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complementary PWM signals in Python

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borna.barisic

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Hello, my name is Borna Barisic and i am from Croatia. If you don't mind i have a question about Raspberry Pi's PWM.

I have build a DC step down Buck converter with half H bridge and i am using IR2113 driver.
Schematics for that circle shown in (DC Buck schematic attachment).

I have watched PWM lessons for Raspberry Pi and thanks to them i managed to make a program in Python to generate two PWM signals from Raspberry Pi, but i cant make those PWM signal out of phase.

If i am right, for example, if i put frequency f = 1 Hz that means that time of whole period is T = 1 sec, time half of period is t=0.5 sec. In python program i used a time.sleep(t) function in while loop so when one of PWM signal (in this case buck_PWM_H,representing high signal) already starts, second PWM (buck_PWM_L,representing low sigal) waits for half a period and then starts. Then i would get two signals that would be out of phase (when "H" is 1 "L" is 0).
Well to be honest it did not worked that way.
Just to say regardless of time or frequency those two PWM signals were always in phase.

After that said i was wondering if you could help me by telling me how to generate (with Python code) two complementary PWM signals. I hope that i made myself clear enough for you to understand me. I am sorry for my grammar mistakes.


I will show you my Python code.

Code Python - [expand]
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import RPi.GPIO as GPIO 
import time 
 
GPIO.setmode(GPIO.BOARD)
GPIO.setup(11,GPIO.OUT)[/B]
GPIO.setup(12,GPIO.OUT)
 
f = input ("Frequency: ")                              /here im setting up a frequency of PWM
T = 1/f                                         /i need to find time of a period 
t = T/2                                         /to make signals out of phase i need half period to delay other pwm (at least i thought)
 
buck_PWM_H = GPIO.PWM(12,f)                     /setting pin 12 for output PWM High side 
buck_PWM_L = GPIO.PWM(11,f)                     /setting pin 11 for output PWM Low side (this signal have to be delayed for half a period)
 
buck_PWM_H.start(0)                                     /i have put zero because i want PWM to start when i input variable D
buck_PWM_L.start(0)                                 /same here 
 
while(1):
    D = input("Duty cycle[%]: ")        /In this while loop i wanted to start pwm signals when variable D is known 
    buck_PWM_H.ChangeDutyCycle(D)   /and i put a time.sleep function because i thought it would make two signals 
    time.sleep(t)                       /(buck_PWM_H and buck_PWM_L) out of phase, but i was wrong.
    buck_PWM_L.ChangeDutyCycle(D)       /In case D is 0 then PWM signals are shut down
    if D == 0:
        break
 
buck_PWM_H.stop()
buck_PWM_L.stop()
GPIO.cleanup()


Best regards from Croatia and keep up the good job.
 

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  • DC Buck schematic attachment.jpg
    DC Buck schematic attachment.jpg
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Last edited by a moderator:

Hi,

i doubt a Python software PWM with delay() will ever run satisfactory.

Almost any microcontroller/processor has built in PWM, or at least a timer interrupt hardware.


Klaus
 

Thank you for your advice, I am familiar with PWM in PIC microcontrollers, but i was hoping to make 2 complementary PWM signals in python.

Best regards

Borna
 

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