"Upgrading" a DC-DC Converter
Inducktion, Thu Mar 14 2019, 06:50AMHi everyone!
So, to upgrade a DC-DC converter;
I already replaced the fets with better rated ones.
Next on the list is the inductor....
The converter runs at 150 kHz, though I can change it to run at 225 kHz. As it stood the original inductor was rated at 15 uH.
If I want to increase the power it can handle, I obviously need to change the inductor to something better.
What core material would be better? Ferrite? Powdered iron? Some sort of fancy alloy?
Should I shoot for higher permeability? Is there a general guideline towards what material to not use?
thanks!
Re: "Upgrading" a DC-DC Converter
Conundrum, Thu Mar 14 2019, 07:19AM
Good tip, thanks.
My own experiments use a hand made primary constructed from twisted clock wire. Its a pig to wind but worth it!
Incidentally I am using the described modification to build a battery charger so watch this space! Actually more like "More POWAAAAH Igor!!"
Conundrum, Thu Mar 14 2019, 07:19AM
Good tip, thanks.
My own experiments use a hand made primary constructed from twisted clock wire. Its a pig to wind but worth it!
Incidentally I am using the described modification to build a battery charger so watch this space! Actually more like "More POWAAAAH Igor!!"
Re: "Upgrading" a DC-DC Converter
2Spoons, Fri Mar 15 2019, 12:23AM
post a schematic.
2Spoons, Fri Mar 15 2019, 12:23AM
post a schematic.
Re: "Upgrading" a DC-DC Converter
hen918, Fri Mar 15 2019, 08:10PM
Upgrading a commercial DC-DC converter is basically impossible unless it has been really badly designed in the first place. A MOSFET may seem to be of a lower rating than an alternative, however there are many factors that go into the selection of the MOSFETs and the other components, not just price. For a DC-DC converter, the design process goes like this:
Power & voltage rating changes Topology, component size/package (including transformers, inductors, etc), frequency, current/voltage rating of switches.
Frequency changes Inductance of inductors, turns on transformer, switching component speed, number of cores in high current wire (litz), Gate driver selection and design.
The switching frequency changes everything, from losses to the compensation network in the feedback, if you increase it, you are likely to make things worse. Losses depend on both gate charge and the Rds(on) of the switches, a higher switching frequency is likely to increase peak currents and switching losses, leading to rapidly overheating MOSFETs, MOSFETs that have a lower Rds(on) will have a higher gate charge, lowering conduction losses whilst increasing switching losses.
No (power) inductor operating at more than 1kHz should be powdered iron.
If you really want to go ahead you should increase the frequency and drop the inductance to maybe 10 µH whilst ensuring the resistance is lower. Keep the material the same (N87 or N97 or similar). Don't be surprised if things get hotter.
hen918, Fri Mar 15 2019, 08:10PM
Upgrading a commercial DC-DC converter is basically impossible unless it has been really badly designed in the first place. A MOSFET may seem to be of a lower rating than an alternative, however there are many factors that go into the selection of the MOSFETs and the other components, not just price. For a DC-DC converter, the design process goes like this:
Power & voltage rating changes Topology, component size/package (including transformers, inductors, etc), frequency, current/voltage rating of switches.
Frequency changes Inductance of inductors, turns on transformer, switching component speed, number of cores in high current wire (litz), Gate driver selection and design.
The switching frequency changes everything, from losses to the compensation network in the feedback, if you increase it, you are likely to make things worse. Losses depend on both gate charge and the Rds(on) of the switches, a higher switching frequency is likely to increase peak currents and switching losses, leading to rapidly overheating MOSFETs, MOSFETs that have a lower Rds(on) will have a higher gate charge, lowering conduction losses whilst increasing switching losses.
No (power) inductor operating at more than 1kHz should be powdered iron.
If you really want to go ahead you should increase the frequency and drop the inductance to maybe 10 µH whilst ensuring the resistance is lower. Keep the material the same (N87 or N97 or similar). Don't be surprised if things get hotter.
Re: "Upgrading" a DC-DC Converter
Inducktion, Sat Mar 16 2019, 01:22AM
^^^ Is the schematic, albeit it has a tacked on op amp for constant current control as well.
^^^ is the true schematic, though it's really blurry and I apologize. Again, the only difference between the two, is the op amp circuit added on.
