Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Flyback open loop transfer function in LTSpice

Status
Not open for further replies.

steinar96

Full Member level 1
Joined
Oct 9, 2009
Messages
95
Helped
24
Reputation
48
Reaction score
24
Trophy points
1,288
Activity points
1,989
Greetings.

I have a flyback with a pwm controller in LTSpice. I'd like to plot the open loop transfer function in order to aid me in designing the control loop compensation. How is this best performed in LTSpice. Can i somehow sweep the feedback input pin of the controller or do i measure the output stage somehow independently ?

Assistance greatly appreciated.
 

crutschow

Advanced Member level 5
Joined
Feb 22, 2012
Messages
3,790
Helped
903
Reputation
1,804
Reaction score
888
Trophy points
1,393
Location
L.A. USA Zulu -8
Activity points
21,359
Typically you open the loop and do an AC gain/phase (Bode plot) simulation from input to output.
 

steinar96

Full Member level 1
Joined
Oct 9, 2009
Messages
95
Helped
24
Reputation
48
Reaction score
24
Trophy points
1,288
Activity points
1,989
I have opened the loop. But if i sweep the input (the comp pin) i do not get the bode plot i'm expecting. It starts at -140dB
 

LvW

Advanced Member level 5
Joined
May 7, 2008
Messages
5,824
Helped
1,739
Reputation
3,480
Reaction score
1,338
Trophy points
1,393
Location
Germany
Activity points
39,134
I have opened the loop. But if i sweep the input (the comp pin) i do not get the bode plot i'm expecting. It starts at -140dB
Yes, simply opening the loop will destroy the dc bias point. For such a task there are specific simulation procedures that have been discussed - also in this forum - several times.
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
47,873
Helped
14,125
Reputation
28,507
Reaction score
12,808
Trophy points
1,393
Location
Bochum, Germany
Activity points
277,843
I guess you are looking for the continuous time transfer function of the control loop, including modulator and power stage? It can't be easily "measured" in LTSpice, at least not in AC simulation.

It has to be derived from a transient simulation with suitable sine or step stimulation. You would be able to add an auxilary measurement circuit to your simulation setup that filters the response signals and determines it's magnitude and phase, similar to a network analyzer.

Usually, flyback transfer function is put in the calculation according to models derived in literature.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top