of my circuit
 

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jogzzz

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Your zip file decompressed is a single file ending in .ms14.
The first line contains "MSMCompressedElectronicsWorkbench".
An unfamiliar type.

Can you reply to this same thread and post a schematic? Then you are likely to get more informed replies.
sorry about that, I posted my ms14 file , I've replied with screenshots of my circuit. Thank you for the tip.
 

jogzzz

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555 lacks many features necessary for an H-bridge
of any decent voltage / power level. From drive
strength and levels, to shoot-through control.

Is the extra dime for a proper PWM IC too much,
or is it verboten to use a sensible part for the
application?
This wasn't made clear to us, it stated that any PWM method could be used but no single chip solution can be used( this was cryptic as I'm not sure if this meant no single chip solution for the entire circuit or just the PWM) but we did use the 555 timer to control the speed of a motor in a single direction prior to lockdown and this was the only technique shown to us. its a fundamental class so I'm assuming we're supposed to use what was shown to us. I did replace my PWM circuit with a function generator on multisim, using a square wave and under lower frequencies the circuit performed as expected but I could not replicate this with my 555 timer PWM circuit. Thank you for taking the time to respond.
 

danadakk

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One of the key issues in H Bridge design, and gate drive has
no overlap where two devices in bridge in vertical leg on
therefore shorting the power supply to ground. Which of
course, at minimum, take out the whole system or blow a fuse.

Most modern day processors have PWMs where this is controllable
'to set the "dead band" time of the PWM.

Here you can see the dead band is set to 10 clocks and observe that
the PH1 and PH2 phases do not overlap by that amount.

1600697079315.png

You can do this with discrete logic, its involved. Or in software, which is not so
precise as a HW approach.

So a 555 timer(s) is a tough way to approach this.





Regards, Dana.
 

    jogzzz

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KlausST

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Hi,

but when i added in my own PWM I keep getting simulation errors.
This is useless information as long as you don´t show/say what exact errors you get.

I don't have access to lab technicians or lecturers to help
Lucky you, who lives in 2020. When I was in engineer school we had no internet. Usually not even good datasheets. When we needed a NE555 or it´s datasheet we need to drive to the next bigger city..
We had no online datasheets, no application notes, no online example schematics, no forum, no youtube videos. No circuit simulators.
So please use the internet. All information is available for you within seconds.

Compared to a couple of decades ago it´s really simple for you.
--> When a MOSFET does not turn ON as expected in your simulation, then you just need to show the driving signals on a scope view. That´s all.

--> when an output voltage is not like expected you also may use a virtual ampere meter to show whether the current is in the expected range.
I think your teachers can expect that you use voltmeter, amperemeter, scope on a simulator.

***
Circuit issues:

The HIGH side MOSFETS are drawn the wrong way round. D <--> S

Both MOSFETS turn ON voltage is specified up to 4V for only 250 microamperes. Thus you may not get saturated condition when driving with just 5V.
Read datasheet.
On the other hand if you use two MOSFETS with smaller V_GS_on you run into cross conduction problems, like mentioned by others.

NE555 output HIGH voltage may be just 5V - 1.4V = 3.6V. Read datasheet.

Almost any H-Bridge manufacturer (single chip, hybrid, discrete) has online documentation about working principles and calculations. The tell you the advantages and disadvantages...
****

Later in your job:
You will be responsible for getting complete informations. Either you ask your it or will have to do it "trial and error" style.
Noboby will give you the latest datasheets, nobody will show you new techniques... you need to look for them on your own.

****
My recommendations.
Read datasheets and application notes.
Rectifiy the known issues of your circuit.
Do simulation.
Find debug methods on your own (V, I measurement)
Then - it´s quite usual that you need help from others - show what you did so far.
(A schematic is not enough, you need to provide error messages and measurement resuslts)

Klaus
 

jogzzz

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thank you so much for taking the time to respond, The simulation just didn't run, it just said a simulation error occurred, this puzzled me as the circuit did function when I used a function generator instead of the 555 timer PWM. I have immense respect for engineers that completed their degree before the age of the internet, especially in electronic engineering. I cant even imagine how difficult it must have been and i sincerely thank you for sharing your knowledge with me and I will take your advice in future.

The multisim version that I've used does allow to change the parameters of the 555 timer which is what I did to get a desired output , using an oscilloscope the output was 5V peak.
--- Updated ---

One of the key issues in H Bridge design, and gate drive has
no overlap where two devices in bridge in vertical leg on
therefore shorting the power supply to ground. Which of
course, at minimum, take out the whole system or blow a fuse.

Most modern day processors have PWMs where this is controllable
'to set the "dead band" time of the PWM.

Here you can see the dead band is set to 10 clocks and observe that
the PH1 and PH2 phases do not overlap by that amount.

View attachment 164242

You can do this with discrete logic, its involved. Or in software, which is not so
precise as a HW approach.

So a 555 timer(s) is a tough way to approach this.





Regards, Dana.
thank you so much for replying , everything I read thus far does suggest that the 555 timer is not a good PWM method for the h bridge, i will try this approach now.
 

danadakk

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Take a look at this thread.




Regards, Dana.
 

FvM

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I understand that the design objective is to demonstrate a principly working bidirectional pwm control, not to achieve a specific level of performance.

I believe that the circuit can work with a few modifications applied.

1. Use CMOS version of 555 to get full 0 to 5V drive level.
2. Use logic level MOSFET or increase the supply voltage for guaranteed MOSFET turn on.
3. (possible improvement) Design a basic dead time generation method. Most simple would be RD gate circuit for fast turn off - slow turn on.
Make use of the simulator to visualize transistor voltages and currents, also peak and average power dissipation. Determine if critical states like bridge shoot through are avoided and if the performance is sufficient for a real motor driver.
 

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