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# FLYBACK_CONVERTER calculations help

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#### Siddharth_saxena

##### Newbie level 6
can anyone help me out with my fly back converter calculation ?

Solution

im finding difficulty while calculating capacitor value

#### KlausST

##### Super Moderator
Staff member
Hi,

I guess: yes. But without details...

You need to provide some electrical specifications.
And you need to show what you have done so far. Don´t expetc others do your job.

Did you do an internet search, a forum search .. see the links in the box "similar threads" below.

Klaus

#### c_mitra

can anyone help me out with my fly back converter calculation ?
just tell us what you have done yourself. Next you tell where you have got stuck. I guess it is an assignment and you are expected to do it yourself, right?

### Siddharth_saxena

Points: 2

##### Super Moderator
Staff member
Are you talking about the common DCR snubber across the primary winding? Then it resembles the behavior of a buck-boost converter:

1) The primary produces a current kick at Turn-off.
2) Diode steers current surge so it charges capacitor.
3) Resistor discharges capacitor during second half of the cycle.

The goal is to select RC values so that their snubbing action generates minimum heat and minimum stress on components.

### Siddharth_saxena

Points: 2

#### Siddharth_saxena

##### Newbie level 6

im finding difficulty while calculating capacitor value

#### FvM

##### Super Moderator
Staff member
I'd say, you have more problems to select a meaningful resistor value...

What are the transformer parameters in your simulation? Without transformer leakage inductance, you don't need a snubber at all. Capacitor value depends on energy stored in leakage inductance and acceptable overvoltage. Resistor must be able to discharge the capacitor til next pulse.

### Siddharth_saxena

Points: 2

#### Siddharth_saxena

##### Newbie level 6
I'd say, you have more problems to select a meaningful resistor value...

What are the transformer parameters in your simulation? Without transformer leakage inductance, you don't need a snubber at all. Capacitor value depends on energy stored in leakage inductance and acceptable overvoltage. Resistor must be able to discharge the capacitor til next pulse.
i took leakage inductance as 2uH, turns ratio is 1:1.
--- Updated ---

just tell us what you have done yourself. Next you tell where you have got stuck. I guess it is an assignment and you are expected to do it yourself, right?
not really

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#### FvM

##### Super Moderator
Staff member
i took leakage inductance as 2uH, turns ratio is 1:1.
Related to which main inductance?

#### Siddharth_saxena

##### Newbie level 6

Is my waveform is correct ?
oscilloscope1 :across switch
oscilloscope2 :across channel A -R3
across channel B- R4
oscillloscope3 :across R6

#### Siddharth_saxena

##### Newbie level 6
Related to which main inductance?
Sir transformer leakage reactance

##### Super Moderator
Staff member
R6 (looks like 20 ohms) drains the capacitor very quickly after Turn-off. I believe you can select a higher ohm value so that discharge lasts the remainder of the cycle. Maybe 100 ohms.

I'm sorry, I see now R6 is 20k. Then it may be more effective if you increase the capacitor value. A reasonable volt level to charge it is 1.5 or 2 times the supply voltage. Remember a higher volt rating puts more stress on the cap, as well as costs more money. And higher voltage causes your resistor to need a higher Watt rating, also more money

I'm looking again... What polarity is applied to the transformer primary? Your schematic makes it appear that positive goes to the lower end.

Last edited:
Siddharth_saxena

### Siddharth_saxena

Points: 2

#### Easy peasy

it's 20k.

swap the mosfet and the DC source - it's drawn wrong

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