SMPS for heaters

[TUBESMAN]

I'd like to use a switching supply to run the heaters in a stereo PP power amp (6.3V at around 6-7A). I want a SMPS because I think it will be easier to allow operation over a 100V-240V mains voltage range, lower cost, smaller size, less heat. I have a concern, noise from the supply feeding into the audio from whatever methods it might sneak in. I know most of it will be 50kHz and higher in frequency. So, anyone have an OEM PS they like? What level of filtering and shielding would you suggest? Or is this a bad idea no matter how you cut it?

[pergo]

i recently built an hybrid car amp with triode VAS.
Heater 6.3v come from a LM2678. It works at 250kHz.
I use 33uH+2200uF+100nF (near pins) for filtering. Absolutely no noise at all.
Everything is through hole design - no SMD.
Top layer is only for ground planes.

Very cheap, easy circuit. 0 problems.

[TUBESMAN]

sounds interesting. What current level is it working at and I guess your input voltage is DC.

[pergo]

Yeah, only dc-dc. That amp use several SMPS.

1- DC-DC from 12/14v to +-30V PWM regulated (sg3525) @80kHz. +-30V is filtered by LC with 330uH+3000uF each rail.

2- The second is the heater power supply, take the +30V and get 6.3V by LM2678 (250Khz) filtered by LC 30uH+2200uF+100nF

3- The third is the triode HV, 250V, from +30V step up by LM3478 (adj. frequency set 100kHz) filtered by a mosfet capacitance multiplier (RC cell on the gate is 10k+82uF)


Each smps is not-synchronized from other but i don't have any audible noise or hum. I can synchronize for better performances in future.

My heater current draw is less 1A (6H30 or similar)...not very much, but i'm very happy for the results.

[tomchr]

Originally Posted by TUBESMAN

I'd like to use a switching supply to run the heaters in a stereo PP power amp (6.3V at around 6-7A).

Actually, I'm building a Universal Filament Regulator using National Semiconductors' LM22673-series of SimpleSwitchers. Granted, those are DC-DC converters so the idea is to run the switchers from two 6.3 V windings in series. An AC-DC converter would be even more versatile, but also of much higher complexity due to the voltages involved. In addition, you'd have to design a transformer for the galvanic isolation. It's not an overly complicated task (my final project in college was an AC-DC converter), but the magnetic components are tricky to find.

That said, I'm getting truly excellent results with my switchers. I'm using them on a directly heated tube (300B), where I would have expected any impact of switching noise to be the worst. I've compared the switcher with a linear regulator and there isn't any significant difference between the two. In fact, I seem to recall the switcher to be slightly better.

I'm measuring a few mV of 500 kHz ripple on the outputs of my switching supplies when drawing 1.5 A at 5.0 V.

I went with switchers as I thought it was fairly ridiculous that I'd have to dissipate more power in the linear filament regulators than the amp would be able to deliver to the speakers. At >90 % efficiency, the switchers barely get lukewarm....

~Tom

[TUBESMAN]

I'm faced with the idea of either multiple transformers (or taps) for the range of mains voltages or go with a SMPS for heaters. I'm not yet knowledgeable on the design of many parts of such a supply. I have 3 books in my Amazon wish list on the design of SMPS, but they will cost me over $150 and many months to absorb.

I get from the reply's that its no big deal using them the way I have in mind.
I've seen OEM units all ready to run from Jameco that are near my voltage and current needs, and some are less than $20. Winding transformers is not an option.
What I might do is take one of the Jameco units with too much voltage, and feed it into a switcher of my own making to get the rail where I want it. That would be better than feeding it into a linear reg for the same purpose.
The other advantage the pre-made units have is UL, CE and ROHS certification.
So, a slight change in my question, who makes OEM SMPS's you like in the 7.5V, 6-7A output region?

[tomchr]

Originally Posted by TUBESMAN

What I might do is take one of the Jameco units with too much voltage, and feed it into a switcher of my own making to get the rail where I want it. That would be better than feeding it into a linear reg for the same purpose.

That certainly is an option. One could also entertain the thought of buying a 5 V supply of sufficient amperage and changing the feedback network to increase the output voltage to 6.3 V.

The other advantage the pre-made units have is UL, CE and ROHS certification.

The certs are mostly relevant if you're looking to go into production with your supplies. For a one-off hobby project, they don't matter as much.

So, a slight change in my question, who makes OEM SMPS's you like in the 7.5V, 6-7A output region?

Honestly, I haven't looked at the market in recent years, so I can't really give any recommendations.

Just one note, though... Many of the commercially available supplies have rather high output voltage ripple. I'd probably either filter the output or put a post-regulator on it. The former will be the most efficient.

~Tom

[TUBESMAN]

I would like to sell this project that is why certs are useful.
Modifying the feedback for a different output voltage is an idea I will look at.
I think I can rig a spectrum analyzer to the output and decide if extra filtering is needed.
All good information. I thank you all.