Building your own power supply: An actual example

[goodpsusearch]

Re: Building your own power supply: An actual example

At high loads the output inductor started to buzz. I thought the regulation loop was oscillating but hooking up the scope revealed what was really going on - 2.1vpp of 50Hz ripple! My caps are too small, both primary and secondary. I knew i wasn't gonna get away with just 2x 330uF secondary caps, but the 2x 680uF primaries i thought will be enough. Even at a moderate load there's like 400mvpp.

You don't specify the load that made the 2x 680uF primaries insufficient.

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

The same 400W load i've used till now.

However it's not that - the ferrite core of the output inductor is saturating (needs more turns) and effectively turns it into a piece of wire, which isn't too effective at filtering ripple... I was able to get much better results by messing around with the air gap, under 100mV at 50W load. So, need to take the core apart and put some more turns on it.

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

Okay, i can say i've figured out the source. Due to the messy wiring the mains electric field is making its way inside the feedback loop. For some reason it's worse with a ferrite core output inductor at low loads, but at max load it's pretty much the same whatever i use. Looks like some PCB design is in order before i can start troubleshooting.

I have fixed the main PCB dimensions to 100x200mm. This should be enough room to enable me to leave some space and decide on the output inductor after everything else is installed. The only limitation is the max height of 70mm, so the daughterboards and heatsink will need to reflect this. However, the bottom of the case is plastic, i can cut a little bit so 80mm fans will fit perfectly.

I have also decided upon the power transistors i'll be using for the final version. Might as well be doing it right so i'm gonna be running a pair of those.

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

Almost completed assembly of the controller PCB. I'll post details, pics, and the layout tomorrow. I have to say that i made a little oopsie - somewhere along the way the scale got bugged up on the vertical axis. Which means that my trances ended up a little taller, and the hole pitch no longer is what it was supposed to be.

I found out only after i started drilling the board, so i let it be. I did have to persuade the TL494 a fair bit... but it all fit together eventually.

Now, i have a little problem. I got my IRFP360s. For some reason, they run really hot with not much load at all. I mean, i could go on forever at 60W load without a fan on the 2SK2341s or IRF740s, and the heatsink wasn't even getting warm. Now, it gets hot. Quickly. I have added 15 ohm gate resistors because the ringing was getting out of hand on those big FETs - it went all the way down to turning the thing off and then on again during a switching cycle - however, they didn't fix anything. I used to run with no gate resistors at all on the smaller ones. While this wasn't a good idea as ringing develops, the gate voltage remained high enough at all times for it to not matter. Even on the IRFP360s Vgs has a spike to 12v and then settles at 10v. And the datasheet specs are given at Vgs=10v. So they should work great. I have no idea why they don't.

Now, one little detail. I've never got from this shop two transistors that have identical writing. Every time i buy a pair, for some reason, the two MUST have a different font. It's like the clerk picks them up that way. And they usually have an just an "i" instead of the familiar International Rectifier logo with a diode symbol in the O. So far they have all worked fine but now i think i got fakes. I'll post some pics tomorrow.

Anyway, i also bought two pairs of IRF740s. Looks like four weak cheap FETs are better than two big and more expensive ones.

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

Here's how my IRFP360s look like. They both look dodgy IMO. What do you think?

[Khron]

Re: Building your own power supply: An actual example

If they're from the pre-Vishay-acquisition, then they should look like it says on the last page of the datasheet (2004). The Vishay version doesn't specify how the markings should look...

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

Thanks, but i already had that datasheet. Anyway i think i found the issue. Due to my driving method fall time is slow. With the big capacitances of those fets, it's even slower. I have to get that bipolar driver supply going.

[b700029]

Re: Building your own power supply: An actual example

That's the Siliconix/Vishay logo, but the one on the left looks like it was stamped on upside-down and the labeling is also misaligned. All the pictures of IRFP360s I could find look like the one on the right.

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

Thanks. I've sped up the fall time a bit (still not using bipolar driver supply tho), and the result is.... the same. I'm puzzled.

The losses are far too great to only be rise/fall time related. It's like they are constantly hitting breakdown voltage. And they are (supposedly) 400 volt devices, while i got by with 250 volt ones in insulated TO-220 packages - how's that for heat dissipation - and yet they ran COLD at this load level, and could go for about a minute at 400W without a fan till things got really crispy. The IRFP360s heat up about the same like when i tried IRF640s (200 volt) just for kicks... Those ran for a few minutes but got really really hot and then one of them died. I don't want the same to happen with the IRFP360s because they ain't cheap.

