# ATX power supply turns off when used with DIY boost converter

I am trying to build my own step-up boost converter. I have my connections as shown in the following schematic (except for the one circled in red.)

First I powered up my step-up module with a breadboard power supply. Everything works as expected when there is no load on the output.

When the output voltage reads 12V with no load, I connected a 12V 0.5A DC fan across the output. The output voltage dropped upto 6V. This is my code(if it is necessary.)

#define F_CPU 8000000
int feedbackPin = 3;
int potPin = 2;
int gatePin = 1;
int pwm=0;
int v1 = 0;
int v2 = 0;

void setup() {
//Clear OC0A & OC0B on Compare Match, set OC0A/OC0B at BOTTOM
TCCR0A = 2 << COM0A0 | 2 << COM0B0 | 3 << WGM00;
//Normal Waveform genetation mode , timer uses clock source clkI/O/(No prescaling)
TCCR0B = 0 << WGM02 | 1 << CS00;
// PWM1A enables PWM mode based on comparator OCR1A in Timer/Counter1 and the counter
//value is reset to \$00 in the CPU clock cycle after a compare match with OCR1C register value
//0 << COM1A0 makes Timer/Counter Comparator A disconnected from output pin OC1A.
TCCR1 = 0 << PWM1A | 0 << COM1A0 | 1 << CS10;
GTCCR = 1 << PWM1B | 2 << COM1B0;

pinMode(feedbackPin,INPUT);
pinMode(potPin,INPUT);
pinMode(gatePin,OUTPUT);
analogWrite(gatePin,pwm);

}

void loop() {
v1 = map(v1,0,1024,0,255);
v2 = map(v2,0,1024,0,255);
while(v2 > v1){
v1 = map(v1,0,1024,0,255);
v2 = map(v2,0,1024,0,255);
if(pwm<250){
pwm += 1;
analogWrite(gatePin,pwm);
}
}
while(v2 < v1){
v1 = map(v1,0,1024,0,255);
v2 = map(v2,0,1024,0,255);
if(pwm > 0){
pwm -= 1;
analogWrite(gatePin,pwm);
}
}
}


I understand that what needs to happen to control the output voltage is changing the frequency of the PWM signal. But what I do is increasing the duty-cycle. Could this be the reason it fails to maintain the voltage constant upon connecting a load?

My biggest problem as the title says is when I connecct the +3.3V and +5V and ground to an ATX power supply.

Everything works as before when there is no load across the output, but as soon as I connect the fan I used earlier across the output my ATX power supply turns off.

I have no clue why this happens. I would like to know why this happens.

Also is it really necessary to have the capacitor circled in red?

• PWM control is a perfectly valid way to control the output voltage. – JRE Jun 26 '20 at 15:02
• What is the PWM frequency? – JRE Jun 26 '20 at 15:03
• According to the code I have used and the datasheet it's 250kHz.I haven't check it using an oscilloscope. – AfiJaabb Jun 26 '20 at 15:09
• Part number for the inductor please and a link to its data sheet. – Andy aka Jun 26 '20 at 15:24
• If the inductor is saturating, because it isn't rated for the current, then it'll act as a short. If you short the ATX power supply, does it turn off? – Aaron Jun 26 '20 at 16:03