Effective ways to improve the efficiency of power adapters

　　Effective ways to improve the efficiency of power adapters

　　1. Input filter and rectifier circuit

　　1. Factors related to common mode inductance loss

　　(1) Wire warp size (2) Magnetic ring material (3) Winding method

　　Calculation formula: Number of turns = √ L/ A

　　The relationship between inductance and rated current:

　　Rated current (A): 1 3 6 10 12 15

　　Inductance (mH): 8-23 2-4 0.4-0.8 0.2-0.3 0.1-0.15 0.0-0.08

　　2. The total capacity of safety capacitors (X capacitor and Y capacitor) is less than 0.47uF, otherwise the leakage current will exceed 0.5mA

　　If the emc test fails, you can add magnetic beads to the pins of the safety capacitor, or increase the capacitance appropriately.

　　X capacitance range: 0.01---0.47uF/250V---280VAC

　　Y capacitance range: 2200PF---0.1uF, 250V-280VAc

　　3. Rectifier bridge: its current = (3-5) times the output current of the power adapter, or use parallel rectifier bridges to reduce heat loss.

　　Selection principle: (1) The forward voltage drop is small; (2) The reverse withstand voltage is twice the rectified output voltage; (3) The reverse recovery is fast;

　　I=P0/VdcE, (E: rectifier efficiency, Vdc: rectifier output voltage, P0: output power)

　　4. Input filter capacitor:

　　(1) Withstand voltage Uc>=1.5*1.414Ui;

　　(2) Good frequency characteristics;

　　(3) Low ESR internal resistance;

　　(4) Capacity: C=0.88-3.6uF/W

　　Capable of withstanding large ripple current and small size electrolytic capacitors.

　　(5) Good temperature characteristics: -40℃---105℃;

　　(6) Leakage current;

　　(7) Long life.

　　2. European power adapter power factor PFC circuit:

　　(1) Switching loss: including the switching loss of the boosting diode and the switching tube; it is determined by the reverse recovery time and current of the diode, and the stored charge and distribution parameters of the switching MOS tube;

　　(2) Conduction loss: the forward voltage of the diode is determined by the conduction resistance of the switch tube;

　　(3) Boost inductor loss: composed of magnetic loss (iron loss) and copper loss. Since the continuous mode improves the current resistance of the diode by 2 times that of the discontinuous mode, the discontinuous mode or the critical mode is commonly used for low-power power supplies to minimize switching losses. Choose a diode and MOS tube with a higher rated current to reduce conduction loss.

　　(4) Single-stage Pfc inductor design:

　　L=(1.41Vs*Ton = 5*Vs*Vs*E*Ton/Po; where Ton=0.673T, T=1/f, Vs is the lowest effective value of input voltage, En: efficiency

　　PFC capacitor Co=429*10-6*Iav, Iav=2Po/E (Vo+300);

　　(5) Interleaved PFC inductor design:

　　L1=L2=Vin*1.414*Dp/(I1-I2) *fs= 90*1.414*0.68/5.4fs, where fs is the oscillation frequency and (I1-I2) is the current increment.

　　Dp=0.68, Vin is the lowest input voltage.

　　Third, the power conversion part

　　1. Switching MOS tube: conduction loss and switching loss; the switching circuit uses soft switching mode;

　　(1) Choose CoolMOS tube;

　　(2) The driving power must be satisfied;

　　(3) The rising edge of the driving waveform is required to be steep;

　　(4) The drive series resistance is appropriate and not too large;

　　(5) The driving positive bias voltage is appropriate;

　　(6) The dead time control is reasonable.

　　(7) The stored charge of the MOS tube should be small; the reverse recovery time of the parasitic diode should be short;

　　(8) Withstand voltage 1.5 times VDC;

　　(9) Withstand voltage 4 times the working current.

　　2. Design of high frequency transformer:

　　Magnetic components should consider: additional losses caused by eddy current, skin effect, proximity effect, etc., which are composite functions of voltage, current, frequency, energy, number of turns, magnetic leakage, core air gap, temperature, processing technology, materials, etc. . When a high-frequency current passes through the wire, according to the right-hand rule, a counterclockwise magnetic field line will be generated on the wire, and the magnetic field line will also cause eddy currents, which increases the wire surface current and offsets the central current. This current flows on the wire surface. The phenomenon of no current flowing in the center is called the skin effect.

　　We choose the magnetic core to consider the following factors:

　　(1) High magnetic permeability;

　　(2) The coercive force should be small, and the hysteresis loss should be small;

　　(3) High resistivity and low roaming loss;

　　(4) The saturation magnetic induction should be high; commonly used soft magnetic materials are: ferrite, magnetic powder core, and amorphous alloy.

　　(5) The iron loss and copper loss of the transformer should be treated in a balanced manner;

　　(6) Try to choose a large-sized magnetic core;

　　(7) Reduce the leakage inductance, reduce the insulating layer between the windings, increase the height ratio of the windings, and increase the width of the windings; use thin and long magnetic cores instead of fat magnetic cores;

　　(8) Winding with three-layer insulated wire, using sandwich winding method, without adding tape, the volume will be small, and the leakage inductance will naturally be small. For multiple output power supplies, the winding with the highest output power must be close to the primary winding;

　　(9) Magnetic core selection: POT pot-shaped magnetic core, good magnetic permeability, good transfer energy, can greatly reduce EMI, the disadvantage is that the heat dissipation is not good, the temperature rises, suitable for low power; Rm magnetic core coupling ability, good heat dissipation effect , Suitable for high-power power supplies above 100w, the disadvantage is large space; EE core area is large, the output power is also large, but the leakage inductance is large, and the volume is large.

　　(10) The following problems should be avoided: In a unipolar converter, the energy storage recovery of the transformer is not complete, the demagnetization is not complete, the positive and negative volt-second products of the transformer are not equal, and the transformer is saturated. We must avoid saturation of the transformer inductance. ; In the bipolar converter, due to asymmetric drive and asymmetric switching parameters, the DC bias of the equivalent transformer is saturated, the magnetic induction density Bw is too high, and the leakage inductance of the transformer is too large; the absorption circuit parameters are not properly selected;

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