Simulate a step-down chopper circuit using LTSpice

In summary, the third point is that the -15V voltage supply for the LM741 is connected to +15V. This causes the LM741 to not trip, and incorrect voltage supplies can lead to incorrect results.
  • #1
Fatima Hasan
319
14
Homework Statement
Written below.
Relevant Equations
-
Simulate the circuit shown below sung LTspice.I tried to simulate it , but I didn't get the corrected result. Here's is my attempt:

q1.png

q1.png

The result (voltage in the collector of the transistor) :

q1.png


Could someone let me know where is my mistake?
 
Last edited:
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  • #2
This looks like a continuation of your previous thread using MultiSim:

https://www.physicsforums.com/threa...ep-down-chopper-using-multisim-matlab.988525/

First of all, this is not a "step-down chopper circuit", so I may end up fixing your thread title. You also never answered my question in your other thread about what function the opamp is serving in this circuit. It is not being used as an opamp, it is being used as a ______________. Please try to answer that question.

On your LTSpice circuit, there are a number of errors. First, the -15V voltage supply for the LM741 is connected to +15V. Please reverse that voltage supply so that the LM741 is running between +/-15V power supplies.

Second, you have 0V going into the "-" input of the 741, but the triangle wave generator going into the "+" input is ramping 0V to 5V, so the 741 will not trip. You need to offset the "-" input to somewhere in the middle of 0V to 5V if you want to see a tripping behavior to drive the output transistor.

Finally, it is not good practice to draw the wire to the "-" input as you have done. When the wire to the connection point runs along the body of the IC symbol like that, you can't be sure that the wire is reliably connected to the "-" input point of the opamp. Instead, draw the wire coming at a right angle out of the opamp triangle symbol (to the left) for a little ways, before you put in a right-angle bend in the wire to go down to the voltage supplies below.

Can you make those fixes and then show us what the quiescent voltages are for all of the nodes in the circuit before you turn on the triangle ramp generator? Thank you.
 
  • #3
berkeman said:
First, the -15V voltage supply for the LM741 is connected to +15V. Please reverse that voltage supply so that the LM741 is running between +/-15V power supplies.
Actually, looking closer I see that you have listed the "voltage" of that battery as "-15V", so it might actually be okay. It is very misleading to use a symbol like that battery symbol and set it at a negative voltage, IMO. It would be better to turn the battery upside down with a +15V output, in order to make your -15V supply. Maybe it's just an LTSpice style issue...

1588867203211.png
 
  • #4
berkeman said:
Maybe it's just an LTSpice style issue
No, you can draw, connect, and define these as in other schematic capture SW. .
 
  • #5
DaveE said:
No, you can draw, connect, and define these as in other schematic capture SW. .
Yeah, but a battery symbol means something else to me, versus a generic round symbol for a voltage source. Once I see a battery symbol, my mind automatically looks for the longer line side and associates it with "+". Then changing the "value" of that symbol to a negative voltage gives me vertigo.

I don't think I've ever used a battery symbol in a Spice simulation, but I could be wrong. Hopefully LTSpice has traditional voltage source symbols available, like I use in Micro-Cap Spice software:

1588884079775.png
 
  • #6
berkeman said:
LTSpice has traditional voltage source symbols available
Yes, I think it does (I haven't used it in several years, though). Typically the battery symbol is a only a DC source, and the circular symbols are more complicated waveforms (or just DC, if you want). It's just a difference in style.
 
  • #7
berkeman said:
First of all, this is not a "step-down chopper circuit", so I may end up fixing your thread title.

BTW @Fatima Hasan -- After looking at the circuit again (assuming appropriate fixes to the schematic), I may be able to see how one could call this a "chopper step-down" circuit, but a more accurate description would be a "PWM" analog voltage generation circuit, but only if there is a diode and a capacitor added in the right place. :wink:

Have you had a chance to work on this project any more?
 
  • #8
berkeman said:
BTW @Fatima Hasan -- After looking at the circuit again (assuming appropriate fixes to the schematic), I may be able to see how one could call this a "chopper step-down" circuit, but a more accurate description would be a "PWM" analog voltage generation circuit, but only if there is a diode and a capacitor added in the right place. :wink:

Have you had a chance to work on this project any more?
I tried to make the fixes you mentioned, but could you please explain further how to fix the third point ?
3.png

(V3 =15 V, V2=15V )
The result :
3.png
 
  • #9
The circuit looks much better, thank you. :smile:

The key here is that the opamp is being used as a comparator, to compare the two input voltages from the voltage divider on the "+" input and the sawtooth waveform on the "-" input. As you move the reference voltage that you put into the "+" input by varying those two voltage sources, you will trip the comparator earlier or later on the rising ramp of the sawtooth waveform. That will give you a varying duty cycle on the "pulse width modulated" (PWM) output of the comparator.

