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What are the most commonly used formulas in the design of 10 power supply? This article helps you sort out clearly

release time:2022-03-17Author source:SlkorBrowse:804


--Maximum duty cycle d of -MOSFET switch tubemax

Where: Vor is the reflected voltage of the secondary side refracted to the primary side, and the reflected voltage is 135 V when the input is AC 220V; VminDC is the lowest DC voltage after rectification; VDS is the voltage between the D and S poles when the MOSFET power tube is turned on, generally taking 10V.


---Peak current IPK of transformer primary winding is:

Where η is the conversion efficiency of the transformer; Po is the rated output power, and the unit is W.

---Transformer primary inductance lP为:

Where: Ts is the period (s) of the switch tube; LP unit is H .

---The air gap lg of the transformer is:

Where: Ae is the effective cross-sectional area of the magnetic core (cm2); △B is the change value (T) of the working magnetic induction intensity of the magnetic core; Lp unit is h, IPK unit is a, lg unit is mm .

---Transformer core

Flyback converter usually has low power, so ferrite core is generally used as transformer core, and its power capacity AP is:


AQ is the window area of the magnetic core, in cm2;

Ae is the effective cross-sectional area of the magnetic core, in cm2;

Po is the nominal output power of the transformer, in W; Fs is the switching frequency of the switch tube;Is the maximum magnetic induction intensity of Bm magnetic core, in t; δ is the current density of the coil wire, usually 200 ~ 300 A/cm2, and η is the conversion efficiency of the transformer;

Km is the window filling coefficient, generally 0.2 ~ 0.4;

KC is the filling coefficient of the magnetic core and 1.0 for ferrite.

According to the obtained AP value, the magnetic core with a slightly larger margin is selected. Generally, the magnetic core with a larger window length-width ratio is selected as much as possible, so that the effective window use coefficient of the magnetic core is higher and the leakage inductance can be reduced.

---Transformer primary turns NP

Where: △B is the change value (t) of magnetic induction intensity of magnetic core, the unit of Ae is cm2, and the unit of Ts is S.

---TraNSformer secondary turns ns

Where: VD is the forward voltage drop of the rectifier diode on the secondary side of the transformer.

---Selection of power switch tube

The minimum voltage stress UDS of the switch:

Generally, the MOSFET power tube whose inter-DS breakdown voltage should be slightly larger than the calculated value of formula (9) should be selected.

---Copper consumption PCU of winding

The resistance values of primary and secondary windings can be calculated by the formula of winding resistance R. When it takes time to calculate the copper of primary winding, the current is calculated by the peak current IPK of primary winding. It takes time to find the copper of the secondary winding, and the current is calculated by the output current Io.

---Magnetic core loss

The core loss depends on the working frequency, the working magnetic induction intensity, the working state of the circuit and the performance of the selected core material. For bipolar switching transformer, core loss Pc:

Where: Pb is the core loss per unit mass (w/kg) at working frequency and working magnetic induction intensity; Gc is the core mass (Kg).

For unipolar switching transformer, the loss of magnetic core is about half of that of bipolar switching transformer because the magnetic core works in the half area of hysteresis loop. Total transformer loss is the sum of total copper loss and core loss.

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