Should the voltage regulator diode of the flyback power supply control chip be connected in parallel?

kal9623287

Should the voltage regulator diode of the flyback power supply control chip be connected in parallel? [Copy link]

 

In the figure below, a Zener diode is added to the control chip of the flyback power supply, the power supply and the ground to protect the IC from damage due to overvoltage. Should the Zener diode be connected in parallel as shown in the figure? Why?

bigbat

Not all of them work. Usually, there is a voltage regulator design inside the chip, which is mainly used to ensure the normal operation of the chip's control circuit.

maychang

"Should the Zener diodes shown in the figure be connected in parallel? Why?"

It is usually not necessary to use this Zener diode.

kal9623287

bigbat posted on 2022-6-21 16:49 Not all of them work. Usually, there is a voltage regulator design inside the chip, which is mainly used to ensure the normal operation of the chip's control circuit

If so, what are the dangers?

For example, the voltage on the transformer Vcc winding cannot be designed so accurately. For example, when the output is unloaded, the Vcc winding is 14V. If the main output power is relatively large, Vcc may rise to 18V, 20V or even higher. In addition, the Vcc voltage fluctuates with the power supply power.

Can the voltage regulator tube be held?

kal9623287

maychang posted on 2022-6-21 17:14 『Should the Zener diode be connected in parallel as shown in the figure? Why? 』 Usually there is no need to use this Zener diode.

Thanks for your answer, I have a question

If so, what are the dangers?

For example, the voltage on the transformer Vcc winding cannot be designed so accurately. For example, when the output is unloaded, the Vcc winding is 14V. If the main output power is relatively large, Vcc may rise to 18V, 20V or even higher. In addition, the Vcc voltage fluctuates with the power supply power.

Can the voltage regulator tube be held?

maychang

kal9623287 posted on 2022-6-22 14:09 Thank you for your answer. I have a question. If it is combined, I want to know what the harm is. For example, the voltage on the transformer Vcc winding cannot be designed so accurately, ...

The transformer windings in the first post have the same-name terminals marked incorrectly. The "point" end of the winding connected to the voltage regulator should be at the top, that is, the end connected to the 10-ohm resistor.

maychang

kal9623287 posted on 2022-6-22 14:09 Thank you for your answer. I have a question. If it is combined, I want to know what the harm is. For example, the voltage on the transformer Vcc winding cannot be designed so accurately, ...

Therefore, the output winding produces the same voltage per turn as the winding that powers the chip, and the ratio of the voltages output by the two windings is equal to the turns ratio.

If the flyback power supply is a regulated output, then the output of the winding that powers the chip is also regulated. No matter whether the output is unloaded or loaded (the flyback power supply usually does not allow the output to be unloaded). Unless the output of the flyback power supply is variable (can be adjusted over a wide range), there is no need to worry about the chip supply voltage being too high. Moreover, the chip supply voltage does not need to be very stable. For this, you can refer to the UC3842 manual, for example.

maychang

kal9623287 posted on 2022-6-22 14:09 Thank you for your answer. I have a question. If it is combined, I want to know what the harm is. For example, the voltage on the transformer Vcc winding cannot be designed so accurately, ...

You can refer to typical flyback switching power supply control chips such as UC3842~UC3845 to see whether such a voltage regulator is connected in parallel in the typical application circuit in its manual.

208971

Parallel connection prevents instantaneous surges during startup or other aspects, which is good for reliability. It can be removed if cost requirements are high.

kal9623287

maychang posted on 2022-6-22 14:38 Therefore, the output winding and the winding that powers the chip generate the same voltage per turn, and the ratio of the voltages output by the two windings is equal to the turns ratio. If the reverse...

Thanks a lot for answering

OK, go check out the UC3842 manual

不亦心

kal9623287 posted on 2022-6-22 14:09 If you want to know what the harm is, for example, the voltage on the transformer Vcc winding cannot be designed so accurately, for example, when the output is unloaded, the Vcc winding...

Many chips rely on VCC voltage for ovp protection, and PSR chips rely on this winding for voltage regulation. You cannot add a voltage regulator tube in this way. The problem of voltage drifting with load you mentioned is caused by the poor design of transformer winding distribution and large leakage inductance. Only by optimizing the transformer can the problem be solved at the source. If a voltage regulator tube must be added to absorb the leakage inductance peak energy, a resistor should be connected in series with the voltage regulator tube in the figure, and the voltage regulator tube value should also be larger than the theoretical voltage of the winding.

kal9623287

Bu Yixin published on 2022-6-25 14:35 Many chips rely on VCC voltage for ovp protection, and PSR chips rely on this winding for voltage stabilization. You can't add a voltage regulator tube like this. The voltage you mentioned fluctuates with the load...

