# Vin for calculation of number of turns in transformer pirmary

Discussion in 'The Projects Forum' started by kender, Dec 12, 2012.

1. ### kender Thread Starter Senior Member

Jan 17, 2007
263
0
Subject: Input voltage for calculation of transformer pirmary number of turns

Colleagues,

I'm making calculations for a push-pull transformer and following equations in Microchip application note AN1207 (pp.51ff).

${\Delta} B = \frac{V_P T_{ON}}{N_P A_{core}}$ eq.175

$N_P=\frac{V_{DC,min}-V_{Q,on}}{A_{core}f_{PWM} {\Delta} B}D_{max}$ eq.176

where
$V_{DC}$ is the input voltage
$D_{max}$ is the max PWM duty cycle (between 0 and 0.5 for push-pull topology)

Why do they use $V_{DC,min}$ and not $V_{DC,max}$ in this calculation?
I though that higher voltage across the primary will cause higher magnetic flux. Higher magnetic flux can saturate the core.

Any suggestion, insight or reference is really appreciated!

Cheers,
- Nick

2. ### crutschow Expert

Mar 14, 2008
13,009
3,233
I agree that it should be $V_{DC,max}$. Don't know why they used $V_{DC,min}$.