Switching power transformer problems

shaorc

Switching power transformer problems [Copy link]

 

As shown in the figure, this is the circuit diagram of a single-ended flyback switching power supply. The input +vin is usually 36v. In the two outputs of n3n4, the voltage drop from the positive end to the negative end is about 20v, and n2 outputs a stable 12v voltage. Question 1: From the transformer data in the figure, we can see that the number of turns of the coils n1 and n3n4 is the same, but the voltages corresponding to n1 and n3n4 are different. Why is the turn ratio 1:1? Question 2: Since there is an absorption circuit composed of r3 and c10, why do we need to add an absorption circuit composed of r4 and c8?

maychang

"Question 1: From the transformer data in the figure, we can see that the number of turns of the coils n1 and n3n4 is the same, but the voltages corresponding to n1 and n3n4 are different. Why is the turns ratio 1:1? "

The DC voltage of each winding of the flyback switching power supply transformer is not proportional to the number of turns, and the conclusion that the AC voltage ratio of the power frequency transformer is equal to the turns ratio cannot be applied to it. Please refer to relevant books for calculation of the specific output voltage of the secondary winding after rectification.

maychang

The reason why the switching power supply (specifically the flyback switching power supply in the first post) can achieve voltage regulation is by changing the pulse duty cycle. Since the output voltage can remain unchanged (so-called voltage regulation) when the DC input voltage (+Vin in the first post) changes, it can be seen that the winding turns ratio is not equal to the DC voltage ratio.

maychang

"Question 2: Since there is a snubber circuit composed of R3 and C10, why do we need to add a snubber circuit composed of R4 and C8?"

The figure only uses resistors and capacitors in series to form an absorption circuit. The usual practice is to also use diodes to form a so-called RCD absorption circuit. Without a diode, the resistor consumes more power and the absorption is not better than using a diode.

maychang

"Question 2: Since there is a snubber circuit composed of R3 and C10, why do we need to add a snubber circuit composed of R4 and C8?"

There is no harm in using two groups of absorption circuits (including diodes). On the contrary, it can still protect the switch MOS tube when one group of absorption circuits fails. In addition, by using two groups of absorption circuits (including diodes), the power dissipation of the resistors in the absorption circuits can be relatively small, and the corresponding resistors are relatively small in size and easy to buy.

shaorc

Maychang published on 2022-1-10 15:16 『Question 2: Since we already have a snubber circuit composed of R3 and C10, why do we need to add a snubber circuit composed of R4 and C8? 』 Using two sets (including diodes...

"Using the diode absorption circuit, the power dissipation of the resistor is relatively small"

As shown in the figure in this post, it is the rcd absorption circuit. Its function is to transfer the excessive drain-source voltage to the capacitor C1 through D1 for energy absorption when Q1 is turned off to form a back electromotive force. If the figure shows an AC power input, the excessive drain-source voltage energy can also be fed back to the AC input power supply, and the function of the resistor R1 is to allow the energy in C1 to have a release circuit. At this time, the power dissipation on the resistor is smaller than when the diode is not needed?

shaorc

maychang posted on 2022-1-10 15:08 "Question 1: Looking at the transformer data in the figure, we can see that the number of turns of the n1 and n3n4 coils is the same, but the voltages corresponding to n1 and n3n4 are different. Why is the turns ratio 1: ...

"The DC voltage of each winding of the flyback switching power supply transformer is not proportional to the number of turns. Please refer to relevant books for calculation."

【1】Calculation and design of transformer windings for DC input, DC output, DCDC single-ended flyback switching power supply like this,

Is it mentioned in the book "Mastering Switching Power Supply Design" (2nd Edition) by Sanjaya Maniktala?

【2】If it is a single-ended flyback switching power supply with DC input and DC output, it will chop the DC input at high frequency to make it AC, and then output it to the secondary side through transformer coupling, and then rectify it for output. Does the single-ended flyback switching power supply with AC input and DC output also have such a chopping process?

maychang

shaorc posted on 2022-1-11 08:59 "Using the absorption circuit of the diode, the power dissipation of the resistor is relatively small." As shown in the figure, this is the rcd absorption circuit, and its function is to form a reverse when Q1 is turned off...

