Multiple voltage SMPS regulation question - Page 1
Forum section: electronics / projects

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: December 25, 2022, 02:22:05 am
content: I have a multiple voltage PSU for HIFI amp as advertised +/-36V +/-15V and 12V all three isolated. How the regulator takes account of regulating each at the same time?Lets forget the low voltage ones, if the load is alternating from +36 to -36 how it can regulate alternatively? The regulator has a TL431.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 25, 2022, 05:23:16 am
content: multiple secondary winding transformer is key... regulator only on off at primary, transformer windings ratio will do the job. try retrace it (rev eng) to know. ymmv.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: December 25, 2022, 05:40:18 am
content: Thank you giving consideration for the subject. If you look at the picture, there is an optocoupler and an adjust, so the regulation is taken from the secondary. I have seen Hifi intended SMPS that as you describe use the primary, they are classified as non regulated.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 25, 2022, 06:02:24 am
content: yes primary is controlled from secondary by taking one reference at secondary through opto to the high voltage bjt/fet on primary. some topology use this way, usually using low voltage controller ic. there are high voltage controller ic in other topology resides in primary. try retrace it (rev eng) to know. ymmv.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: December 25, 2022, 06:59:55 am
content: It is new under warranty, I cannot open it. If Audiophiles prefer non regulated type there must be a reason. This is my first split supply by single PWM. When using two seperate, at low frequencies, each loads and unloads alternately half wave rectified and results poor bass.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 25, 2022, 10:20:08 am
content: the only one thats regulated is the one that its sensing, the other just follow and usually within spec of operating range. how its sensing will differ based on psu requirements. the only way to know is retrace it (rev eng) to know. if its under warranty, put it back together and enjoy the show... ymmv. ps: and you dont have to destroy anything to rev eng, not even to desolder... all you have to do is just look at the traces where they all go and draw schematics... and you've voided the warranty since the time you opened the enclosure... ymmv.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 25, 2022, 11:44:30 am
content: With multiple secondaries off a single transformer, you just regulate based on one of them (the one accuracy of which is most important). Rest will have poor load regulation.

author: magic
author_rank: Super Contributor
timestamp: December 25, 2022, 12:26:58 pm
content: They could also use some average of multiple rails or regulate 72V across the two 36V rails.None of it changes a fundamental property of such PSU: if one rail is loaded more than another, its voltage decreases more than the other's. So it is impossible for all rails to stay in regulation simultaneously, no matter how the regulator works.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 25, 2022, 05:47:43 pm
content: On the other hand, one needs to realize the two parts to regulation:1) line regulation, against changes in input voltage2) load regulation, against changes in internal voltage drops due to load currentIf the design is of high efficiency, voltage drop due to load changes can be quite small. Input voltage variations OTOH can be pretty large if it isn't an active PFC device and is designed for different voltages (e.g., anything from 200 to 240V).Even if only one output voltage is used for feedback, it provides line regulation for all outputs. It's just the load regulation part that's wrong.For active PFC design, the input boost stage would provide its own regulation and make stable input voltage from wide range of mains voltage (typically like 80 - 250 VAC).

author: Thunderer
author_rank: Regular Contributor
timestamp: December 25, 2022, 08:16:00 pm
content: Actually, it may be that no voltage is regulated at all.But, if the +/-36V is the monitored one, the other lower voltages do not create any drop at all, as they are used to power some tens of mA (preamps and small control).Now, an audio amplifier does not need regulated power supply. Why? Because the transients power reservoirs are the caps already on the amplifier PCBs. They are there to take care of small duration power peaks.Now, if your amplifier does create big voltage variation at the output of the power supply, then the power supply rating is inadequate, too small for your amplifier.What Audiophiles believe and what really happens in electronic circuits is not always in line with the reality about how electronics work. But, this is another's topic subject.It is true, most commercial amplifiers (last decades) were provided with sub-dimensioned power supply, but it was for economic reasons (low price). And some of them sound weak/bland. Some others can sound good. It is a question of luck, matching with speakers (low or high SPL).

