Author Topic: Transformer tap switching between series and parallel  (Read 3731 times)

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Offline nemail2

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Re: Transformer tap switching between series and parallel
« Reply #50 on: June 13, 2021, 01:07:51 pm »
My guess is (not sure I'll explain it properly)  that there is a 'shift' in the secondary voltages w.r.t. to your system ground which is interfering with the way your ideal-bridge IC is functioning.

Can you look at the current being drawn from an ordinary bridge-rectifier and the ideal-rectifier?  Also, 'scope the actual secondaries (scope ground=output 0V), you might see a difference between the ordinaryBR and idealBR.

yes, something like a "shift" i was also putting together in my mind from what i have seen and measured. only way i currently could explain why C2 seems to be completely uneffective.

current being drawn from the positive output terminal of the rectifiers to the rest of the circuit you mean?

scope the secondaries: scope ground to the negative output of the rectifiers and then measure directly a the top terminal of the transformer and then the center where the middle terminals of the transformer are connected together?

also: do you think there could be a way around this? i really like the possibility of the space saving switching of the windings with a P-FET and not having to worry about the contacts of a switching relay at all but having to stick with a standard diode based rectifier for this seems too much of a drawback for me. i'd then have to cope with heatsinking on the rectifier again + deal with the voltage drop. the zero voltage drop came in quite handy...
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Offline nemail2

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Re: Transformer tap switching between series and parallel
« Reply #51 on: June 13, 2021, 05:39:14 pm »
just found this thread on the LT forums - maybe related to this issue...? https://ez.analog.com/power/f/q-a/115706/lt4320-with-center-tapped-transformer

i also posted this question there, it is currently in the moderation queue though...

edit: did some measurements.

Ch1: IC10 pin 2
Ch2: IC10 pin 7
Ch3: IC10 pin 6

SDS00008: everything connected as in schematics, no button pressed
SDS00009: IC10 pin 6 (positive output) disconnected from C1 and thus from the rest of the circuit. Voltage flattens out at around 29V as ist should be
SDS00010: everything conencted as in schematics, button for top winding disconnection pressed. the output of the whole circuit falls to about 9.5V (not seen in scope shot), the output of IC10 drops a liiiiittle bit and the mosfet gate drive changes a tiny bit as well

pins 3 and 4 of IC10 (ground mosfet gate driving pins) look perfectly square, non of that weird business which is going on at the positive mosfet gates. not sure what's that about but as that doesn't really change even with pin 6 of IC10 disconnected from the rest of the circuit, where the voltage stabilizes at the expected leve, i reckon that's just as it should be?

geez, I'm out of ideas.. anyone got any clue? please :)

« Last Edit: June 13, 2021, 09:45:52 pm by nemail2 »
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Offline nemail2

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Re: Transformer tap switching between series and parallel
« Reply #52 on: June 13, 2021, 10:50:33 pm »
dios mio, can this be the solution?
https://www.diyaudio.com/forums/group-buys/333844-ideal-bridge-rectifier-gb-post5911437.html

i am scared to try, though  :-DD especially on the toroidal transformer which exactly specifies how to put the windings in series or in parallel...
« Last Edit: June 13, 2021, 10:55:46 pm by nemail2 »
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Offline fcb

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Re: Transformer tap switching between series and parallel
« Reply #53 on: June 14, 2021, 12:31:16 pm »
I've spent a few minutes simulating the LT4320 in roughly your configuartion in LTSpice (see attached). Including reversing one of the secondaries.

TLDR; LT4320 won't work with a split supply.
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Offline nemail2

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Re: Transformer tap switching between series and parallel
« Reply #54 on: June 14, 2021, 06:38:50 pm »
I've spent a few minutes simulating the LT4320 in roughly your configuartion in LTSpice (see attached). Including reversing one of the secondaries.

