Hello,
I'm designing a +-15V dual rail low noise power supply, 200mA mini per rail. A first version was the classic design with tapped transfo and a pair of LM317 and LM337 regulators. But because of the price and size, I want to make it with no transfo.
I'll use a 9Vdc or 18Vdc SMPS. First idea was to use buck / boost dual (positive and negative) converters. I find some IC's like LTC3265 (perfect ICs but not enough current, and really expensive!!), LM27762, TPS6513x, LT8335... But not easy to choose the good one and the corresponding architecture.
Do you have any recommendations, sources, schematic, to help me to design it well?
Thanks a lot!
I'm designing a +-15V dual rail low noise power supply, 200mA mini per rail. A first version was the classic design with tapped transfo and a pair of LM317 and LM337 regulators. But because of the price and size, I want to make it with no transfo.
I'll use a 9Vdc or 18Vdc SMPS. First idea was to use buck / boost dual (positive and negative) converters. I find some IC's like LTC3265 (perfect ICs but not enough current, and really expensive!!), LM27762, TPS6513x, LT8335... But not easy to choose the good one and the corresponding architecture.
Do you have any recommendations, sources, schematic, to help me to design it well?
Thanks a lot!
Hi Ben,
Never use Buck-Boost in audio as they are noisy switchers. When talking about low noise, the SMPS has no place.
You can realize the +/-15V with some of TI ultra low noise LDOs . In this table - https://www.ti.com/power-management/linear-regulators-ldo/products.html#sort=238max;desc& , you can choose the best ultra low noise LDO for either positive or negative rails.
After your +/-18V SMPS, use good filtering, then use the desired LDOs for +/-15V. Don't forget the good SMPS shielding.
Never use Buck-Boost in audio as they are noisy switchers. When talking about low noise, the SMPS has no place.
You can realize the +/-15V with some of TI ultra low noise LDOs . In this table - https://www.ti.com/power-management/linear-regulators-ldo/products.html#sort=238max;desc& , you can choose the best ultra low noise LDO for either positive or negative rails.
After your +/-18V SMPS, use good filtering, then use the desired LDOs for +/-15V. Don't forget the good SMPS shielding.
A common mode choke in the output from a SMPS will filter out the noise but will work best if the switching frequency is relatively high - >50kHz.
I guess the same would apply to a buck/boost converter but find one with a high switching frequency and be prepared to enclose it in a metal cover (eg thin copper sheet from Amazon)
On the other hand, for a15v 200ma suppy, small dual 15v 5-6watt toroidal transformers are snall and cheap.
I guess the same would apply to a buck/boost converter but find one with a high switching frequency and be prepared to enclose it in a metal cover (eg thin copper sheet from Amazon)
On the other hand, for a15v 200ma suppy, small dual 15v 5-6watt toroidal transformers are snall and cheap.
You might want to look at the LT1965 and LT3015 which are newer and lower noise and very low dropout voltage. Available in 5pin TO220 package.
Also, instead of a toroid, how about a pcb mounting trafo?
see here circuit based on TPS7A39 https://www.patreon.com/posts/gotovye-bloki-su-104730643
The MeanWell PD-2515 costs $16.92 at Mouser (link). Sometimes it's in-stock at Amazon, other times not. PD-2515 an SMPS, 107 x 61 millimeters in size (4.2" x 2.4"), takes in 115VAC or 230VAC, and puts out both +15.0VDC and -15.0VDC at 800 milliamps each. Because it's an SMPS, it's fully self-contained, you don't need an external power transformer. The Mouser sales pages includes a link to MeanWell's engineering datasheet.
If you wish, you can follow PD-2515 with a pair of diyAudio Store SMPS_DC_filter cards (here's their page) for HF noise removal. Or, if you seek even more elaborate filtering, substitute a pair of "AmyAlice" filter cards, which you assemble yourself as a true DIY project. Forum Search will lead you to the AmyAlice discussion thread if you're interested.
If you wish, you can follow PD-2515 with a pair of diyAudio Store SMPS_DC_filter cards (here's their page) for HF noise removal. Or, if you seek even more elaborate filtering, substitute a pair of "AmyAlice" filter cards, which you assemble yourself as a true DIY project. Forum Search will lead you to the AmyAlice discussion thread if you're interested.
What you seem to mean here is no 50/60Hz power transformer. Line-operated switching supplies DO have a transformer, it's just smaller and the power goes through it at a much higher frequency. The chips you discuss operate on something like 60VDC or less, so you'd still have the problem of how to get that DC voltage, isolated from the power line. Making a power-line-operated SMPS requires greater than average skill, as well as excellent attention to safety, so overallis not recommended. At such low power, the usual 50/6Hz transformer is a much easier design.
Mark Johnson wrote:
I'm sure these filters do well individually, but if you connect them to the standard +/- output supply with the common ground, the "positive" side of the negative filter will be shorted out by the ground connection of the positive filter. This can be fixed by "inverting" the polarized components and swapping the positive and negative terminals of both input and output, but it seems like a kludge and can add to confusion and the chances of incorrect connections.
I've been frustrated by the lack of power supplies with dual outputs (two independent, unconnected outputs with same voltage and current rating). This separation of common ground would make the addition of such filters easy (and also, further linear regulation using two identical regulator circuits), and the ground connection would be at the final output of the whole thing.
It appears the problem for SMPS manufacturers is regulation, being able to detect the voltage outputs and feedback to the PWM circuit, but I think that could still be done with independent outputs with a minimal addition of extra components. This is mainly needed for power amplifiers, where the old-fashioned 50/60Hz transformers get really big and heavy.
Although it may feel like a Scary Dangerous Monster to some people, several members of these Forums have successfully built dual (bipolar) power supply filters using two Store PCBs, a little ingenuity, and a lot of LabelMaker (link) labels to remind themselves which is the positive supply PCB and which is the negative. In every case the builders got it right on the first try, without damaging the SMPSs or the polarized capacitors or the unidirectional TVS diodes. It can be done, it has been done, it will be done again.
Delightfully, viewing the situation from the opposite direction reveals a silver lining: there may well be a niche market for a new product here, available to ambitious entrepreneurs: Engineer a single PCB which contains two SMPS filters, one for (V+ and GND), another for (V- and GND), meticulously labeled to make it crystal clear where the positive input connects, where the positive output connects, where the negative input connects, and where the negative output connects. Now the Scary Monster is slain and incinerated; one board does both jobs. You might even decide to simply make two copies of the existing Store circuit (schematic is public domain), reducing the project scope so it's nothing more than a PCB layout job.
Delightfully, viewing the situation from the opposite direction reveals a silver lining: there may well be a niche market for a new product here, available to ambitious entrepreneurs: Engineer a single PCB which contains two SMPS filters, one for (V+ and GND), another for (V- and GND), meticulously labeled to make it crystal clear where the positive input connects, where the positive output connects, where the negative input connects, and where the negative output connects. Now the Scary Monster is slain and incinerated; one board does both jobs. You might even decide to simply make two copies of the existing Store circuit (schematic is public domain), reducing the project scope so it's nothing more than a PCB layout job.
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