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HVM12 Diode Recovery Time

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Conundrum

Sun Feb 22 2015, 06:55AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Intriguing idea:

Power IR LED transmitters ssometimes have an external resistor to discharge the LED's capacitance..

I added 10K one to my heli and achieved 40 feet range with a trijunction 0.3W LED

johnf

Mon Feb 23 2015, 08:57AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

A quick note
trr
time to recover while the diode slows conduction and conduction stops

the diode is becoming a resistor during this time this = heat
all of this due to junction capacitance that varies with applied reverse voltage.

for low frequency rectifiers this is in the nf region
high speed rectifiers are in the pf range

so impedance of supply "R" times junction capacitance is a time constant that is a constant so the higher the freq the higher the dissipation

all for now

Patrick

Mon Feb 23 2015, 04:18PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

johnf wrote ...

so impedance of supply "R" times junction capacitance is a time constant that is a constant so the higher the freq the higher the dissipation

Ha! Im starting to remember now ! I knew capacitance played a role. And the older series of HV diodes weren't all that great for other reasons too.

Ash Small

Mon Feb 23 2015, 07:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

It's a bit like the 'mesh resistance' in a 'fet. The electrons have to be pulled out of the valence band in silicon diodes, to overcome the junction voltage. Schottky's don't have a valence band as they have metal junctions, so have no recovery time or 'on' time.

High speed diodes tend to have gold doped junctions for this reason.

Dragon64

Tue Feb 24 2015, 05:31AM

Registered Member #1438 Joined: Sat Apr 12 2008, 12:57AM
Location: Canada
Posts: 218

Patrick wrote ...

Dragon64, can you share how it will be used ?

this helps.
And

In any case, I think Conundrum may have a good idea, either that or choose better diodes and/or understand the circuit better. Ive series-ed many HER-108 diodes with good results. Ive done this past 28kV, I derate by 60%.

I planned to use them for a ~20kHz voltage multiplier with several stringed up. I was afraid of getting spikes in supply that feeds the multiplier if the diodes were too slow and those HVM's are cheap and fairly rugged.

Considered stringing up UF4007F's but at only 1kv ratings, it would require 30 of them stringed (input = 9kv | 9*2*1.41 = ~25kv with 5kv headroom) which is not really practice for oil immersed multiplier I'm planning.

I already bought the alternative diode for the multiplier which was costly

(36 x 2CL2FL)

Antonio wrote ...

The HVR-1X diodes seem to be the same thing. The datasheet mentions 50 ns of recovery time, and shows a test circuit.

I believe the 50ns is for the UX-FOB that's on the same datasheet as the HVR-1X diodes

The HVR-1X I'm still not that sure as it's listed as "Standard Recovery".

Patrick

Tue Feb 24 2015, 06:36AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Dragon64 wrote ...

I planned to use them for a ~20kHz voltage multiplier with several stringed up. I was afraid of getting spikes in supply that feeds the multiplier if the diodes were too slow and those HVM's are cheap and fairly rugged.

This might need a testing apparatus as conundrum suggests.

Conundrum

Wed Feb 25 2015, 05:51AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Maybe I should macgyver a diode curve "tracer" using that old flat CRT in my junkbox.

I still have a dead B/W TV so in theory could use this as the driver by adjusting HV and supplying a dummy coil to keep they flyback happy.

Re. multipliers, it would be interesting to make a test CW using 10M *2 resistors across each diode and would be a lot safer as well due to the capacitors having something to discharge through.
For something running from a CCFL tube driver as I have done in the past each stage would generate 2*VPeak so that would be 1.8KV*2 = 3.6KV yielding a current of 0.018uA.
The main reason for using 10M resistors is that they are rated up to 1KV so under oil they should work fine.

Dragon64

Thu Feb 26 2015, 07:58AM

Registered Member #1438 Joined: Sat Apr 12 2008, 12:57AM
Location: Canada
Posts: 218

Conundrum wrote ...

Re. multipliers, it would be interesting to make a test CW using 10M *2 resistors across each diode and would be a lot safer as well due to the capacitors having something to discharge through.
For something running from a CCFL tube driver as I have done in the past each stage would generate 2*VPeak so that would be 1.8KV*2 = 3.6KV yielding a current of 0.018uA.
The main reason for using 10M resistors is that they are rated up to 1KV so under oil they should work fine.

The multiplier has already been build once but because of some carelessness of dealing with the multiplier (such as the accidental arcing bypassing the resistive load between the last stage and the ground due to mineral oil being influenced by the high voltage

and the underrated diodes (20kV - 20mA 2CL20KV). I plan to eliminate this oil problem (not sure what this phenomenon is called) by building the tower vertically oil immersed hopefully minimizing exposure of any of the leads to air.

Since the original intention of the multiplier was for driving an x-ray tube (BX-1A), and with the tube drawing ~1-2mA at 1A filimant current, the capacitors and the diodes have to be rated for a minimum of 25mA drawing about 220W from the ZVS.

I did purchase a couple 2kohm (x2) ceramic resistors across each diode. Previously strings of several 2Mohm 1/2W * 4 but could not handle the power from the multiplier and failed (a water resistor seems like a more cost effective choice if I was to go down this route).

Conundrum

Fri Feb 27 2015, 08:22AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Yeah, this would work.
I also observed that oil has a tendency to move under HV so you might consider making the whole assembly mostly gastight with a small vent to equalize pressure if needed.
Indium on glass or even BiInSn makes an excellent seal and has been used on Geiger tubes which is why you never, EVER solder to the pin.

An interesting idea is to put crowbar tubes across the capacitors to prevent them failing due to overvoltage.
Usual voltage is 7KV and they can be seriesed with care with Vmax approx 21KV

EDIT: Ordered some diodes, this time they are guaranteed good 7ns ($$!) but rated to 1.2KV and designed specifically for SMPS's.

Arjan EMM

Fri Feb 27 2015, 05:21PM

Registered Member #149 Joined: Sun Feb 12 2006, 09:11AM
Location: Netherlands
Posts: 12

Hello everyone. It's quite easy to get a descent Trr measurement if you have a scope and function generator. Connect the diode to the function generator in series with a 50 ohm resistor to ground. connect the scopeprobe in the middle.
You would expect to see a squarewave from V+ to 0. instead you see a massive shootthrough all the way to V- before recovery to 0.
Measure the length of the shootthrough and you have your recoverytime.

The picture i added is a mur1100erl, 75 ns, 1kv, 1A diode from fairchild being tested.
Fuction generator is set at 100 Khz. The bottom trace shows two markers set at 75ns distance. upper trace is 1 uS/div, bottom one 50ns/div.

In the same test a 1n5408 shows about 2.5 uS for example. a general purpose diode.

1425057598 149 FT169216 Wp 20150227 17 48 56 Pro

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