TLDR; LT4320 won't work with a split supply.

thanks for your time and effort!
regarding the result: well, crap...

any ideas? I really liked the possibility to switch with a FET instead of a relay and also not having to heatsink the bridge rectifier...
I'm out of ideas, obviously. If I had to decide between cooling the BR or going with a relay instead of a FET for switching taps, I guess I'd go with the relay. And I think that's probably not a smart choice. But I just really don't want to deal with any additional major heating within the case and having to worry about airflow around the BR and stuff.

edit:
I revisited that diode topic in my mind and was wondering (again, but harder), why the rectifier diodes in my scenario got so hot, before I chose to use the LT4320 controller.
I was using these https://www.mouser.at/datasheet/2/427/vs-16edu06-m3-1769326.pdf and they had around 80-100°C under full load which was "only" 4A (currently I am aiming towards 5A). Looking at the datasheet, they should have around 0.9V of Vf drop each and that multiplied with 4A gives me 3.8W which then again leads to 6,46°C temp rise according to the datasheet (RthJ to solder pad). I even had heatsinks on them back then but still they were cooking.
Could this be due to the 20000uF of capacitance right after them? The transformer in use is afaik not underrated (4.17A) so I don't think there should be too much ripple which would lead to constant capacitor charging/discharging at high currents...?

Thanks!
« Last Edit: June 14, 2021, 07:08:37 pm by nemail2 »
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Offline fcb

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Re: Transformer tap switching between series and parallel
« Reply #55 on: June 14, 2021, 07:41:47 pm »
I think there are a fewof options if you care about heat dissipation over BOM cost, and don't want to visit switching techniques.  But probably the easiest is to give each secondary it's own LT4320 and effectively build two isolated rectifier circuits, stack the outputs and then feed then switch between them with MOSFET's.

If you wanted to use a few more MOSFET's then I'm sure you could arrange some sort of circuit that allowed you to parallel the two seperate rectified outputs (for more current) or stack them (for more voltage).  Or perhaps just use a DPDT relay.  Of-course if you had a DPDT relay, you could just switch the transformer secondaries and have a vastly reduced circuit with a single LT4320 and no requirement for more MOSFET.
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Online Ian.M

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Re: Transformer tap switching between series and parallel
« Reply #56 on: June 14, 2021, 08:04:46 pm »
Another alternative for a center-tapped bridge is to simply use separate dual ideal diode controllers for the positive and negative side MOSFET rectifiers.

I've attached a sim of the negative side of such an arrangement using a LTC4354 controller.  Unfortunately LTspice doesn't seem to have a model for a positive side dual ideal diode controller that's as simple and easy to use as the LTC4354, although A.D. do sell some.
 
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Offline nemail2

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Re: Transformer tap switching between series and parallel
« Reply #57 on: June 14, 2021, 08:05:54 pm »
I think there are a fewof options if you care about heat dissipation over BOM cost, and don't want to visit switching techniques.  But probably the easiest is to give each secondary it's own LT4320 and effectively build two isolated rectifier circuits, stack the outputs and then feed then switch between them with MOSFET's.

If you wanted to use a few more MOSFET's then I'm sure you could arrange some sort of circuit that allowed you to parallel the two seperate rectified outputs (for more current) or stack them (for more voltage).

That both sounds good but also rather expensive and quite board space intensive. I think that would be a classic example of "overdoing" stuff if I chose to do it that way for my intended purpose (although I'm kind of tempted tbh). I know I said I'd like to keep the ideal diode controller stuff, just an example of how undecided I am. I mean if the diodes would settle at around 50 or 60°C at full load, I guess I'd go for that + the P-FET for switching the second tap. probably way cheaper (P-FET + diodes way cheaper than ideal diode controller + SPDT relay). only thing I'm a bit worried, is whether it is allowed to use only one winding of a toroidal transformer.. the datasheet claims otherwhise... https://www.mouser.at/datasheet/2/410/VPT36_4440-781806.pdf

Or perhaps just use a DPDT relay.  Of-course if you had a DPDT relay, you could just switch the transformer secondaries and have a vastly reduced circuit with a single LT4320 and no requirement for more MOSFET.

That's how it is now, and it is working (see screenshot). But I'm worried about the current and arcing the relay has to withstand when switching taps under full load.

Regarding the hot BR diodes, before I was using the LT4320:
Playing with LTSpice showed me that the bridge rectifier diodes have to deliver way more current than what the load is sinking - can this be the reason why my diodes were heating up that much at only 4A load?
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Offline nemail2

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Re: Transformer tap switching between series and parallel
« Reply #58 on: June 14, 2021, 08:16:44 pm »
Another alternative for a center-tapped bridge is to simply use separate dual ideal diode controllers for the positive and negative side MOSFET rectifiers.