As far as upgrading the fets, yes, I know that typically higher rated fets have greater switching losses due to increased gate charge; that's why I tried to get fets with essentially the same switching characteristics as the originals, but better.
The new fets are these, * by Vishay. Gate capacitance of 5100 pF, and the original fets were *, with a gate capacitance of 5000 pF.
Otherwise, the thunderfets are better in almost every way compared to the originals, AND, are known to be genuine.
Additionally, I chose a new inductor material; Magnetics-inc makes a variety of toroids with a material called KoolMu, designed for low loss and high saturation, so I figured it would be a good choice.
Inducktion, Sat Mar 16 2019, 01:22AM
^^^ Is the schematic, albeit it has a tacked on op amp for constant current control as well.
^^^ is the true schematic, though it's really blurry and I apologize. Again, the only difference between the two, is the op amp circuit added on.
As far as upgrading the fets, yes, I know that typically higher rated fets have greater switching losses due to increased gate charge; that's why I tried to get fets with essentially the same switching characteristics as the originals, but better.
The new fets are these, * by Vishay. Gate capacitance of 5100 pF, and the original fets were *, with a gate capacitance of 5000 pF.
Otherwise, the thunderfets are better in almost every way compared to the originals, AND, are known to be genuine.
Additionally, I chose a new inductor material; Magnetics-inc makes a variety of toroids with a material called KoolMu, designed for low loss and high saturation, so I figured it would be a good choice.
Re: "Upgrading" a DC-DC Converter
2Spoons, Sat Mar 16 2019, 03:38AM
KoolMu is a terrible choice for your application. Compared to 3C90 ferrite, its specific power loss is ten times higher, even for the best KoolMu (125u) - based on 200kHz @0.1T. That means 10x the core heating compared to plain old 3C90.
Use a ferrite core with an air-gap.
2Spoons, Sat Mar 16 2019, 03:38AM
KoolMu is a terrible choice for your application. Compared to 3C90 ferrite, its specific power loss is ten times higher, even for the best KoolMu (125u) - based on 200kHz @0.1T. That means 10x the core heating compared to plain old 3C90.
Use a ferrite core with an air-gap.
Re: "Upgrading" a DC-DC Converter
Inducktion, Sat Mar 16 2019, 03:53AM
What type of material should I shoot for? I have some ferrite toroids, I think they use W ferrite (
Or is an air gap a requirement for use with a DC-DC converter?
Edit:
This is the original toroid material:
(under KS090-125A)
Inducktion, Sat Mar 16 2019, 03:53AM
What type of material should I shoot for? I have some ferrite toroids, I think they use W ferrite (
Or is an air gap a requirement for use with a DC-DC converter?
Edit:
This is the original toroid material:
(under KS090-125A)Re: "Upgrading" a DC-DC Converter
2Spoons, Sun Mar 17 2019, 10:00PM
'W" material has a high mu, so with no air-gap (ie toroid) it will saturate easily. Its really meant for transformers and common mode chokes.
The toroid in your link is a 'pressed powder' type, and so effectively has a distributed air-gap - resulting in less nH/n^2 and higher saturation flux.
If you are trying to upgrade your converter I assume you want more power, so you can expect more loss too - unless you choose lower loss materials. Sendust looks about the same as KoolMu, and both are worse than ferrite (power grades) for power loss. Remember commercial design decisions also include cost as a parameter (sometimes its the highest priority), and that gets balanced against things like efficiency. Your priorities might be different - have a look at TI's webbench power converter designer - it has a knob you can adjust to balance size, cost and efficiency.
Inductors for this type of power converter need to be designed. Start with target output power, then calculate the peak current requirement and inductance you need. Pick a core material suitable for the frequency you will operate at, and that is obtainable in a core shape you like. I would pick a shape that comes pre-gapped if possible as it makes everything simpler - I like the RM am P style cores, but they may not come big enough for higher power systems. Power throughput is pretty much a function of frequency and core volume, so knowing the first two you can gauge how big a core you need (scan though the datasheets). Once you pick a core you can calculate the number of turns, and the wire gauge that will fill the bobbin ( to get minimum DCR). If the wire gauge is bigger than 2x skin depth you should consider multi-filar winding or litz wire, otherwise you are just wasting copper.
