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| Resistance of MOSFET vs IGBT/SCR ? |
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| ricko_uk:
Hi, with reference to the attached schematic, the capacitor is charged to high voltage and then discharged into the coil. As switching device I can use a MOSFET, an IGBT or an SCR. I need to keep the rate of change in current across the coil as high/fast as possible. If I use a mosfet it obviously has a internal resistance (albeit very low) which would decrease the rate of change of the current through the coil. Using an IGBT or a SCR has a voltage drop across it but not a resistance as such. Two questions: 1) does the IGBT/SCR have some equivalent characteristic to the mosfet internat on resistance? 2) does the IGBT/SCR cause the rate of change of the current to decrease like the on resistance of a mosfet would (compare to an ideal "mechanical-like" switch)? UPDATE: there is a type in the schematic. The cap is mF not F! :) Many thanks :) |
| schmitt trigger:
One thing is certain, if you employ an SCR, you won't be able to turn it off with the circuit as shown. |
| exe:
--- Quote from: ricko_uk on March 27, 2020, 12:58:09 pm ---1) does the IGBT/SCR have some equivalent characteristic to the mosfet internat on resistance? --- End quote --- Well, what I do if I need to compare mosfet and bjt: I calculate dropout at the current of interest. Dropout is a figure of merit. If you want to get "resistance" for bjt, then it's Vdropout/Icollector (or Iemmiter). Of course for bjt it varies a lot with current, but I usually calculate it for the worst scenario. EDIT: or just compare them by dissipated power, that's even better. |
| Zero999:
To summarise the above. A MOSFET behaves like a resistor when turned on, with the current increasing linearly with current, whilst an SCR and IBGT roughly drop a fixed voltage, irrespective of the current. High voltage MOSFETs have a relatively high on resistance (if I remember rightly resistance goes up to the square of voltage rating, everything else being equal) so the higher voltage rated parts drop quite a voltage, at any significant current. For high voltages an IGBF/SCR give low on losses, than MOSFETs. The crossover point depending on whether MOSFETs or SCRs/IGBTs have the lower on loss is around 600V. |
| ricko_uk:
Thank you all! :) So am I correct in saying that the IGBT does not slow down the ramping up of the current through the inductor (while the MOSFET clearly does)? Thank you |
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