I've attached a sim of the negative side of such an arrangement using a LTC4354 controller.  Unfortunately LTspice doesn't seem to have a model for a positive side dual ideal diode controller that's as simple and easy to use as the LTC4354, although A.D. do sell some.

Thanks but I'm not sure if I understand this correctly - I don't need any negative voltage, I only need to switch between series and parallel connection (or at least between series and single) of the transformer taps. Would your proposed solution still work?

Thanks!
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Online Ian.M

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Re: Transformer tap switching between series and parallel
« Reply #59 on: June 14, 2021, 08:28:44 pm »
The problem with the LT4320 is that the load current must be purely between its OutP and OutN terminals, and cannot use a center tap of a secondary feeding In1 and In2 for a split supply.

My proposed LTC4354 circuit is the negative side of a classic bridge rectifier + secondary center tap split supply, which is best regarded as separate two diode full wave rectified positive and negative supplies.

Any split supply circuit can be used for half/full positive voltage supply - just move the ground from the center tap to the former negative rail and connect all the former ground pins together and to the center tap.
 
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Online bdunham7

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Re: Transformer tap switching between series and parallel
« Reply #60 on: June 14, 2021, 08:37:08 pm »
Regarding your heating concerns, are the transformer and voltage regulators going to be in the same enclosure?  If so, I can't imagine the heat from the bridge rectifier being significant compared to the heat from those.  Perhaps just reassessing what a proper, robust bridge would be might dissuade you from pursuing an ever more complex solution here.

https://www.allelectronics.com/item/fwb-352/35-a-200-piv-bridge-rectifier/1.html
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Offline nemail2

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Re: Transformer tap switching between series and parallel
« Reply #61 on: June 14, 2021, 09:16:23 pm »
The problem with the LT4320 is that the load current must be purely between its OutP and OutN terminals, and cannot use a center tap of a secondary feeding In1 and In2 for a split supply.

My proposed LTC4354 circuit is the negative side of a classic bridge rectifier + secondary center tap split supply, which is best regarded as separate two diode full wave rectified positive and negative supplies.

Any split supply circuit can be used for half/full positive voltage supply - just move the ground from the center tap to the former negative rail and connect all the former ground pins together and to the center tap.

thanks! I will try to wrap my head around this. any DaveCad sketch would be appreciated, though :D

Regarding your heating concerns, are the transformer and voltage regulators going to be in the same enclosure?  If so, I can't imagine the heat from the bridge rectifier being significant compared to the heat from those.  Perhaps just reassessing what a proper, robust bridge would be might dissuade you from pursuing an ever more complex solution here.

https://www.allelectronics.com/item/fwb-352/35-a-200-piv-bridge-rectifier/1.html

yes, they are going to be in the same enclosure. the thing is that:
1) there shouldn't be much heating of the diodes at all in the first place (mathematically, that would be 1,4V * 5A = 7W * 3K/W(1) = 21°C + 25°C (Tambient) = 46 °C)
2) when I had the design built up with a diode bridge rectifier, the diodes got around 100 °C hot, even with a heatsink

Note 1: 3K/W is for GBU8 with a 4x4x0.15inch copper plate mounted

So I'm wondering why they even got so hot and back then I just decided to go with the ultra-cool ideal diode controller and just don't bother with that issue anymore.
Back then I was using TO263AC-3 diodes (VS-16EDU06-M3/I) like mentioned before and even with those there shouldn't have been significant heating at the specified load of 4A.

The cooling solution of the pass transistors is mature or even overdone a bit. I have two BD139/TIP3055 Darlingtons mounted on a huge heat sink with silpads and even a 80mm fan. silent operation kinda guaranteed :-) heatsink: https://www.mouser.at/ProductDetail/ohmite/cr201-75ve/?qs=EU6FO9ffTweI3RXL9kLvyw==&countrycode=DE&currencycode=EUR

Regarding cooling of the diodes: other than slapping some heatsink on them there is no solution currently in my design nor much of free space in the case. I guess if I'd be using some GBU10 or something like that (or even a bigger rectifier like you suggested), I'd be able to put some heatsink on + somehow put that heatsink into the airflow of the main cooler. But first I'd really like to understand why the diodes got so hot in the older version of this design.

i have attached a screenshot of how the diodes were placed back then. on top of them i had also some small heatsinks (once I even tried with one larger BGA heatsink which covered all four of them). I was assuming that all the vias and copper should be able to dissipate quite some heat but still the diodes warmed up to 80-100°C...
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Online bdunham7