Does that help?
types L, R, P, F and T would all work for you. R, P and F seem to be available in more sizes. L looks like its intended for high frequency converters.
I'd say any of the 3C9x ferrites would be OK.
Final choice comes down to price, availability and any special requirements.
2Spoons, Sun Mar 17 2019, 10:00PM
'W" material has a high mu, so with no air-gap (ie toroid) it will saturate easily. Its really meant for transformers and common mode chokes.
The toroid in your link is a 'pressed powder' type, and so effectively has a distributed air-gap - resulting in less nH/n^2 and higher saturation flux.
If you are trying to upgrade your converter I assume you want more power, so you can expect more loss too - unless you choose lower loss materials. Sendust looks about the same as KoolMu, and both are worse than ferrite (power grades) for power loss. Remember commercial design decisions also include cost as a parameter (sometimes its the highest priority), and that gets balanced against things like efficiency. Your priorities might be different - have a look at TI's webbench power converter designer - it has a knob you can adjust to balance size, cost and efficiency.
Inductors for this type of power converter need to be designed. Start with target output power, then calculate the peak current requirement and inductance you need. Pick a core material suitable for the frequency you will operate at, and that is obtainable in a core shape you like. I would pick a shape that comes pre-gapped if possible as it makes everything simpler - I like the RM am P style cores, but they may not come big enough for higher power systems. Power throughput is pretty much a function of frequency and core volume, so knowing the first two you can gauge how big a core you need (scan though the datasheets). Once you pick a core you can calculate the number of turns, and the wire gauge that will fill the bobbin ( to get minimum DCR). If the wire gauge is bigger than 2x skin depth you should consider multi-filar winding or litz wire, otherwise you are just wasting copper.
Does that help?
types L, R, P, F and T would all work for you. R, P and F seem to be available in more sizes. L looks like its intended for high frequency converters.
I'd say any of the 3C9x ferrites would be OK.
Final choice comes down to price, availability and any special requirements.
Re: "Upgrading" a DC-DC Converter
Inducktion, Mon Mar 18 2019, 12:32AM
That all makes sense, thank you. My biggest issue is just trying to find where to buy the ferrites and cores; I've been looking on eBay since I can't seem to find them on arrow or newark reliably. Is there a website or someplace I should be looking specifically, or am i just not finding the right category on arrow/newark?
Inducktion, Mon Mar 18 2019, 12:32AM
That all makes sense, thank you. My biggest issue is just trying to find where to buy the ferrites and cores; I've been looking on eBay since I can't seem to find them on arrow or newark reliably. Is there a website or someplace I should be looking specifically, or am i just not finding the right category on arrow/newark?
Re: "Upgrading" a DC-DC Converter
2Spoons, Mon Mar 18 2019, 02:34AM
those are nz versions of US sites but i'm sure you'll get redirected. Search 'ferrite' then pick the category that looks most like 'cores' while avoiding anything that is for EMC or RF
I used these searches on ebay
ferrite core E -emi -rf -supression -toroid -filter -supressor -toroidal
354 results
ferrite core rm -emi -rf -supression -toroid -filter -supressor -toroidal
107 results
ferrite core p -emi -rf -supression -toroid -filter -supressor -toroidal
286 results
You may also want to search by ferrite type instead of core shape eg N87 (TDK/Epcos)
Your search-fu needs practice, young grasshopper.
2Spoons, Mon Mar 18 2019, 02:34AM
those are nz versions of US sites but i'm sure you'll get redirected. Search 'ferrite' then pick the category that looks most like 'cores' while avoiding anything that is for EMC or RF
I used these searches on ebay
ferrite core E -emi -rf -supression -toroid -filter -supressor -toroidal
354 results
ferrite core rm -emi -rf -supression -toroid -filter -supressor -toroidal
107 results
ferrite core p -emi -rf -supression -toroid -filter -supressor -toroidal
286 results
You may also want to search by ferrite type instead of core shape eg N87 (TDK/Epcos)
Your search-fu needs practice, young grasshopper.
Re: "Upgrading" a DC-DC Converter
klugesmith, Mon Mar 18 2019, 06:45AM
There are people who say that if a SMPS doesn't get hot, it's unnecessarily expensive.
A SMPS designer I am not, but here's a chart of core loss vs. temperature (at 100 kHz and 1 k gauss) for the F, P, and R materials that 2Spoons referred to.
