Electronics > Projects, Designs, and Technical Stuff

Resistance of MOSFET vs IGBT/SCR ?

<< < (3/4) > >>

ricko_uk:
Thank you all! :)
I will address each one below.

Duak,
Sometimes the inductor is a transformer with a large number of secondary turns like an ignition, to create a spark in a spark plug.
1) Am I correct in thinking that the energy stored in the inductor is dissipated into the spark of the spark plug?
2) if not, an antiparallel diode across the capacitor as shown in the new schematic attached here, would allow the energy to dissipate across the inductor's resistive part. Is that correct?
3) what are the pros and cons of placing an anti-parallel diode across the capacitor vs placing it across the inductor (like in classic motors/solenoids switching)?

Ref the circuit loop being important, what do you mean? I assume the smaller the loop the lower the parassitic inductance? Is that correct?

Ref the peak currents, it varies on the application and the load characteristics. Peak currents anywhere from couple of amps to 40-50A.

Zero999,
that is just the capacitor rating. The peak voltage is 300V.

David,
thank you for the additional comments

Thank you all :)

schmitt trigger:
What Zero was asking, and certainly I also would like to see, is the size of a 1 Farad, 2000 volt rated cap.

1 Farad is supercap territory, at very low voltage.
And 2 Kv is glass cap territory, at very low volumetric capacity.

How do you obtain both ratings simultaneously?

ricko_uk:
sorry everybody, in the schematic it is a 1mF not 1F capacitor.

duak:
G'day Ricko,

Are you working on a Ignition system then?

1) Am I correct in thinking that the energy stored in the inductor is dissipated into the spark of the spark plug?

Sure, but 300 V is not enough to cause a spark plug to work unless the air gap is exceedingly small.  Supposing the inductor is a transformer, like an ignition coil, then a good bit of the energy is dissipated in various resistances and other loss mechanisms.  It also gets dissipated in the resistances of the various other components in the circuit.  Note that sparks and arcs have quite low voltage drops because of their relatively low resistances.

2) if not, an antiparallel diode across the capacitor as shown in the new schematic attached here, would allow the energy to dissipate across the inductor's resistive part. Is that correct?

Yes, but two things. Q1 will have to stay conducting to allow the current to continue to flow unti it reaches zero.  If you shut the switch off while current is flowing, L1 will generate a voltage that will try to maintain that current. In this case positive relative to mains-gnd and could be enough to damage Q1.  Secondly, inductive circuits have a time constant of L/R where L is the inductance and R is the series resistance.  Oddly enough, reducing the series resistance causes the time constant to increase so that it takes longer to discharge the inductor.  A diode has a low resistance, so while it prevents C1 from charging in reverse, it extends the discharge time, something that may be good thing.

3) what are the pros and cons of placing an anti-parallel diode across the capacitor vs placing it across the inductor (like in classic motors/solenoids switching)?
Putting the diode across the inductor creates a circuit that doesn't require Q1 to continue to conduct and also protects it from being subjected to a higher or even damaging voltage.

Ref the circuit loop being important, what do you mean? I assume the smaller the loop the lower the parassitic inductance? Is that correct?

Yes, the inductance of a circuit is a function of the size of the area enclosed by the circuit's loop.  ie., A larger area leads to more inductance. 

ricko_uk:
Thank you again Duak,

You are right about "Oddly enough, reducing the series resistance causes the time constant to increase so that it takes longer to discharge the inductor."! I didn't think of that. I assume that is the reason why they make spark plugs or spark plug cables with resistor inside...? I thought it was only to reduce the electrical noise in the car's electrical system...

But then if I add R1 in the circuit as shown in the new schematic attached, then the same resistor increases the time constant of the capacitor and decreases the time constant of the inductor... How does it work? I assume there is a "minimum" point of balance?

If so how do I find the correct value? With calculation and experimentally (as calculations might not take in consideration all parassitic effects)?

Thank you again :)

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod