Understanding SMPS and Its Components: Key Switching, Inductors, and More

[NexusN]

Hello everyone,
I am here again to ask something, though maybe silly to be raised,
to obtain a clearer picture of why exactly we are implementing the SMPS in the way we are doing.

Let me list them to save your time in reading:
Don't bother to answer them all, it would be my honor to have you sharing some ideas with me regarding one to two points.

1. What is the key switching components in a SMPS?
In my understanding it is the semiconductor switch adopted in the buck or boost topology, is it true?

2. I seem to understand the reason for a high frequency on/off, to reduce the size of the transformer and also the capacitors needed as a temporary energy storage, but why do we need a inductor? What exactly we would like to obtain from the inductor?

3. It looks we can implement a simple step down transformer together with the high frequency switch to provide the voltage convert, what is wrong with this configuration and how topologies like buck and boost solve it?

4. What roles exactly do buck or boost play in a SMPS?
I know they are used to generate an output voltage as desired by controlling the duty cycle, but I saw from a passage in Wiki(The Power Factor), buck and boost are some sorts of Power Factor Correction methods, this struggled me a lot.
Buck is a voltage converter? Or it is actually just the PFC part in a SMPS.

5. For a Buck topology, working in CCM, the Voltage relationship between Vin and Vout is simply given by D=Vo/Vin, where D(Duty-cycle) is the on time divided by period of the cycle.
However, on further reading, the D is not set at will, in which we need to take care of the volt-second law, in other words, the duration of On time and Off time are somehow fixed or we will get progressive current escalation to infinity or depletion to zero after some time.
Then how exactly we manipulate the circuit to get the desired values of D and thus the voltage gain?
Is this done by dealing with the values of the capacitor?
Thank you.

I understand that I have these questions probably because I missed out some critical ideas about SMPS designs, please let me know.
I will continue to update this post and may answer myself in case I find a possible solution. Please correct me if I am wrong in my answer.

Thank you so much for your kind attention.

 

[Zryn]

This looks a lot like a research assignment that could be at least partly answered from reading textbooks.

1. What is the key switching components in a SMPS?
In my understanding it is the semiconductor switch adopted in the buck or boost topology, is it true?

Semiconductor is a broad term, there's really only two (maybe three?) main ones, can you name them? A semiconductor alone is susceptible to problems that can be solved with the use of resistor and/or capacitors. Look up the term snubber.

2. I seem to understand the reason for a high frequency on/off, to reduce the size of the transformer and also the capacitors needed as a temporary energy storage, but why do we need a inductor? What exactly we would like to obtain from the inductor?

The high frequency is a trade off between the size of the components and the power losses of the components. What energy does the capacitor store and what energy does the inductor store. Could you leave one or both of them out of the circuit?

Since the inductor is the foundation of the converter, I would recommend some reading to answer "why do we need a inductor".

3. It looks we can implement a simple step down transformer together with the high frequency switch to provide the voltage convert, what is wrong with this configuration and how topologies like buck and boost solve it?

What happens when you pass a DC signal through a transformer? What happens when you pass a DC signal through converter?

4. What roles exactly do buck or boost play in a SMPS?
I know they are used to generate an output voltage as desired by controlling the duty cycle, but I saw from a passage in Wiki(The Power Factor), buck and boost are some sorts of Power Factor Correction methods, this struggled me a lot.
Buck is a voltage converter? Or it is actually just the PFC part in a SMPS.

Basic buck and boost deliver highly efficient power conversion at the desired output voltage level by controlling the duty cycle. You can add circuitry so a SMPS appears as a resistance instead of an impedance. How will this affect the PFC?

5. For a Buck topology, working in CCM, the Voltage relationship between Vin and Vout is simply given by D=Vo/Vin, where D(Duty-cycle) is the on time divided by period of the cycle.
However, on further reading, the D is not set at will, in which we need to take care of the volt-second law, in other words, the duration of On time and Off time are somehow fixed or we will get progressive current escalation to infinity or depletion to zero after some time.
Then how exactly we manipulate the circuit to get the desired values of D and thus the voltage gain?
Is this done by dealing with the values of the capacitor?
Thank you.

The duty cycle for a basic buck converter is fixed since the output voltage is fixed.

The duty cycle is not fixed in a circuit that requires a variable voltage (or current) output, such as a lab power supply. In these cases the converter components are designed to accommodate a range of values, so that the converter will always be in CCM.

In this instance the duty cycle will be monitored and changed automatically by a PID controller.

 

[NexusN]

This looks a lot like a research assignment that could be at least partly answered from reading textbooks.



Semiconductor is a broad term, there's really only two (maybe three?) main ones, can you name them? A semiconductor alone is susceptible to problems that can be solved with the use of resistor and/or capacitors. Look up the term snubber.



The high frequency is a trade off between the size of the components and the power losses of the components. What energy does the capacitor store and what energy does the inductor store. Could you leave one or both of them out of the circuit?

Since the inductor is the foundation of the converter, I would recommend some reading to answer "why do we need a inductor".



What happens when you pass a DC signal through a transformer? What happens when you pass a DC signal through converter?



Basic buck and boost deliver highly efficient power conversion at the desired output voltage level by controlling the duty cycle. You can add circuitry so a SMPS appears as a resistance instead of an impedance. How will this affect the PFC?



The duty cycle for a basic buck converter is fixed since the output voltage is fixed.

The duty cycle is not fixed in a circuit that requires a variable voltage (or current) output, such as a lab power supply. In these cases the converter components are designed to accommodate a range of values, so that the converter will always be in CCM.

In this instance the duty cycle will be monitored and changed automatically by a PID controller.

Thank you very much for your answers, it is so nice of you.
I think I have to spend some time to understand and do a little search for them.
I have been reading some materials and currently a book named "Switching Power Supply A to Z".
It looks my understanding of SMPS is still very little, I will continue reading it for more ideas.