Flyback transformer turns ratio basic question

[ku637]

Hello experts,

I just wanted to design my first flyback converter.. so after some initial search i obtained a pre built flyback converter transformer whose characteristics are

f=100kHz, Lprim=290uH, Np/Ns=14 , Np/Naux=10 This transformer was designed to be used with a 12/3A SMPS operating from 85-265VAC

So after some study i thought it will be better to do a a design around UC3845 and experiment..

So i built a PCB and used a generic schematics,

VIN =85-265VAC
VOUT=12V
Iout desired =3A
Feed back TL431+OPTO
OSC FREQ=100kHz
SENSE Resistor i put 2.2Ohm i.e it will give 1/2.2Ohm =454 mA primary peak current.
Auxillary winding is generating 16V to power IC as expected.
MOSFET and other components sized as per requirements to best of my knowledge.

And when i observed at 85 VAC input, i got pretty 12V which can load up to 1.5A.. i don't know how can i improve it up to 3A capability..

But then i tried another possibility

I just changed the output resistor divider to obtain VOUT=5V
but now the load capacity is dramatically poor it can't give even 500mA..

I tried to observe the waveform using a 100x probe at the VDS, it appears that the converter is running in DCM,

I tried to increase the fsw to 150 kHz, as in DCM i thought P=[1/2*L*(Ip)²]*fsw no improvement in load capacity

I tried to decrease sense resistor value to 1 Ohm i.e Ip is set to 1A (i.e 1V/1 Ohm) still it cant load up to 500mA and the converter appears pretty instable.

So my question is,

1. Whether it is possible to use a transformer designed for 12V output to work for 5V output?
2. Will the turns ratio have much impact to prevent it?

From Pressman

The rules for an inductor with more than one winding are as follows:The primary to secondary ampere-turns ratios are conserved (not the voltage ratios, as was the case with a true transformer). For example,if the primary is, say, 100 turns and the current when Q1 turns “off” is 1 amp, then we have developed 100 ampere-turns in the primary. This must be conserved in the secondaries. With, say, a single secondary winding of 10 turns, the secondary current will be 10 amps (10T×10A = 100 ampere-turns). In the same way, a single turn will develop 100 amps or 1000 secondary turns will develop 0.1 amps.So where do we stand with regard to voltage? Well, to the first order,there is no correlation between primary and secondary voltages. The secondary voltage is simply a function of load. Consider the 10-turn 10-amp (100 ampere-turns) secondary winding example mentioned above. If we terminate the winding with a 1-ohm load, we will get 10 volts. What is more striking because the 10 amps must be con-served is that if we terminate it with 100 ohms, we will get 1000 volts!
This is why the flyback topology is so useful for generating high volt-ages (don’t try to open circuit this winding because it will destroy the semiconductors). With several secondary windings conducting at the same time, then the sum of all the secondary ampere-turns must be conserved.

 

[FvM]

As far as I see, the output current is limited by the low primary inductance in combination with primary peak current limit.

 

[ku637]

Dear Fvm,

Thank you for the reply.. But can u please share your opinion on

Whether it is possible to use a transformer designed for 12V output to work for 5V output?
Will the turns ratio have much impact to prevent it?

Thanks

 

[FvM]

You already showed in your experiment that flyback converter operation at 5V output isn't "prevented". Obviously the turns ratio (together with other parameters like primary inductance and allowed primary peak current) impacts the available output current with different voltages. For an exact answer, refer to the flyback converter design equations derived in Pressman's book.

 

[chuckey]

I would think that for maximum power transfer the on/off time of the transistor should be 50/50, so the power loaded into the primary inductance can be unloaded into the secondary when the transistor is off. If due to the feedback, the transistor is only on for, say, 10% of the time, its fairly obvious that the power transfer will be substantialy lower. I would guess the design should allow for, say +10% overvoltage on the output voltage, so the transistor can be throttled back a bit for ordinary operation, but has something in hand when the input voltage is low.
Frank

 

[treez]

if you give (in very brief list form) the following info, then it is easy to see straight away what should be your turns ratio...

Vin = xxx
vout = xxx
pout = xxx

if they have min and max's then give the range.

I would use info to optimise your duty cycle at max power/min vin...then I would work out your turns ratio from that.