Flyback Transformer Design

yhhhy

Flyback Transformer Design [Copy link]

 

uo=12v,po=60w,n>=85%,switching frequency f=65khz,ui is 90-264Vrms

How to determine duty cycle, Vds, and turns ratio?

maychang

There is a post in this forum called "Switching Power Supply Design (3rd Edition) PDF Scanned Version 540 Pages", which is now on the first page. There is a detailed calculation process in it.

maychang

The fourth chapter of the book is entirely about the flyback converter, first discussing the working principle, then the working mode (the fourth section specifically discusses the discontinuous working mode), and then the design principles and design steps. The sixth section uses a typical circuit to give a design example.

yhhhy

maychang published on 2023-5-26 08:40 The fourth chapter of the book is entirely about flyback converters, first the working principle, then the working mode (the fourth section is specifically about the discontinuous working mode), and then the design principles and...

Teacher, how do we get the rated voltage of the switch tube to be 200V? Why is Vds in formula (4.4) 120V?

yhhhy

picture

maychang

yhhhy posted on 2023-5-26 09:23 Teacher, how do you get the rated voltage of the switch tube to be 200v? Why is Vds in formula (4.4) 120v?

[How do we get the rated voltage of the switch tube to be 200v here?]

The 200V here is not the "rated voltage", but the maximum voltage that the switch tube is allowed to withstand.

In addition, as a voltage unit, the letter V must be capitalized. As a power unit, the letter W must be capitalized. As a frequency unit, the letter H must be capitalized, and the letter z must be lowercase.

maychang

yhhhy posted on 2023-5-26 09:23 Teacher, how do you get the rated voltage of the switch tube to be 200v? Why is Vds in formula (4.4) 120v?

[Why is Vds 120v in equation (4.4)? ]

There is only Vdc in formula (4.4), but no Vds.

Vdc in formula (4.4) represents the DC power supply voltage, which is given as 60V in this example.

yhhhy

Teacher, as shown in the figure above, how do we determine to choose a switch tube with a rated voltage of 200V? How do we determine the maximum voltage stress Vms that the switch tube bears when it is turned off in formula (4.4) to be 120V?

maychang

yhhhy posted on 2023-5-26 10:45 Teacher, as shown in the figure above, how to determine the switch tube with a rated voltage of 200V? In formula (4.4), the maximum voltage that the switch tube can withstand when it is turned off should be...

[As shown in the figure above, how to determine the switch tube with a rated voltage of 200V? ]

It is selected based on formula (4.4) plus a certain margin.

maychang

yhhhy posted on 2023-5-26 10:45 Teacher, as shown in the figure above, how to determine the switch tube with a rated voltage of 200V? In formula (4.4), the maximum voltage that the switch tube can withstand when it is turned off should be...

[How to determine the maximum voltage stress Vms that the switch tube bears when it is turned off in formula (4.4) as 120V? ]

There are two terms on the right side of equation (4.4). The first term is the DC power supply voltage, which is given as 60V in this example. For your design, it is a DC voltage after rectification and filtering with an effective value of 90~264V. The second term is the output DC voltage when the switch is turned off (5V for this example and 12V for your design), multiplied by the turns ratio. This term is usually called the reflected voltage, which is the voltage reflected from the secondary output voltage (the diode voltage drop should be added) to the primary voltage through the transformer (the turns ratio plays a role here) when the switch is turned off. This is the maximum voltage that the switch can withstand in the ideal state (120V. Here, the reflected voltage is equal to the power supply voltage, that is, the turns ratio is considered according to the output DC voltage and the DC power supply voltage). In fact, the peak voltage generated by the leakage inductance when the switch is turned off should also be considered (after being absorbed by the RCD circuit, the text after the equation estimates it to be 0.3 times the power supply voltage). Then, a 30% margin is also considered.

不亦心

The parameters of the transformer of the same power supply are not fixed, and neither are the design components of the switching power supply. It is a process of finding a relatively optimal solution for the components you can get based on the constraints of design indicators, cost, volume, structure, etc.

yhhhy

maychang published on 2023-5-26 11:29 [How to determine the maximum voltage stress Vms that the switch tube withstands when it is turned off in formula (4.4) as 120V? ] There are two terms on the right side of formula (4.4). The first...

Teacher, in formula (4.4), the maximum voltage and turns ratio that the switch tube can withstand are unknown. Which equations should be combined to calculate that the maximum voltage that the switch tube can withstand is 120V?

maychang

yhhhy posted on 2023-5-26 15:48 Teacher, in formula (4.4), the maximum voltage and turns ratio of the switch tube are unknown. Which equation should be combined to calculate the maximum voltage of the switch tube...