Can you clarify what the numbers shown next to the V1 ramping supply symbol mean? I'm not sure they mean the same thing as in my Micro-Cap Spice program. Do they mean you are using a sawtooth ramp that goes from 2.5V to 5V? I think you used to have the ramp going between 0V and 5V -- either is fine, but you need to keep in mind that you will adjust the comparator input voltage to different places along that sawtooth input ramp to get different output duty cycles out of the comparator into the output transistor stage.
1589065214843.png
 
  • #10
berkeman said:
The circuit looks much better, thank you. :smile:

The key here is that the opamp is being used as a comparator, to compare the two input voltages from the voltage divider on the "+" input and the sawtooth waveform on the "-" input. As you move the reference voltage that you put into the "+" input by varying those two voltage sources, you will trip the comparator earlier or later on the rising ramp of the sawtooth waveform. That will give you a varying duty cycle on the "pulse width modulated" (PWM) output of the comparator.

Can you clarify what the numbers shown next to the V1 ramping supply symbol mean? I'm not sure they mean the same thing as in my Micro-Cap Spice program. Do they mean you are using a sawtooth ramp that goes from 2.5V to 5V? I think you used to have the ramp going between 0V and 5V -- either is fine, but you need to keep in mind that you will adjust the comparator input voltage to different places along that sawtooth input ramp to get different output duty cycles out of the comparator into the output transistor stage.View attachment 262459
DC offset : 2.5
Amplitude: 5
Carrier frequency = 500 Hz
Modulation index = 0.2
 
  • #11
Fatima Hasan said:
DC offset : 2.5
Amplitude: 5
Carrier frequency = 500 Hz
Modulation index = 0.2
Interesting. What's a Modulation Index in this context?

Anyway, did you understand my comments about how the rising sawtooth waveform defines a pulsewidth for the output of the comparator circuit? I think that's what the homework problem is trying to guide you toward.

So how can you optimize setting the input comparator voltage and your sawtooth ramp voltage input to give you the ability to set this circuit between 0% and 100% PWM output? And can you figure out where to put a diode and smoothing capacitor on the tranistor output circucit to let you set the output voltage with your PWM control circuit? :wink:

(sorry that this is such a hard learning exercise with so few resources available to you during the pandemic. Please just keep trying your best to answer our hints and questions to guide you to a better understanding of circuits like these. Hang in there! :smile: )
 
  • #12
berkeman said:
Anyway, did you understand my comments about how the rising sawtooth waveform defines a pulsewidth for the output of the comparator circuit? I think that's what the homework problem is trying to guide you toward.
Just to confirm , what I understood that to change the pulse width for the output of the comparator, I have to change the values of the two voltage sources which are connected to the '+' input of the comparator in order to change the pulse width. When the modulation index (for example = 0.2 ) , we have to divide 0.2 by the amplitude of the sinewave which is 5V to get the value of the 2 voltage sources which are connected to the '+' input of the comparator , since the modulation index is the ratio of the amplitude of the sine wave to the value of the '+] input of the comparator. Am I right ?
 
  • #13
Yes, I think you are getting closer to understanding how to work with this circuit.

I still don't understand the definition of "modulation index" in this context.

I would just use one voltage source for the "+" input to the comparator circuit. You can ramp that in your Spice simulation to show how the PWM output.

Is the overall goal to show the transfer function between the "+" input to the comparator to a lowered PWM output voltage? If so, you still need to add a rectifying diode and smoothing capacitor to the output circuit.

Do you have any instructor or TA resources available to answer these types of questions? :smile:
 
  • #14
berkeman said:
I still don't understand the definition of "modulation index" in this context.
from http://electriciantraining.tpub.com/14184/css/Modulation-Index-101.htm :
qqqq.png


berkeman said:
Is the overall goal to show the transfer function between the "+" input to the comparator to a lowered PWM output voltage? If so, you still need to add a rectifying diode and smoothing capacitor to the output circuit.

