Totem-pole gate driver VS Active pull down VS Fast Fet turn off

[green]

good test green
so i think it's safe to say the problem is not the gate ringing
maybe some sort of complex impedance happening at the first turn on moment in that RLC rabbit hole(coil, damping, diode and mosfet capacitance, connections and packages,....)
by the way why your gate voltage on the scope is reverse?
it's an N mosfet and current starts when gate voltage falls in your pics, why is that?

Recording turn off. Coil current(-1.1V)1.1A at start of turn off, decays to zero current in about 1.2us. Gate V starts at zero on scope(+12V on gate), goes to -12V(zero V on gate)at turn off. Scope ground is at +battery.

I've been recording turn off. Should I record turn on?

[green]

I made a fitting for the scope probe exactly as portrayed in the YouTube video that Altra recommended in post 36. This was directly across the 0.1 ohm resistor.

The TLC timer already had a 22uF tantalum bead across the pins to the 8V supply, but I added a 4.7uF thin film ceramic capacitor. The ringing on the tail of the switchoff edge of the drive pulse seems much less.

Ringing still apparent on the switchoff of the Mosfet as seen using the better probe connection, but does seem less.The large initial ring seems to coincide with the flyback peak at 0.4uS

I think I am going to accept the result as all seems clean after 1.5 - 2.0uS and I don't need to sample that early anyway. Instead I am going to look into ways of eliminating the slow recovery characteristics of the preamp.

One thing I might quickly look at is the same Mosfet but driven by a complementary emitter follower drive.

[ATTACH]54497[/ATTACH]

Eric.

Have you tried recording across the .1ohm resistor with one end of the coil connected to + supply not the resistor? One end of resistor still connected to + supply. Should be zero signal unless noise pickup.

[Mr.Jaick]

Recording turn off. Coil current(-1.1V)1.1A at start of turn off, decays to zero current in about 1.2us. Gate V starts at zero on scope(+12V on gate), goes to -12V(zero V on gate)at turn off. Scope ground is at +battery.

I've been recording turn off. Should I record turn on?

got it
thanks

[green]

I see I put Rd across the coil instead of across the coil and the 1ohm resistor in schematic(mosfet driver4). New schematic(mosfet driver4a)

[Ferric Toes]

I see I put Rd across the coil instead of across the coil and the 1ohm resistor in schematic(mosfet driver4). New schematic(mosfet driver4a)

This is the arrangement that I use.

The 300uH coil is of course shunted by its own self capacitance and the capacitance of the coax cable. My coil plus cable resistance is at the moment 5.6 ohms.

Eric.

[green]

This is the arrangement that I use. [ATTACH]54502[/ATTACH]

The 300uH coil is of course shunted by its own self capacitance and the capacitance of the coax cable. My coil plus cable resistance is at the moment 5.6 ohms.

Eric.

Hi Eric
See your schematic has same error as my(mosfet driver4). Rd across coil not across coil and current resistor. Shouldn't show any Rd current if Rd is just across the coil. I had it across both when I did the test, just drew it wrong in the schematic.

Couldn't convince my self what was right so a did a test with Rd both places. Need to play some more now that I see oscillation when Rd just across coil. Doesn't show decay current so I'm not sure it matters.

[green]

Hi Eric
See your schematic has same error as my(mosfet driver4). Rd across coil not across coil and current resistor. Shouldn't show any Rd current if Rd is just across the coil. I had it across both when I did the test, just drew it wrong in the schematic.

Couldn't convince my self what was right so a did a test with Rd both places. Need to play some more now that I see oscillation when Rd just across coil. Doesn't show decay current so I'm not sure it matters.

Another try with Rd across coil only. Looks better, Rd clips on and I think wasn't making good connection last try #7.

[Ferric Toes]

Another try with Rd across coil only. Looks better, Rd clips on and I think wasn't making good connection last try #7.

Hi Green, I always have used the circuit as in your 'Mosfet driver 4' and a 0.1 ohm resistor so that the coil current is substantially the same as without it. I can then see the coil current growth in the inductance; whether it is reaching its resistive maximum or still limited by the inductive growth; or in other cases how long it is flat topped. Rarely do I look at the fall time in detail, and particularly where the drive and current is approaching zero.

