Problems building Boost Converter...

[skillz21]

Hi
I am trying to build a boost converter (hoping you figured that out by now). Here is my Arduino sketch.


const int sensorPin = A0;
const int pwmPin = 3;
int potvalPre;
int potval;
void setup()
{
  pinMode(3, OUTPUT);
  Serial.begin(9600);
 
}

void loop()
{
    potvalPre = analogRead(sensorPin);
    potval = potvalPre / 4;
    Serial.println(potval);
    analogWrite(3, potval);

}


So... I'm using the standard boost converter circuit, with a 15 nano farad capacitor, 30n06l as the switch. my problem is, the voltage range is from about 4.8v to 5.2v.... What's going on? This voltage range is not with the smooth turning of the potentiometer... How do I fix it? BTW, with a load (led) the voltage doesn't change at all.... figured I'd try to fix that later.... thx for your help.

[skillz21]

Ok, im scared i was just playing around with the wires... all of a sudden something started buzzing and the voltage shot up to 60v..... luckily the capacitor i'm using is rated for 400v..... plz help i have no idea what i'm doing


Edit: i think i broke the mosfet

[skillz21]

Okay then... same code, when the duty cycle is between 99% and 1% (i am assuming, I don't have an oscilloscope) the voltage goes straight up to 65 volts and the mosfet (assuming again, i'm not sure whether a capacitor or an inductor could buzz) starts buzzing... What is going on? please help...


Edit: it's actually the inductor buzzing... why?
Edit: i tried a coil, it buzzes, could you use it as a speaker?

[skillz21]

Any ideas?

[Whales]

I'm using the standard boost converter circuit

The general circuit topology might be standard, but we need to know the exact details of how you have hooked everything up.  Please draw a full schematic, including how you are powering it and attaching the Arduino.

my problem is, the voltage range is from about 4.8v to 5.2v

Which voltage?  The output?  I presume you are powering your circuit with 5V, so that means it's not boosting at all.

BTW, with a load (led) the voltage doesn't change at all..

Are you using a resistor on the LED?

void loop()
{
    potvalPre = analogRead(sensorPin);
    potval = potvalPre / 4;
    Serial.println(potval);
    analogWrite(3, potval);

}

Is the pot sampling the booster's output voltage, or is it a completely separate (ie run off 5V)?  If the latter then you have made an 'open loop' booster that will work, but it won't maintain a "constant" output voltage, instead varying depending on load.  If the former: you have done your feedback the wrong way around (positive instead of negative) and you'll need to make sure you "smooth" the signal being analogRead() by the Arduino, or it will just be random numbers due to the 500Hz switching.

Also note that the default Arduino PWM speed (analogWrite() PWM speed) is 500hertz.  Your boost converter will run much better at much higher frequencies (I'm aim for somewhere into the tens to hundreds of kilohertz). 

Edit: i tried a coil, it buzzes, could you use it as a speaker?

Yes.  That's all a common speaker is, plus a paper diaphragm and some stuff to support it.

You'll be hearing the 500Hz PWM being made by your Arduino.

[skillz21]

Hi
So, yes, the voltage range is the output and I am powering it from my computer. I am also using a resistor on the LED. I attached the schematic. And a picture of the inductor... because I don't know the value of it. Also, how did I manage to get the output up to 60v?
Thx


PS, the inductor image is a bit blurry, I used my webcam, let me know if you need a clearer picture, also, ignore all the unnecessary components in the background, including the Nano. Oh yeah, it's also upside down.... sorry.

[skillz21]

What do you guys think?

[Neilm]

The picture you have posted shows the value of the inductor - 220uH.

I would guess that the voltage went up to 60V either as it started running at the correct speed for the circuit or your load fell off. It might be that you have to drive the circuit slighly differently to build the voltage up. I have built boost circuits for outputs of 150V for a low voltage insulation tester. 

One thing you have not said is what voltage you actually want out of boost converter.

I would suggest you use a simulation tool (TinaTI, LTspice etc) to simulate what you actually want before you build it.

[Whales]

I would suggest you use a simulation tool (TinaTI, LTspice etc) to simulate what you actually want before you build it.

There's a few things here that would be hard to simulate for beginners, especially if you're just using parts you have handy rather than ones you can find good spice models for.  Anyway, the circuit's already made, and changing stuff in the model wouldn't be any harder/easier than changing it in real life; so I don't see any many advantages to modelling it.

(1) You don't have any capacitive decoupling next to your microcontroller or the pot.  The voltage on both is likely going to oscillate around as the current draw of the boost circuit changes.  A big (eg 100uF) capacitor would be useful.

(2) The 60V is likely the voltage that your diode fails at.  The way you have designed your boost circuit: if you have no load attached to the output of the booster it will keep boosting the voltage forever (or atleast until something fails). 

[skillz21]

But there is no variation in the output voltage, even when i turn the potentiometer.... why?

[Whales]

What voltage is it stuck at now?

[skillz21]

I haven't made any changes yet.... but I was asking why it's just stuck at 60v and not changing as I turn the potentiometer... I can hear the inductor vibrating at different frequencies, but there is no difference in the output voltage...

[krystian]

Maybe its boosting because frequency is too low. PWM on Adruino is around 340Hz (?). Even small pulse width is enough to fully charge the coil. You can change pwm frequency by changing register values on MCU or write your own software pwm with delays or asm("NOP") function. And you need to calculate parts of your converter. Easiest way of doing that is to use online dc/dc converter calculator .

[Whales]

Your boost circuit can only charge the capacitor on the output side.  It cannot drain it.

It other words: no matter what position the pot is in, you are still operating the same transistor + inductor + diode combo.  This 3 part combo can only push energy from the left side of your diagram to the right.  No current can flow backwards through the diode, so the voltage on the output capacitor never reduces.

You need to connect a load to fix this.  The load will drain the output capacitor, dropping the voltage.

Make sure to look up how capacitors work if you're not familiar with how they charge and discharge.

[skillz21]

but i followed the usual boost converter schematic... what did i do wrong?

[Whales]

"Building" and "controlling" a power supply are two very different things.  You have only done the former.

I've mentioned possible solutions are few times:

 (1) Load
 (2) Negative feedback

You need to do some of your own research.