Ok, the second part of my first exam was to add this fully controlled full wave rectifier (single phase) to the buck circuit discussed in the other thread! In the exam, part 1 was the buck circuit and part 2 is this rectifier!

So for this part we were asked to:

a) Draw the schematic for the requested topology (fully controlled full wave rectifier) and evaluate the firing angle that ensures (or assures, not sure) the needed voltage at the buck input.

b) Plot the input waveform at the rectifier. Knowing that the current at the rectifier output is 120% of the buck output current. evaluate the 'Irms' at the rectifier input.

First question, before anything else, is that I don't fully understand if the rectifier is to be placed after or before the buck converter! I think it should be after the buck converter, but I'm not sure!

I tried to assemble this circuit on LTSpice but I getting errors.

 

As the voltage across C1 is DC, albeit with some ripple, I can see no point in a bridge rectifier after that.
However, I have no idea where it would be useful.

 

I definitely vote to put controlled rectifier at the input. So you can use the AC mains voltage and set the firing angle that ensures 80V at the input.

 

I definitely vote to put controlled rectifier at the input. So you can use the AC mains voltage and set the firing angle that ensures 80V at the input.

You mean to place the rectifier between the 80V voltage source and the buck circuit?

The 80V are DC voltage. Why would it need to be rectified? But I think it's the only option... :s I'll change the circuit in LTSpice!

 

No, instead of DC voltage (80V) try using an AC mains voltage 230V for example. And then choose the firing angle in the rectifier is such way so you are able to obtain this 80V DC at the output of a rectifier (input voltage for a buck converter). But I think that some additional filtering will be needed.

 

No, instead of DC voltage (80V) try using an AC mains voltage 230V for example. And then choose the firing angle in the rectifier is such way so you are able to obtain this 80V DC at the output of a rectifier (input voltage for a buck converter). But I think that some additional filtering will be needed.

Ok, but that means to place the rectifier between the AC mains voltage source and the Buck converter, right?

 

I'm not being able to make this circuit work at all. An .op simulation gives 0V and 0A at every point of the circuit! Please help!

 

Ok, but that means to place the rectifier between the AC mains voltage source and the Buck converter, right?

Something like this:

I'm not being able to make this circuit work at all. An .op simulation gives 0V and 0A at every point of the circuit! Please help!

I am not familiar with LTspice but you may read this:
https://www.tu-eshop.com/index.php?route=tublog/blog&id=37

For DC analysis you should set DC offset for SINE.

 

Hello...

This is what I have but as I said, all points in the circuit are with 0V or 0A. Can't figure out the problem!

 

Did you try to set a non-zero DC offset component for SINE as I suggested above?

 

At zero time, the input voltage is zero (op ~ time=0), and also the thyristors are controlled (in my opinion) is not correct. The control period should be 10 msec or 20 msec. I tend to the second meaning.

 

Ok, I did the following to test it out. I splitted the circuit in two, Rectifier and Buck and they worked, apparently, correct. I'll upload here the 3 files I used. Rectifier, Buck and Rectifier + Buck.

I hope the parameters are not a problem. The control pulses are like this in the circuit of this post (ignore the above because I changed nodes names and other stuff):

p1 and p1 are swiching at (0º+alpha) and (120º+alpha)
p3 and p4 are switching at (240º+alpha) and (360º+alpha)

I'm not sure this is correct for a single phase, fully controlled, full wave rectifier, but the output waveform looks rectified!


PS: I'll be back in an hour. Lunch time!

 

I'm not sure this is correct for a single phase

This is not corrent.
For me, the correct situation look like this

 

Did you try to set a non-zero DC offset component for SINE as I suggested above?

I'm sorry I didn't reply to you. Busy work day! No, I have not yet tried! I'll try it if after look to @Jony130 circuit I still can't make it work!

This is not corrent.
For me, the correct situation look like this

@Jony130 you changed switching frequency to 50Hz. Can't the input signal be 50Hz and the switching frequency be 150kHz? Isn't usually the switching frequency quite higher than the input signal frequency?

 

Hi,

Are you really using SCR's to control the rectifiers? You can not turn them off once they are turned on. The current must go to zero.
That means if you have 50Hz input AC then you can only turn on once per half cycle, and it will turn off at the zero crossing of the current.

