9 V To 48 V DC-DC Converter Design

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Samuel Groner

Well-known member
Joined
Aug 19, 2004
Messages
2,940
Location
Zürich, Switzerland
Hi

Got asked to design a little phantom power box to power two channels on a film camera (which does have XLR inputs) from a 9 V battery. After some searching I found the LT1073 to be a good match for this kind of application and I came up with the following circuit: [removed] and [removed].

The circuit is basically an adaption of the "1.5V to 5V Very Low Noise Step-Up Converter" shown on page 14 of the datasheet. As this is my first homebrew switching design I'd appreciate the opinion of the experts on this.

For the inductor I planned on the Wilco ITS471-VM (shielded torroid). What about the critical 100 uF capacitor? Linear recommends the Sanyo OS-CON type, but I did not find a source for them. Would using a 1 uF polyester film in parallel with a standard (Panasonic FC) low-ESR capacitor give similar results?

Thanks for your input!

[Edit: current revision of schematic 48V_box_r1.pdf and 50 mA version: 48V_DC-DC_50mA_r1.pdf]

Samuel
 
[quote author="Samuel Groner"]Got asked to design a little phantom power box to power two channels on a film camera (which does have XLR inputs) from a 9 V battery. After some searching I found the LT1073 to be a good match for this kind of application[/quote]
You may want to use a different switcher. The absolute maximum rating on the SW1 pin of the LT1073 is only 50V, and for 48V output you will most certainly get transients above that. Furthermore, the part's switching frequency (15..23kHz) is a bit too close to the audio band for my liking.

I don't know of any step-up switchers which can generate 48V by themselves. You can either use a chip like the LT1073 and have a cascoded FET in its switch line to extend the voltage range, or use a switch controller chip with an external hi-voltage FET. I'm going the latter route myself, with the LT1738. For a battery-powered application, keep in mind that high-current FETs have high gate capacitance which needs to be charged/discharged every switching cycle, so usually it's best to take the smallest N-FET that will fit your voltage/current budget.

JDB.
 
Thanks for your answers. Yes, the frequency is low and the voltage a bit high, but I'd really like to go with that IC as anything else I've found is more complicated and usually SMD-based. It doesn't need to be super-clean for this application, I'd say.

I don't mind going a bit lower in voltage--what would you recommend as maximum? Or would a zener be fast enough to catch at least the dangerous part of the transients?

Samuel
 
Do you really need five-WATT Zeners? :shock: -Seems to me that 400mW types will be just fine & dandy.

I've thought about this a LOT recently, and the thing about the 48V phantom is that it will NOT pick up in a differential or balanced line, because it's common-mode, and anything common mode is rejected.

However...

Inside the Microphone, the common-mode voltage is *summed* (not canecelled) from the two legs, and then used to power the microphone. For that reason, you *DO* want nice, clean, ripple-free power, otherwise ripple -while it doesn't get induced into the mic preamp from the cable coupling- will re-accumulate inside the microphone. Most mics I've looked at recently also rely on electolytic filtering, and this will be fine for 'typical' 50/60Hz-derived phantom power-supplies, it might be vulnerable to noise from HF switching types...

If it's pretty high-frequency ripple, then the electrolytics will have a hard time filtering it out, so I'd INSIST on something with 'poly-wotsit' film capacitors... or even a healthy dosing of ceramic... but relying on aluminium electrolytics for HF surpression might be a mite optimistic, especially as they get older. (might work cleanly on the bench, but after a year of field service, could get a bit 'whistly'...)

Also, perhaps the further judicious use of ferrite beads here & there wouldn't do any harm... -Just a thought...

Apart from that, -first-rate ingenuity! -Fine work!

Keith
 
Do you really need 5 W Zeners?
Seems like there's much more peak current flowing than one would think--according to the THAT "Phantom Menace" AES paper it is up to 2 A with the 10 ohm limiting resistors as shown in case the user decides to short the input while phantom power is on. How often these currents occur in real world is questionable, but designing for them is surely not the worst idea, especially in the given situation where potential sonical degradation due to the protection circuitry is of little relevance.

I've thought about this a lot recently, and the thing about the 48 V phantom is that it will not pick up in a differential or balanced line, because it's common-mode, and anything common mode is rejected.
I wouldn't rely on things being truly common mode at a few 100 kHz too much. Getting it clean in the first place is probably the right thing to do. I think I'll pick up your suggestion of adding a film capacitor somewhere, though I assume that the electrolytics aren't all that bad here because we have some very high series resistance in the filter.

Samuel
 
A peak 2A into the zener would be utterly impossible from an approx. 48V feed through two 6.8k resistors... On center-tap-fed transformer assemblies, the available current would be slightly higher perhaps, but never 2A. To generate 2A peak through 6.8KΩ, you'd need a peak voltage of 1,360V... even with the zener replaced by a hard short! -Betcha this never happens! :wink:

Even so, the duration of any peak current would be much too small to have a destructive (heating) influence, the key part of which is surely going to be RMS, more than peak.

Nothing actually wrong with 5W devices, but you'd need 4 of them, and 20Watts worth of Zener diode protection per mic input is expensive, bulky and -as far as I can see- unneccesary. I'd put money on the fact that you can use 'regular' sized 400mW zeners and never EVER pop one. -Certainly, I see this input protection arrangement used on lots of transformerless mic pres and I've never seen a 5W zener needed for the job.