"If the picture shows an AC power input, the excessive drain-source voltage energy can be fed back to the AC input power supply."

I don't know about this and I can't tell from the picture.

There is no indication in this figure that "energy is fed back to the AC input power supply".

maychang

shaorc posted on 2022-1-11 08:59 "Using the absorption circuit of the diode, the power dissipation of the resistor is relatively small." As shown in the figure, this is the rcd absorption circuit, and its function is to form a reverse when Q1 is turned off...

"The function of resistor R1 is to allow the energy in c1 to have a release loop. At this time, the power dissipated in the resistor is smaller than when the diode is not needed?"

Yes. With a diode, the resistor only dissipates the energy stored in the transformer primary leakage inductance. Without a diode, the resistor, in addition to dissipating the energy stored in the transformer primary leakage inductance, also bears the voltage across the transformer primary during the entire cycle.

If there is a diode, the diode is turned off during the conduction of the power switch tube. If there is no diode, the resistor will also dissipate energy during the conduction of the power switch tube.

maychang

shaorc posted on 2022-1-11 09:21 "The DC voltage of each winding of the flyback switching power supply transformer is not proportional to the number of turns. Please refer to relevant books for calculation." [1] Like this DC input...

[1] Is the transformer winding calculation and design for a DC input, DC output, DCDC single-ended flyback switching power supply mentioned in the book "Mastering Switching Power Supply Design" (2nd edition) by Sanjaya Maniktala?

The flyback converter is not discussed in detail in this book. However, the flyback converter is developed from the Buck-Boost circuit. To understand the operation of the flyback converter, just look at the Buck-Boost circuit. Note that the Buck-Boost circuit is also distributed in several chapters in this book. For example, the Buck-Boost circuit is discussed on page 31, but it is discussed again on pages 92 and 197.

maychang

shaorc posted on 2022-1-11 09:21 "The DC voltage of each winding of the flyback switching power supply transformer is not proportional to the number of turns. Please refer to relevant books for calculation." [1] Like this DC input...

【2】If it is a single-ended flyback switching power supply with DC input and DC output, it will chop the DC input at high frequency to make it AC, and then output it to the secondary side through transformer coupling, and then rectify it for output. Does the single-ended flyback switching power supply with AC input and DC output also have such a chopping process?

In your sentence, both the front and back mentioned the flyback switching power supply. The only difference is that the front mentioned DC input and the back mentioned AC input.

The AC input is usually rectified and filtered to become approximate DC, and then this approximate DC is used to power the DC-DC converter. As for the chopping process, it is the same. The only difference is that the AC input adds the AC rectification and filtering process.

se7ens

The two outputs of N3 and N4 have added D2 and D4 voltage regulators to see if this is the reason why the output voltage is clamped at 20V

shaorc

maychang posted on 2022-1-11 10:42 "If the picture shows AC power input, the excessive drain-source voltage energy can be fed back to the AC input power supply." I don't know about this...

In the diagram on the sixth floor, if the input +Vin is replaced with a DC voltage input formed after rectification and filtering of the AC mains, then C1 and R1 in the diagram on the sixth floor are equivalent to being connected to the bus.

【1】When Q1 is turned off, the transformer primary winding (including leakage inductance) generates a back electromotive force, which passes through D1 and enters C1 for storage. What I mean is, can the energy stored in C1 be fed back to the bus?

[2] Because in the absorption circuits I have seen, the capacitance of the capacitors is at the level of several nf, such as 1nf and 2.2nf. Can capacitors with such small capacitance absorb voltage spikes?

shaorc

maychang posted on 2022-1-11 10:47 "The function of resistor R1 is to allow the energy in c1 to have a release loop. At this time, the power dissipated in the resistor is smaller than when the diode is not needed? ' Yes. ...

"Without the diode, the resistor, in addition to dissipating the energy stored in the transformer primary leakage inductance, also carries the voltage across the transformer primary throughout the cycle."

When there is a diode, that is, RCD topology, the diode is turned off during the conduction of the power switch tube. At this time, the input voltage Vin passes through the primary winding, then through the switch tube to the ground, without passing through the RCD circuit. At this time, the resistor has no power dissipation. Is this the reason why the resistor in the RCD circuit will not bear the voltage across the primary of the transformer during the entire cycle?

maychang

shaorc posted on 2022-1-13 09:33 In the figure on the sixth floor, if the input +Vin is replaced with a DC voltage input formed after the AC mains is rectified and filtered, then C1 and R in the figure on the sixth floor ...

