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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
I've recently realised that I'll need to increase frequency from the 25kHz I was originally planning to run at in order to get more current from the multiplier, but what are the factors that actually limit the upper frequency?
Chiefly, the reverse recovery time - Trr - which is the time the diode takes to transition between blocking mode and conduction mode.
When you see a diode described as 'fast' or 'ultra-fast,' it is Trr that is being refered to.
Where diodes are not fast enough, they will overheat and fail.
As a rule of thumb, a 70 nanosecond diode will be good up to 50kHz.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
Ash Small wrote ...
I've recently realised that I'll need to increase frequency from the 25kHz I was originally planning to run at in order to get more current from the multiplier, but what are the factors that actually limit the upper frequency?
Chiefly, the reverse recovery time - Trr - which is the time the diode takes to transition between blocking mode and conduction mode.
When you see a diode described as 'fast' or 'ultra-fast,' it is Trr that is being refered to.
Where diodes are not fast enough, they will overheat and fail.
As a rule of thumb, a 70 nanosecond diode will be good up to 50kHz.
I hope I'm not going too far off topic here, but I've read that Shottky diodes have no reverse recovery time.
I've also read that they have a reverse recovery time of the order of ~100 picoseconds, and I've also read that they can operate at frequencies of 200kHz to 2MHz.
This is all in the Wikipedia article on Shottky diodes.
Are you able to clarify any of this, as, to a newbie like me, it all seems a bit confusing and contradictory?
Also, would silicon carbide Shottky diodes be suitable. This will run in a vacuum, so cooling and outgassing are also factors here.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Proud Mary wrote ...
Ash Small wrote ...
I've recently realised that I'll need to increase frequency from the 25kHz I was originally planning to run at in order to get more current from the multiplier, but what are the factors that actually limit the upper frequency?
Chiefly, the reverse recovery time - Trr - which is the time the diode takes to transition between blocking mode and conduction mode.
When you see a diode described as 'fast' or 'ultra-fast,' it is Trr that is being refered to.
Where diodes are not fast enough, they will overheat and fail.
As a rule of thumb, a 70 nanosecond diode will be good up to 50kHz.
I hope I'm not going too far off topic here, but I've read that Shottky diodes have no reverse recovery time.
I've also read that they have a reverse recovery time of the order of ~100 picoseconds, and I've also read that they can operate at frequencies of 200kHz to 2MHz.
This is all in the Wikipedia article on Shottky diodes.
Are you able to clarify any of this, as, to a newbie like me, it all seems a bit confusing and contradictory?
Also, would silicon carbide Shottky diodes be suitable. This will run in a vacuum, so cooling and outgassing are also factors here.
Do you have any other practical suggestions?
"High voltage Schottky" usually means a few hundred volts, though I've heard about silicon carbide types said to operate at 1.2kV. You should ask a real Schottky person.
But I have practical suggestions!
1. Design your box of tricks around components which are inexpensive and readily available.
2. The Chinese ebay sellers popular with 4HV members usually have three speeds of HV diode. (a) 100ns, (b)150ns, (c) unspecified.
The 100ns ones should be good to 100kHz, but Trr of a diode can be influenced by its associated circuitry, which is why the 'rule of thumb' figure I gave above is lower.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
. "High voltage Schottky" usually means a few hundred volts, though I've heard about silicon carbide types said to operate at 1.2kV. You should ask a real Schottky person.
But I have practical suggestions!
1. Design your box of tricks around components which are inexpensive and readily available.
2. The Chinese ebay sellers popular with 4HV members usually have three speeds of HV diode. (a) 100ns, (b)150ns, (c) unspecified.
The 100ns ones should be good to 100kHz, but Trr of a diode can be influenced by its associated circuitry, which is why the 'rule of thumb' figure I gave above is lower.
The main disadvantage of using SiC Shottkys will be the number/space required, although from what I've read they run a lot cooler and have less reverse leakage current.
I may stick with the N27 cores as they are good to 100kHz or more, and see how the 100ns Chinese diodes perform.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Grenadier wrote ...
If you just want a hardy HV supply, an xrt + a variac might be a good option.
Ha, if I knew where to get an x-ray transformer in Australia I would jump at the chance with my fists full of cash. Same goes for a distribution trans or PT.
@HighVoltageChick: I think my problem is the fact that mine is made from junk with hardly any tools and no instruments. And no knowledge :D
@Proud Mary: From reading through the Vishay multipliers document, I think I'll try and build a 5 or 6 stage full wave tripler powered by my 6kV NST using 25kV ceramic caps of roughly 10-20nF (probably have to stack cheap caps in parallel) and similarly rated diodes plus an output resister to protect them. Although I'm worried when the document mentions:
"The full-wave circuit, however, requires that the secondary side of the transformer be capable of withstanding high voltages (approximately 1/2 of the output voltage)."
Registered Member #2018
Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74
Teslacoils are fun I really recommend you to not give up on these. I built several SSTCs, VTTCs and SGTCs, and I can't get enough of these fascinating devices. Here is an example from my latest tube coil with 2 GU81Ms:
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
XravenorX wrote ...
@Proud Mary: From reading through the Vishay multipliers document, I think I'll try and build a 5 or 6 stage full wave tripler powered by my 6kV NST using 25kV ceramic caps of roughly 10-20nF (probably have to stack cheap caps in parallel) and similarly rated diodes plus an output resister to protect them. Although I'm worried when the document mentions:
"The full-wave circuit, however, requires that the secondary side of the transformer be capable of withstanding high voltages (approximately 1/2 of the output voltage)."
My little NST definitely can't handle 50kV :(
I think this is true of some FW voltage doubler topologies, but not of the FW C&W driven by a centre-tapped transformer
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New guy with newbie questions.
I really recommend you to not give up on these. I built several SSTCs, VTTCs and SGTCs, and I can't get enough of these fascinating devices. Here is an example from my latest tube coil with 2 GU81Ms:
