One of the very important aspect to look at in dealing with quasi resonant flyback transformer is the saturation. A quasi-resonant flyback transformer is not actually a transformer that we usually know. A transformer will transfer energy from the primar`y to the secondary at real time. However, the flyback converter will store energy on the primary and then transfer that energy to the secondary somehow. This action makes a flyback transformer prone to saturation if not properly been designed.
Detailed Quasi Resonant Flyback Transformer Saturation Analysis
Quasi-resonant flyback saturation is can be verified by the two equations below.
Where;
B – is the computed magnetic flux density
Ipeak – this is the peak current seen on the primary
Lp – primary inductance
V – peak voltage across the primary winding
t – time period when the current reaches the peak level from zero
Np – number of primary turns
Ae – core effective cross-sectional area in square meter
The computed magnetic flux density should be lower than the minimum flux density rating of the core. The operating temperature must be considered as well, as the higher the core temperature, there is a tendency that the flux rating will decrease. When the core saturates, it loses magnetic property and the current will go crazy. Catastrophic failure will follow.
How to Know in Actual Testing if the Transformer Saturates?
Using an oscilloscope, measure the primary current of the transformer. A good transformer will have a primary current that will linearly ramp up. Refer to below image.
A saturated transformer on the other hand, the primary current will rise exponentially. Refer to below figure.
How to Prevent Transformer from Saturation?
Based on the above equations, the flux density is dependent to the peak current, inductance, primary turns and the cross-sectional area.
Select a Core with a Bigger Cross-Sectional Area
During core selection, consider the one which has the biggest cross-sectional area that can fit to the space requirement. With a bigger core dimension, it can inhouse a greater number of primary turns.
Selecting the Right Inductance
The peak current is dictated by the voltage applied to the primary winding as well as the size of the inductance of the transformer. To limit the peak current means to increase the inductance value. However, for a quasi-resonant flyback, the inductance I value plays an important role so the converter will operated in the DCM mode to attain a soft switching. Therefore, a balance between the normal operation and the peak current should be considered during the design stage. I have made a quick template to know the parameters needed for a quasi resonant flyback to operate in its desired operation. Read the article “Quasi Resonant Flyback Operation Checker and Design Tool – Excel”. Also visit “Quasi Resonant Flyback Design Tool – Automated and Hassle Free”.
Number of Primary Turns
During the design stage, it is the primary winding number of turns that dictate the margin on saturation.
Wind as many turns to the primary. Maximize the effective cross-sectional area.
Give Margin to Flux Density
As a rule of thumb, I always set the maximum B to 60% only. This will give ample design margin.
Bcomputed <= 60% of Brating