模擬 DC-DC boost

youshang

Q:想要確認POP 模擬後，要怎麼知道跑出來結果是合理的？
POP模擬一直取樣到 tran 模擬的最初幾個 cycle
那時候只有 clk 建立， boost 系統的其他值都還沒起來
可能是因為這個原因，AC 模擬的 gain 一直是 -300dB





照直覺我想要避開前面不穩定的區間
所以我調整 POP 模擬的設定值，先 delay 500個 cycle 在取樣
但 POP 模擬結果一直都一樣，都是如上圖處於 initial 的結果 ( vout = 0 )


AC 模擬一直沒辦法建立

A:
1.按F8，點選下圖紅色框框的部分，將Use snapshot from previous transient analysis的選項打勾。

2.接著請先執行一次Transient analysis。

3.再請直接執行POP analysis，將可以得到穩態的Vo輸出。


4.再接著執行AC analysis。


Q2:
現在可以先跑一次 tran 存 snapshot
然後再跑 AC
這樣模擬 AC 沒問題
但是這樣就沒辦法針對某元件設變數
然後掃描變數值來看其對 AC 的影響( ex.  R1的電阻值={rcc}，然後掃變數 rcc )
看起來跑 multi-step 不能吃到 tran 存的snapshot ？
請問這個問題有辦法解決嗎？



為什麼這個電路(附件)直接執行POP模擬會抓不到穩態請問要如何解決?
為什麼點選了Use snapshot from previous transient analysis能使波形順利跑出來?


A2:

SIMPLIS原廠工程師針對問題回覆如下:

The schematic contains two Piecewise Linear (PWL) sources.  If there is no snapshot available from a previous transient simulation, the PWL sources will be held at their constant values defined at t=0.  So the input voltage source V9 will be held at 0V and the load current I1 will be held at 0A.  Therefore, the POP analysis converges to having an output voltage Vout of 0V.

But if you have run a transient simulation first, and without modifying the schematic, and you have selected to "Use snapshot from the previous transient," then the PWL sources will be held constant at its last value of the transient simulation.  Hence, V9 will be 5V and I1 will be 1A during the POP analysis.  In addition, all capacitor voltages and inductor currents at the end of the last transient simulation will be used as the initial conditions for the corresponding capacitor and inductor to start the POP analysis.

This strange behavior of the PWL sources are due to some old behavior of the simulator.

A better way is to assign good initial conditions to the capacitors and inductor currents instead of leaving them uninitialized.  For example, the output capacitor should be initialized to about 10.8V and the input inductor should be initialized to about 2.5A.  You can also accomplish this by clicking Simulator | Initial Conditions | Back-annotate after the finish of the transient analysis.  Having good initial conditions are helpful for the simulation of switching regulators.  The POP analysis usually converges quite rapidly if there are good initial conditions.

If you really want to use a soft-start approach to bring the switching regulator up to the correct operating condition, you should take the following two steps.

A) Replace the PWL Sources by periodic pulse sources with very long pulsewidths and a very long periods so that, for the purpose of the POP analysis and the transient analysis, V9 will be rising from 0V to 5V for a short period of time and after that it becomes 5V during the POP analysis and the transient analysis.  You should also replace the PWL load current source with a pulse current source with very long pulsewidth and long period.

B)  Increase the number of "Cycles Before Launching POP" to make sure the circuit will converge close enough to the steady-state operation before allowing the POP analysis to make the prediction/correction.  In your case, I have raised this option to 1500 cycles.

I have attached a schematic to show the steps taken for A) and B).