The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design Languages >> Verilog-AMS >> verilog-A dependent source NOT converge
https://designers-guide.org/forum/YaBB.pl?num=1411221053

Message started by ywguo on Sep 20^th, 2014, 6:50am

Title: verilog-A dependent source NOT converge
Post by ywguo on Sep 20^th, 2014, 6:50am

Hi Guys,

I wrote one voltage controlled voltage source with verilog-A, which is used in a transient analysis of one buck model. The spectre simulator could not get initial point for transient analysis. The VCVS code is shown below.

Code:

`include "constants.vams"
`include "disciplines.vams"

module Epwm(in, out);
input in;
electrical in;
input out;
electrical out;

parameter real vm = 4.0 from (0:inf);

analog begin
@(initial_step) begin
V(out) <+ (V(in)>0.9) ? 0.9 : (V(in)<0.1) ? 0.1 : V(in)/vm;
end

V(out) <+ (V(in)>0.9) ? 0.9 : (V(in)<0.1) ? 0.1 : V(in)/vm;
end

endmodule

After I replaced it with a spectre VCVS, the buck model works. The spectre VCVS is shown below.

Code:

E0 (d 0 vx 0) vcvs gain=0.25 min=0.1 max=0.9

What's wrong with my verilog-A code? How do I improve it?

Thanks in advance.
Yawei

Title: Re: verilog-A dependent source NOT converge
Post by Ken Kundert on Sep 21^st, 2014, 8:11am

Get rid of the initial step code. You are contributing the same thing to the output twice on the initial step. That creates an impulse at t=0 that is probably the source of you convergence issues.

-Ken

Title: Re: verilog-A dependent source NOT converge
Post by Geoffrey_Coram on Sep 23^rd, 2014, 2:23pm

I'm also a little concerned about continuity at the breakpoints:

V(out) <+ (V(in)>0.9) ? 0.9 : (V(in)<0.1) ? 0.1 : V(in)/vm;

At V(in) = 0.9 plus a bit, the output is 0.9
At V(in) = 0.9 minus a bit, the output is V(in)/vm = 0.9/4 = 0.225

I don't think that's what the VCVS does.

Title: Re: verilog-A dependent source NOT converge
Post by boe on Sep 24^th, 2014, 2:06am

Geoffrey_Coram wrote on Sep 23^rd, 2014, 2:23pm:

You are right, this is not what the VCVS does and such model behavior causes convergence issues.
- B O E