Diodes on ac voltage source

[Noodles141]

I have been given the task of analyzing an analog circuit, we are asked to make a square wave(pulse) voltage source connected in series with a diode and a resistor. We are asked to comment on the voltage waveform across the resistor. I get a very odd waveform and not rectangular. i feel like this is not normal at all and have no clue why it would be doing this. Also we are asked what kind of problems this could cause, not to sure under what circumstances, just a general question. I'm not too sure what problems it could cause in general but under certain circumstances like say a edge triggered system needs a square wave input this kind off waveform would mess with the triggering obviously.

 

[ericgibbs]

hi noodles.

Can you post your circuit diagram and also mark it with the square wave amplitude and period.??

E.

 

[wizard]

I have been given the task of analyzing an analog circuit, we are asked to make a square wave(pulse) voltage source connected in series with a diode and a resistor. We are asked to comment on the voltage waveform across the resistor. I get a very odd waveform and not rectangular.View attachment 75411 i feel like this is not normal at all and have no clue why it would be doing this. Also we are asked what kind of problems this could cause, not to sure under what circumstances, just a general question. I'm not too sure what problems it could cause in general but under certain circumstances like say a edge triggered system needs a square wave input this kind off waveform would mess with the triggering obviously.

Yes as Eric said please post your Circuit diagram. This way I'll be able to Simulate it for you and put the result here.

 

[Noodles141]

as for the amplitude and period, its rather irrelevant to the question plus i haven't had a look at what it is since it isn't needed But here a comparison of the waveform of the resistor compared to the waveform of source

 

[ericgibbs]

hi n141,

Clue, have you tried using higher speed diodes in your simulation.?

E

 

[alec_t]

At such high switching speeds what you are observing is the effects of the diode's parasitic parameters. Can you deduce which part of the waveform distortion is due to which parameter?

 

[Noodles141]

@ericgibbs:No i havent, the question asks us to use that specific diode, would it reduce the parasitic capacitance and inductance and make the waveform look more like a square wave function.
@alec_t:Ok this makes a lot of sense i thought it has something to do with capacitance since it looked a little like a charging curve of a cap, thank you very much, this will help me describe the graph

 

[MikeMl]

Read the 1n914 data sheet. Understand the Ct and Trr Parameters. How do they correlate with what the sim shows...

 

[Noodles141]

if i change those value in the diode model database for the diode in the diagram i can get the circuit to produce and almost perfect square wave function. I understand that Ct is the junction capacitance but i don't know what Trr is. Also it seems increasing Ct will help a great deal in producing a square wave and decreasing Trr will help but not much, Ct seems to be the main factor in my sim.

 

[wizard]

but i don't know what Trr is.

Reverse Recovery Time:
If a diode is conducting in a forward condition and immediately switched to a reverse condition, the diode will conduct in a reverse condition for a shot time as the forward voltage bleeds off. The current through the diode will be fairly large in a reverse direction during this small recovery time.

After the carriers have been flushed and the diode is acting as a normal blocking device in the reversed condition, the current flow should drop to leakage levels.

This is just a generic description of reverse recovery time.

 

[crutschow]

A Schottky diode has essentially zero reverse recovery time so works well as a high speed rectifier.