I had come up with a divider network to scale the output to about 3V.You can use a microcontroller and a pretty simple one for a single voltage. You would use the on board ADC to control the pwm.
It's not easy.
You would have to develop a divider network on the output side to scale the voltage to the range the uC can accept.
Then you need to develop the code to read the ADC and generate the PWM to track that and also be accurate.
That doesn't begin to help with the switcher which will be a substantial learning curve and probably should be designed first.
I've not done it. I've repaired them and know a small amount about them.
Your design seems to be odd, what is it for?
Many of the newer generation of microcontrollers have built-in features that faciliatate PWM signal generation. I know for a fact that ATMEL's AVR series has a PWM function that is very straight forward to implement. I believe that PICs are similarly endowed. Most of the AVR micros have a built in analog comparator that can come in handy for comparing the output for the purpose generating an error signal for use in adjusting the PWM under software control.I had come up with a divider network to scale the output to about 3V.
By reading the feedback into the ADC, how can i make the controller output the required pwm automatically.
Many of the newer generation of microcontrollers have built-in features that faciliatate PWM signal generation. I know for a fact that ATMEL's AVR series has a PWM function that is very straight forward to implement. I believe that PICs are similarly endowed. Most of the AVR micros have a built in analog comparator that can come in handy for comparing the output for the purpose generating an error signal for use in adjusting the PWM under software control.
The purpose of the feedback is to let the uC know when to terminate the on time of the PWM to keep the charge on the output filters at the desired level. Using higher voltages means less on time.
Your challenge is in the percentage of regulation. You may have arranged a divider to drop the output voltage to 3 volts, but you also have to think of the number of bits in the A to D converter that reads this voltage.
An 8 bit converter is good to one part in 256. For 5 volts FS, that's 19.5 mv/bit. If +/- 20 mv is good enough regulation, there you are. Keep in mind that the actual output voltage is 133.33 times the voltage out of the divider. That means that the output regulation will be 133 times as coarse as the control, or +/- 2.6 volts. If the output need a tighter control, then you will need to use a better A to D.
10 bits gives four times the resolution, so your output can be regulated to +/- 65 mv. And so on. This part of the boost converter may drive the selection of the uC.
If you have access to some other microcontroller that's supported by your school or you get it cheap use it. Learning on one will teach you the basics of all of them.readadc = analogRead(analogPin); // read the input pin for adc
//Here you could calculate what to write to the PWM, there are ways to compact it and reduce //code size.
analogWrite(pinforpwm, val); //write the calculated PWM value, you could do the calculation //here for compactness.
// Note: analogRead values go from 0 to 1023, analogWrite values from 0 to 255
This of course would be looping over and over as fast as possible.
The arudino is easy to use since it's an all in one system with a free IDE (compiler, editor, uploader,ect). http://www.arduino.cc/
It is not the only one, it's the one I'm familiar with. The clones can be much cheaper and cheaper still if you buy a kit and build it.
atmega168 used in the arduino and clones does 10 bit adc (0-1023) , the pwm is 8 bit (0-256)
It has several adc, pwm and digital I/O pins.
You read the ADC and it is proportional to the voltage on the output. You'd use that to figure what to tell the PWM to go to and then set a pin to output that.
This is NOT a working program, there is more to it but the critical lines are reading an adc value and writing a pwm value.
If you have access to some other microcontroller that's supported by your school or you get it cheap use it. Learning on one will teach you the basics of all of them.
The value of my inductor is 124mHWhat is the value of your inductor?
Is your catch-diode a schottky-type? What is its part number?
What is the value of your load resistor?
What is your PWM frequency?
What MOSFET are you using?
These are but a few of the things that would be needed to help us determine where any design weaknesses my reside.
sorry there is a typoI suspect that your inductor is a little on the high side.
You have indicated that the inductor is 124 milliHenries (mH). I think that 124 microHenries (uH) would be closer to the value you would need for 40KHz. Another important parameter for your inductor is that it have as low a DC resistance as possible. It would be a good idea if you could measure the DC resistance using an ordinary ohmeter and see what you get. A fraction of an ohm is desireable.
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by Gary Elinoff
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