What is the function of R1 in the upper left corner of the picture? Has EA simplified the structure of the error amplifier here?

西里古1992

What is the function of R1 in the upper left corner of the picture? Has EA simplified the structure of the error amplifier here? [Copy link]

 

Does it limit the current of the voltage regulator? I didn't see the function of R1 in the book, so it is not involved in the voltage regulation. And EA is the error amplifier. Is it simplified by using a triangle + EA? This is not the error amplifier structure, is it a comparator?

maychang

【 What is the function of R1 in the upper left corner of the picture? 】

R1 provides current for the voltage regulator tube VDz. Without this current, the voltage across the voltage regulator tube is zero.

maychang

[Has EA simplified the structure of the error amplifier here?]

Of course it's simplified. At least the power pins are not shown.

maychang

[This is not an error amplifier structure, it is a comparator]

This EA must be an operational amplifier, not a comparator.

hjl2832

R1 and VDZ1 together form a reference voltage source. As you can see in the figure, the reference voltage is connected to the positive input terminal of the op amp, and the feedback voltage is input to the negative input terminal of the op amp. The error signal of the transistor output voltage is amplified by the op amp, and then the conduction degree of the transistor is controlled from the output terminal of the op amp to achieve the function of variable resistor voltage regulation.

西里古1992

maychang posted on 2023-3-8 18:32 [Has EA simplified the structure of the error amplifier here?] Of course it has been simplified. At least the power pins are not drawn.

Teacher, I have read many materials saying that the adjustment tube (transistor) of linear power supply generates a lot of heat and has low efficiency because it works in the amplification state. Why can LDO only work in the amplification area and not in the saturation state?

maychang

Sirigu1992 posted on 2023-3-9 09:23 Teacher, I have read a lot of information saying that the adjustment tube (transistor) of the linear power supply generates a lot of heat and has low efficiency because it works in the amplification state. Why is LDO only...

[Why LDO can only work in the amplification area and not in the saturation state]

When working in saturation state, the voltage gain of the transistor or op amp will drop to a very low level (the input changes but the output remains basically unchanged), and there will be no negative feedback, so the output cannot be adjusted.

西里古1992

maychang posted on 2023-3-9 09:27 [Why can LDO only work in the amplification area and not in the saturation state] When working in the saturation state, the voltage gain of the transistor or op amp will...

Yes, yes, this question is stupid. I still don’t understand the several working states of the transistor and why the linear power supply can adjust the output.

西里古1992

maychang posted on 2023-3-8 18:31 [ What is the function of R1 in the upper left corner of the picture? ] R1 provides current for the voltage regulator VDz. Without this current, the voltage across the voltage regulator is zero.

Because, without this resistor R1 connected, due to the fact that the linear op amp is disconnected, the voltage regulator tube has no voltage and is broken.

maychang

Sirigu1992 posted on 2023-3-9 10:15 Is it because, without this resistor R1 connected, given the reason that the linear op amp is disconnected, the voltage regulator tube has no voltage and is broken?

If there is no resistor R1 (open circuit), there will be no current in the Zener diode (the bias current at the input of the op amp is extremely small and can be ignored), and the voltage across the Zener diode will be zero.

In order for the voltage across the Zener tube to be the rated value, current must flow through the Zener tube. This current value is specified in the manual. If the current through the Zener tube changes and deviates from the value specified in the manual, the voltage across the Zener tube will also change, but the change is relatively small.

maychang

Sirigu 1992 posted on 2023-3-9 10:11 Yes, yes, this question is stupid, but I still don’t understand the various working states of the triode and the reason why the linear power supply can adjust the output.

Note: Even for a switching power supply, its feedback loop is approximately linear. The switch tube works in saturation and shutdown state, but there must be a filter circuit to filter the current pulse into a DC with very small ripple.

西里古1992

maychang posted on 2023-3-9 10:54 If there is no resistor R1 (open circuit), there will be no current in the Zener diode (the bias current at the input of the op amp is extremely small and can be ignored), and the voltage across the Zener diode is zero. ...

OK, I understand. So the value of resistor R1 is determined by the operating current in the specification of the voltage regulator. Teacher, the information here introduces the advantages of linear power supplies, such as fast response speed, low noise and small output ripple. For the time being, I understand that fast response speed means that there is no need for a control IC to adjust the duty cycle like a switching power supply. It can be adjusted by the amplification characteristics of a transistor, a variable resistor. How should the other two be understood specifically? Why is there low noise and small ripple?

maychang

Sirigu1992 posted on 2023-3-9 11:33 OK, I understand, so the resistance value of resistor R1 is determined by the working current in the specification of the voltage regulator. That teacher, the information here introduces the advantages of linear power supply...

[The information here introduces the advantages of linear power supplies, such as fast response speed, low noise and small output ripple, etc.]

These three points are correct.

There must be a low-pass filter in the switching power supply. Since the feedback loop includes this low-pass filter link, its response speed cannot be very high. At least, the response time cannot be shorter than one cycle of the switch tube saturation shutdown. In fact, the response time is much longer than the PWM cycle, at least dozens of PWM cycles.

The noise is low and the ripple is small because there is no noise and ripple generated by the saturation shutdown of the switching tube in the linear power supply.

maychang

Sirigu1992 posted on 2023-3-9 11:33 OK, I understand, so the resistance value of resistor R1 is determined by the working current in the specification of the voltage regulator. That teacher, the information here introduces the advantages of linear power supply...

Therefore, it is common to see a design where the front stage is a switching power supply and the back stage is connected in series with a linear power supply. The front stage uses a switching power supply to avoid the low efficiency of the linear power supply in reducing a higher voltage to a lower voltage. The back stage uses a linear power supply to avoid the higher noise and ripple of the switching power supply. At this time, the voltage drop at the input and output of the linear power supply can be designed to be quite small, so the efficiency is acceptable.

西里古1992

maychang posted on 2023-3-9 10:56 Note: Even for a switching power supply, its feedback loop is approximately linear. The switch tube works in saturation and shutdown states, but there must be a filter circuit to...

Does this current pulse refer to the current passing through the switch tube?

maychang

Sirigu1992 posted on 2023-3-9 14:47 Does this current pulse refer to? The current passing through the switch tube

What does this current pulse refer to?

It may be the current passing through the switching tube, the current in the energy storage inductor, or the current induced by the switching tube through the transformer.

1nnocent

maychang posted on 2023-3-8 18:31 [ What is the function of R1 in the upper left corner of the picture? ] R1 provides current for the voltage regulator VDz. Without this current, the voltage across the voltage regulator is zero.

Teacher, can this R1 be regarded as a current limiting resistor (compared to directly short-circuiting R1)? Provide the appropriate current to the Zener diode.

maychang

1nnocent posted on 2023-3-9 17:19 Teacher, can this R1 be regarded as a current limiting resistor (compared to directly short-circuiting R1)? Provide the appropriate current for the Zener diode.

This resistor must be large enough to prevent the Zener diode from being damaged by excessive current when the input voltage is the highest, and to ensure that sufficient current flows through the Zener diode when the input voltage is the lowest. In this sense, it can be regarded as a current limiting resistor.