Practical Tips | Talking about Flyback Converters

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1. Let's first understand the flyback converter
1. The basic circuit of flyback:

2. Working principle:

The polarity of the primary and secondary windings of the transformer is opposite, which is probably the origin of the name Flyback:
a. When the switch tube is turned on, the current in the primary inductor of the transformer begins to rise. At this time, due to the relationship between the secondary terminals, the output diode is cut off, the transformer stores energy, and the load is provided with energy by the output capacitor.
b. When the switch tube is cut off, the voltage induced by the primary inductor of the transformer is reversed. At this time, the output diode is turned on, and the energy in the transformer is supplied to the load through the output diode, while charging the capacitor to replenish the energy just lost.

3. The evolution of the flyback circuit:
It can be regarded as an isolated Buck/Boost circuit:

4. In the flyback circuit, the output transformer T not only realizes electrical isolation and voltage matching, but also has the function of storing energy. The former is the property of the transformer, and the latter is the property of the inductor. Therefore, some people call it an inductor transformer, and sometimes I also call it an asynchronous inductor.

2. Flyback working mode:
1. DCM (discontinuous current mode) & CCM (continuous current mode)
According to whether the secondary current drops to zero, flyback can be divided into two working modes: DCM and CCM. The two modes have their own characteristics. The waveforms in the following two working modes (ideal waveforms).

Waveforms of the flyback converter operating in CCM

Waveforms of the flyback converter operating in DCM

3. Working mode:

1) Voltage and current waveforms
2) Use the inductor transformer model to indicate the working process; the primary and secondary sides can also be discussed separately, using a voltage source to replace the intermediate conversion.

4. Source of waveform oscillation:

1) The oscillation when the switch is turned off comes from the leakage inductance;

2) The intermittent oscillation is mainly caused by the primary inductance, because there is no reflected voltage clamping. We can treat the reflected voltage as a voltage source.

5. What is the voltage that the switch tube is subjected to when the actual situation is not ideal?

a. (1) The switching tube voltage is divided into several parts:

Vds=VDC+VRo(N*Vo)+Vlk

b. (2) There is nothing to explain about VDC. VRo is the voltage reflected to the primary side by the transformer (N*Vo) when the primary switch is turned off and the secondary diode is turned on. In addition to the voltage controlled by the transformer, there is also the leakage inductance voltage Vlk that cannot be controlled. Since it is the leakage inductance voltage, it is of course related to the leakage inductance of the transformer. This voltage is what we hate!

c. (3) What if the leakage inductance voltage is limited? The RCD absorption clamp circuit uses capacitors to absorb, diodes to clamp, and resistors to consume the leakage inductance energy. The design principle is to allow the RCD to consume the leakage inductance energy and play its due role, but it should not be too strong to become a load on the primary side that consumes the excitation inductance energy.

6. Transformer, voltage and current waveforms, diode reverse recovery:

1) The flyback transformer is more of an inductor than a transformer, but we cannot deny that it is a transformer;

2) Transformers all have more or less leakage inductance, which exists relative to the primary side and also relative to the complex side;

3) Two oscillations in the voltage waveform:

a. In CCM mode, only when the switch is turned off, the leakage inductance causes oscillation (Lk and C); b.
In DCM mode, the secondary current is zero, and the primary inductance loses NVo clamping, causing oscillation (Lm and C);

c. All oscillations, even the basic switching of power conversion, are sources of EMI;

5) At the same time, we know that the diode has a reverse recovery problem:

a. In CCM mode, this problem will occur;

b. In DCM mode, since the current is about to reach zero, this problem basically does not exist;

3. Let’s talk about Vds again:

Vds=VDC+VRo(n*Vo)+Vlk;

I would like to add two points to this formula:

a. If the voltage drop of the secondary rectifier tube is also taken into account, it should be n*(Vo+VDsec); because VDsoec is very small, it is ignored, provided that n is not large.

b. When we design, the input voltage and output voltage are fixed, and we can't do anything about it, but the turns ratio is designed by ourselves. Sometimes, in order to accommodate the standard tube, when our voltage is high, we have to slightly adjust the turns ratio to meet our design.

4. Let's talk about the intermittent and continuous issues:
1. The constraint relationship between the primary and secondary sides:
a. Uo=D/（1-D）Vin This is the voltage constraint formula of Flyback. There is no load parameter here, which means that the output voltage is theoretically not affected by the load. This most common formula is only valid under CCM.
b. If in DCM mode, the primary and secondary sides are constrained by power conservation, so the output voltage formula is:
Vo=VinTon*Sqrt(R0/2TLp) The output voltage is related to the load size, and the voltage is inversely proportional to the load. Therefore, in principle, the open loop cannot run without load, don't forget it.