【1】When Q1 is turned off, the transformer primary winding (including leakage inductance) generates a back electromotive force, which passes through D1 and enters C1 for storage. What I mean is, can the energy stored in C1 be fed back to the bus?

The answer is no. The energy stored in C1 cannot be fed back to the busbar because there is no current channel. There is no current channel because there is a rectifier diode in the middle. As long as the rectifier tube (usually a bridge rectifier, but no matter which type of rectifier is used, energy cannot be returned from the DC side to the AC side) is included.

maychang

shaorc posted on 2022-1-13 09:33 In the figure on the sixth floor, if the input +Vin is replaced with a DC voltage input formed after the AC mains is rectified and filtered, then C1 and R in the figure on the sixth floor ...

If the rectification is not done by diodes, but by using a bridge circuit consisting of four MOS tubes or bipolar tubes, the energy on the DC side can be returned to the AC side. However, this is no longer rectification, but inversion - the conversion of DC into AC is called inversion.

maychang

shaorc posted on 2022-1-13 09:33 In the figure on the sixth floor, if the input +Vin is replaced with a DC voltage input formed after the AC mains is rectified and filtered, then C1 and R in the figure on the sixth floor ...

[2] Because in the absorption circuits I have seen, the capacitance of the capacitors is at the level of several nf, such as 1nf and 2.2nf. Can capacitors with such small capacitance absorb voltage spikes?

The so-called "voltage spike" lasts for a very short time (the shorter the duration, the higher the voltage), and the energy is not large (the energy stored in the leakage inductance of the primary to the secondary of the transformer), so a capacitor of several nF is enough. The energy added by the capacitor must be released by the resistor at the end of this switching cycle. As for how large the capacitor should be, of course, it must be calculated based on the energy stored in the leakage inductance of the primary to the secondary of the transformer and how much the voltage across the capacitor is allowed to rise.

maychang

shaorc posted on 2022-1-13 09:33 In the figure on the sixth floor, if the input +Vin is replaced with a DC voltage input formed after the AC mains is rectified and filtered, then C1 and R in the figure on the sixth floor ...

[2] Because in the absorption circuits I have seen, the capacitance of the capacitors is at the level of several nf, such as 1nf and 2.2nf. Can capacitors with such small capacitance absorb voltage spikes?

From this, we can see that the leakage inductance should be minimized when winding the transformer. The smaller the transformer leakage inductance, the less energy is stored, the less energy needs to be absorbed when the power tube is turned off, and the less energy is dissipated in the resistor, which can improve the efficiency of the flyback converter. Therefore, the leakage inductance of the flyback transformer is an important parameter, and the process often uses the primary-secondary clamp winding method to reduce the leakage inductance.

maychang

shaorc posted on 2022-1-13 09:45 "Without a diode, the resistor not only dissipates the energy stored in the transformer primary leakage inductance, but also bears the voltage across the transformer primary during the entire cycle." ...

"Is this the reason why the resistor in the RCD circuit does not see the voltage across the transformer primary during the entire cycle?"

Yes.

In addition, as mentioned above, the peak voltage generated by the transformer leakage inductance lasts for a very short time, accounting for only a small part of a switching cycle of the power switch tube. Without the diode, the resistor will be subjected to voltage throughout the entire cycle: the voltage across the primary of the transformer, and power will be dissipated because of the transformer primary voltage at both ends. With the diode, the effective value of the voltage across the resistor is much smaller than without the diode.

maychang

shaorc posted on 2022-1-13 09:33 In the figure on the sixth floor, if the input +Vin is replaced with a DC voltage input formed after the AC mains is rectified and filtered, then C1 and R in the figure on the sixth floor ...

Generally speaking, diode rectification (regardless of which type) cannot return energy from the DC side to the AC side.

Converting DC energy into AC energy is called inversion. Inversion is another issue, which also involves multiple circuits and requires the use of controllable devices such as bipolar transistors or MOS transistors, which is even more complicated than the switching power supply that converts DC into DC.