I'm trying to design a flyback DC-DC converter which can provide up to 24V DC from a normal 12V DC lead-acid battery that every car has. It can be used to run radios, small lights, relays, horns and other 24V accessories(multiple output) from a 12V vehicle.
I have done research on flyback topology and design the circuit diagram. I couldn't get the 24V DC from the output, after I assembly all the components. Is my design having problem or I selected the wrong flyback transformer?
Your turns ratio is wrong. The transformer you are using is designed for mains input. You need your turns ratio to be roughly equal to Vin:Vout. This keeps the duty cycle in a reasonable range and makes sure you dont end up with huge flyback pulses on the primary or too much current on the secondary. I am also not sure what flavour of FET you are using. You need this to have a low ON resistance and a low Qg to get the effiency high. Remove the 100 Ohm gate resistors - this is only there to reduce EMC, but it will kill the efficiency of the converter. You need to tweak the current sense resistor in the source of the FET to put more (or less) power into the circuit to get more (or less) power out. The saturation current of the primary needs to be higher than the current set by this resistor, as does the Ids of the FET.
simonbramble:
You mean I should change the flyback trasformer or change the design circuit? I can't find the other flyback transformer which is under my requirement in the market anymore.
I am using IRF540N (IRF 540N MOSFET N-CHANNEL).
I am newbie in this area, please give me some guide and advice.Thank you.
ronsimpson:
if i use inductor to replace the flyback transformer, then this design will became boost converter.
i thought LM2585 is for flyback regulator nor flyback converter.
Of course, you know you won't get that, right? Your converter isn't going to be 100% efficient, so you're either going to have higher input current or lower output current. (You can probably get close to 80-90%, in theory at least.)
......if i use inductor to replace the flyback transformer, then this design will became boost converter.
i thought LM2585 is for flyback regulator nor flyback converter.
The LM2585 can also be used as a boost converter. See last paragraph on page 18 of the data sheet and Figure 35, which shows a 12V to 24V, 0.6A boost regulator.
Be sure and follow the circuit board layout guidelines on page 24 when you build the circuit.
unless you want multiple outputs or an isolated output, I would go for a standard boost converter. This uses a single coil instead of a transformer and single coils are much easier to find. Half the battle with designing flyback converters is finding the correct transformer. Coiltronix do 'Versa Pacs' and there are equivalents from Wurth and Coilcraft, but go for a single inductor if you can. Your FET looks OK. See my design guide above for how to design a boost converter
Cutshow,
That is in part due to the fact, (look at post #1) he takes 12 volts to make 24 volts.
In a simple inductor boost up you take 12 volts to make a second 12 volts that sits on top of the first 12 volts. Half of the power comes for free.
I was able to generate first output(24VDC) with S1 and 0V,but further add on S2 and 0V,the circuit showed error and can't generate both outputs(24VDC). For the second output, I only connect the zener diode(1N5818) and capacitor. Should I add the feedback loop for second output or add opto-coupler to isolate the output voltage?
I was able to generate first output(24VDC) with S1 and 0V,but further add on S2 and 0V,the circuit showed error and can't generate both outputs(24VDC). For the second output, I only connect the zener diode(1N5818) and capacitor. Should I add the feedback loop for second output or add opto-coupler to isolate the output voltage?
attached is my schematic diagram.
S1 is pin 9 from transformer 74014, and S2 is pin7.
when I switch off the 12V DC power supply, the voltage from second output will drop very slowly(0.01VDC~0.02VDC), meanwhile the voltage from first output is drop normal..at the same time, the temperature from IC chip is high.
what is the root cause for this problem?how to fix it?
please advice. Thank you.