The current situation is that the transformer winding distribution is indeed a design problem. The leakage inductance is large. How to compensate for it without redesigning the transformer? If the parallel diode cannot solve the problem or is harmful, what good suggestions are there?

That is to say, in order to better protect the IC, or the voltage difference between Vcc empty and full load is too large and needs to be regulated, how should we connect it?

不亦心

kal9623287 published on 2022-6-27 07:34 The current situation is that the transformer winding distribution is indeed a design problem. The leakage inductance is large. How to compensate for it without redesigning the transformer...

Didn't we mention above that the resistor in series with the voltage regulator is to absorb the leakage inductance spike, not to clamp VCC, so it looks a little different from the usage of ordinary voltage regulators. In addition, you can change the VCC rectifier diode to a slower tube, which can effectively reduce the leakage inductance spike.

kal9623287

Bu Yixin posted on 2022-6-27 11:55 Didn’t it say above that the resistor in series with the voltage regulator is to absorb the leakage inductance spike, not to clamp VCC, so it looks slightly different from the usage of ordinary voltage regulators...

It is to reduce the leakage inductance spike

Change to a slower speed tube, 1N4007 is ok?

kal9623287

Bu Yixin posted on 2022-6-27 11:55 Didn’t it say above that the resistor in series with the voltage regulator is to absorb the leakage inductance spike, not to clamp VCC, so it looks slightly different from the usage of ordinary voltage regulators...

Similar to the principle of linear voltage regulation, a resistor and a voltage regulator, NPN transistor, are used to assemble a simple linear voltage regulator.

Teacher, please give me some advice. Is the following improvement possible?

不亦心

kal9623287 published on 2022-6-27 14:25 Similar to the principle of linear voltage regulation, use a resistor and a voltage regulator, NPN transistor, to assemble a simple linear voltage regulator. Teacher, please give me some advice, below...

You can use 4007 rectifier tube.
The output is fixed voltage, so there is no need to add linear voltage regulator. The transistor can be short-circuited. Generally, this is done only when the output is adjustable.
Whether you can add a transistor depends on whether your specific chip uses VCC for other protection.

kal9623287

maychang posted on 2022-6-22 14:55 You can refer to typical flyback switching power supply control chips such as UC3842~UC3845. Whether the typical application circuit in its manual is connected in parallel with such a voltage regulator...

I checked the manual of UC342-3845, and it does not mention parallel connection of this voltage regulator circuit.

The problem I'm having now is

The power supply voltage to UC3842 is unstable, or the voltage fluctuates

The voltage regulator added here, from the analysis, why doesn't it work?

Also, you said on the 7th floor that the voltage generated by each turn of the output winding and the winding that powers the chip is the same, and the ratio of the voltages output by the two windings is equal to the turns ratio. Is this power supply for the chip related to the transformer winding?

kal9623287

Bu Yixin published on 2022-6-25 14:35 Many chips rely on VCC voltage for ovp protection, and PSR chips rely on this winding for voltage stabilization. You can't add a voltage regulator tube like this. The voltage you mentioned fluctuates with the load...

Now there is a situation that there is a 12-13V voltage fluctuation on pin 7 of 3842, and there is no 5V voltage output on pin 8. What should I do?

maychang

kal9623287 posted on 2022-6-28 08:00 I checked the manual of UC342-3845, and there is indeed no parallel connection of this voltage regulator circuit in the manual. The problem I am facing now is the power supply of UC3842...

"Is this power supply to the chip related to the transformer windings?"

The three windings in the first post are wound on the same iron core. The voltage rectified by the winding that powers the chip is obviously related to the winding.

maychang

kal9623287 posted on 2022-6-28 08:00 I checked the manual of UC342-3845, and there is indeed no parallel connection of this voltage regulator circuit in the manual. The problem I am facing now is the power supply of UC3842...

"The voltage regulator added here, from the analysis, why doesn't it work?"

I didn't say "no" on the 3rd floor, I said "not necessary".