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

[Mr.Jaick]

waltr
I don't know what to say but this reply truly made my day.
YOU ARE simply AMAZING
I read your post like ten times and everytime a bigger smile on my face, absolutely loved it
you have always helped me , i won't forget that.

so it's quite hard to balance the performance / power dissipation ... hmmm

again thanks for having me, my generation is SO lucky to be able to get advice from top end industry engineers like this.

[Mr.Jaick]

Current is constant for about 3 times the target Tau.

I never know how you guys determine a specific target's Tau
is it empirical or calculated?

[Monolith]

My comment -Changing to the MUR460 seemed to make a small difference, but will look into that later.

The difference appears to be in the reverse leakage current with the HER208 being higher. The voltage on the drain/cathode junction settles down between pulses at least 50V lower than the MUR460. I haven't yet confirmed this by looking at the data sheets. Nothing else seems to change, so it matters little. I have, though, put a 1Meg bleed resistor to ground so that with either diode the voltage is brought to zero before the next Tx pulse.

Eric.

Attached is the graph showing the Drain to Source capacitance of the Mosfet IRF740.
At about 200V is what I call the sweet spot, where the Coss starts increasing fast, as the voltage diminishes.
At 0 Volt the Coss is highest.

[Xtrem]

for example the SD2000 which doesn't use any limiter and its TX is far below any Tau valeu with that 0.4ohm coil

There is in use a very nice auto snubber circuit, that hold the flyback lower than
200V... see in the schematic the irf710 act as resistor in parallel with the high
vltage (200vdc) electrolytic capacitor.

[6666]

There is in use a very nice auto snubber circuit, that hold the flyback lower than
200V... see in the schematic the irf710 act as resistor in parallel with the high
vltage (200vdc) electrolytic capacitor.

Where do we find it ? thanks.

[Mechanic]

Where do we find it ? thanks.

It's in the sd2000 schematic, the circuit that controls the irf710.
It works as I built it ages ago on breadboard while testing some transmit circuit stuff.
This way you don't need to worry about using the avalanche diode in the mosfet to clamp the flyback.

Cheers Mick

[6666]

It's in the sd2000 schematic, the circuit that controls the irf710.
It works as I built it ages ago on breadboard while testing some transmit circuit stuff.
This way you don't need to worry about using the avalanche diode in the mosfet to clamp the flyback.

Cheers Mick

Thanks I will try and dig it up.

[Qiaozhi]

Attached is the graph showing the Drain to Source capacitance of the Mosfet IRF740.
At about 200V is what I call the sweet spot, where the Coss starts increasing fast, as the voltage diminishes.
At 0 Volt the Coss is highest.

That's an interesting observation, although it's difficult to determine the exact values from looking at the graph in the datasheet.

With reference to Appendix D of "The Voodoo Project", where I investigated the use of a series diode in a PI TX circuit, there is an equation for Coss effective. In the case of the IRF740, Coss effective was calculated as 211pF at a voltage of 320V. This appears to more or less match the graph that you posted.

Coss effective is defined as a fixed capacitance that would give the same charging time as the output capacitance of a mosfet while VDS is rising from zero to 80% VDS with VGS = 0V.

[waltr]

I never know how you guys determine a specific target's Tau
is it empirical or calculated?

Target Taus have been calculated in threads on this forum. However, due to not knowing the exact Allow conductivity calculated values are approximate. We have also empirically determine Tau values for common targets such as coins.
Check threads by Green where he post Log/Lin plots of decay to determine Tau.
Here is one on gold nuggets:
https://www.geotech1.com/forums/atta...2&d=1547586775

[Monolith]

That's an interesting observation, although it's difficult to determine the exact values from looking at the graph in the datasheet.

With reference to Appendix D of "The Voodoo Project", where I investigated the use of a series diode in a PI TX circuit, there is an equation for Coss effective. In the case of the IRF740, Coss effective was calculated as 211pF at a voltage of 320V. This appears to more or less match the graph that you posted.

Coss effective is defined as a fixed capacitance that would give the same charging time as the output capacitance of a mosfet while VDS is rising from zero to 80% VDS with VGS = 0V.

Intrigued by your calculation for "Coss effective" I looked at some datasheets.
Looking at the Coss of IRF740B or IRF740AS series and comparing with the diode MUR460 Junction capacitance, it becomes obvious, that the diode may not always be of much benefit.
Reading a lot of datasheets and comparing the many available parts in detail has become imperative.

[Mr.Jaick]

There is in use a very nice auto snubber circuit, that hold the flyback lower than
200V.

that's correct
but what i'm looking into is:
they didn't use any method to flat top coil current at some point
I think Bruce said "bugger that" to all high conductive targets

[Mr.Jaick]

Target Taus have been calculated in threads on this forum. However, due to not knowing the exact Allow conductivity calculated values are approximate. We have also empirically determine Tau values for common targets such as coins.
Check threads by Green where he post Log/Lin plots of decay to determine Tau.
Here is one on gold nuggets:
https://www.geotech1.com/forums/atta...2&d=1547586775

found it

[Qiaozhi]

Intrigued by your calculation for "Coss effective" I looked at some datasheets.
Looking at the Coss of IRF740B or IRF740AS series and comparing with the diode MUR460 Junction capacitance, it becomes obvious, that the diode may not always be of much benefit.
Reading a lot of datasheets and comparing the many available parts in detail has become imperative.

From a combination of calculation and simulation it appears that the diode eliminates about 50% of the mosfet capacitance, but how much improvement you see in practice depends on what you're trying to achieve. Either way, the diode doesn't make things worse.

Considering the so-called "sweet spot" sounds like it may assist in finding the point of maximum benefit.

[Ferric Toes]

From a combination of calculation and simulation it appears that the diode eliminates about 50% of the mosfet capacitance, but how much improvement you see in practice depends on what you're trying to achieve. Either way, the diode doesn't make things worse.

Considering the so-called "sweet spot" sounds like it may assist in finding the point of maximum benefit.

The MUR460 also benefits from holding upwards of 100V reverse potential on the drain/cathode junction reducing the capacitance to around 15pF.

[KingJL]

The MUR460 also benefits from holding upwards of 100V reverse potential on the drain/cathode junction reducing the capacitance to around 15pF.

[ATTACH]54627[/ATTACH]

Eric, as your previous thread on the subject had shown, not all diodes are suitable for producing these effects. In my own experiments, I have successfully used the through hole mur460 and BYV28-200 (as used in the SD2000). For SMD's I have successfully used the MURS320, MURS340, and MURS360. I keep a stock of about 10 each BYV28-200's, MURS320, and MURS340 on hand. Simulation shows that almost any diode will work, but in the real world they don't... I have tried general purpose diodes with very disappointing outcomes. I once tried a 1004 because simulation showed that it would work... IT DIDN'T!! Eric, I guess all you can do is keep putting valid information forward with supporting real world measurements/results and hope that some use it to their benefit.