HOW TO CALCULATE SMPS FLYBACK TRANSFORMER DESIGN

 A switched-mode power supply (switching-mode power supply, switch-mode power supply, switched power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently

SMPS stands for switch-mode power supply unit. The properties of an SMPS transformer are highly dependent on the frequency in which they operate. High switching frequency opens up the possibilities to choose smaller SMPS transformers these high frequency, SMPS transformers uses ferrite cores.

The transformer core design is the most important thing in an SMPS transformer construction. A core has a different type of A_L (Ungapped core inductance Coefficient) depending on the core material, core size, and core type. Popular type of core material are N67, N87, N27, N26, PC47, PC95, etc. Also, the manufacturer of ferrite cores provides detailed parameters in the datasheet, which will be useful while selecting the core for your transformer

 

 

Bobbin

A bobbin is the housing of cores and windings. A bobbin has an effective width which is essential to calculate the wire diameters and the construction of the transformer. Not only this, a bobbin of a transformer also has a dotted mark which provides the information of primary windings.

 

Primary winding

The SMPS transformer winding will have a primary winding and a minimum of one secondary winding, based on the design it might hav more secondary winding or an auxiliary winding. The primary winding is the first and innermost winding of a transformer. It is directly connected to the primary side of an SMPS. Usually number of winding on primary side is more than other windings of the transformer. Finding the Primary winding in a transformer is easy; one just needs to check the dot side of the transformer for the primary winding. It is generally situated across the high voltage side of the mosfet.

In an SMPS schematic, you can notice the high voltage DC from the high voltage capacitor connected with the primary side of the transformer and the other end is connected with the power driver (Internal mosfet drain pin) or with a separate high voltage MOSFET's drain pin.

 

Secondary winding

Secondary winding converts the voltage as well as the current on the primary side to the required value. Finding out the secondary output is a bit complex as in some SMPS designs the transformer usually has multiple secondary outputs. However, the output or low voltage side of an SMPS circuit is generally connected to the secondary winding. One side of the secondary winding is the DC, GND and the other side is connected across the output diode.

As discussed, an SMPS transformer can have multiple outputs. Therefore an SMPS transformer can also have multiple secondary windings.

 

 

 Some people may still be confused about how the rules for the SMPS transformer or commonly called ferrite transformers.

Previously there were several rules for selecting wire diameters based on the frequencies used.

After knowing the wire that is used based on the frequency of the MOSFET driver that is used now we see first the formula determines the primary winding (Np).

Description

Npri = primary winding

Vin = Effective Input Voltage

f = frequency

Bmax = maximum flux density 1200 - 2000

Ac = Effective Cross Sectional Area (see datasheet of each type of transformer core (example EI33 = 1.19, ETD39 = 1.25).

For example, the above formula is specifically for push-pull topology.

For example, I want to make an inverter transformer with specifications:

Effective Vinput = 12v

Minimum Vinput = 10,5v

Max output = 330v

Normal Vout = 220v

Driver frequency = 50KHz or 50000Hz

then the primary (Np) and secondary (Ns) are?

I previously set the Bmax I want to use is 1600 (range Bmax 1200-2000) and uses EI33 core type with Ac 1.19

Np = (12v x100000000) / (4x50000x1600x1,19)

Np = 3.15 my turn rounds to 3 turns

check the Bmax value again not to be less than or more than the adjusted range above. then if I turn rounds into 3 (Np = 3).

Bmax = (12v x100000000) / (4x50000x3x1,19)

Bmax = 1680 (still in the range 1200-2000)

Now we calculate Primary Turn (Ns). because the output voltage can be adjusted or stabilized by setting the percentage duty cycle, if there is a voltage input drop to a minimum of 10.5v, the duty will increase to a maximum of 98% to maintain normal output stability at 220v or at least above 200v. For that N = 330v / (10,5v x 98% duty) = 32 so

Ns = N x Np

Ns = 32 x 3

Ns = 96 turns

To optimize the voltage-current, you can double the number of wires in accordance with the power and capacity of the bobbin.

Suppose you make a transformer with an output power of 300 watts, then 300watt / 220v = 1.4Amper and see the wire that can be used with a 50KHz frequency in the AWG table above. for example the wire used is 0.5mm then 1.4 Amp / 0.5 = 2.8 or rounded down or upward to be 2 or 3 double wire and of course the coil must be 2 or 3 times more primary wire used than secondary because Vin 12v then 300w / 12v = 25A so that the primary transformer wire does not overheat it must be joined into several wires in accordance with the bobbin ability or the capacity of the transformer bobbin.