capacitorltspicespice
I have a very simple circuit that includes a capacitor meant to provide a few seconds of battery-like backup to a few LEDs when power is removed. In real life, it does exactly that. However, when I simulate it in LTSPICE it seems that after the input voltage is removed the capacitor doesn't "pick up" to power the circuit. Being rather new to LTSPICE, is there something about how to simulate it that I'm missing?
The circuit, as drawn, does not work.
Furthermore, even if it did work, it is doing so by horribly abusing the transistors base-emitter junction diode, which can potentially damage the transistor.
It's worth noting that it is possible to make the circuit work. As far as I can tell, the person drawing the diagram simply forgot a diode:
The pull-up resistor also needs to be somewhat smaller, in order to turn Q1 on hard enough that the generated current waveform is appropriately symmetric. It's 10K in the above image, but 1K works much better.
According to my experiences, all simulator are just simulator, they rely on how precise your models are, and how precise your circuits are, such as if you take all the parasitic factors into consideration. This is particularly true for power supply designs.
In my opinion, LT's demo design (not just LTSpice demo circuit, i mean demos with demo board) usually have high quality. So i recommend you to reference their demo boards design. I think them are more reliable than the LTSpice demo circuit.
In your posted .asc file, you'd better go to the similar demo board: DC1961A - LT8309ES5/LT3748EMS Isolated Demo Board | 36V ≤ VIN ≤ 72V; VOUT = 12V @ 5A.
Update:
I don't mean simulator has no use, they are very useful indeed, they can help you to verify your ideas before throwing so much money. But if you want to find a reliable reference demo, demo board may be more reliable.
Best Answer
You might think that setting your PWL to zero makes it open circuit. It doesn't. It's generating a low impedance zero, that discharges the capacitor.
If you want to simulate removing it, then you need a switch in series with it that opens at some time, or a variable resistor that you set to a very large value.
Series diodes are often used in real life so that a collapsed main power supply doesn't drag down its backup supply. You could do the same here.