Help with Flyback Control

Thread Starter

samus aran

Joined Nov 22, 2020
9
I am doing a simulation of a flyback circuit using LTspice (I am just new in LTspice). I created the simulation circuit based on what I have learned on the internet.
My main problem is how can I translate the current mode control diagram shown on the attached image to an actual circuit model. I have drafted the initial design (attached file), but I got stuck in the controls. Any help would be appreciated. Usually, I will just use a PWM controller for that, but I want to try to use an actual circuit for my model.
 

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Thread Starter

samus aran

Joined Nov 22, 2020
9
What I meant was this came from an original circuit with a PWM controller IC, but I want to learn how to design the controller on component level using LTspice.
 

Thread Starter

samus aran

Joined Nov 22, 2020
9
Each of the blocks in the schematic is a recognizable component. How much deeper were you imagining the need to go?
Yes, I tried to put the actual components but it did not work. Maybe there is something wrong with my model or the spice directives. That is why I am asking for help. See 1st image for what I did, and the 2nd image for the M1 gate voltage.
 

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Papabravo

Joined Feb 24, 2006
16,195
I notice two things right off the bat:
  1. the "A" devices A1 & A2 have no visible parameters set. Was it your intention to use the default values?
  2. You are trying to debug the whole thing at once
What you need to do is divide and conquer, especially if you are new to LTspice. If it were me I would first establish the operation of the SR flip-flop and the switch driving the primary.
 

ronsimpson

Joined Oct 7, 2019
1,246
It is very hard to get a UC3842 to work (making it from nothing).
Here I used a LT1242 which is a UC3842.
Because this circuit takes a long time to start up I added V2 and D7 to jump start it. ".IC" command should work but not today LOL.
You will need to hit "ESC" key a couple of times with the mouse over the schematic to get it to simulate faster.
1606366922539.png
 

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Thread Starter

samus aran

Joined Nov 22, 2020
9
It is very hard to get a UC3842 to work (making it from nothing).
Here I used a LT1242 which is a UC3842.
Because this circuit takes a long time to start up I added V2 and D7 to jump start it. ".IC" command should work but not today LOL.
You will need to hit "ESC" key a couple of times with the mouse over the schematic to get it to simulate faster.
View attachment 223406
Well, thank you for that. But I thought that I can model the current mode control with just basic components. So, does that mean that the current mode control is really hard to model in LTspice?
 

ronsimpson

Joined Oct 7, 2019
1,246
I tried to load your file but the RS Flip Flop did not come through. I could not check how A1 and A2 are working.
The 3842 current mode controller is a little hard to make. First example is the LT1017 you are using. It is about 100x too slow compared to the real part.
 

Papabravo

Joined Feb 24, 2006
16,195
Well, thank you for that. But I thought that I can model the current mode control with just basic components. So, does that mean that the current mode control is really hard to model in LTspice?
Modeling anything with an oscillator can be challenging. In a real circuit there are things that are difficult to model with "ideal" components. Simulation is a combination of art and science and it takes time and effort to develop the artistic side.
 
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