FlyBack Transformer Design

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SACHIN C

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Dear All,
How to Design DC-DC Flyback Transformer with two output?
Details are as mention below,
1)Vin = 5V
2)Vout1 =15V
3)Vout2 =15V
4)Switching Frequency =70KHz
5)Io1 =2A
6)Io2 =2A
7)Duty Cycle =0.5


Thanks & waiting for knowledge enhancement,:thinker:
Sachin
 

Can we use SG6858 control IC for Flyback Topology as Maximum Duty Cycle is 0.7 ?



Thanks & waiting for knowledge enhancement,:thinker:
 

Can we use SG6858 control IC for Flyback Topology as Maximum Duty Cycle is 0.7 ?
Click to expand...
0.5 < 0.7, isn't it?

But you'll face the problem that SG6858, and most flyback controllers designed for off-mains SMPS won't start operation with 5V input voltage.
 

FvM,
Yes that is the problem,at the start up as output voltage is not build up the duty cycle will be almost 0.7 instead of 0.5.When the output reached to final voltage value the control loop gets activated and duty cycle 0.5.
How can we overcome duty cycle issue as our output final voltage will get build up when input is 90Vrms,so upto 90Vrms duty cycle 0.7?



Thanks & waiting for knowledge enhancement,:thinker:
 

Your output spec adds up to 60W. This requires that your 5V supply provide upwards of 36 A, at a 50% duty cycle. A tall order.

You will need to make sure that total impedance is no more than 1/7 ohm, during the input cycle.

Instead it may be better to consider designing two flybacks which turn on at opposite times. Then the 5V supply would provide 18 A continuously, which is less of a chore.

Or perhaps a full H-bridge topology, sending AC square waves to two transformers (or a tapped transformer if you want the outputs to be connected).
 

BradtheRad,
Specification for Flyback Topology Modified which are as follows,
1)Vin = 90Vrms to 300Vrms
2)Vout1 = 18VDC/2A
3)Vout2 =5VDC/1A
4)Vout3 =3.3VDC/0.5A
5)Vout4(Bias) =20VDC/0.3A
6)Fsw =70Khz
7)Duty Cycle =0.5

Thanks & waiting for knowledge enhancement,:thinker:
 

It makes no sense to specify a duty cycle of 0.5 independent of input voltage.

If you make a transformer for duty cycle of 0.5 with 90 V input voltage (sufficient load for continuous mode assumed), you get about 0.23 duty cycle at 300 V.
 

FvM,
So at high input voltage say 300Vrms the Control IC will change duty cycle from 0.5 to 0.23 to make output voltage constant?

Do i need to change Control IC?
I am having TL494 & UC3844 with me.

Circuit Checked at 100V input(Not Continuous stored in Capacitor) & external control supply was connected to the SG6858,Output Voltage was proper as per turns ratio.
But,when supply at input say 50Vrms provided continuously with external supply to SG6858,frequency changes from 70KHz to 2KHz?

Thanks & waiting for knowledge enhancement,:thinker:
 
Last edited:

It is possible (theoretically) to design a transformer with the proper turns ratio, and proper inductance, such that it will let you vary the duty cycle within a range above and below the 50 percent mark.

From my experiments with a simulation, it works for the transformer to have a 9:1 step-down ratio. (This is to output 18V at 2A, your Vout1 supply.)

Duty cycle is 67 percent when the supply is 90V.
37 percent when the supply is 300 V.
 

BradtheRad,
But,the frequency changes with duty cycle as we increase the input voltage.
Do i need to change the SG6858 with another control IC as i didn't understand the Green Mode provided in SG6858?
Can you provide me some tips or guide me regarding transformer design for multiple output Flyback Topology in Continuous & Discontinuous Mode?

I know the difference between Continuous & Discontinuous Mode but,don't know how it's reflect theoretically?

Present Turns Ratio selected are as follows;
17:2:1:2 Viz correspondes to Voltage as (90Vrms to 300Vrms): 18Vdc : 5Vdc : 20Vdc(Bias Winding)

Observation : Varying feedback pin of SG6858 frequency varies but ON-Time remains same.

Thanks & waiting for knowledge enhancement,:thinker:
 
Last edited:

SACHIN C said:
BradtheRad,
But,the frequency changes with duty cycle as we increase the input voltage.
Do i need to change the SG6858 with another control IC as i didn't understand the Green Mode provided in SG6858?
Click to expand...

The frequency can be changed without necessarily changing the duty cycle, and vice versa.

However you will need to experiment with both frequency and duty cycle if you construct a real transformer.

Sorry I have no direct experience with smps control IC's.


Will the loads be constant? If so then it will make your overall job easier.
However if one of the loads increases, then it will draw more amperes from its supply, which can alter flux field dynamics, which can reduce the voltage of the other supplies. The voltage may drop below the threshold needed by your regulating devices.

I've dismantled transformers but I have not constructed any with a metal core. All I can suggest is that you make your transformer easy to experiment with, to add or remove turns, etc.

It will be a complicated job to design a transformer with four secondaries. So it may turn out to be easier for you to construct it with a one or two secondaries, and split off the additional supplies.
 

Do we need to use Schottky diode at the output of flyback because of it small forward voltage drop & fast recovery time or normal diode can be used?

Voltage drop will matter but fast time matters in flyback?


Thanks & waiting for knowledge enhancement, :thinker:

- - - Updated - - -

BradtheRad said:
It will be a complicated job to design a transformer with four secondaries. So it may turn out to be easier for you to construct it with a one or two secondaries, and split off the additional supplies.
Click to expand...

Splitting supply will yield us same or different ground?
As our requirment is different ground for each supply?

Do we need to use Schottky diode at the output of flyback because of it small forward voltage drop & fast recovery time or normal diode can be used?

Voltage drop will matter but fast time matters in flyback?


Thanks & waiting for knowledge enhancement, :thinker:
 

SACHIN C said:
Splitting supply will yield us same or different ground?
As our requirment is different ground for each supply?
Click to expand...

If you divide one supply into two, then yes, they will have a common rail, either the ground or the positive.

Which do you require, common grounds, or isolated?


You can expect less of a drop with a fast recovery diode.

However the more important thing is the speedy recovery.

If your frequency is slow enough, you can get by with ordinary diodes.
 

Do we need to connect MOSFET driver IC with SG6858, as output source & sink current from SG6858 Flyback Control IC will be less?

But usually in SMPS why MOSFET Driver IC or external Totem pole circuit not required eventhough the SMPS control IC is not capable to drive MOSFET >100mA,because of which MOSFET turn-ON will be slower?



Thanks & waiting for knowledge enhancement, :thinker:

- - - Updated - - -

BradtheRad said:
Which do you require, common grounds, or isolated?
Click to expand...

Required isolated ground for each output.
 

Dear All,
Why we cannot use Ferrite Material Core for 50Hz application instead of Laminated Cores?
IS it because of Viberating Sound or Magnetic Property?


Thanks & waiting for knowledge enhancement,:thinker:
 

Why we cannot use Ferrite Material Core for 50Hz application instead of Laminated Cores?
Click to expand...
Who told you can't? It's pretty possible, but not reasonable. You simply have to calculate number of turns according to a meaningful Bmax value. Because Bmax of ferrite is about 1/4 of laminated iron core, consider a fourfold core size.
 

FvM,
So after calculating proper Bmax we can use ferrite core for 50Hz?

Can we use Bmax for 50Hz application 2000Tesla.

Thanks & waiting for knowledge enhancement,:thinker:
 

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