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Re: Transformer tap switching between series and parallel
« Reply #62 on: June 14, 2021, 09:45:55 pm »
yes, they are going to be in the same enclosure. the thing is that:
1) there shouldn't be much heating of the diodes at all in the first place (mathematically, that would be 1,4V * 5A = 7W * 3K/W(1) = 21°C + 25°C (Tambient) = 46 °C)
2) when I had the design built up with a diode bridge rectifier, the diodes got around 100 °C hot, even with a heatsink

Note 1: 3K/W is for GBU8 with a 4x4x0.15inch copper plate mounted

So I'm wondering why they even got so hot and back then I just decided to go with the ultra-cool ideal diode controller and just don't bother with that issue anymore.
Back then I was using TO263AC-3 diodes (VS-16EDU06-M3/I) like mentioned before and even with those there shouldn't have been significant heating at the specified load of 4A.

Something went wrong in your thermal design or calculations, but it's not uncommon for rectifiers to have a Tj quite a bit over 100C if you don't derate them.  Is there any room on your heatsink?  Just bolt on the part I linked and I'm very sure your problem will be solved forever.  You could also use TO-220 sized Schottky rectifiers if you want to reduce voltage drop a bit.  If you stick with your split-CT design with a one diode voltage drop, these would work well for all 5 diodes and might fit on your heat sink more easily.

https://www.mouser.com/datasheet/2/395/SRAF1620_SERIES_I1512-1918579.pdf
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Offline nemail2

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Re: Transformer tap switching between series and parallel
« Reply #63 on: June 14, 2021, 09:53:01 pm »
yes, they are going to be in the same enclosure. the thing is that:
1) there shouldn't be much heating of the diodes at all in the first place (mathematically, that would be 1,4V * 5A = 7W * 3K/W(1) = 21°C + 25°C (Tambient) = 46 °C)
2) when I had the design built up with a diode bridge rectifier, the diodes got around 100 °C hot, even with a heatsink

Note 1: 3K/W is for GBU8 with a 4x4x0.15inch copper plate mounted

So I'm wondering why they even got so hot and back then I just decided to go with the ultra-cool ideal diode controller and just don't bother with that issue anymore.
Back then I was using TO263AC-3 diodes (VS-16EDU06-M3/I) like mentioned before and even with those there shouldn't have been significant heating at the specified load of 4A.

Something went wrong in your thermal design or calculations, but it's not uncommon for rectifiers to have a Tj quite a bit over 100C if you don't derate them.  Is there any room on your heatsink?  Just bolt on the part I linked and I'm very sure your problem will be solved forever.  You could also use TO-220 sized Schottky rectifiers if you want to reduce voltage drop a bit.  If you stick with your split-CT design with a one diode voltage drop, these would work well for all 5 diodes and might fit on your heat sink more easily.

https://www.mouser.com/datasheet/2/395/SRAF1620_SERIES_I1512-1918579.pdf

dunno what went wrong. isn't large capacitance after the rectifier diodes affecting them negatively in terms of heat generation?
I'd have to put another heatsink in, the current one is occupied with the two TIP3055 and i don't think it could carry the additional heat from 5 diodes...
Attached a photo of the situation
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Online bdunham7

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Re: Transformer tap switching between series and parallel
« Reply #64 on: June 14, 2021, 10:20:07 pm »
dunno what went wrong. isn't large capacitance after the rectifier diodes affecting them negatively in terms of heat generation?
I'd have to put another heatsink in, the current one is occupied with the two TIP3055 and i don't think it could carry the additional heat from 5 diodes...
Attached a photo of the situation

If you can line them all up along the top, I doubt that they would have much impact on a fan cooled heat sink like that.  Or bolt a small aluminum plate to the top that overhangs and then bolt them to the underside.

