ST公司自激式开关电源设计
1 Power Transformer Design Calculations
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l The specifications:
–V AC= 85~265V
l Line frequency: 50~65Hz
–V O= 5V
–I O= 0.4A
Taking transient load into account, the maximum output current is t as
I O(m a x)= 1.2I O= 4.8 A
1.1Switching Frequency
The system is a variable switching frequency system (the RCC switching frequency varies with the input voltage and output load), so there is some degree of freedom in switching frequency
lection. However, the frequency must be at least 25kHz to minimize audible noi.
Higher switching frequencies will decrea the transformer noi, but will also increa the level
of switching power dissipated by the power devices.
The minimum switching frequency and maximum duty cycle at full load is expresd as
f S(m i n)= 50 kHz
D m a x= 0.5
where the minimum input voltage is 50kHz and 0.5, respectively.
1.2 STD1LNK60Z MOSFET Turn Ratio
The maximum MOSFET drain voltage must be below its breakdown voltage. The maximum drain
voltage is the sum of:
l input bus voltage,
l condary reflected voltage, andpractidoing
voltage spike (caud by the primary parasitic inductance at maximum input voltage).
The maximum input bus voltage is 375V and the STD1LNK60Z MOSFET breakdown voltage is 600V. Assuming that the voltage drop of output diode is 0.7V, the voltage spike is 95V, and the大的英文
margin is at least 50V, the reflected voltage is given as:
V fl= V(B R)DS S–V m arg i n–V D C(ma x)–V s p k= 600 –50 –375 –95 = 80 V
The Turn Ratio is given as
where,
V fl= Secondary reflected voltage
V(BR)DSS= MOSFET breakdown voltage
V margin= Voltage margin
V DC(max)= Maximum input bus voltage
V spk= Voltage spike
V f= Voltage drop
N = Turn Ratio
N p= Primary Winding Turns
N s= Secondary Winding Turns
1.3Primary Current
l Primary Peak Current is expresd as:
l Primary Root Mean Square (RMS) Current is expresd as
where,
I ppk= Primary peak current
V O= Voltage output
I O(max)= Maximum current output
η= Efficiency, equal to 0.7
D max= Maximum duty cycle
V DC(min)= Minimum input bus voltage
I prms= Primary RMS current
1.4Primary Inductance
Primary Inductance is expresd as
如梦令翻译where,
V DC(min)= Minimum Input DC voltage f s
(min)= Minimum switching frequency D max
= Maximum duty cycle
f s(min)= Minimum switchin
g frequency
I ppk= Primary peak current
For example, if Primary Inductance is t to 5.2mH, the minimum switching frequency is:
1.5Magnetic Core Size
One of the most common ways to choo a core size is bad on Area Product (AP), which is the product of the effective core (magnetic) cross-ction area times the window area available for the windings.
Using a EE16/8 core and standard horizontal bobbin for this particular application, the equation ud to estimate the minimum AP (in cm4) is shown as
where,
L p= Primary Inductance
I prms= Primary RMS current
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k u= Window utilization factor, equal to:
–0.4 for margin wound construction, and
–0.7 for triple insulated wire construction
B max= Saturation magnetic flux density
T = Temperature ri in the core
1.6Primary Winding
1.6.1 Winding Turns
The effective area of an EE16 core is 20.1mm2(in the core’s datasheet). The number of turns of primary winding is calculated as
where,
N p= Primary Winding Turns
V DC(min)= Minimum Input DC voltage
D max= Maximum duty cycle
f s(min)= Minimum switchin
g frequency
B = Flux density swing
A e= Effective area of the core
1.6.2 Wire Diameter
The current density (A J) allowed to flow through the chon wire is 4A/mm2. The Copper
diameter of primary wire is expresd as
where,imb
d p= Diameter of primary winding wire
I prms= Primary RMS current
A J= Current density
1.6.3 Number of Primary Winding Turns per Layer
The EE16 bobbin window is about 9mm, so if the enamel wiring chon has a 0.21mm outer
diameter and a 0.17mm Copper diameter, the number of turns per layer is expresd as
where,
N p1= Layer 1 Primary Winding Turns
N p1= 42 turns per layer, 4 layers needed
N p= 168 (total turns for all 4 layers)
1.6.4 Practical Flux Swing
Using the N p= 168 value, the practical flux swing is expresd as
where,
B = Flux density swing
V DC(min)= Minimum input bus voltage
阿房宫赋翻译
D max= Maximum duty cycle
f s(min)= Minimum switchin
g frequency
A e= Effective area of the core
N p= Primary Winding Turns
1.7Secondary Winding
Using triple insulation wire with a 0.21mm Copper diameter, the number of turns of condary winding is expresd as香港大学研究生申请
where,
N s= Secondary Winding Turns
N p= 168 (total turns for all 4 primary winding layers) N p=
Primary Winding Turns
N = Number of turns per primary winding layer
1.8Auxiliary Winding
1.8.1 Winding Turns
The MOSFET gate voltage at minimum input voltage should be 10V to conduct the MOSFET
completely. For this application, the optocoupler is powered by the fly-back method, so the number of auxiliary winding turns of auxiliary winding is calculated as
where,
V g= Gate voltage
V DC(min)= Minimum input bus voltage
N a= Auxiliary Winding Turns N p
= Primary Winding Turns V o=
Optocoupler voltage
V F= Fly-back voltage
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N s= Secondary Winding Turns
1.8.2 Wire Diameter
With the auxiliary winding turns t to 11 (N a=11), the enamel wire chon has a 0.21mm outer
diameter and a 0.17mm Copper diameter. The Copper diameter of primary wire is expresd as
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1.9Gap Length
The gap length tting is bad on the number of primary winding turns and primary
inductance during the manufacturing process.
