I've been working on a 12 to 5 volt buck converter, but I've run into a problem. The buck converter needs to operate at a duty cycle less than 50%. I've looked at many articles and run many simulations and I've used both the formulas ((r1 +r2)/(r1+2*r2) and r1/(r1+r2)) to calculate the required resistance, but when I set it up on a breadboard, it refuses to work and is usually around 8.3 volts. I'm using a standard astable 555 circuit with a diode in parallel with r2. Any ideas? Thanks.

 

Welcome to AAC!

Post a schematic of your circuit and give us more details for your buck regulator (e.g. current requirement). Duty cycle of the switch typically depends on load.

 

How about this?

 

Here's a schematic of the circuit.

 

Have you actually checked the mark/ratio of the '555 output or are you assuming it is wrong because the output voltage is higher than you want?
The output voltage will depend on the mark/space ratio, but also on the frequency, the inductor, the output capacitor and the loading on the output.

 

Is the mark/ratio the duty cycle? The duty cycle should be (5/12)%. The current draw should be at most 2 amps.

 

How about this?
View attachment 114123

I'll try this out.

 

Here's a schematic of the circuit.

You need 2 diodes pointing in opposite directions and a resistor for each. The 2 resistors determine the charge period and the discharge period. The diodes are basically switches - one resistor only does anything during charging, the other resistor only does anything during discharge. Since the 2 resistors have a common connection; you can insert a pot and make the ratio variable.

The typical textbook example of a 555 PWM control uses 2 of them (or a 556). A monostable is triggered by a variable speed astable - a fixed on period triggered more often fills more of the total time period. This version is *MUCH* easier to arrange a feedback voltage to control it.

 

Is the mark/ratio the duty cycle? The duty cycle should be (5/12)%. The current draw should be at most 2 amps.

Having a duty cycle of 5/12 doesn't mean that you will automatically get 5V with 12V in.
It also depends on the other things I mentioned in post #5.