The filter cap size does affect the diodes to a certain extent because it narrows the conduction window by reducing the per-cycle voltage droop.  That means that your average current has to flow in less time--so if the conduction time is 33% at full load, your peak current will be 3X full load.  This means your voltage losses are higher and the diode ends up dissipating more than 3X the average during the peak--and therefore more than if the current was constant.  I think your original diodes were not underrated, just a combination of small and minimally heat sinked--and you want them to run cool.  If you look at the datasheet for the Schottky I linked, you'll see that at 12A Vf is 0.6V, so since  you have two diodes conducting at any one time, you have 4*0.6*2 = 4.8 watts dissipated at full load.  5 watts can cook small parts even with some PCB heat sinking, but 4 TO-220s on a fan cooled aluminum heat sink, not likely.
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Offline nemail2

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Re: Transformer tap switching between series and parallel
« Reply #65 on: June 15, 2021, 08:21:58 pm »
If you can line them all up along the top, I doubt that they would have much impact on a fan cooled heat sink like that.  Or bolt a small aluminum plate to the top that overhangs and then bolt them to the underside.

that top area you mean? quite nice idea I must admit, although I'd be concerned of the long wires + maybe some interference with the pass elements...? This PSU is quite precise, only around 1-8mV absolute error from 0-24V, settable in 1mV steps and I wouldn't want to introduce a huge error/interference source into that, if that'd even be a concern... (i honestly don't know).

BUT probably the biggest issue is, that there is very little clearance to the top of the case there, so hot rectifiers would very likely be melting the case there (assuming they'll still get considerably hot while being on such a heatsink, which they probably wouldn't)...

Anyway, I guess I'll figure out some way for this to happen.

The filter cap size does affect the diodes to a certain extent because it narrows the conduction window by reducing the per-cycle voltage droop.  That means that your average current has to flow in less time--so if the conduction time is 33% at full load, your peak current will be 3X full load.  This means your voltage losses are higher and the diode ends up dissipating more than 3X the average during the peak--and therefore more than if the current was constant.  I think your original diodes were not underrated, just a combination of small and minimally heat sinked--and you want them to run cool.  If you look at the datasheet for the Schottky I linked, you'll see that at 12A Vf is 0.6V, so since  you have two diodes conducting at any one time, you have 4*0.6*2 = 4.8 watts dissipated at full load.  5 watts can cook small parts even with some PCB heat sinking, but 4 TO-220s on a fan cooled aluminum heat sink, not likely.

Thanks! Is there some rule of thumb formula how I can calculate the conduction time and the peak current? I wasn't able to find anything on the internet, maybe I googled wrong...
What about a load which rapidly switches a high current (to be precise, I have a very cheap electronic load from aliexpress which does this)? That load seems to be switching to achieve the set current draw, I could imagine that this load could be maybe drawing loads of current more from the caps than it should which in turn causes the caps to be recharged much "harder" by the diodes?

I guess I'll give it another shot with the diodes, veeery likely with Schottkys like you suggested. Any advice on the reverse voltage rating with a 12/24VAC toroidal transformer? Would something like 40V be sufficent?
Unfortunately every try costs quite some significant bucks and time, unless I go ahead and desolder all the small, expensive parts and reuse them on the next prototype (not knowing whether an what damage I might have caused to them buy desoldering/resoldering them).
Sometimes I think I should have separated the power board from the analog board so I could respin them independently but then again I always think "doesn't matter, it will be the last revision"  :-DD

Thank you again for your help!
« Last Edit: June 15, 2021, 08:24:34 pm by nemail2 »
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Offline nemail2

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Re: Transformer tap switching between series and parallel
« Reply #66 on: June 15, 2021, 10:09:36 pm »
https://www.mouser.com/datasheet/2/395/SRAF1620_SERIES_I1512-1918579.pdf

How about 5 of these? https://www.mouser.at/ProductDetail/Diodes-Incorporated/SDT30100VCT?qs=5666Nh5RPmax9ZPmI%2FKCtg%3D%3D
On the Mouser product page it says single but in the datasheet there seem to be two diodes - but I guess I could use them in parallel as one diode, probably they are meant to be used that way as the datasheet nowhere mentions it being two diodes in one case...

The diodes you suggested are kind of unavailable, so I searched the cheapest >=65V >=15A TO220 schottky diodes and this model is what i have found...
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Online bdunham7

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Re: Transformer tap switching between series and parallel
« Reply #67 on: June 15, 2021, 10:29:04 pm »
that top area you mean? quite nice idea I must admit, although I'd be concerned of the long wires + maybe some interference with the pass elements...? This PSU is quite precise, only around 1-8mV absolute error from 0-24V, settable in 1mV steps and I wouldn't want to introduce a huge error/interference source into that, if that'd even be a concern... (i honestly don't know).