Bu Yixin on the 11th floor is right: the parameters of the transformer are not fixed, and neither are the design component selection of the switching power supply.

Your problem cannot be solved by simultaneous equations. You can first choose the reflected voltage as the DC power supply voltage (note that this is not calculated, nor is it the final value), then the voltage that the switch tube bears (not considering the leakage inductance spike, nor the voltage tolerance margin) is estimated to be 120V. The leakage inductance spike is estimated to be 30% of the power supply voltage (note that the transformer winding method has not been determined, so we don’t know how much leakage inductance there is), and then consider the tolerance, and you can conclude that the switch tube should withstand a voltage of at least 200V.

After calculating the switch tube current, you can now select the switch tube. Switch tubes are not like resistors. You can choose as high a withstand voltage as you want. It is very likely that the switch tube you selected can withstand 180V, and the current is not a problem. Then, you can change the turns ratio and try to reduce the reflected voltage, or choose another type of high-voltage switch tube (such tubes are usually more expensive and the current capacity may be slightly smaller). After repeated attempts, you can determine the switch tube model, and also determine the transformer core model, primary turns (inductance), turns ratio, etc. The same is true for transformer core selection. After selecting a certain type of core, the window may not fit, so you can only change to a larger core. If the window is only a little different, you can also consider slightly increasing the operating frequency... After the primary inductance of the transformer is determined, the air gap must be considered (the flyback converter uses a ferrite core, so an air gap must be added). In short, the choice of tubes and cores... may be the result of repeated estimates and weighing.

maychang

yhhhy posted on 2023-5-26 15:48 Teacher, in formula (4.4), the maximum voltage and turns ratio of the switch tube are unknown. Which equation should be combined to calculate the maximum voltage of the switch tube...

The calculation of the transformer primary and secondary wire diameters may also be the result of repeated calculations. The diameter of the enameled wire (electromagnetic wire) does not change continuously, not to mention that multiple strands of wire (two, three, four...) are sometimes used in parallel winding in switching power supplies. It is difficult to calculate the window area occupied by the wire after parallel winding (multiple strands of wire may overlap and cross). This requires estimation, verification, and finally actual winding.

yhhhy

maychang posted on 2023-5-26 16:15 The calculation of the primary and secondary wire diameters of the transformer may also be the result of repeated calculations. The diameter of the enameled wire (electromagnetic wire) does not change continuously, not to mention the switch...

I've learned a lot, thank you for your careful answer!

led2015

For this flyback transformer design problem, we can follow the steps below.

1. Calculate the output current Io. With given values of Uo and Po, the output current can be calculated using the formula Io=Po/Uo, that is:

   Io = Po / Uo = 60W / 12V = 5A

2. Calculate the input power Pi. Ideally, the input power and output power should be equal. Considering the efficiency of the transformer, we assume that its minimum efficiency is 85%, then:

   Pi = Po / Eff = 60W / 0.85 = 70.59W

3. Calculate the on time Ton and off time Toff of the switch tube.

Since the switching frequency is 65kHz, the period T = 1/f = 15.38us. The duty cycle D can be inferred from other parameters, where Ton and Toff can be calculated using the following formula:

Ton = D × T Toff = (1 - D) × T

4. Determine the maximum leakage induction voltage Vds of the switch tube.

For the flyback transformer, the maximum leakage induction voltage Vds of the switch tube is very important and should be determined according to the AC input voltage, transformer turns ratio, duty cycle and rated current of the switch tube. Here, we assume that a field effect tube is used and a device of about 60V is used, so the value of Vds should be slightly greater than 60V.

5. Determine the transformer turns ratio N.

When the sensitive load changes, the output voltage Uo needs to be able to quickly adjust to the target value. To this end, we need a suitable transformer turns ratio to provide the required feedback control.

For a flyback transformer, the transformer turns ratio N is usually between 1:0.3 and 1:0.6. Here, a turns ratio of 1:0.5 is assumed.

Based on the above parameters, the circuit schematic and component parameters of the flyback transformer can be designed, and simulation and experimental verification can be performed. It should be noted that there are many other factors to consider in actual applications, such as EMI, the selection of solid-state relays, etc.

Ming8686

It is recommended to read the book Switching Power Supply Design

不亦心

See for yourself

AN-4150, Design Guidelines for Flyback Converters Using FSQ-series Fairchild Pow.pdf (555.23 KB, downloads: 9)

2023-5-27 15:32 上传

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