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Old 27th April 2013, 11:43 PM   #121
rmb is offline rmb  United States
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The voltage dividing resistors are essential. The technique has been around for years. Use it...especially since you are dealing with voltages that kill. This may save you the trouble of an "avalanche" if a diode fails.
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Old 28th April 2013, 03:37 AM   #122
Magz is offline Magz  United States
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The voltage dividing resistors are essential. The technique has been around for years. Use it...especially since you are dealing with voltages that kill. This may save you the trouble of an "avalanche" if a diode fails.
Thanks for the advice. Can you point me to a source of 1Mohm, 2W wirewound resistors with a 1200V rating?
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Old 28th April 2013, 05:50 AM   #123
tomchr is offline tomchr  United States
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Interestingly, the datasheet from Cree shows that the Ir is quite steady with voltage and temperature up to at least 1000V per diode and 125 degC. Must be the SiC.
You actually don't have that data because it doesn't register on the graph. There's no way the leakage current is constant with temperature. That's fundamental semiconductor physics.

I looked at Digikey's selection of 2 W resistors. It looks like they top out at about 350-500 V, so you may need to connect two resistors in series across each diode to meet the voltage spec. Welcome to high voltage design.

You mention 1200 V. Why? I thought you had a string of four diodes across 2500 V. That's "only" 625 V per diode. Or am I missing something? With 625 V, you'll need two resistors in series. At 1200 V, you'll need three or four depending on the resistor of choice.

~Tom
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Last edited by tomchr; 28th April 2013 at 05:53 AM.
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Old 28th April 2013, 05:51 AM   #124
Magz is offline Magz  United States
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These 2M, 1500V, 1W MOX HV resistors will probably suffice if needed.

RNX0382M00FKEE Vishay Dale | RNX2.00MBCT-ND | DigiKey
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Old 28th April 2013, 06:03 AM   #125
Magz is offline Magz  United States
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Originally Posted by tomchr View Post
You actually don't have that data because it doesn't register on the graph. There's no way the leakage current is constant with temperature. That's fundamental semiconductor physics.

I looked at Digikey's selection of 2 W resistors. It looks like they top out at about 350-500 V, so you may need to connect two resistors in series across each diode to meet the voltage spec. Welcome to high voltage design.

You mention 1200 V. Why? I thought you had a string of four diodes across 2500 V. That's "only" 625 V per diode. Or am I missing something? With 625 V, you'll need two resistors in series. At 1200 V, you'll need three or four depending on the resistor of choice.

~Tom
It's actually a string of 5. My diodes are 1200V, my caps are 1250V, why would I want to put a resistor across them rated less? Seems like a step backward to me, when I could use a single MOX resistor rated at 1500V. Fits nicely in the space, too.
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Old 28th April 2013, 06:35 AM   #126
Magz is offline Magz  United States
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Now, you can choose to ignore physics, oversize everything, do some hand-waiving, and hope for the best. But in a $10k amplifier build operating at 2500 V with more than enough energy to kill you twice without breaking a sweat, I think you need to slow down and understand the problem rather than relying on a plug-n-play approach.


~Tom
By the way, this quote is classic. I've been researching the topic for weeks, building the circuit, experimenting with it, measuring the results, reporting the findings, and seeking explanations for them. Hardly handwaiving and plug-n-play, is it?

Give me some credit already.
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Old 28th April 2013, 07:14 AM   #127
Magz is offline Magz  United States
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You actually don't have that data because it doesn't register on the graph. There's no way the leakage current is constant with temperature. That's fundamental semiconductor physics.

~Tom
Yes, I'm aware that the graph runs into the abscissa and that Ir is temperature dependant. My point here is that even at much higher voltages than my diodes will ever see, where the data is visible, the difference in Ir between 25C and 75C is only 2 or 3X, and the lines are converging and the slope decreasing as the voltage decreases. Is that much of a difference really something to worry about when I have 700V of headroom per diode?

I'm trying to understand the fundamentals here, not just follow a cookbook.
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Old 28th April 2013, 12:48 PM   #128
Magz is offline Magz  United States
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Following up on this a little further, if you look again at the datasheet plot (post #120), at a temp of 175C (clearly a worse case than I will ever see, but useful because it's easy to see on the graph), the Ir increases from ~10uA to ~25uA from 500V to 1000V. Assuming the highest voltage drop diode in my worst stack stayed unchanged and the lowest changed by the full 2.5X, my calculated 27V difference in the worst stack at 600V (calculated assuming the diodes all changed linearly at the same rate) would become a 67.5V difference. I have 700V headroom per diode, or 10X that amount.

Where is the flaw in this logic? In this case it looks to me like adding 20 resistors per bridge would be an unnecessary expense, complication, and potential additional source of failure. Perhaps that's why the more recent editions of the ARRL handbook for ham radio amateurs recommend against adding resistors (or caps for that matter) to diode strings?

Again, this is not an attempt to be obstinate or argumentative, it's an attempt to understand why I should add complication and potential problems for what seems like little to no benefit.

It seems to me the next experiment is to run the bridge up through several voltage levels, measure the voltage differences between diodes, plot the data and see what it tells us about the change in voltage drop difference per unit applied voltage. Or as one of my grad school profs used to always insist "plot the data, plot the data, plot the data!".

Last edited by Magz; 28th April 2013 at 01:11 PM. Reason: added reference
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Old 28th April 2013, 01:48 PM   #129
MelB is offline MelB  Canada
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Magz:
Found a great application note from SGS-Thompson on using fast diodes in series and explaining resistors and diodes. Don't know if you have seen this:
http://www.datasheetcatalog.org/data...cs/mXyzyuu.pdf
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Old 28th April 2013, 03:25 PM   #130
Magz is offline Magz  United States
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Magz:
Found a great application note from SGS-Thompson on using fast diodes in series and explaining resistors and diodes. Don't know if you have seen this:
http://www.datasheetcatalog.org/data...cs/mXyzyuu.pdf
Yes, I ran into that in my digging, and thanks for sharing it anyway!

I don't argue with the basic principles here, diodes can vary a lot from batch to batch, and there's no guarantee a customer will use diodes that are all from the same batch so they need to make recommendations based on the worst case scenario - for example, the datasheet for the diodes I'm using show Vf as 1.4V typical, 1.8V max. I'm pretty sure a 1.4Vf diode and a 1.8Vf diode might have some huge differences in reverse current and voltage drop.

The 40 diodes I have all were within .007V for Vf - do they still need to be treated with the worst case scenario? That's what I'm trying to find out. If you surf the Ham Radio forums the consensus now seems to be that if the diodes are from the same batch, then resistors aren't needed; the caveat is that they are not talking specifically about SiC Schottky diodes, rather they focus on standard Si diodes.
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