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verilog-A dependent source NOT converge (Read 2327 times)
ywguo
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verilog-A dependent source NOT converge
Sep 20th, 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
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Ken Kundert
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Re: verilog-A dependent source NOT converge
Reply #1 - Sep 21st, 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
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Geoffrey_Coram
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Re: verilog-A dependent source NOT converge
Reply #2 - Sep 23rd, 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.
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boe
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Re: verilog-A dependent source NOT converge
Reply #3 - Sep 24th, 2014, 2:06am
 
Geoffrey_Coram wrote on Sep 23rd, 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.
You are right, this is not what the VCVS does and such model behavior causes convergence issues.
- B O E
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