# Impact of Transformer Ratio to Performance of Transformer

Based on my initial calculation, the primary inductance of my transformer will be around Lp = 351uH. This 351uH is designed with switching frequency of 55kHz for AC-DC flyback converter (DCM). By using LT SPICE, I will get result as below:

• Primary inductance, Lp = 351uH
• Switching frequency, fsw = 55kHz
• Primary peak current, Ipri pk = 2.30A
• Primary RMS current, Ipri rms = 0.91A
• Secondary peak current, Isec pk = 8.58A
• Secondary RMS current, Isec rms = 3.42A

From the initial Lp, I calculate minimum number of primary turns using below equation: From above, I will get Np = ~ 24 turns (assume Bmax = 0.3T, Ae = 0.000120)

From Np = 24 turns, I calculate the secondary winding using below equation (assume Vout = 24V, Vdiode = 1V, Vr = 90V) From that, I will get Nsec = ~ 7 turns

Therefore Npri = 24 turns, Nsec = 7 turns.

Then suddenly I found this document AN-4193 Design Guideline for Flyback Travel Adapter using FAN602 and it stated that: Based on that document, I checked Np for both during (a) primary peak & (b) during transient/fault condition. By that I will get different number of turns with the same turns ratio as below:

• By using equation 14, Np will be ~ 24 turns, Ns will be ~ 7 turns, Nsp=0.2916
• By using equation 15, Np will be ~ 30 turns, Ns will be ~ 9 turns, Nsp=0.3000

Remark

1. For equation 15, I'm using Bmax = 0.41T & IDS LIM = 4.09A (IDS LIM is based on current limit setting 0.9V/0.22 = 4.09A)
2. My transformer has an air gap ~ 0.247mm

My question:

1. As you can see above, both transformer A & B will have same turn ratio with different number of turns between primary & secondary. Will this impact the transformer performance? (ex. saturation level, peak current)?
2. As you can see, my secondary peak current is 8.58A & secondary RMS current is 3.42A. Will this secondary RMS current 3.42A is able to go up until 8.58A without saturation? If not, what parameter should I improve?
3. If this power supply is suddenly loaded with 3.7A for short duration, can this transformer handle it without saturation (same question as #2)
4. Can problem #2 & #3 be improve by changing topology from normal flyback DCM (fixed frequency) to quasi-resonant flyback (frequency change based on output load)?

Thanks.

• What about the inductances? Are they equal (i.e. adjusted air-gap)? What's the usage of these transformers (i.e. SMPS power transformer or current transformer or something else)? – Rohat Kılıç Feb 11 at 8:33
• If the transformer is designed for a Flyback, keeping the same ratio, will keep the reflected voltage similar so don't need to change the rated voltage of the secondary diode. – Delphesk Feb 11 at 8:54
• @RohatKılıç, I updated some information. – Lutz Fi Feb 11 at 9:54
• The main advantage of the QR Flyback is to keep a good efficiency at low load because the frequency is variable Vs fixed on time Flyback. QR Flyback is able to work at DCM. – Delphesk Feb 11 at 10:34
• The important thing point of view secondary is to choose the right wire section to limit the losses and the number of cable in parallel to avoid the skin effect. The peak current is the consequence of the primary energy transfer, if you want to deliver a certain amount of power to the load you cannot avoid it. Just make sure the core is not saturated on the primary side ( Np*Ippk/ Rcore+Rgap) – Delphesk Feb 11 at 10:56

Will this impact the transformer performance? (ex. saturation level, peak current).

All other things being equal.....

1. The transformer with the higher number of turns on the primary will be less likely to saturate its core. This is because the magnetization inductance is greater.
2. The peak current is usually defined by the load and, more turns means higher leakage inductance and resistance hence, the transformer with the least number of turns will handle load current more effectively.

Will this secondary RMS current 3.42A is able to go up until 8.58A without saturation?

Saturation is due to primary current

If this power supply is suddenly loaded with 3.7A for short duration, can this transformer handle it without saturation (same question as 2)

Saturation is due to primary current

• I updated some information. – Lutz Fi Feb 11 at 9:55
• I updated my answers. Please don't make wholesale changes any more. – Andy aka Feb 11 at 10:07
• Ya, thank you! Thanks for your explaination. – Lutz Fi Feb 25 at 12:54