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Old 26th May 2011, 11:04 PM   #11
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You could do 7.5V(1.25A) for 801As.
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Old 27th May 2011, 12:10 AM   #12
tomchr is offline tomchr  United States
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Quote:
Originally Posted by SpreadSpectrum View Post
You could do 7.5V(1.25A) for 801As.
Certainly.

~Tom
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Old 27th May 2011, 12:53 AM   #13
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But the more I look at mains supplied switchmode, the more I realize that sourcing the magnetic components is a royal pain.

~Tom
I have found the same problem with high power class d amps.
I now wind my own.
In most cases it works out cheaper than buying in specials.
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Old 27th May 2011, 03:20 AM   #14
tomchr is offline tomchr  United States
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Quote:
Originally Posted by nigelwright7557 View Post
I have found the same problem with high power class d amps.
I now wind my own.
In most cases it works out cheaper than buying in specials.
I was referring to the cores and bobbins needed for winding my own magnetics when I commented on the lack of availability. For anything but the simple IC regulated supplies, I would plan on winding my own magnetic components. But I have yet to find a good source for the parts. The vendors I have found haven't had any data on what they actually had for sale.

The low-voltage regulator ICs are so much easier to work with as the magnetic components needed are available commercially and the components are generally well spec'ed.

~Tom
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Old 27th May 2011, 12:42 PM   #15
TheGimp is offline TheGimp  United States
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National has a part out that works for buck/boost (Zeta) with a Constant On Time regulator which is discussed in their POWER designer application note #130.

http://www.national.com/assets/en/ap...esigner130.pdf

The design is for a fixed output 600ma regulator, but since it has an external pass transistor and uses a resistor divider for feedback it should be straight forward to change it to selectable output and greater current capacity(bigger FET and Inductor?). Minimum V-out is 1.25 (limited by the internal ref). But could be made lower by using acouple of tricks like post regulation (drop voltage with a series schottky diode).

How far can one go on the reange of voltage boefore the inductor has to change?
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Old 27th May 2011, 04:19 PM   #16
tomchr is offline tomchr  United States
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Quote:
Originally Posted by TheGimp View Post
National has a part out that works for buck/boost (Zeta) with a Constant On Time regulator which is discussed in their POWER designer application note #130.
Nifty...

Quote:
Originally Posted by TheGimp View Post
How far can one go on the reange of voltage boefore the inductor has to change?
There are quite a few variables to play with. Inductance, (Vin-Vout), Vout, switching frequency, inductor current ripple, etc. I'll need to set up a spreadsheet and find a minimum number of inductance values that will work with the output voltages listed. Hopefully, I can reduce it to just one inductance. Given that National makes the LMZ-series with the internal inductor, I'm guessing I'll be able to find a good compromise.

~Tom
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Old 28th May 2011, 03:34 PM   #17
Zeta4 is offline Zeta4  United Kingdom
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Hi Tomchr

Interesting thread. Have you seen the SM regulator modules from TI
particularly the PTH/PTN series. They are ready built modules and
look like they could cover the requirements of most DHTs. Only problem
might be that while they have a soft start feature Im not sure if it is
"soft" enough and I dont know if it can be altered. Might be other
problems I havent thought of.
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Old 29th May 2011, 06:48 PM   #18
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Zeta4 View Post
Have you seen the SM regulator modules from TI
particularly the PTH/PTN series. They are ready built modules and
look like they could cover the requirements of most DHTs. Only problem
might be that while they have a soft start feature Im not sure if it is
"soft" enough and I dont know if it can be altered.
I hadn't seen those. They're pretty neat. However, not all that flexible and actually rather expensive. Not only are the modules expensive, they also require two expensive external ceramic caps (100 uF, X5R is not cheap). The ripple voltage is also higher than I'd like (1 % of Vout). But it is pretty impressive that they can crank 45 W out of a small module like that. 95 % efficiency... Nuttin' wrong with that.

Usually "soft start" means it takes about 1 ms for the regulator to start up... I'm planning to extend that to about 5 seconds. - Though, if you want it to be different, just change the soft-start cap... In my experience, a 5 second start-up is enough to avoid hitting the current limiter when powering up into a cold 300B filament.

The PTH series goes up to 5.5 V - but only 2.25 A so not enough for a 2A3, but enough for the other low-voltage tubes. The PTN-series can deliver enough current (3 A), but only go down to 3.0 V, so again the 2A3 lose out...

This is exactly what I ran into in the previous incarnation of my filament regulator. The National LMZ-series are awesome and have rock solid performance, but they only go to 5.5 V, so the 6.3 V tubes will need a different regulator. The regulator I used for the 6.3 V couldn't deliver the output current needed by a 300B, so I ended up with two different designs. Nothing wrong with that, but it did make me wonder if it wasn't about time that someone threw together a design that could handle all the voltages. I'd much rather have one design that support all the needed voltages with a simple tweak of the passives.

~Tom
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Old 30th May 2011, 03:08 AM   #19
tomchr is offline tomchr  United States
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It turns out I can support the range of common tube voltages (1.5, 2.5, 4.2, 5.0, 6.3, 7.5, and 12.6 V) with a single inductor value. For some of the output voltages, I may have to increase the switching frequency from the nominal 500 kHz to keep the inductor ripple current within reasonable limits (250-ish mA). But that's easily done by swapping out a resistor.

Latest schematic and board layout are attached. The component values on the schematic are for 5.0 V out. The board is etching as I type... Just a quick prototype before I commit more serious money to a fab run.

~Tom
Attached Images
File Type: png FilamentReg_1p0.png (48.9 KB, 847 views)
File Type: png FilamentReg_1p0_Schematic.png (40.0 KB, 857 views)
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Old 30th May 2011, 05:52 AM   #20
tomchr is offline tomchr  United States
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Default First hack...

Folks,

The first results are in. I assembled the circuit as shown on the schematic, except I omitted D1, C1~C5 and just fed the circuit from a lab supply. I also didn't have any 68 kOhm resistors in 0603 package for R2, so I used 47 kOhm. As a result, I had to accept a higher switching frequency of approx 800 kHz versus the 500 kHz I designed for. Oh, well...

At Vout = 5 V; Iout = 2 A, the regulator shows a few mVpp of ripple voltage. The start-up time is about 5~6 seconds. Not bad for a day's work, eh?!

~Tom
Attached Images
File Type: jpg FilamentReg_1p0_Proto.jpg (290.5 KB, 835 views)
File Type: jpg FilamentReg_1p0_Ripple_5V_2A.jpg (182.5 KB, 788 views)
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