How to increase Reflected Output Voltage (VOR)?

Posted by: HighHopes on 03/30/2014

my input DC voltage is fixted, 60Vmin, 180Vmax and 12V output. Although the power requirement is only 15W, I keep having to chose the very largest TOPSwitch in order to keep the junction temperature under 100C. I'm reading through the application notes and it seems that the solution to my problem is to increase VOR. But how to do this? Seems that my only option is to change the turns ratio of the transformer.

with Np/Ns = 4, the range of secondary voltage would be roughly 15Vto 45V. From here the dutycycle works to create 12V output and the flyback converter isprimarily a buck converter.

Can I arbitrarily select a turns ratio of 10, for secondary voltage range of 6V to 18V and ask the flyback converter act as a buck/boost to achieve desired 12V output?

Comments

I read your entry several times but still I do not get what you are trying to tell me. I am sorry

You know VOR is the voltage that appears on the primary winding during the flyback period. It is equal to VOUT multiplied by (NP/NS). We are not counting the forward voltage drop of the output rectifier.

So if you like to change the VOR, you need to change your transformer turn ratio.

If you need to adjust your VOUT with the actual transformer, you need to adjust the feedback to set your VOUT. If you can do that, still you need to check your design for proper operation during all conditions.

I suggest you to download PI Expert software and create a design with this program. a

http://www.powerint.com/en/pi-expert-download

let me ask my question in a different way.

input: 60Vdc

output: 12V

if i chose a transformer with a 10:1 turns ratio, design a flyback converter topology, will that be a problem for TOPswitch-HX ?

Is not a question that I can answer right away and say yes or not

In order to get the right answer you need to run some calculations.

The simplest and easier way to find out is to download PI expert. Then you enter your design variables and adjust the turn ratio as you wish. You need to enter different VORS until you get the turn ration you want. Then you check the deign and will know if your transformer is suitable for your application

By the way, is not only the turn ratio of the transformer. You need to enter the core type you are using and check for proper flux density and power capability of the core.

i think i answered my own question. reading design DER-188 i see similar 85Vdc_min and transformer 10:1 which would give 8.5V_min on secondary. so the dutycycle will move to create the 12V desired. good.

now my next problem.. how to cool this thing... got more reading to do.

I am glad you found you an answer

how to decide turns ratio for output voltage 5 volt

how to measure voltage coming at primary if i m using flyback transformer???

Hello jagrutiraut3,

it depends which IC you use. The best way to do the design is to use our PIxls. you can adjust the turn ratio by tuning the Vor of the design. the tool will tell if the design is proper or not. below is the link:

https://piexpertonline.power.com/site/login#

what do you mean by voltage coming at primary? Do you mean the voltage reflect from secondary to primary during the MOSFET turns off?

Best Regards

No............voltage coming from input side at primary winding.

one of the tranformer manufacturer asked me about this voltage

Hello jagrutiraut3,

you can use the voltage probe to measure the voltage at primary winding. During the MOSFET is turned on, the voltage across the primary winding should be your Bulk voltage substract the Von of the MOSFET(very small though, can be ingnore). I don't if you mean this. then the maximum voltage should be your maximum peak input voltage.

Best Regards