Auxiliary output and poor cross-regulation, Flyback converter

[strape81]

Hi all,

I am currently working on a 65W flyback converter which is controlled by a NCP1217A PWM controller.

The problem is the auxiliary output which supplies the PWM controller. When the main outputs are not loaded, I measure a 12,8V voltage at the auxiliary one but when they are fully loaded then the auxiliary output is measured at 22Volt which is unacceptable (datasheet NCP1217A: Vccmax=16V).

I have two questions:

1) Is a Zener diode the best solution in order to clamp the Vcc-voltage at 15V or should I work more on something else, for example transformer?

2) Can this cross regulation problem cause problems to the short-circuit-protection function of the controller? The converter at no circumstance enters the protection mode when a short occurs.

some infos:

The flyback transformer has a sandwich winding in order to reduce leakage inductance: (from bottom to upper layer) PrimA - Sec1 - Sec2 - PrimB - Vcc
Lprim=930uH , Lpleak=10uH , Nprim=58turns, Nsec1=10turns, Nsec2=14turns, Nvcc=10turns

The auxiliary circuit is quite known: Resistor - Diode - el.Capacitor
R=10ohm, C=47uF


Thanks in advance,

Efstratios

 

[goldsmith]

Hi Efstratios
It seems a bit strange . but you can use a zener diode across the vcc pin of your driver . or a resistor in parallel with auxiliary winding . but i think a simple zener will be better . ( 15 volts ) ( by the way i hope that you know a flyback converter shouldn't be without load ! )
Best Wishes
Goldsmith

 

[strape81]

Thanks goldsmith for answering my question...about the load u are right...normally I have a light load of 150mA at the outputs