# Flyback DC/DC Converter

Discussion in 'The Projects Forum' started by jcesar203, Apr 4, 2013.

1. ### jcesar203 Thread Starter New Member

Dec 19, 2012
9
0
Hello,

I am building a Flyback DC/DC converter for my Power Electronics class and I would like to know where can I buy the coupled inductor.

The calculations of the design are as follow (ideally):

Vin = 24V
Vo = 12
Switching F = 100kHz
Pout = 1W = Pin
Iout = 83.33mA
Iin = 41.67mA
N2/N1 = 1:2
Duty Cycle = 50%

I want to operate the converter at 50% and use the coupled inductor to step down the 24V down to 12V.

The coupled inductor calculations are as follow:

Lm > 72mH
L1 = 1uH
L2 = 0.250uH
R1 = 1 ohm
R2 = 0.250 ohm

I cannot find the coupled inductor with a turn ratio of 2:1. Can somebody help me find the coupled inductor?

Thank you.

2. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,222
Coupled inductor, It is the transformer we are talking about Switchmode transformers are not always easy to find. Where in the world are you located. And do have any frequency specs regarding switching frequency

3. ### JMac3108 Active Member

Aug 16, 2010
349
66
Well, actually the OP is correct in calling it a coupled inductor rather than a transformer

A flyback works by storing energy in the primary inductance, then allowing it to "fly back" and transfer over to the secondary side. It is nothing more than two inductors wound on the same core. Its customary to call it a flyback transformer, but its really is a set of coupled inductors.

Now a forward converter or related topologies - those use an actual transformer.

4. ### jcesar203 Thread Starter New Member

Dec 19, 2012
9
0
I am located in the United States. The frequency is the same as the switching frequency of 100kHz. I can choose a smaller value of switching frequency if possible. I found some coupled inductors in Coil Craft but they are not 2:1 they are 1:1. Therefore I will have to change my duty ratio to 33.3%.

If anybody can guide me to get the 2:1 coupled incuctor or flyback transformer it would be awesome. The reason I want 2:1 is because I would like my flyback dc dc converter to operate at 50%.

Thank you.

5. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,222
Last edited: Apr 6, 2013
6. ### crutschow Expert

Mar 14, 2008
13,523
3,392
Unlike a buck switching regulator, the flyback output voltage is not mainly determined by duty cycle, it also is significantly affected by the load.

This difference is because the buck inductor always sees the output voltage pushing back against the inductor current, so the inductor acts to give an output voltage that is the average of the input pulses (in continuous mode), with only a small affect from the load current (mainly due to inductor resistance).

A flyback acts more like a boost switching regulator. The flyback has a diode in the output so there is no push back from the output voltage when the input is high and charging the inductor (since at the point the output is the opposite polarity and blocked by the diode). Thus the output voltage is not necessarily the average value of the input pulses. Actually with a light or no load the output voltage can become very high, limited mainly by the breakdown voltage of the switching transistor.

So whether you use a 1:1 or 2:1 coupled inductor, the output will still not necessarily be what you want with a 50% duty-cycle.

7. ### nigelwright7557 Senior Member

May 10, 2008
488
71
I built my own using an etd54 core.
I put 60 turns on the primary and 15 on the secondary to get about 40 volts from a 320VDC supply.
Don't forget to gap the core.

8. ### jcesar203 Thread Starter New Member

Dec 19, 2012
9
0
Theoretically it should work because of the relationship of Vout/Vin = [D/(1-D)]*(N2/N1). I also calculated the inductance values of the coupled inductor. I know I will not get exactly 12V output with 50% duty cycle but close to 12V should be good enough. It is a school project and I have also simulated on Simulink.

9. ### jcesar203 Thread Starter New Member

Dec 19, 2012
9
0
Can you please provide me the link to where you bought the core?

10. ### crutschow Expert

Mar 14, 2008
13,523
3,392
The problem is that Vin is not the supply voltage. It is the flyback voltage at the transistor drain when the transistor turns off, and this can be much higher than the supply voltage due to the stored energy in the inductance of the coupled inductor.

11. ### jcesar203 Thread Starter New Member

Dec 19, 2012
9
0
What do you recommend I should modify from my design?

12. ### crutschow Expert

Mar 14, 2008
13,523
3,392
If you want a regulated voltage from a flyback converter then you need negative feedback from the output to control the PWM duty-cycle. The feedback will need a compensation network to form a stable control loop.

13. ### jcesar203 Thread Starter New Member

Dec 19, 2012
9
0
The project is an open loop flyback converter, so what you are saying is that I will not achieve the requirements of the project unless I use a feedback controller?

14. ### crutschow Expert

Mar 14, 2008
13,523
3,392
For a fixed load and input voltage you can select a PWM duty-cycle to get your desired output voltage. But any change in load or input voltage will change the output voltage. If you want a stable output voltage with change in load, you will need a feedback loop.

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15. ### jcesar203 Thread Starter New Member

Dec 19, 2012
9
0
I understand, thank you.

By the way do you have any recommendation where I can buy a through hole coupled inductor?

16. ### crutschow Expert

Mar 14, 2008
13,523
3,392
Not off hand. You might try a search of switching transformer manufacturers.