BUT probably the biggest issue is, that there is very little clearance to the top of the case there, so hot rectifiers would very likely be melting the case there (assuming they'll still get considerably hot while being on such a heatsink, which they probably wouldn't)...

I doubt the wires would cause an issue at mains frequency, although you never know until you try!  If the top is tight, my suggestion of an additional plate might work.  Just cut a 2 or 3mm thick aluminum plate and attach part of it to the heatsink and have part of it hang over and screw the diodes on the bottom side, then insulate the topside of everything to keep the case cool  You could even bend the plate down towards the board to keep the wires short.

[/quote]
Thanks! Is there some rule of thumb formula how I can calculate the conduction time and the peak current? I wasn't able to find anything on the internet, maybe I googled wrong...
What about a load which rapidly switches a high current (to be precise, I have a very cheap electronic load from aliexpress which does this)? That load seems to be switching to achieve the set current draw, I could imagine that this load could be maybe drawing loads of current more from the caps than it should which in turn causes the caps to be recharged much "harder" by the diodes?

I guess I'll give it another shot with the diodes, veeery likely with Schottkys like you suggested. Any advice on the reverse voltage rating with a 12/24VAC toroidal transformer?
[/quote]

Rules of thumb are not all that accurate in a case like yours because there are too many unknowns--it's easier just to measure them. For example, how much does the peak voltage sag under full load?  You would have to check with a scope to really know.  But you should know that the AC RMS current will be higher than the DC current (1.5-3X)  which has implications for your transformer selection and that the filter cap size does have an effect on this ratio, just not a big one in any design with reasonably sized capacitors. 1-2mF per ampere of output seems reasonable.

As far as your fast switching load, it shouldn't really put any significant stress on your capacitors or rectifiers, but the linear regulator may have an inadequate frequency response and may not be able to keep up, so you would want a decoupling capacitor right at the input of the switching supply.  And some linear regulators don't like capacitive loads because it slows down their feedback response and can cause oscillation, so....  Again, check with a scope.

A reverse voltage of 40V would probably be just barely enough, but I'd go with 60V just for some safety margin.  This increases your forward voltage and power dissipation, but probably not enough to matter.
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Online bdunham7

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Re: Transformer tap switching between series and parallel
« Reply #68 on: June 15, 2021, 10:36:25 pm »

How about 5 of these? https://www.mouser.at/ProductDetail/Diodes-Incorporated/SDT30100VCT?qs=5666Nh5RPmax9ZPmI%2FKCtg%3D%3D
On the Mouser product page it says single but in the datasheet there seem to be two diodes - but I guess I could use them in parallel as one diode, probably they are meant to be used that way as the datasheet nowhere mentions it being two diodes in one case...

The diodes you suggested are kind of unavailable, so I searched the cheapest >=65V >=15A TO220 schottky diodes and this model is what i have found...

Those will work.  There are two diodes in each case, but there's no equivalent common anode design so you may as well just parallel the two cathodes.  One drawback of this part is that the metal tab is not insulated but is connected to pin 2, which makes attaching it to a heat sink a little trickier. 
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Offline nemail2

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Re: Transformer tap switching between series and parallel
« Reply #69 on: June 15, 2021, 10:42:59 pm »
that top area you mean? quite nice idea I must admit, although I'd be concerned of the long wires + maybe some interference with the pass elements...? This PSU is quite precise, only around 1-8mV absolute error from 0-24V, settable in 1mV steps and I wouldn't want to introduce a huge error/interference source into that, if that'd even be a concern... (i honestly don't know).

BUT probably the biggest issue is, that there is very little clearance to the top of the case there, so hot rectifiers would very likely be melting the case there (assuming they'll still get considerably hot while being on such a heatsink, which they probably wouldn't)...

I doubt the wires would cause an issue at mains frequency, although you never know until you try!  If the top is tight, my suggestion of an additional plate might work.  Just cut a 2 or 3mm thick aluminum plate and attach part of it to the heatsink and have part of it hang over and screw the diodes on the bottom side, then insulate the topside of everything to keep the case cool  You could even bend the plate down towards the board to keep the wires short.

