N-channel vs P-channel MOSFETs

[CAS]

Just a small side question when it comes to the MOSFETs - I was going through some parts I have here at work and was looking at the specs of various MOSFETs. Would it be an advantage to have a slightly higher Rds and so you can have lower output capacitance? I have a few different MOSFETS that have variations of specs with regard to Rds and Coss. It seems the higher resistance you get lower capacitance and visa versa. So higher resistance vs higher capacitance. Whats better?

[Silver Dollar]

I was thinking the same thing. Add some resistance to get a low capacitance FET. The trade off is less
current in the coil but a quicker sample so you can see smaller targets.

Eric Foster adds resistance in series with the coil to "flat top" the current to speed up the coil.
" I have a 27R series resistor in the TX circuit so the current flat tops in 13uS."

It seems lower flyback voltages are OK too if your trying to go fast...

[kt315]

Eric Foster adds resistance in series with the coil to "flat top" the current to speed up the coil.
" I have a 27R series resistor in the TX circuit so the current flat tops in 13uS."

this way was released in Eric's c-scope 4pi 6pi detectors, those coils has 1mH inductivity (milli-), no need to add the resistance in series.

[Davor]

Actually constant current drive separates charging pulse from flyback, and improves response of fast decaying targets. Both Whites and a certain manufacturer from OZ are holding some freshly issued patents with constant current drive by virtue of reducing the charging voltage to zero after initial pulse. Both approaches make sense, as there is no futile heating of resistors/transistors/etc. if you simply short a coil when it is fully charged.

I'm fully in line of thinking to pick higher Rds as it is inversely related with Coss, but a small problem is that devices that do that fell out of vogue for the past 10 years or so. You may employ a series diode to disconnect the drain capacitance from a coil after flyback with great success, and keep the recommended and cheap MOSFETS in your designs.

[CAS]

I'm fully in line of thinking to pick higher Rds as it is inversely related with Coss, but a small problem is that devices that do that fell out of vogue for the past 10 years or so. You may employ a series diode to disconnect the drain capacitance from a coil after flyback with great success, and keep the recommended and cheap MOSFETS in your designs.

I have been going through all the old stock in the workshop and have found a box full of assorted MOSFETs. There are probably about 8 or 9 that have the Voltage/current specs needed but all have different Rds values. I was really just asking as I dont want to waste time trying them if someone else has already found that there is no appreciable difference in lower Coss but with a higher Rds. I am looking to try to improve the MPP on small targets.

[Davor]

In that case your best bet is a combination of a diode in series with drain, and constant current or resistor limited charging. A diode like MUR460 is a very common thing in most shops.

[CalebG]

Does it just need the ultrafast diode? Im working on the surf pi, added what I think your describing into the schematic below. On your drawing it also has a resistor network, wasnt sure if that was needed on the surf pi?

[Davor]

Your schematic is correct.
Fast recovery diodes (<100ns) should do. Also current and voltage handling must be sufficient. They are usually less expensive than mosfets, and their common use is in switching PSU. So it is by no means rocket science.
The resistor in series with drain is a part of minipulse schematic, and I kept it for no particular reason. A network in parallel with a coil with another diode, a capacitor etc. is my attempt on a partially energy saving snubber/pulse-shaper, and is by no means required for Surf.