They're either not what it says on the tin, or there's something else that i'm missing. Regardless, i'm going to hook up the two pairs of 740s in parallel, and if that works fine, then that's what i'm using. The differences in capabilities are marginal, and the two pairs of 740s still have roughly half the gate capacitance of 360s, so rise/fall times are likely to be faster too.

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

Fixed. The transistors are fine. I found that the secondary of the transformer was ringing like crazy at 250kHz! Obviously most of that made its way to the primary as well and disturbed the switchers. The culprit was the output inductor, i was again testing one with a lot of turns on a pair of T106-26 iron powder cores. Replaced it with a ferrite core one and all the ringing has gone away. Transistors are now cold.

Of course i could've tried using a snubber. But instead of wasting power why not actually fix the offending part?

Morale: When in doubt, scope around.

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

200W and they aren't even getting warm. Even at 400W level they stay comfortable with only a slow running fan. Now all i need is a nice PCB design and i'm all done.

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

I now have my first protection circuit. I put a LM339 and a TL431 on my protoboard, made an undervoltage protection circuit and set it to cut in at 70 volts. I then shorted the power supply. Click. Zero volts. No popped fuse.

Cut the power to the LM339... and it came right back up! Wooohooo! It'll also need a delayed powerup, because due to the soft start the power supply will not start at all if the LM339 is connected from the beginning , but hey, it works.

I'll also decrease the value of the soft start capacitor, 4 seconds is way too long. It'll be more like 1 second or so. A slow soft start won't help if the power supply is trying to start on a short circuit. In the final version the delayed powerup of the protection circuit won't be a problem because current limiting will step in before the LM339 is enabled.

[severach]

Re: Building your own power supply: An actual example

An IR2110 will be happy to drive those MOSFETs for you.

Different FABs and different lines will have different laser marking equipment so the finished parts look different. Often the case plastic, leads, tabs, and shear marks are different too. It comes from buying companies and from buying from different equipment suppliers through the years.

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

An IR2110 will be happy to drive those MOSFETs for you.

For one, IR2110 costs the same as one IRFP360. Which means "not quite cheap". Secondly, it's easy to blow up. I've followed lots of class D experiments on diyaudio and all of them have resulted in several failed 2110s. This is why in my amp i went for discrete drivers. Third, my controller will be on the secondary side. The gate drive transformer gives me isolation from the primary. An IR2110 would not.

Different FABs and different lines will have different laser marking equipment so the finished parts look different. Often the case plastic, leads, tabs, and shear marks are different too.

Thanks for clearing my doubts on this.

[ben7]

Re: Building your own power supply: An actual example

Did you ever finish the design of this? and did you ever take pics? :/

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

It's still sitting around somewhere. Made a couple other power supplies in the meantime. That Class-D amp module i was talking about, i still have it around, it works, and i still haven't gotten around to building the higher power version even though the PCB design is 100% complete since.... last year.

[ben7]

Re: Building your own power supply: An actual example

That Class-D amp module i was talking about, i still have it around, it works, and i still haven't gotten around to building the higher power version even though the PCB design is 100% complete since.... last year.

Um, lol!

[rogfanther]

Re: Building your own power supply: An actual example

A theoric question :

Suppose I buy a core from a store, or take one from some China atx psu. Probably they will not have references for the magnetic parameters for transformer design.

Is there ( and can it be used ) a "generic" table for core properties , so that one would use it, some conservative values for flux density and the like, and the little differences would be taken care of by the control/compensation part ?

Or the bigger cores have so much bigger specifications that one just need to calculate number of turns/wire diameter and be done with it ?

Thanks

[Th3_uN1Qu3]

Re: Building your own power supply: An actual example

A theoric question :

Suppose I buy a core from a store, or take one from some China atx psu. Probably they will not have references for the magnetic parameters for transformer design.

Is there ( and can it be used ) a "generic" table for core properties , so that one would use it, some conservative values for flux density and the like

All cores are standard sizes. You can simply take a ruler to it, note down the dimensions and you're likely to find it on Ferroxcube's site. If not, try Epcos, TDK and Micrometals.

[rogfanther]

Re: Building your own power supply: An actual example

I mean, the properties are just all geometric, different makers ( like, Siemens or Ferroxcube versus Chinaboom Inc. ) won´t change them that much ?