# Design of the hottest single ended flyback switchi

2022-10-19
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Transformer design of single ended flyback switching power supply the transformer of single ended flyback switching power supply is essentially a coupling inductor, which is responsible for energy storage, voltage transformation, energy transmission and so on. The following summarizes the transformer design of single ended flyback converter working in continuous mode and intermittent mode

1. Known parameters

these parameters are determined by the designer according to the needs of users and the characteristics of the circuit, including: input voltage Vin, output voltage Vout, output power pout of each circuit, efficiency, switching frequency fs (or cycle T), voltage withstand VMOS of the main switch of the line

2. Calculation

in the flyback converter, the secondary side reflected voltage, that is, the sum of the flyback voltage VF and the input voltage, cannot be higher than the withstand voltage of the main switch, and at the same time, there must be a certain margin (assumed to be 150V here). The flyback voltage is determined by the following formula:

vf=vmos vindcmax-150v

the relationship between the flyback voltage and the output voltage is determined by the turn ratio of the primary and secondary sides. Therefore, after the flyback voltage is determined, the turn ratio of the primary and secondary sides can be determined

np/ns=vf/vout

in addition, the maximum duty cycle of the flyback power supply appears in the state of minimum input voltage and maximum output power. According to the magnetic balance of the transformer in the steady state, there can be the following formula:

vindcmin dmax=vf (1-dmax)

when the maximum duty cycle is set, when the switch is turned on, the primary current is IP1, and when the switch is turned off, the primary current rises to IP2. If IP1 is 0, it means that the converter works in intermittent mode, otherwise it works in continuous mode. From the conservation of energy, we have the following formula:

1/2 (ip1+ip2) will also start the key new functional materials and device research and development and utilization, advanced structure and composite utilization and other projects Dmax vindcmin=pout/

general continuous mode design, we make ip2=3ip1

in this way, we can calculate the primary side current of the converter, from which we can get the primary side inductance:

LP = Dmax vindcmin/fs IP

for continuous mode, ip=ip2-ip p1=2ip1; For intermittent mode, ip=ip2

the required iron core can be calculated by the awae method:

awae= (LP IP22 104/bw K0 kJ) 1.14

in the above formula, aw is the window area of the magnetic core, the unit is cm2

ae is the cross-sectional area of the magnetic core, the unit is cm2

lp is the primary inductance, the unit is h

ip2 is the primary peak current, the unit is a

bw is the working magnetic induction intensity of the magnetic core, and the unit is t

k0 is the effective use coefficient of the window, which is determined according to the requirements of the safety regulations and the number of output channels, Generally, 0.2~0.4

kj is the current density coefficient. Generally, 395a/cm2

select the appropriate magnetic core according to the obtained awae value. Generally, try to choose the magnetic core with a large window length width ratio, so that the window effective use coefficient of the magnetic core is high, and the leakage inductance can be reduced

with the magnetic core, the number of turns on the primary side can be calculated. According to the following formula:

npc919 body line is smooth =lp IP2 104/bw ae

then the number of turns on the secondary side can be calculated according to the turn ratio relationship between the primary and secondary sides that will cause some problems in the experimental process. Sometimes the number of turns is not an integer, so some parameters should be adjusted to make the number of turns on the primary and secondary sides appropriate

in order to avoid saturation of the magnetic core, we should add an appropriate air gap into the magnetic circuit, and the calculation is as follows:

lg=0.4 NP2 AE/lp

in the above formula, LG is the length of the air gap, the unit is cm

NP is the number of turns on the primary side,

AE is the cross-sectional area of the magnetic core, the unit is cm2

LP is the inductance on the primary side, and the unit is h

so far, the main parameter design of the single ended flyback switching power transformer has been completed. We should check whether the window area is large enough and whether the loss and temperature rise of the transformer are acceptable after the design is completed. At the same time, the clamping method can also be adopted in the manufacture of transformer) there are still some process problems that need attention

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