Question regarding the voltage of the auxiliary winding of this flyback transformer...

szabikka

Joined Sep 3, 2014
94
Hello everyone!

I have started to learn AC to DC SMPS design and I have a question right at the beginning. Consider the following transformer in an SMPS with a control IC, that has a recommended maximum Vcc voltage of 9V and an absolute maximum Vcc voltage of 16V: Myrra 74023 transformer datasheet. The datasheet says that the secondary, marked S4 has a voltage of 18V and the auxiliary has a voltage of 15V. Am I right, that these mean maximum voltages? So, if I set up my control circuit as such, that I want to get 12V at this secondary (e.g.: with a TL431 reference set to 12V and an optocoupler), then I will see about 10 V at the auxiliary? Am I on the right track or I have the chance of burning my control IC because the auxiliary outputs 15V no matter what?

Dodgydave

Joined Jun 22, 2012
10,139
All secondarys will reduce by the same gain, so 18 to 12v is a 25% reduction, so 15v will reduce to approx 11v.

AlbertHall

Joined Jun 4, 2014
11,695
The control pin voltage is internally regulated to 5.8V.
From the datasheet:
When the externally fed current charges the
CONTROL pin to the shunt regulator voltage of 5.8 V, current
in excess of the consumption of the chip is shunted to SOURCE
through resistor R E as shown in Figure 2. This current flowing
through R E controls the duty cycle of the power MOSFET to
provide closed loop regulation.

szabikka

Joined Sep 3, 2014
94
All secondarys will reduce by the same gain, so 18 to 12v is a 25% reduction, so 15v will reduce to approx 11v.
Thanks Dodgydave! That was my guess too, but wanted to make sure. It's just that I'm used to building power supplies with regular transformers. These flyback circuits were a mystery for me, but I'm starting to get a grasp of their working.

crutschow

Joined Mar 14, 2008
28,537
These flyback circuits were a mystery for me, but I'm starting to get a grasp of their working.
As you may know, the flyback transformer stores energy in its inductance when the transistor (switch) is on and inductive current builds in the primary, and then transfers that energy to the other windings by transformer action when the transistor turns off.

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szabikka

Joined Sep 3, 2014
94
As you may know, the flyback transformer stores energy in its inductance when the transistor (switch) is on and inductive current builds in the primary, and then transfers that energy to the other windings by transformer action when the transistor turns off.
Yes, I know this and I think it's quite fascinating. There is current flowing in the secondary when the primary side is switched off. Kind of like black magic.