[SOLVED] voltage sesing circuit issue

[andreas157]

hi all. im trying to simulate this voltage sensing circuit for the output voltage of a boost converter. its an LM358 + an IL300 linear optocoupler.

generally speaking the circuit seems to work fine in all the range of my output voltage [0-350V]
the only issue i have is that for a voltage >~ 280V i got a noticeable voltage ripple.

this is what i got for Vin = 200V Vsense = 2V(red) (fast response, going quickly to steady state)

and this is what i've got for Vin = 350V Vsense = 3.5V (red) (seems like vsense its not going to steady state, causing a lot of ripple)

any ideas of how to have faster response and avoid any errors caused by this voltage ripple? thanks :-D

 

[tgootee]

Try putting about 1 pF in parallel with R2. You can adjust the value for suitability. You almost always need to low-pass filter an op amp input, so that the input slew rate can't exceed the amp's capability. In simulations, you should also always adjust the input source's rise time to be somewhat realistic, or at least make it non-zero. (I would try different op amps, too.)

In a real circuit, you would also need a small X7R ceramic capacitor connected very close to pin 8 and to ground, and an electrolytic cap in parallel with it. You might also need at least a small resistance in the feedback path, close to the negative input pin, to damp any stray L or C. R3 should probably also be mounted close to the negative input pin. And, of course, don't make any "enclosed loop area" with your input and input ground conductors. Otherwise it might make a very good antenna, especially with the large-value resistance in the loop.

 

[andreas157]

ahhh thanks for that. tried an exponential source and everything is fine :grin: i'll take into consideration the other suggestions during the design. thanks again