12VDC to 24VDC flyback converter(multiple output)

[peggy]

Hi,

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?

Attach are my design and the info of flyback transformer.
https://obrazki.elektroda.pl/11_1312757763.gif
https://www.farnell.com/datasheets/323239.pdf
IC chip is UC3843AN(current mode) and flyback transformer is myrra 74014.
View attachment 74014.pdf

PLS HELP and advice!!! Thank you.

 

[YAS_HASHI]

I think 74014 is not for 12V to 24V.
It's for 100V to 24V.

 

[murali_dece]

Hello friend, no problem you can still use that transformer. As per your design you can use Aux winding (pin-1 & pin-2) as primary winding and make parallel both secondary windings together (pins-7,9 $ pins 8,10) to get more ampere. In this way you can use your transformer as step up transformer. finally you have to play with feedback network resistors and comp resistor to fine tune your power supply on trail and error basis.
I Wish your Success

 

[peggy]

very appreciate for all reply.

YAS_HASHI:
You mean this flyback transformer cannot work when the voltage less than 100V?(since this transformer only available for 100V to 24V).I can't find the other flyback transformer which is under my requirement in the market anymore.

murali_dece:
Thank you for your advice. By the way, how about +E and drain winding( pin-6 & pin-4) or just ignore it?
The power supply flows to the transformer is DC voltage, but the transformer can only take in AC voltage, so does this circuit work? Please advice! Thank you.

 

[xaccto]

Hi, for 12V to 24V you really should be recommended to wind your own transformer.
or even **broken link removed** - switched capacitor
(I really must try this one myself oneday)

if you google for terms like car amplifier inverters DCDC, you should find some hits.
Big car amps use DCDC inverters to step 12V to +-35V for example.
They are also, invariably push-pull.
You can get tips off those designs.

The Transformer you have, for doubling your input voltage will require such a short duty cycle I wonder whether it
would be viable, and expect high ripple....
You mention running 24v radios........so you won't want much ripple, or noise! I expect you would need to further filter the 24V with a choke.

 

[Orson Cart]

What about a simple boost converter done with a National Semi simple switcher IC? very easy and convenient. LM2585
Regards, Orson Cart.

 

[RCinFLA]

Since you don't need isolation from output to input (as depicted by your feedback and common ground) you can get by a lot simplier with just a boost converter with a single coil. Going from 12v to 24v is only a doubling which is easily done with a boost coil.

A flyback is a coupled inductor. Energy is stored in the core during primary side charging then transfered to secondary output on discharge. Having a turns ratio gets the duty cycle of the primary side to a more optimum range.

 

[peggy]

RCinFLA
how to generate multiple output by using single coil? i only found the flyback converter able to generate multiple output.

 

[murali_dece]

Hello Peggy, single coil boost converter suggested by other friends is also a good idea any how you already spent some money on this one and make it to work. regarding pin4 & pin6 ignore them and be carefull while your circuit is fully operational these two pins will provide morethan 70V. you must be very carefull while testing your circuit. your doubt about transformer operation with DC is genuine, actually dc current flow through primary winding (in your case pin-1 & pin-2) and stops at MOSFET, pulse width modulator output operates the MOSFET in such a way it makes primary winding DC currents are Sqare Pulsed which results in alternating magnetic flux to yeild energy transformation from primary to secondary winding. but voltage convertion depends on turns ratio. go ahead with your circuit and learn practically, be cautious once again.

 

[Orson Cart]

How many o/p's do you require? can they be common ground or do you require full isolation between all ouputs and to the input?

 

[peggy]

hi all, thank you for all reply and advice.

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?
Please advice.Thank you

orson cart,
2 output (24VDC,0.5mA)

murali_dece,
What is mean for parallel both secondary windings together (pins-7,9 $ pins 8,10)?connect together for pin 7&9?

 

[murali_dece]

Hi, I am happy to hear from you, I suggested that only to get single output with more amps, since you need multiple outputs dont join them you will get dual output voltage with rated current. I think you succeeded if one output came means defnitely second one also will come, only mistake is while joining second winding (S2 & 0V) they are out of phase and giving zero voltage, the dots in the secondary winding of transformer datasheet represents orientation of winding. you just dont join the secondary windings you will get dual output. finally forgot about number of output windings any one output voltage feedback is sufficient.

I think you are clear now
sorry for confusing earlier

 

[Raza]

Hi peggy,
Please give me the output voltages you need and the current of each output, I can calculate the winding turns for you for a given size of Ferrite Core. Hope I may be able to help you.
Raza

 

[peggy]

Raza,
24VDC, 0.5mA x 2

murali_dece,
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.

 

[murali_dece]

Hi, Peggy,
don't worry it is normal, voltage from first output drops quickly due to output capacitor discharges through resistors used for feed back, while in second output there is no path to discharge for filter capacitor. if you apply any load to the both outputs, both voltages will drop at the same rate.
finally to find root cause for ic over heating send your final circuit with component values to analyse.

regards,
murali

 

[Raza]

HI peggy,
Myrra 74014 has DC 140 volts input and 18 volts output capable of handling 12 watts output. If possible for you to get core E25/13/7 I can give you the turns and wire number so you can wind it yourself.

 

[peggy]

murali_dece,
attached is my final circuit design. There is any way to discharge for filter capacitor for second output (same rate from first output)?I thought no path to discharge for filter capacitor for second output will cause the IC overheat.

 

[murali_dece]

hi peggy,
i am suspecting that failure of snubber circuit to primary winding may result in over voltage of IC in your case, when switching transistor is in off state energy stored in primary winding generate reverse voltage more than operating voltage Vin in applications of operating voltage more than 10V. in your case operating voltage is 12V so snubber circuit is must. Snubber circuit is a combination of fast recovery diode connected with parallel connection of resister and capacitor, the snubber network is connected parallel to primary winding and serves as discharge path to high frequency regenerative ripples.
one more observation i was found in your circuit is feedback network, R1 and R2 values justifying for 19.8 V regulation, for 24V regulation you have to change your R1 Value as 370K ohm.
For feedback you have to use allways metal film resistors they have 1% tolerance and their body colour is blue,
regards,
murali

 

[peggy]

hi murali_dece,

I use 0.22uF capacitor, 2k resistor and 1n4148(fast switching diode) for snubber circuit, but the temperature of IC still high. I no sure which components i select wrongly? fast switching diode is same with fast recovery diode?
By the way, Can i use 37k and 2k for feedback network ? what is different between use (37k & 2k) and (370k & 20k)?
Thank you very much for helping clear my doubt.

 

[murali_dece]

Dear Peggy,
Regarding feedback network it is simple, we have to provide 1.23V at feed back pin at our required output voltage. suppose you are using R2=20K ohm and assume 1.23V established across it then current flowing through resistor is 0.0615mA. Now your regulation/ required output voltage is 24V and current required to pass through feedback network is 0.0615mA, So total feedback network reistance=24V/0.0615=390244 ohm=390K ohm, Now R1= 390K-R2=370K.
I was shown the calculation in this way scince many hobbyist's alter the R1 value to change smps voltage, they keep the R2 original value as it is, if we know the R2 value this is the way to calculate R1. Otherwise one direct formula is given in datasheets.
regarding temperature check the voltage existing across the IC and current flowing through primary of transformer may be it is morethan 3A. otherwise it is normal.