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Old 23rd October 2007, 04:02 AM   #1
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Default A new DHT power supply idea

Instead of having to use a separate winding for each DHT I was thinking about building modular "off-line" switching supplies. The outputs are isolated from the inputs so a single AC supply is all thats needed. Heck they could even be run straight from the mains supply.

Most of the offline switching chips operate at 100Khz or less but I would try for at least 1Mhz. Speeds in the Mhz range are becoming more common now. So does this sound like a good idea?
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Old 23rd October 2007, 04:45 AM   #2
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http://pmillett.com/hf_fil.htm
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Old 24th October 2007, 10:44 PM   #3
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computer monitors and tv's have been running the crt filaments from the flyback for a very long time. there was a time when tv's used the main filament supply, but that was discontinued in the '70's and '80's.......

with switching supplies, you reference your regulation through an optoisolator, usually sensing from your highest secondary voltage (or just pick one secondary voltage you will always use, and sense from that one. overdesign the primary side. then you can add windings and rectifiers on the secondary to get whatever voltages you need....... SMPS can be very flexible. your frequency limitation is determined mostly by the speed of your rectifiers. also schottky diodes are rarely rated at more than 60V PIV.
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Old 24th October 2007, 11:11 PM   #4
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All of the off the shelf SMPS chips produce square waves. Experimentation with square waves in the 250 KHz range on DHT filaments produced some IMD distortion products that fell back into the audio range. The resulting sound had an unusual quality that was not easily identified, but all agreed that it was not good.

Pete's circuit produces reasonable sine waves by adding capacitance across the transformers primaries. This reduces the efficiency by forcing the mosfets to operate in the linear region. This obviously works (as he has proven) at low power levels but may cause unexpected mosfet death syndrome at higher power levels. Instant mosfet explosion will result if tried in an off line converter (as me how I know this).

The easy solution is to use DC on the filaments, but that makes this whole idea redundant. It is possible to "modulate" an SMPS that operates at a high frequency (say 1 to 2 MHz) with a sine wave at a lower frequency (say 100 to 200 KHz). This is the avenue that I am currently exploring.
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Old 25th October 2007, 01:44 AM   #5
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Tubelab what I was thinking is a simple switcher with a clean DC output and a wide range AC input. So you could buy a cheap transformer and feed as many isolated modules as you want. No more custom transformers with a half dozen secondaries.

Fairchild has a good offline smps simulator http://www.fairchildsemi.com/whats_n...s_toolkit.html

The only problem that might come up is needing a custom transformer for the switcher
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Old 25th October 2007, 03:17 PM   #6
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Seperate winding? I use one winding for both output tubes in my PP6B4G amp. No humpots. Shared cathode resistor on center tap. Did I do wrong?
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Old 25th October 2007, 03:43 PM   #7
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With matched tubes, sharing a winding and cathode resistor is fine. If you can supply a few volts of variable bias (positive, negative or both), you can handle mismatched ones too.
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Old 25th October 2007, 04:31 PM   #8
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Curious, I never thought of it that way. If I were to use seperate wires off the B+ capacitor to each tube, I could balance mismatched tubes with a wirewound dropping resistor or 5w pot? Plate voltage being off a bit being less a sin than unbalanced current in the output transformer.
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Old 25th October 2007, 05:53 PM   #9
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Quote:
Originally posted by tubelab.com
This reduces the efficiency by forcing the mosfets to operate in the linear region.
Actually it's not linear - it works as a zero-voltage-switched resonant converter, with each side turning on for maybe 30-45 degrees of the sine wave. So efficiency is OK. The biggest problem I had was actually the buck converter section that regulates output voltage, with the diode getting hot. A synchronous design would be better, or a bigger diode! Peak circulating currents here are wicked high, so that limits output power. It would be better to start with a higher input voltage, and custom-wind a transformer.


Quote:
Originally posted by astouffer
Tubelab what I was thinking is a simple switcher with a clean DC output and a wide range AC input. So you could buy a cheap transformer and feed as many isolated modules as you want. No more custom transformers with a half dozen secondaries.
I've used isolated DC-DC converters several times for DHT filament supplies. It does allow you to use a single supply to feed several isolated filaments. They seem to work pretty well, though they do tend to induce some HF junk.

My 813 amps (http://www.pmillett.com/813_se_triode_amps.htm) generates 48VDC from a conventional linear supply then uses DC-DC converters to get the 10V for the 813 filaments. Another amp I have running now does a similar thing, starting with +/24V (from a SMPS) to generate low voltage for filaments in a stereo amp.

Pete
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Old 25th October 2007, 06:22 PM   #10
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Quote:
Originally posted by astouffer
The only problem that might come up is needing a custom transformer for the switcher
You can get cores from cws bytemark in low qty. A good book like
Switching Power Supply Design by Pressman contains a good magnetics design section.

It's not too too hard with a bit of practice and research.
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