Sorry, the forum's automatic link feature is having trouble with space characters in the middle of the URL.
We see that material P is formulated for minimum loss at 70 to 80 degrees C, and R for minimum loss at 100 to 110 degrees C.
At work we use a ton of buck converters with outputs like 0.9 volts at 13 amps, with a surface mounted inductor out of a catalog. Recently I was surprised to learn that they run at 2 MHz! Now you have me wanting to look up the inductor part number, and see if its core material and/or air-gappiness is given.
Hint, please, about how you edited pictures into your post. Thanks!
klugesmith, Mon Mar 18 2019, 06:45AM
There are people who say that if a SMPS doesn't get hot, it's unnecessarily expensive.
A SMPS designer I am not, but here's a chart of core loss vs. temperature (at 100 kHz and 1 k gauss) for the F, P, and R materials that 2Spoons referred to.
https://www.mag-inc.com/Media/Magnetics/File-Library/Product%20Literature/Ferrite%20Literature/Ferrite-Core-Material-Selection-Guide.pdf?ext=.pdfSorry, the forum's automatic link feature is having trouble with space characters in the middle of the URL.
We see that material P is formulated for minimum loss at 70 to 80 degrees C, and R for minimum loss at 100 to 110 degrees C.
At work we use a ton of buck converters with outputs like 0.9 volts at 13 amps, with a surface mounted inductor out of a catalog. Recently I was surprised to learn that they run at 2 MHz! Now you have me wanting to look up the inductor part number, and see if its core material and/or air-gappiness is given.
Hint, please, about how you edited pictures into your post. Thanks!
Re: "Upgrading" a DC-DC Converter
Inducktion, Wed Mar 20 2019, 12:30AM
Got N87 material cores, with 2 mm gap. I can buy ones with varying gap sizes to try out as well. Found them on arrow, thankfully.
Also, arrow has free 1 day shipping! Very useful.
And Kludge, whatcha mean how I edited pictures in?
There's a button near the top when you post a reply that lets you insert pictures directly into your posts, if that's what you're asking bout?
Inducktion, Wed Mar 20 2019, 12:30AM
Got N87 material cores, with 2 mm gap. I can buy ones with varying gap sizes to try out as well. Found them on arrow, thankfully.
Also, arrow has free 1 day shipping! Very useful.
And Kludge, whatcha mean how I edited pictures in?
There's a button near the top when you post a reply that lets you insert pictures directly into your posts, if that's what you're asking bout?
Re: "Upgrading" a DC-DC Converter
2Spoons, Wed Mar 20 2019, 02:15AM
Ergh! Just had a look at the Arrow site for the first time ever. No wonder you had trouble finding stuff - they must have the worst site layout and part filter I've seen.
2Spoons, Wed Mar 20 2019, 02:15AM
Ergh! Just had a look at the Arrow site for the first time ever. No wonder you had trouble finding stuff - they must have the worst site layout and part filter I've seen.
Re: "Upgrading" a DC-DC Converter
klugesmith, Wed Mar 20 2019, 05:06AM
Oops, I missed that. Let's try it now.
But it doesn't replace the old way, which has been broken for a few months,
to upload images to 4hv server. Readers saw an appropriate-size thumbnail (~ 400px) inline, and could click on if they wanted to see full size image. Just clicked in the dialog at the bottom of composing window, called "Attach file(s) / image(s)". Since the failure, the list of Allowed file types is empty.
The alternative, which you demonstrated, is better than nothing. But doesn't it depend on an outside link, which might not work tomorrow or two years from now? And most users wanting to post need to engage a third party to host the image, and also the thumbnail if they want to do it properly.
klugesmith, Wed Mar 20 2019, 05:06AM
Oops, I missed that. Let's try it now.
But it doesn't replace the old way, which has been broken for a few months,
to upload images to 4hv server. Readers saw an appropriate-size thumbnail (~ 400px) inline, and could click on if they wanted to see full size image. Just clicked in the dialog at the bottom of composing window, called "Attach file(s) / image(s)". Since the failure, the list of Allowed file types is empty.
The alternative, which you demonstrated, is better than nothing. But doesn't it depend on an outside link, which might not work tomorrow or two years from now? And most users wanting to post need to engage a third party to host the image, and also the thumbnail if they want to do it properly.
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