Flyback LED Driver

[jman]

Hello Everyone

i'm currently working with a Flyback LED driver 40W output power.

i have some question regarding the Flyback Transformer.

1.) i don't quite understand what is a flyback Voltage / Flyback Reflected Voltage i want to know how to properly calculate that voltage,
i have read some white notes it is only assume around 77-100V i just want to know how did they arrived with kind of assumption.

2.) my transformer heats up to 60C what could have causing these heat? im using EFD25 Core, i want to lessen the heat dissipation any advice?

3.) same as for the Mosfet i'm also having heat problem i also want to lessen the heat i'm using a 7N60 Mosfet.

Thanks in advance guys.

[Skimask]

Guessing what the schematic looks like....

[nickm]

The reflected voltage is the voltage that is imposed across the transformer when the switch turns off.  It will be the output voltage multiplied by the turns ratio.  The flyback voltage is the voltage that is imposed across the leakage inductance of the transformer when the switch turns off and it will look like a spike.  The you switch needs to handle the reflected voltage plus the flyback voltage.

What frequency are you switching at and what core material are you using? 40W is a lot to put through an EFD25.

Try creating a spreadsheet which estimates losses based on Rdson, Qg, etc. to get a rough idea on which ones will do better in your application. 

[Kremmen]

The inverse of the turns ratio, to be sure.

[jman]

The reflected voltage is the voltage that is imposed across the transformer when the switch turns off.  It will be the output voltage multiplied by the turns ratio.  The flyback voltage is the voltage that is imposed across the leakage inductance of the transformer when the switch turns off and it will look like a spike.  The you switch needs to handle the reflected voltage plus the flyback voltage.

What frequency are you switching at and what core material are you using? 40W is a lot to put through an EFD25.

Try creating a spreadsheet which estimates losses based on Rdson, Qg, etc. to get a rough idea on which ones will do better in your application.

Thanks this help me understand how to select proper mosfet
my switching frequency is 50Khz and the core is actually EFD30 my mistake

[jman]

The inverse of the turns ratio, to be sure.

what do you mean by this sir? 

[Neilm]

The reflected voltage is why fly-back transformers tend to have subber networks across them. Have a look at this http://www.fairchildsemi.com/an/AN/AN-4147.pdf

The inverse of the turns ratio, to be sure.

what do you mean by this sir? 

The turns ratio is usually expressed as 1:n (primary:secondary). When dealing with the reflected voltage, you need the value of the n:1 hence the inverse of the turns ratio

[Marco]

The reflected voltage is why fly-back transformers tend to have subber networks across them.

The leakage induction is why you have the snubber network, the reflected voltage plus input voltage is simply what your switch will have to be able to deal with period (at least with 1 switch, with 2 switches the peak voltage across the switches can be reduced to Vin).

[jman]

The reflected voltage is why fly-back transformers tend to have subber networks across them.

The leakage induction is why you have the snubber network, the reflected voltage plus input voltage is simply what your switch will have to be able to deal with period (at least with 1 switch, with 2 switches the peak voltage across the switches can be reduced to Vin).

i agree with you sir, but may question is how do i calculate the reflected voltage?

[Odysseus]

i agree with you sir, but may question is how do i calculate the reflected voltage?

Referring to the image above, you first need to know the forward voltage of your LED string at rated current.  As an example, lets assume one with 50V, i.e. the voltage across the output capacitor and LED string averages 50V when operating normally.  Next, scale this voltage by the turns ratio of your transformer.  Again, let's assume a value of 3:1, i.e. 30 turns on the primary side with the FET switch, and 10 on the secondary with the LEDs (or some other equivalent ratio.  So, the reflected voltage in this case is 150V. 

Now, this isn't the voltage seen by the switch. You still need to add the bulk voltage on your input capacitor.  For a mains operated device in the US, this will peak at 170V.  Thus, the theoretical peak voltage across the FET is 170V + 150V = 320V.  HOWEVER, this STILL doesn't include the effect of ringing due to the leakage inductance of the transformer and the parasitic capacitance of the mosfet, transformer, and the output diode.  At worst, without any snubbing, this MORE THAN DOUBLE the peak voltage on the FET to above 640V!

Engineering products for the European market gets even uglier.  It's not uncommon to use BJTs or FETs rated for 1kV or more.

2.) my transformer heats up to 60C what could have causing these heat? im using EFD25 Core, i want to lessen the heat dissipation any advice?

3.) same as for the Mosfet i'm also having heat problem i also want to lessen the heat i'm using a 7N60 Mosfet.

As already requested, in order to answer your second question, I need to know the material your core is made of, not just it's size. Also, a schematic with your current component values would really be helpful, especially for your third question.

By the way, 60C is nothing to be worried about. The converters I've worked on (60W Incandescent LED replacements) are designed to operate nominally above above 90C.  The datasheets for ferrite materials reflect this trend.  The permeability and loss parameters are optimized for high temperature operation, and actually under perform significantly at room temperature. Look at this as an example.  Note that the core loss parameters are specified at 100C.

[jman]

Thanks for your response sir it's what i needed

kindly see attached picture for the 2nd question,

the core material is MnZn TP4A

[T3sl4co1l]

The feedback winding, after the diode, needs a filter cap to ground.  Assuming you are using a common control IC like UC3842.

But you don't want that either, because constant voltage is incorrect for an LED.  You need current feedback from the LED, like this.

http://seventransistorlabs.com/Images/LED_Light.png

Tim

[jman]

Yes tim you are correct I just forgot to draw it, my mistake..  thanks anyway