Question asked by Tarzan on Sep 30, 2014
Latest reply on Oct 9, 2014 by bkennedy

Since I don't have experience using isolated DC/DC,

It would be great if anyone could give me some advice.

(sorry to ask many questions at the same time)

#1 Question,

I'm trying to calculate Transform ratio by using the following formula

Ns/Np=(Viso+Vd)/(Vdd1min*D*2)

But I don't know what duty cycle should be.

it says  Duty should be 0.3, but I dont where this number come from.

#2 Question,

In order to select Trans inductance, Cout and L1 inductor,

Are there any calculating formulas?

The datasheet recommend value like 400uH for Trans, 47uF for Cout and 100uH/400uH for L1.

Should I use these figures?

Since the output current for my model could be less than 10mA so I'd like to use the smaller capacitor and inductor.

#3 Question,

The datasheet says that it's okay to use 5V to 3.3V for Ns/Np=2.

But I don't think Ns/Np is calculated in 1.2 for 5V to 3.3 V.

Could you make sure if it's correct?

=================================================================

[From page 15]

For a similar 3.3 V input to 3.3 V output, isolated single power

supply and with V DD1 (MIN)  = 3.0 V, the turns ratio is also N S /N P  =

1. 2. Therefore, the same transformer turns ratio N S /N P  = 2 can be

used for the three single power applications (5 V to 5 V, 5 V to

1. 3.3 V, and 3.3 V to 3.3 V).

=================================================================

#4 Question,

It looks like the most low power isolated DC/DC incorporate Fryback converter.

But ADUM3070 is push-pull converter(using two FET inside the IC).

I think Flyback can reduce external components like Inductor on the second side.

So I just wonder what is the merit of push-pull converter for the low power isolated DC/DC.

Best regards,

Tarzan