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: December 25, 2022, 08:59:27 pm
content: I also thought as "magic" to be 72v regulated, if so, the TL431 anode should ring with -36v. It is grounded. The optocoupler receives the voltage of the TL431 cathode and compares with the voltage of +36v via a zener and a resistor.The primary impedance is in series with the secondaries. That is, if a load is applied on any output, the primary higher impedance falls because of little mains rectified reservoir capacitor. The regulator then works mainly to regulate the primary, on condition, the secondaries are preloaded to decrease the secondary diodes impedances and they are, by wounded high temperature 1W resistor on each output.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: December 25, 2022, 11:32:09 pm
content: The dynamic impedance of -36v is a disaster. Worst can happen if there is a capacitor between +&-36v. The decrease of -36v then increases the +36 which is unloaded. The regulator will sense the increase and pull down the -36v also which in turn increase further the +36v as positive feedback. The mutual +/-36v capacitance can arise by by weak grounding. Now, I understand why Audiophiles prefer the non regulated version or else it will still require an armada of reservoir capacitors on -36v.I will modify the regulator to make it as "magic" suggested, the whole 72v. the +36v links the optocoupler by 18v zener and 1k resistor with 1.2v across. I need to double the resistor 47k that feeds the TL431 from 36v and double the voltage of the soft start capacitor.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: December 26, 2022, 03:51:38 am
content: This is the error amp. The two diodes represent the led of optocoupler, 10M resistor is to make the model TL431 function. I forgot the soft start capacitor.And bellow is the 72v with the ground to be -36v.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: December 26, 2022, 05:37:43 am
content: The first solution can't work, the voltage of the TL431 exceeds the 36v.Much simpler solution, is to link the ground of the circuit to -36v via a 36v zener and touch nothing else.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: December 28, 2022, 12:01:05 am
content: By doubling the measured voltage, I also doubled the loop gain. On the other hand the error voltage due to load remains the same as the amp loads only on either rail.Should I decrease the gain or leave it as it is? The gain can be decreased by 1k resistor in series with the optocoupler's diode.

author: timenutgoblin
author_rank: Regular Contributor
timestamp: December 28, 2022, 10:31:10 am
content: Quote from: Kokoriantz on December 25, 2022, 02:22:05 amI think I found the PSU that you're referring to:https://m.aliexpress.com/i/33026435934.htmlHere is an image of the +/- 48VDC variant from the AliExpress website:U1 looks like a PWM controller (HVDC). U2 looks like an opto-coupler. Q1 looks like a switching MOSFET. There is a bridge rectifier next to Q1. T1 looks like a switching transformer. The component designators and layout look very similar to your photo.

author: Circlotron
author_rank: Super Contributor
timestamp: December 28, 2022, 12:09:36 pm
content: Quote from: Siwastaja on December 25, 2022, 11:44:30 amI've seen some PSUs with multiple rails that use a filter choke with each rail wound with it's own separate winding on the shared choke core. The amount of turns for each choke winding is according to that rail's voltage so there is a transformer action between all the windings that helps lock the voltages in a fixed relationship so they all more or less track the single regulated rail.The PSU in the picture doesn't look like it has a filter choke so it's probably a flyback, not a forward converter.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: December 28, 2022, 12:41:31 pm
content: This SMPS uses UC3842 controller that can run at 500khz.I have a modest $9 costing 48v/2A adverized as Led light PSU, has double CM EMI filter and LCL output. May be this one runs at much higher frequency and has soft start to charge high reservoir capacitors that the inductor is omitted. It costs about $30 with shipment only.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: December 29, 2022, 07:09:36 am
content: If you or anyone wants to modify for both polarities regulation here is the picture of what you should do. Cut the track from the G central pin leading to the anode of TL431 and link it by a zener diode 1N4753 36v to the -36V pin. I tried in dynamic with 50 ohm on +36V on and off while analog multimeter measuring the 36v, not a slight movement I could see. I have now a 24b/194k sound card Behringer UMC202, once applied on the amp, I'll do real measurement of noise and dynamic impedance.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 31, 2022, 01:28:36 pm
content: Since when did an audiophile need a reason for anything electrical?

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 31, 2022, 08:08:54 pm
content: he is not one yet... real audiophile will pay 4 digits for the cable and 5 digits for the psu that is synched to the earth's resonance...

author: negativ3
author_rank: Regular Contributor
timestamp: January 01, 2023, 03:46:58 pm
content: Suprised deoxidised copper welding electrodes aren't being coated with hot snot and marketed as OFC 100% nevermind the 99.999% rubbish.