Is there a problem with the direction of current flow?

yhhhy

Is there a problem with the direction of current flow? [Copy link]

 

 

If there is no problem, when the switch is turned on, is the same-name terminal positive or negative?

yhhhy

This is the picture

qwqwqw2088

Suddenly such a sentence

It seems to be talking about a certain topology of switching power supply?

The cause of the problem should be made clear.

Regardless of the forward conversion or flyback conversion topology, when the switch tube is turned on, the same-name terminal is positive or negative relative to the non-same-name terminal.

led2015

In the on state, the potentials of the like-named terminals are the same.

maychang

yhhhy posted on 2023-5-11 21:11 This is the picture

Is there a problem with the direction of the current flow?

Judging from the direction of the arrow in the figure, there is no problem.

maychang

yhhhy posted on 2023-5-11 21:11 This is the picture

[When the switch is turned on, is the same-name terminal positive or negative? ]

When the switch is turned on, the end with the dot is positive relative to the end without the dot.

yhhhy

maychang published on 2023-5-12 07:34 [When the switch tube is turned on, is the same-name terminal positive or negative? ] When the switch tube is turned on, the end with a dot is positive relative to the end without a dot.

Teacher, are the same-name ends of the first coil labeled incorrectly?

maychang

yhhhy posted on 2023-5-12 11:23 Teacher, is the same-name end of the first coil marked incorrectly?

[Is the same-name end of the first coil labeled incorrectly? ]

Which one is the first one? Most of the components in the picture are labeled, and the windings are also labeled. Is it bad to label?

yhhhy

maychang posted on 2023-5-12 11:28 [Is the same-name end of the first coil labeled incorrectly? ] Which one is the [first] one? Most of the components in the figure are labeled, and the windings are also labeled. It is said that the label...

Teacher, is Nr marked incorrectly for the same-name end?

maychang

yhhhy posted on 2023-5-12 20:13 Teacher, is Nr marked incorrectly for the same-name end?

[Is Nr incorrectly labeled with the same name? ]

There is no mistake.

yhhhy

When the switch is off, if the polarity of Np is measured once, is it positive at the bottom and negative at the top? Is the polarity of Nr also the same?

maychang

yhhhy posted on 2023-5-13 15:28 When the switch is turned off, is the polarity of Np positive at the bottom and negative at the top? Is the polarity of Nr also the same?

[When the switch is turned off, measure the polarity of Np once. Is it positive at the bottom and negative at the top? Is the polarity of Nr the same? ]

When the switch is off, the polarity of Np is positive at the bottom and negative at the top, but the polarity of Nr is negative at the bottom and positive at the top. This can also be seen from the markings of the terminals with the same name.

yhhhy

When turned off, the polarity of Nr is positive at the top and negative at the bottom. Why can the current on Nr flow from low potential to high potential?

maychang

yhhhy posted on 2023-5-14 11:43 When turned off, the polarity of Nr is positive at the top and negative at the bottom. Why can the current on Nr flow from low potential to high potential?

When the switch is off, the polarity of Nr is positive at the top and negative at the bottom.

At this time, Nr is a power source for the diode (not marked) and Vi, and its energy source is the magnetic energy stored in the transformer core during the switch-on period. The voltage across Nr is higher than Vi (note: Nr is a power source, the voltage across the two ends is positive at the top and negative at the bottom), so the current direction is as shown on the right side of the first post, from the upper end of Nr (without a dot) through Vi and then through the diode to the lower end of Nr. The diode is in forward conduction at this time.

yhhhy

Thank you for clarifying my doubts

maychang

yhhhy posted on 2023-5-16 14:20 Thank you for clearing up my doubts

The first circuit is a very common way for a forward converter to release the energy stored in the transformer core.

Np and Nr are actually two windings wound in parallel with two wires (with different wire diameters, Np wire is thick and Nr wire is thin). The two-wire winding can maximize coupling with almost no leakage inductance.

maychang

yhhhy posted on 2023-5-16 14:20 Thank you for clearing up my doubts

When the power switch is turned on, the voltage across the secondary winding Ns is as shown in the first figure, the diode D1 is turned on, the current in the inductor L increases, and the output capacitor is charged (while supplying power to the load). The magnetic flux generated by the current in Ns will offset part of the magnetic flux generated by the current in Np.

When the power switch is turned off, the Ns winding has no current due to the reverse direction of D1, and the current in the inductor L decreases. At this time, the current in the inductor L is maintained by D2, and the output capacitor is discharged (while supplying power to the load). However, at this time, the existing magnetic field energy in the transformer core has nowhere to be released. This requires the Nr winding. At this time, the current in the Nr winding is as shown on the right side of the first post. The Nr winding acts as a power source to charge Vi (the magnetic energy in the core is converted into electrical energy and returned to Vi). Therefore, the duty cycle of the power switch in this circuit cannot exceed 50%. If it exceeds, Nr cannot completely release the magnetic energy stored in the transformer core back to Vi during the shutdown of the power switch, which will cause the magnetic flux in the core to continue to increase and eventually enter magnetic saturation.

maychang

yhhhy posted on 2023-5-16 14:20 Thank you for clearing up my doubts

The circuit in the first post has a disadvantage: the two windings Np and Nr are wound in parallel with two wires, and the distributed capacitance is very large. However, the two windings are not at the same potential when alternating current. When the switch tube is turned on, one end of Np is at the positive power supply, and one end of Nr is at twice the power supply voltage. When the switch tube is turned off, one end of Np is still at the positive power supply, and one end of Nr is at the ground potential (negative power supply). In this way, a large current will flow through the distributed capacitance between the two windings.

This problem can be avoided by moving the diode above Nr (with the dotted end of Nr grounded).