Certainly, CMRR will roll off into the high-frequency realm and that probably even makes the inclusion if a film cap an even more sensible addition in the long run.

Good work!

:thumb:

Keef
 
oh i just saw this thread for the first time. I'll write a reply later when I get home, i'm really busy at work right now.. in fact i shouldn't even be reading this forum.. :shock:
 
[quote author="SSLtech"]A peak 2A into the zener would be utterly impossible from an approx. 48V feed through two 6.8k resistors... On center-tap-fed transformer assemblies, the available current would be slightly higher perhaps, but never 2A. To generate 2A peak through 6.8K?, you'd need a peak voltage of 1,360V... even with the zener replaced by a hard short! -Betcha this never happens! :wink:

Even so, the duration of any peak current would be much too small to have a destructive (heating) influence, the key part of which is surely going to be RMS, more than peak.

Nothing actually wrong with 5W devices, but you'd need 4 of them, and 20Watts worth of Zener diode protection per mic input is expensive, bulky and -as far as I can see- unneccesary. I'd put money on the fact that you can use 'regular' sized 400mW zeners and never EVER pop one. -Certainly, I see this input protection arrangement used on lots of transformerless mic pres and I've never seen a 5W zener needed for the job.

Certainly, CMRR will roll off into the high-frequency realm and that probably even makes the inclusion if a film cap an even more sensible addition in the long run.

Good work!

:thumb:

Keef[/quote]

This has been well documented by THAT et al for a white paper they wrote. The source of transient current is stored in the input blocking capacitors so not limited by 6.8Ks but the low ohm series input resistors.

I for one differ by degrees with their worst case assessments (due to my use of smaller input caps etc.), but this is a valid fault vector and also perhaps responsible for damaging other gear plugged into phantom power.

JR
 
AH!!!! -I get it now!

There I was looking at the Phantom supply current path, when there was another path, staring me in the face!

Right then... -As you were.

Keef
 
[quote author="Samuel Groner"]I don't mind going a bit lower in voltage--what would you recommend as maximum?[/quote]
I would say up to 42V output should be safe enough to be reliable in critical situations (one-time-only recordings). If you do want the full 48V and use this chip, have a look at this EDN Design Idea. A 2N7000 or other low-power N-FET is probably fine if you're going to power just one microphone.

JDB.
 
Thanks--cool idea and articel. Even more convenient the whole article as PDF: 090105di.pdf

I don't need the separate 5 V input feed when using the LT1073, as its input is rated up to 12.6 V, right?

Can I replace D2 with a 1N914B? Looks like they'd have similar characteristics.

Samuel
 
[quote author="Samuel Groner"]I don't need the separate 5 V input feed when using the LT1073, as its input is rated up to 12.6 V, right?[/quote]
Right.

[quote author="Samuel Groner"]Can I replace D2 with a 1N914B? Looks like they'd have similar characteristics.[/quote]
The BAS21 has higher max reverse voltage, but that shouldn't be an issue here. Same goes for D1: just use any >60V >200mA Schottky diode.

JDB.
 
Hmm, you could use something like a TL5001 or a UC38xx series current mode controller. The 5001 is simplest since it is voltage mode, although you have to provide your own compensation with both of the chips (R and C in series, choosing correct values of course, from FB to COMP should be good enough) and you can basically run the chip with low frequency response so it acts like an open loop boost. Also, you can run much higher in frequency which allows you to use smaller value (and cheaper) caps for the same filtering. Have you considered a charge pump? They are good at these low loads, although I think they might have poorer noise levels than the boost. Don't exceed your caps ripple current rating if you are using electrolytics. Try the Panasonic FC or FM or FK series,or the United Chemi-Con equivalents, they have good ripple current ratings. Also, you should add a few .1uF ceramic caps in parallel with your output caps.
-Mike
 
Hi,

Samuel, as far as I remember this Sony circuit was designed for phantom powering condensers microphones in a Betacam camera.
This is just what you're searching, are'nt you?
You have to ask a Sony broadcast agency for more info.

Guy
 
Is the Sony part really worth the price?
http://eshop.euras.com/detailsseite.asp?login=12061&pic=1271923&seo=1
http://www.centralpoint.nl/frame.php?url=http%3A//www.centralpoint.nl/p/Sony/146475811/Converter_Dc-dc_%28cd-54%29.html
 
[quote author="guy_4"]Hi,

Samuel, as far as I remember this Sony circuit was designed for phantom powering condensers microphones in a Betacam camera.
This is just what you're searching, are'nt you?
You have to ask a Sony broadcast agency for more info.

Guy[/quote]

If Sony part is for dedicated design (known mic) it may not provide adequate phantom current for all applications.

JR
 
Thanks for the discussion so far. I decided to go with the original plan and not use the Sony part for cost reasons, because I have some of the parts allready ordered and because this would be an easy (I hope) start with switching design.

I drew a more detailed schematic of the current plan: [removed, see first page]

May I ask for a quick sanity check before I start breadboarding? Thanks!

Samuel
 
Back
Top