# Simulating pulse transformers in LTSPICE.

Discussion in 'Electronic Design' started by John Nagle, May 5, 2009.

1. ### John NagleGuest

I'm playing around with a flyback DC-DC converter design, and
I'd like to know if I'm modeling a stock part correctly.

The part is a Pulse Engineering PA1546NL, which is a small
transformer.

Data sheet:

http://ww2.pulseeng.com/products/datasheet.aspx?Datasheet_Id=935

I want to use the winding between pins 4 and 5 as the primary (L1),
1 and 8 as the secondary (L2), and not use the 7-2 feedback winding
(not modeled).

The SPICE model I'm using for the transformer is:

L1 Vcc N003 17µ Rser=2.1
L2 0 N001 247µ Rser=0.7
K1 L1 L2 0.95

The data sheet gives the inductance of L2 as 247uH.
The data sheet doesn't give the inductance for L1, but it gives
the turns ratio as 3.82 : 1. So 247uH / (3.82^2) = 16.9uH, right?
DC resistances are from the data sheet.

I have no idea what the efficiency value, K, should be for one of these
little things. I'm using 0.95 as a guess. Big iron-core transformers
run around 0.98 or so, but I don't know about these little things.
This matters; for the circuit I'm trying, for values much below 0.95, the
switcher won't oscillate.

John Nagle

2. ### John NagleGuest

OK. The leakage inductance from the data sheet is 8uH (measured with the
other coils shorted) vs. 247uH with the other coils open. So

L1 = (1-K^2) * L2
or
K = sqrt(1 - (L1/L2))
K = 0.9837

Is that right?

John Nagle