Thanks! Is there some rule of thumb formula how I can calculate the conduction time and the peak current? I wasn't able to find anything on the internet, maybe I googled wrong...
What about a load which rapidly switches a high current (to be precise, I have a very cheap electronic load from aliexpress which does this)? That load seems to be switching to achieve the set current draw, I could imagine that this load could be maybe drawing loads of current more from the caps than it should which in turn causes the caps to be recharged much "harder" by the diodes?

I guess I'll give it another shot with the diodes, veeery likely with Schottkys like you suggested. Any advice on the reverse voltage rating with a 12/24VAC toroidal transformer?
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Rules of thumb are not all that accurate in a case like yours because there are too many unknowns--it's easier just to measure them. For example, how much does the peak voltage sag under full load?  You would have to check with a scope to really know.  But you should know that the AC RMS current will be higher than the DC current (1.5-3X)  which has implications for your transformer selection and that the filter cap size does have an effect on this ratio, just not a big one in any design with reasonably sized capacitors. 1-2mF per ampere of output seems reasonable.

As far as your fast switching load, it shouldn't really put any significant stress on your capacitors or rectifiers, but the linear regulator may have an inadequate frequency response and may not be able to keep up, so you would want a decoupling capacitor right at the input of the switching supply.  And some linear regulators don't like capacitive loads because it slows down their feedback response and can cause oscillation, so....  Again, check with a scope.

A reverse voltage of 40V would probably be just barely enough, but I'd go with 60V just for some safety margin.  This increases your forward voltage and power dissipation, but probably not enough to matter.
[/quote]


I doubt the wires would cause an issue at mains frequency, although you never know until you try!  If the top is tight, my suggestion of an additional plate might work.  Just cut a 2 or 3mm thick aluminum plate and attach part of it to the heatsink and have part of it hang over and screw the diodes on the bottom side, then insulate the topside of everything to keep the case cool  You could even bend the plate down towards the board to keep the wires short.

Rules of thumb are not all that accurate in a case like yours because there are too many unknowns--it's easier just to measure them. For example, how much does the peak voltage sag under full load?  You would have to check with a scope to really know.  But you should know that the AC RMS current will be higher than the DC current (1.5-3X)  which has implications for your transformer selection and that the filter cap size does have an effect on this ratio, just not a big one in any design with reasonably sized capacitors. 1-2mF per ampere of output seems reasonable.

As far as your fast switching load, it shouldn't really put any significant stress on your capacitors or rectifiers, but the linear regulator may have an inadequate frequency response and may not be able to keep up, so you would want a decoupling capacitor right at the input of the switching supply.  And some linear regulators don't like capacitive loads because it slows down their feedback response and can cause oscillation, so....  Again, check with a scope.

A reverse voltage of 40V would probably be just barely enough, but I'd go with 60V just for some safety margin.  This increases your forward voltage and power dissipation, but probably not enough to matter.

Ok, thanks! I'll try and develop a solution with five schottky diodes then

Those will work.  There are two diodes in each case, but there's no equivalent common anode design so you may as well just parallel the two cathodes.  One drawback of this part is that the metal tab is not insulated but is connected to pin 2, which makes attaching it to a heat sink a little trickier. 

initially i was worried about thermal drift of the two diodes but then i figured there are many of those TO220 devices which have two diodes inside but don't mention that fact in the datasheet. I guess they are supposed to be used as a single diode and there are two in them to meet the Vf goal..? So I guess they'll be ok...

I was also looking for a complementary common anode device but wasn't able to find one either (would have saved some space). but then, more heat in one case so maybe it is better to utilize 5 devices (cases) anyway.

Would you recommend to utilize devices with a plastic package? I was deliberately looking for diodes with a metal tab to ensure better heat transfer. if that wouldn't be of any concern, plastic cases would make mounting easier (and cheaper), of course (i was planning to use quite expensive silpads, just like on the pass elements)...

edit: filtering for plastic packages, these were the cheapest diodes if i wanted single diodes in a case: https://www.mouser.at/ProductDetail/Vishay-General-Semiconductor/VFT2080S-E3-4W?qs=HSPD0Bff7iaqHXrROfvV8A%3D%3D
I'm really not sure whether paralleling two diodes within a case would be a good idea (if they are specified as two single diodes) in terms of thermal runaway and frying themselves eventually...
« Last Edit: June 15, 2021, 10:49:01 pm by nemail2 »
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Online bdunham7

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Re: Transformer tap switching between series and parallel
« Reply #70 on: June 15, 2021, 11:03:37 pm »
edit: filtering for plastic packages, these were the cheapest diodes if i wanted single diodes in a case: https://www.mouser.at/ProductDetail/Vishay-General-Semiconductor/VFT2080S-E3-4W?qs=HSPD0Bff7iaqHXrROfvV8A%3D%3D
I'm really not sure whether paralleling two diodes within a case would be a good idea (if they are specified as two single diodes) in terms of thermal runaway and frying themselves eventually...

