Primary inductance calculation with different Ki
Hello,
I want to design an isolated power supply with 20W output. I choosed the Top255PN as the controlling chip.
The calculated Bp is more than 4200 Guass, so I try to decrease the Ki value to get lower Bp.
When the Ki＞0.7, the Lp value is 2529uH, the value is same as What's calculated by myself.
When the Ki≤0.7， the Lp value changes to 2927uH, in the fomula, the Lp is not affected by the Ki.
Why the Ki valuie changes the Lp value? How does it affect the Lp value?
コメント
Hello，
The Kp tells how continous or discontinous the design is running. Here I set the Kp <1, so the power supply work under the CCM mode.
The kp value will affect the Lp value, the Ki vlaue will not affect the Lp value in the fomula.
The Kp value is the ripple of the primary current, and the Ki value is the external limited reduction factor.
Now I set Ki less than 0.7 for lower limited current, that will get lower Bp.
I don't want to choose a smaller topswitch because of higher efficiency.
That method will work, too.
The KI tells you how continuous or discontinuous the design is running (KI <1 = continuous, Ki >1 = discontinuous). A lower KI will therefore result in a higher value for primary inductance. In order to get out of your situation with high flux density, you need to increase KI and increase the number of secondary turns, which accordingly will scale up the number of primary turns. You can also run a smaller TOPSwitch device for your intended output power. I am appending a copy of a spreadsheet I ran that that fixes the high flux problem, using the same core (EF25) used originally.