When trying to maximise the switchoff rate and reduce delay times, this area is more important and any spurious ringing should be investigated. I had this problem once in an industrial PI detector which failed an official emc test. it was tracked down to a long trace on the pcb from the pulse generator to the Mosfet. The high frequency ringing was also being radiated by the search coil. The layout was changed, the trace made as short as possible, and the detector then passed the test. On the pcb I have now, the trace to the Mosfet gate is about 10mm.

I have another pcb, identical to the one I have been using for these tests except that it has the complementary emitter follower drive. I will do the same tests on that one to see what the switchoff looks like. I have also arranged it so that I can plug in different Mosfets to compare.

On your coil voltage plot, it looks from the flat top, that your device is going into the avalanche condition.

Eric.

[green]

Another try with Rd across coil only. Looks better, Rd clips on and I think wasn't making good connection last try #7.

Tried again this morning. Appears oscillation(mosfet driver7)was caused by a bad scope ground connection not Rd connection. Tried with both CH1 and CH2 ground leads connected to +battery and with only CH1 or CH2 ground lead connected. Doesn't look good with just one connected. Would probably be better if ground leads were shorter.

Hi Eric, looks like we replied at the same time. My circuit does avalanche. With a fast coil it's going to avalanche or I need to use snubber. Haven't seen a difference in operation avalanche or snubbed. Probably need to snub if peak current is high, above 2A? Is your picture reply #45 Tx off? Where was Rd located reply #45?

Think it might be better to put Rd to +battery not across coil. With Rd across coil, volts across current resistor charging includes current thru Rd and coil(not a big error). Discharging, volts across current resistor only during avalanche. Rd across coil and current resistor, volts across current is correct for coil current charging or discharging.

[green]

Think it might be better to put Rd to +battery not across coil. With Rd across coil, volts across current resistor charging includes current thru Rd and coil(not a big error). Discharging, volts across current resistor only during avalanche. Rd across coil and current resistor, volts across current is correct for coil current charging or discharging.

Rd across coil and current resistor, volts across current resistor is correct for coil current charging or discharging. Not true when coil volts is increasing to avalanche at Tx off, coil capacitance charging wouldn't show across the current resistor.

[green]

Still wondering why I don't see the oscillation. Eric uses a .1ohm resistor which makes sense. Need to order one. Any reason a 1/4W metal film wouldn't work, is there a better choice? With leads or chip?

If I see oscillation, how to tell if it's actual current or noise. Including a schematic I think might work. Any other suggestions? Coil volts increased over 5000V/us, noise source? Wondering if I need to order something else.

[Ferric Toes]

I have some interesting results today, but will report fully later. Basically some Mosfets ring much worse than others and also delay the switchoff for some reason. Unplug the Mosfet and my drive pulse is very clean and switches fully off in 250nS with no ring. Plug in a Mosfet and everything changes particularly at the transition period before fully off.

Metal film may be OK but the film is wound as a spiral so must have a small inductance. I use thick film to play safe. Arcol make non-inductive carbon resistors which they say are very good for pulse work. I don't know if their range goes as low as 0.1 ohms.

Eric.

[Qiaozhi]

Basically some Mosfets ring much worse than others and also delay the switchoff for some reason. Unplug the Mosfet and my drive pulse is very clean and switches fully off in 250nS with no ring. Plug in a Mosfet and everything changes particularly at the transition period before fully off.

Have you considered that the ringing may be due to bond wire inductance inside the mosfet package?

[Ferric Toes]

Have you considered that the ringing may be due to bond wire inductance inside the mosfet package?

I found some figures for that. Apparently a TO-220 part has about 7nH of inductance and as much as 10 ohms of gate resistance. A surface mount part would be more like 3nH inductance and 3 ohms gate resistance. There are several other factors which I am looking into e.g. applying more drive voltage than needed means that Cgs will take longer to discharge, and Mosfets with high current capability usually have high gate capacitance. In other words choose a Mosfet that comfortably handles the current required. 5A would be ample for me at the moment. Driver pcb layout is very important to minimise inductance in the forward and return circuit.

Eric.

[green]

Tried stacked transistor drive in spice. Looks like it might work driven with a 555 oscillator at Tx supply. I need 5V logic command, probably stay with UCC27518 until I know more.

I see the circuit is in reply11, probably where I saw it.