ST公司自激式开关电源设计一

ST公司自激式开关电源设计

1       Power Transformer Design Calculations

l       The specifications:

–     VAC = 85~265V

l       Line frequency: 50~65Hz

–      VO = 5V

–      IO = 0.4A

 

Taking transient load into account, the maximum output current is set as

IO (m a x )   =  1.2IO=  4.8 A

1.1       Switching 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 selection. However, the frequency must be at least 25kHz to minimize audible noise.

 

Higher switching frequencies will decrease the transformer noise, but will also increase the level of switching power dissipated by the power devices.

 

The minimum switching frequency and maximum duty cycle at full load is expressed as

                                   fS (m i n )  = 50 kHz

                                   Dma 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       secondary reflected voltage, and

voltage spike (caused 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 margin is at least 50V, the reflected voltage is given as:

Vfl  =  V( B R) DS S – Vm arg i n – VDC( ma x ) – Vsp k  =  600 – 50 – 375 – 95 = 80 V

The Turn Ratio is given as

where,

Vfl = Secondary reflected voltage

V(BR)DSS = MOSFET breakdown voltage

Vmargin = Voltage margin

VDC(max) = Maximum input bus voltage

Vspk = Voltage spike

Vf = Voltage drop

N = Turn Ratio

 

Np = Primary Winding Turns

Ns = Secondary Winding Turns

 

1.3 Primary Current

l             Primary Peak Current is expressed as:

l             Primary Root Mean Square (RMS) Current is expressed as

where,

 

Ippk = Primary peak current

VO = Voltage output

IO(max) = Maximum current output

h = Efficiency, equal to 0.7

 

Dmax = Maximum duty cycle

VDC(min) = Minimum input bus voltage

Iprms = Primary RMS current

1.4 Primary Inductance

          Primary Inductance is expressed as

where,

 

VDC (min) = Minimum Input DC voltage fs (min) = Minimum switching frequency Dmax = Maximum duty cycle

fs(min) = Minimum switching frequency

Ippk = Primary peak current

For example, if Primary Inductance is set to 5.2mH, the minimum switching frequency is:

1.5 Magnetic Core Size

         One of the most common ways to choose a core size is based on Area Product (AP), which is the product of the effective core (magnetic) cross-section area times the window area available for the windings.

Using a EE16/8 core and standard horizontal bobbin for this particular application, the equation used to estimate the minimum AP (in cm4) is shown as

where,

 

Lp = Primary Inductance

Iprms = Primary RMS current

ku = Window utilization factor, equal to:

–     0.4 for margin wound construction, and

–     0.7 for triple insulated wire construction

 

Bmax = Saturation magnetic flux density

T = Temperature rise in the core

1.6 Primary 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,

 

Np = Primary Winding Turns

VDC (min) = Minimum Input DC voltage

Dmax = Maximum duty cycle

fs(min) = Minimum switching frequency

B = Flux density swing

 

Ae = Effective area of the core

1.6.2    Wire Diameter

 The current density (AJ) allowed to flow through the chosen wire is 4A/mm2. The Copper diameter of primary wire is expressed as

   

where,

 

dp = Diameter of primary winding wire

Iprms = Primary RMS current

AJ = Current density

1.6.3    Number of Primary Winding Turns per Layer

 The EE16 bobbin window is about 9mm, so if the enamel wiring chosen has a 0.21mm outer diameter and a 0.17mm Copper diameter, the number of turns per layer is expressed as

where,

Np1 = Layer 1 Primary Winding Turns

Np1 = 42 turns per layer, 4 layers needed

Np = 168 (total turns for all 4 layers)

 

1.6.4    Practical Flux Swing

Using the Np = 168 value, the practical flux swing is expressed as

where,

 

B = Flux density swing

 

VDC(min) = Minimum input bus voltage

Dmax = Maximum duty cycle

fs(min) = Minimum switching frequency

Ae = Effective area of the core

Np = Primary Winding Turns

1.7       Secondary Winding

Using triple insulation wire with a 0.21mm Copper diameter, the number of turns of secondary winding is expressed as

where,

 

Ns = Secondary Winding Turns

Np = 168 (total turns for all 4 primary winding layers) Np = Primary Winding Turns

N = Number of turns per primary winding layer

1.8       Auxiliary 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,

 

Vg = Gate voltage

VDC(min) = Minimum input bus voltage

Na = Auxiliary Winding Turns Np = Primary Winding Turns Vo = Optocoupler voltage

VF = Fly-back voltage

Ns = Secondary Winding Turns

 

1.8.2     Wire Diameter

 

With the auxiliary winding turns set to 11 (Na =11), the enamel wire chosen has a 0.21mm outer diameter and a 0.17mm Copper diameter. The Copper diameter of primary wire is expressed as

1.9       Gap Length

 

The gap length setting is based on the number of primary winding turns and primary inductance during the manufacturing process.

 

Note: In practice, the saturation current value must be ensured. If it is not, then the design activity should be restarted.