LED switching power supply topology BUCKBOOST analysis

cpfpost

LED switching power supply topology BUCKBOOST analysis [Copy link]

 

Hey guys, please help me answer this question. What are the functions of Q1Q2 in the picture 1? Why does BUCKboost need this coil in the picture 2? Why does the electrolysis part in the picture 3 need two? Thank you

maychang

"Why do we need two electrolysis units for this part 3 in the diagram?"

Do you mean EC4 and EC5?

maychang

"What are the functions of 1 and Q1Q2 in the figure?"

Q1 and Q2 seem to be connected to the rest of the circuit only at the positive end of the power rectifier bridge and the negative end of the rectifier bridge (PGND), and there is no third connection. PGND and GND are connected through R13 and R14, and the N-channel MOS tube can only play the role of short-circuiting R13 and R14.

I don't understand the purpose of the circuit in the red box.

maychang

“Why does the 2BUCKboost in the picture need this coil?”

This coil is used to power chip U1, note the AUX label.

maychang

In this circuit, the output end is not isolated from the AC mains, and there is no feedback from the output end to the chip (neither voltage nor current regulation). The meaning is unclear.

cpfpost

Yes

cpfpost

maychang published on 2020-3-20 13:48 "What are the functions of 1 and Q1Q2 in the figure?" Q1 and Q2 seem to have only the positive end of the power rectifier bridge and the negative end of the rectifier bridge...

How to calculate this transformer? It has a center tap, which is different from the transformers we usually see.

maychang

cpfpost published on 2020-3-21 11:25 How to calculate this transformer? It has a center tap, which is different from the transformers we usually see

This circuit is a fairly typical flyback circuit and should be designed according to the flyback transformer.

The so-called "center tap" is nothing more than an autotransformer. When the power tube is on, the current in the upper half of the transformer (i.e., the winding between the MOS tube drain and the positive end of the power supply) rises roughly linearly. When the power tube is off, the upper and lower half of the transformer (i.e., the winding between the MOS drain and the positive end of D6) charge EC4 through D6.

maychang

cpfpost published on 2020-3-21 11:25 How to calculate this transformer? It has a center tap, which is different from the transformers we usually see

As for D7, ZD2, Q6 and the associated components, I don't know the original circuit designer's intention. This part of the circuit seems to be to make the voltage drop between the drain and source of Q6 a fixed voltage, but I don't know why it is like this.

不亦心

LZ is too unkind to not provide detailed information and waste Mr. Maychang's time...

This is a single-pole PFC dimming power supply.

1. Circle 1 active damping circuit prevents the dimmer from triggering peak current oscillation, R8-R11//C10 delays, Q2 shorts R13//14 to reduce losses, and Q1 provides a fast discharge path for C10.

2. Circle 2. There is nothing much to say about the tapped inductor. It is either voltage doubling or current doubling. It seems to be "voltage doubling" here.

3. Circle 3, EC4 is the output capacitor, EC5 is used to reduce the ripple of the unipolar PFC, and cooperate with Q6 to linearly "filter out" the ripple to solve the unipolar PFC flicker problem

不亦心

Let me complain again, the drawing of the LZ makes me want to beat him up

不亦心

maychang posted on 2020-3-20 15:15 In this circuit, the output end is not isolated from the AC mains, and there is no feedback from the output end to the chip (neither voltage nor current regulation). I don't know what it means.

PSR, BCM mode constant current and voltage limiting

maychang

Bu Yixin published on 2020-3-21 13:29 PSR, BCM mode constant current and voltage limiting

oh?

I am very uninformed.

cpfpost

maychang posted on 2020-3-21 12:13 This circuit is a fairly typical flyback circuit and should be designed according to the flyback transformer. The so-called "center tap" is nothing more than an autotransformer...

This is the schematic diagram, with the transformer pins

This is the transformer specification made by our engineers

You are talking about the typical flyback topology. Can the coil on the left be represented as shown in the figure below, except that it is non-isolated, the primary and secondary are grounded, and the calculation method is the same as the flyback method, but the turns ratio is fixed to 1? Another question, if the left side can be represented by the figure below, then should N1 or N2 be used for the calculation of the 5-6 coil? Please advise.

maychang

cpfpost posted on 2020-3-23 22:17 This is the schematic diagram with the transformer pinout. This is the transformer specification sheet made by our engineers. You are talking about a typical...

This diagram that cannot be drawn with red lines is equivalent to the transformer in the diagram on the 14th floor.

For the transformer in the picture on the 14th floor, the "ground" potential is pin 3 (AC ground, DC is the positive terminal of the power supply), and your "center tap" is pin 11, which is not grounded.

maychang

cpfpost posted on 2020-3-23 22:17 This is the schematic diagram with the transformer pinout. This is the transformer specification sheet made by our engineers. You are talking about a typical...

In the circuit on the 14th floor, the drain of the MOS tube is connected to pin 11, and the positive end of the DC power supply is connected to pin 11. The primary of the transformer in the flyback circuit is the winding from pins 3 to 11.

The secondary of the transformer in this circuit is the winding from pin 3 to pin 1 (the number of turns is exactly twice that of pin 3 to pin 11). Note that EC4 is connected to pins 3 and 1 through D6.

maychang

cpfpost posted on 2020-3-23 22:17 This is the schematic diagram with the transformer pinout. This is the transformer specification sheet made by our engineers. You are talking about a typical...

As mentioned above, the primary of the transformer in this circuit is from pin 3 to pin 11, so when calculating the winding 5 to 6, of course, the calculation should be based on the calculation of the winding 3 to 11. Note: The output voltage of the flyback circuit is not directly calculated using the turns ratio.

PowerAnts

The peeling circuit in the lower right corner, it is best to connect a 10R resistor in series with ZD2

PowerAnts

maychang published on 2020-3-20 13:48 "What are the functions of 1 and Q1Q2 in the figure?" Q1 and Q2 seem to have only the positive end of the power rectifier bridge and the negative end of the rectifier bridge...

This IC does not have an input voltage detection terminal. When the instantaneous voltage of the rectified steamed wave is low, in order to maintain the output, the PWM duty cycle may be close to 1. At this time, the instantaneous current is large and will increase the heating of the MOSFET. The circuit function of circle 1 may be to limit the loop current when the voltage is low.

maychang

PowerAnts published on 2020-3-24 11:23 This IC does not have an input voltage detection terminal. When the instantaneous voltage of the rectified steamed bun wave is low, in order to maintain the output, the PWM duty cycle may be close to 1. At this time, the instantaneous voltage...

From the first picture, the 4th pin of the chip is marked as VSNS, and the 5th pin is marked as ISNS. From the circuit, the VSNS pin is connected to the auxiliary winding through R21, C8, and R27. I did not check the instructions of the chip in the first post, but just guessed that this pin may be used for voltage detection.