Those look good.  Plastic cases are fine--the devices can tolerate a significant thermal gradient in this application.  Paralleling diodes can be an issue but not in this case for two reasons--they are thermally bonded and will always be at almost the same temperature and in any case, either one can take the full current.
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Re: Transformer tap switching between series and parallel
« Reply #71 on: June 15, 2021, 11:58:50 pm »
edit: filtering for plastic packages, these were the cheapest diodes if i wanted single diodes in a case: https://www.mouser.at/ProductDetail/Vishay-General-Semiconductor/VFT2080S-E3-4W?qs=HSPD0Bff7iaqHXrROfvV8A%3D%3D
I'm really not sure whether paralleling two diodes within a case would be a good idea (if they are specified as two single diodes) in terms of thermal runaway and frying themselves eventually...

Those look good.  Plastic cases are fine--the devices can tolerate a significant thermal gradient in this application.  Paralleling diodes can be an issue but not in this case for two reasons--they are thermally bonded and will always be at almost the same temperature and in any case, either one can take the full current.
ok thanks!

one last question about something I'm not totally sure about - would i place the fuses like in the screenshot or one on the top and one in the middle after the series connection of the two windings?
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Re: Transformer tap switching between series and parallel
« Reply #72 on: June 16, 2021, 12:28:01 am »
one last question about something I'm not totally sure about - would i place the fuses like in the screenshot or one on the top and one in the middle after the series connection of the two windings?

It doesn't matter as long as the fuse isolates the winding.  In an unbranched circuit, the current has to be the same everywhere, so the fuse will blow in the same manner regardless of where it is.  And if it disconnects one side of the winding, that winding is effectively isolated.  You probably want a primary side fuse as well and I'd spec them so that the primary blows first in case of overloads, while the secondaries are a sort of last resort in case of some internal malfunction like a shorted rectifier that grossly overloads just one winding.
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Re: Transformer tap switching between series and parallel
« Reply #73 on: June 17, 2021, 01:48:37 am »
You probably want a primary side fuse as well and I'd spec them so that the primary blows first in case of overloads, while the secondaries are a sort of last resort in case of some internal malfunction like a shorted rectifier that grossly overloads just one winding.
Yes that maybe an ideal scenario but in practice matching primary fuses for a max secondary output is subject to fuse value availability and a suitable blow specification (fast or slow) and then there are ON inrush currents that also need be considered.
Multiple secondary outputs will make fusing the primary totally impractical.

IMO any output current limiting should be done in silicon in conjunction with secondary fusing close to the transformer.
 
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Re: Transformer tap switching between series and parallel
« Reply #74 on: June 19, 2021, 08:58:50 pm »
It doesn't matter as long as the fuse isolates the winding.  In an unbranched circuit, the current has to be the same everywhere, so the fuse will blow in the same manner regardless of where it is.  And if it disconnects one side of the winding, that winding is effectively isolated.  You probably want a primary side fuse as well and I'd spec them so that the primary blows first in case of overloads, while the secondaries are a sort of last resort in case of some internal malfunction like a shorted rectifier that grossly overloads just one winding.

You probably want a primary side fuse as well and I'd spec them so that the primary blows first in case of overloads, while the secondaries are a sort of last resort in case of some internal malfunction like a shorted rectifier that grossly overloads just one winding.

Yes that maybe an ideal scenario but in practice matching primary fuses for a max secondary output is subject to fuse value availability and a suitable blow specification (fast or slow) and then there are ON inrush currents that also need be considered.
Multiple secondary outputs will make fusing the primary totally impractical.

IMO any output current limiting should be done in silicon in conjunction with secondary fusing close to the transformer.
 

ok thanks! I do have a primary side fuse as well, which is rated for the main toroidal transformer + the second meanwell switching brick (IRM-05-12). In this case, the primary fuse is more like a last resort, I guess.
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