Do you have any instructor or TA resources available to answer these types of questions? :smile:
No it's not required to find the transfer function.
I didn't find any similar circuit to this, but I have the values of the load voltage and current when the modulation index is varying from 20% to 100%.
 
  • #15
berkeman said:
Yeah, but a battery symbol means something else to me, versus a generic round symbol for a voltage source. Once I see a battery symbol, my mind automatically looks for the longer line side and associates it with "+". Then changing the "value" of that symbol to a negative voltage gives me vertigo.

I don't think I've ever used a battery symbol in a Spice simulation, but I could be wrong. Hopefully LTSpice has traditional voltage source symbols available, like I use in Micro-Cap Spice software:

View attachment 262294
I tried to simulate using Spice, but how to determine I don't know how to determine the values of the ac voltage source, how to set the frequency , modulation index , . . .
Here's my attempt:
4.png

4.png

Could you please explain? I didn't use this software before.
 
  • #16
Sorry, you now show a sine V5 AC source into one of the opamp inputs, but I thought this was supposed to be a PWM conversion circuit. You would need to use a sawtooth input into the comparator versus some DC comparison level in order to generate a PWM waveform.

Also, it looks like there is a mistake grounding the base of the transistor? That needs to be deleted before the output transistor stage can toggle.
 
  • #17
berkeman said:
ou would need to use a sawtooth input
I didn't find a sawtooth waveform block in Spice , but I tried to get it using AC voltage source:
111000000.png


I sat the initial value of the voltage to 0,the pulsed voltage to 5 V and the pulse width to 0.2 ( 20%= 0.2 ).
 
  • #18
Fatima Hasan said:
I sat the initial value of the voltage to 0,the pulsed voltage to 5 V and the pulse width to 0.2 ( 20%= 0.2 ).
How to determine the other values ?
 
  • #19
Fatima Hasan said:
I didn't find a sawtooth waveform block in Spice , but I tried to get it using AC voltage source:
Maybe set the TR (rise time) to something long to get a ramp? Try that and use an oscilloscope probe or plot to see what you get...
 
  • #20
Normally in spice you would make a ramp by using the piece-wise linear definition of a voltage source. This feature allows you to build an arbitrary waveform by specifying points (voltage at a specific time) and the source will connect them with straight lines, these can be repeated at a defined frequency.
 
  • #21
berkeman said:
Maybe set the TR (rise time) to something long to get a ramp? Try that and use an oscilloscope probe or plot to see what you get...
When TR is 10 sec.
qbb.png

The voltage waveform across this node is :
qbb.png

Are the other parameters correct ? or is there any mistake in the connections ?
 
  • #22
DaveE said:
Normally in spice you would make a ramp by using the piece-wise linear definition of a voltage source. This feature allows you to build an arbitrary waveform by specifying points (voltage at a specific time) and the source will connect them with straight lines, these can be repeated at a defined frequency.

The voltage parameters:
qbb.png

Here's what I got :
qbb.png
 

1. What is a step-down chopper circuit?

A step-down chopper circuit is a type of power electronic circuit that converts a high voltage input to a lower voltage output. It is also known as a buck converter and is commonly used in applications such as DC-DC power supplies and motor speed control.

2. How does a step-down chopper circuit work?

A step-down chopper circuit works by using a switch (such as a MOSFET or transistor) to rapidly turn the input voltage on and off, creating a series of pulses. These pulses are then smoothed and filtered to produce a lower average voltage at the output.

3. What is LTSpice and how is it used to simulate a step-down chopper circuit?

LTSpice is a simulation software used for electronic circuit design and analysis. It allows users to simulate the behavior of a circuit before building it in real life. To simulate a step-down chopper circuit using LTSpice, the user can input the circuit components, specify the switching frequency and duty cycle, and run the simulation to observe the output voltage and current waveforms.

4. What are the advantages of using a step-down chopper circuit?

Some advantages of using a step-down chopper circuit include higher efficiency, smaller size, and lower cost compared to other voltage conversion methods. It also allows for precise control of the output voltage, making it suitable for a wide range of applications.

5. What are some common challenges when simulating a step-down chopper circuit using LTSpice?

One common challenge when simulating a step-down chopper circuit using LTSpice is selecting the right components and parameters to accurately model the behavior of the circuit. Another challenge is ensuring that the simulation results match the expected theoretical calculations, as there may be variations due to component tolerances or non-ideal behavior. Additionally, understanding and interpreting the simulation results may require some knowledge of circuit analysis and LTSpice functions.

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