2. What are the real waveforms of intermittent and continuous?
In fact, flyback is a very magical topology, simple yet very profound; for the most basic questions, I only understand 30% of them. The small-power Flybck is really the best choice, how small? Before LLC was mastered by people, 200W was useful. Now people above 75W are beginning to consider using LLC, and the efficiency is better.

3. When the switch is turned off, there is an oscillation from the leakage inductance; when it enters DCM, there is also an oscillation, but this oscillation is the oscillation of the main inductor. Sometimes I use this oscillation to judge whether you are in DCM or CCM. This is the method I use, I don’t know if it is correct.

5. Let's talk about the Flyback transformer
1. In the flyback circuit, the output transformer T not only realizes electrical isolation and voltage matching, but also has the function of storing energy. The former is the property of the transformer, and the latter is the property of the inductor. Therefore, some people call it an inductor transformer, and sometimes I also call it an asynchronous inductor.

2. The Flyback circuit does not have an output filter inductor, or more precisely, the flyback circuit can be without an output filter inductor. In Cai Ming's words, why is this? I have to understand that it is because of the first reason that there is actually an inductor in the current loop, so it is not necessary to have one;

3. Flyback's transformer needs to store energy, which is not a property of the transformer, but a property of the inductor;

Transformers need to store energy, so they usually need to open the breath; some friends may not understand what I said before, what is the relationship between storing energy and breath? Because once the breath is opened, the energy is mainly stored in the breath.
6.

7.
The basic principle and characteristics of the flyback converter have been discussed;
it is time to talk about specific techniques:
Let's talk about the transformer first. It is a bit difficult for me to talk about this:
Let's talk about it briefly:
turns ratio, inductance (taking the critical mode of continuous and discontinuous as an example):
1. When I analyzed the switch off voltage before, I talked about the influence of the turns ratio on this. You should determine a suitable turns ratio based on the desired D; after the turns ratio is determined, it is N, which I will not talk about.
2. How to determine the inductance of the transformer?
First deduce the formula:
a. Find D:
D=N*Vo/(Vin+N*vo) to solve D;
b. You have to solve Ipeak, the peak current:
use the power conservation magic weapon:
Ipeak*(1-Krp/2)*D=(Pin=(Po/K))/Vin; Ipeak is solved;
there is this thing Krp here, which is equal to 1 here. This thing seems very professional, but there is actually no mystery. A very simple thing is sometimes complicated, haha;
c. To find the primary inductance:
First recall the two formulas you are very familiar with:
E=N*di fan/dt is the formula, I will not edit it carefully, you know the law of electromagnetic induction;
there is another one you know better, the inductance formula E=Ldi/dt; OK, combined:

3. The primary inductance, you already know it, so the secondary inductance will naturally be known;

8. Following the above results, let's take a look at the current on the primary side:
(There are many names, I can't figure it out clearly, haha)
1. Average current: Iave=(Pin=(Po/K))/Vin;
2. Peak current: Ipeak=Iave/(1-Krp/2)D
3. Current Ripple: triangle I=Ipeal*Krp;
The above is easy to understand, I won't explain it;
4. Current effective value:

6. You can figure out the secondary current by yourself.

IX. Switching frequency fs:
(1) Switching frequency fs=1/(ton+toff):
ton=I_P*L/Vin;toff=I_P*L/(N*Vo);
(2) For more precise calculations, the rise and fall time of VDS should be added:
t_Lleak=Cp*VDS/I_P;t_w=pi*sqrt(Cp*Lp);
fs=1/(ton+toff+t_Lleak+t_w)

X. Let's talk about the purpose of opening the Flyback transformer:
First look at the picture. I find that the picture is the most direct form of presentation. If you don't believe it, look at it:
1. Generate the required inductance; 2. Avoid core saturation; H-----I, hehe XI. I want to say a few more words about current: I'll give you a graph. The graph is the most intuitive. You can see the average value, peak value, etc. by looking at the graph: What is the circuit loss related to? Effective value; Let me explain the most basic concept of effective value (junior high school): Definition: The root mean square value of the instantaneous value of a time variable within a given time interval; How to calculate? : You need the integration you learned in college, express the instantaneous value, and calculate the integral according to the definition.

The effective value is determined based on the thermal effect of electric current. If an alternating current and a direct current pass through resistors of the same resistance and the heat generated in the same time is equal, then the value of the direct current is called the effective value of the alternating current.

Source: Reprinted from the Internet. If copyright is involved, please contact us to delete it.

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