Switching power supply waveform

aq1261101415

Switching power supply waveform [Copy link]

 

This is a DS waveform of a flyback switching power supply. How is the peak voltage in the T1 area generated? How are the oscillations in the T2 and T4 areas generated? And why is the voltage in the T3 area higher than the input voltage? Thank you everyone.

maychang

“How is the peak voltage in the T1 region generated?”

The start time of T1 is the time when the flyback circuit power switch is turned off. The peak voltage on the drain is generated by the primary leakage inductance of the flyback transformer due to the current being turned off. Most of this peak voltage has been absorbed by a certain absorption circuit, otherwise it would be higher.

maychang

“How do oscillations between the T2 and T4 regions occur?”

The damped oscillation in the T2 region is caused by the LC circuit composed of the primary distributed capacitance and primary leakage inductance of the flyback transformer due to the sudden change of the primary voltage. In fact, there is still damped oscillation in the T3 region, but the amplitude is smaller.

The damped oscillation in the T4 region is caused by the fact that the secondary rectifier tube of the flyback circuit has completed charging the output filter capacitor, and both the primary and secondary sides have become high-resistance. The LC circuit composed of the primary inductance (not leakage inductance!) and primary distributed capacitance plus the secondary inductance and secondary distributed capacitance reflected to the primary side is generated due to the sudden change in current.

maychang

“How do oscillations between the T2 and T4 regions occur?”

Since there is damped oscillation in the T4 region, it can be determined that the flyback circuit operates in a current discontinuous state.

maychang

"Why is the voltage in the T3 region higher than the input voltage?"

In the T3 region, the voltage between DS is the DC input voltage plus the voltage reflected from the secondary winding to the primary, so it is greater than the DC input voltage.

maychang

From the end of T4 to the beginning of T1, the power switch is turned on, and both ends of the primary winding are in a low resistance state, so there will be no damped oscillation. It can be seen from the waveform that the duty cycle of the flyback circuit switch is quite small.

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T1 is the moment when the switch is turned off, and the output voltage + leakage inductance spike voltage are superimposed. Because the leakage inductance spike cannot be transmitted in time, an instantaneous high voltage will be generated at the collector of the switch.

aq1261101415

maychang published on 2020-11-16 07:59 "How are the oscillations in the T2 area and the T4 area generated?" The damped oscillation in the T2 area is caused by the primary distributed capacitance of the flyback transformer and the primary...

Thank you, senior. I understand other areas, but I don't quite understand T4 area. I'm still a little confused.

maychang

aq1261101415 posted on 2020-11-16 19:56 Thank you, I understand other areas, but I don’t quite understand the T4 area, I’m still a little confused

This will require a few pictures to describe. I will draw the pictures later, so be patient.

maychang

aq1261101415 posted on 2020-11-16 19:56 Thank you, I understand other areas, but I don’t quite understand the T4 area, I’m still a little confused

A more detailed response is posted here.

"Damped Oscillations in Switching Power Supplies" .

aq1261101415

maychang posted on 2020-11-18 15:15 A more detailed reply is posted here. "Damped Oscillation in Switching Power Supplies".

Thank you so much, very detailed.

cyrs

Thank you for your explanation