Also, yes the rectifiers must be at the input. Buck circuits have DC output already.

 

Ok, but about the switching frequency, can't it be the 150kHz from the previous buck exam problem? That was the sequence of the exam problems. This full wave rectifier was supposed to use the values of the previous problem.

 

Ok, but about the switching frequency, can't it be the 150kHz from the previous buck exam problem? That was the sequence of the exam problems. This full wave rectifier was supposed to use the values of the previous problem.

Hi,

Oh sorry, i thought you meant that the SCR switching was 150kHz. Yes the buck can be 150kHz.

So how do you turn on the SCR's ?

 

Hi,

Oh sorry, i thought you meant that the SCR switching was 150kHz. Yes the buck can be 150kHz.

So how do you turn on the SCR's ?

What I mean is that the first problem of my exam was about a Buck converter using 150kHz for the switching frequency!

Then the second problem was about a rectifier that was added to the Buck converter so I thought that both, rectifier and Buck circuits, were using the same switching frequency. I mean the thyristors switching frequency and the Buck switch frequency! Can't I use 150kHz on both? I assumed this because the exam problem told nothing about it!


Edited;
Ok, I have now a few more details about how the problem was stated in the exam.
It says:

Consider now that the Buck is fed with a full wave rectifier connected to the main network system. Consider also that the Buck plays the rule of a strong inductive load to the rectifier!

a) Draw the schematic of the suggested rectifier topology and evaluate the firing angle that ensures 80V at the Buck input!

b) For those conditions, plot the rectifier input current waveform. Consider that the output current of the rectifier is 120% of the current calculated in the previous problem (Inductor/output current of the Buck problem from the other thread) . Evaluate rectifier rms input current value.

c) Consider now that there is a variation in the Buck load resistance and that the inductor current has now periods of time where it is 0A. Keeping all the other circuit conditions, will there be any impact in the output voltage? Would the value of the load resistance be greater or lower than the one from previous problem (18 Ω)? Evaluate that resistance.

d) If there was no means of changing duty cycle and the only parameter you could change was the firing angle, would this needed to be greater or lower to keep the dissipated power at the load of 100W? Justify with calcs!

 

What I mean is that the first problem of my exam was about a Buck converter using 150kHz for the switching frequency!

Then the second problem was about a rectifier that was added to the Buck converter so I thought that both, rectifier and Buck circuits, were using the same switching frequency. I mean the thyristors switching frequency and the Buck switch frequency! Can't I use 150kHz on both? I assumed this because the exam problem told nothing about it!

Hello again,

Well if they really are SCR's then you can not switch them at 150kHz for several reasons, the most important is that they DO NOT TURN OFF when you say they should, they turn off when the current through them goes to zero. That means if you turn one on at 90 degrees and try to turn it off at 95 degrees, it will still stay on until the current goes though zero which for resistive load would be 180 degrees. So although you only want a 5 degree wide pulse, you get a 90 degree wide pulse.

This is not usually as much of a problem as it seems however, because you just figure out the conduction angle you need and then WAIT until you get that close to 180 degrees (or 360 degrees). So it you need 10 degrees, you wait until 170 degrees, turn on, then turn the drive signal off, then it turns off by itself at 180 degrees so you get a 10 degree wide pulse. The other thing though is that you only get the tail end of a sine wave not a square wave so you have to take the shape of the sine wave into account as well as the current flow angle.

How did you analyze the four SCR circuit so far? Is that here too? It would be good if you mastered that first.

 

Well if they really are SCR's then you can not switch them at 150kHz for several reasons, the most important is that they DO NOT TURN OFF when you say they should, they turn off when the current through them goes to zero. That means if you turn one on at 90 degrees and try to turn it off at 95 degrees, it will still stay on until the current goes though zero which for resistive load would be 180 degrees. So although you only want a 5 degree wide pulse, you get a 90 degree wide pulse.

Hum, ok. I think I'm wasting time here in what concerns to my main goal that is to know how to answer the questions. To build this circuit in LTSpice is, however, secondary! I think I'll leave this circuit in LTSpice rest for a while, while I focus answering the questions! I'm going to read Ned Mohans book and my teacher's notes and see If I can answer them!

How did you analyze the four SCR circuit so far? Is that here too? It would be good if you mastered that first.

I think I didn't understood the question!