Offline full-bridge SMPS....need help

[Eva]

Danko don't you have a fuse in the mains input of your SMPS? It's a must.

[Danko]

Here's the full schematic:
http://sziget.mine.nu/~danko/aramkor...matic_full.png
It uses a small PCB-mounted 6VA transformer for "house-keeping".
Here's a close-up:
http://sziget.mine.nu/~danko/aramkor.../schematic.png
Here's the PCB layout:
http://sziget.mine.nu/~danko/aramkor/testbed/16/pcb.png

WARNING!! The parts' values not same values I have soldered on the PCB. I changed them when I was experimenting. I haven't updated the schematic yet.
Oh, another important thing! "R5" is the slope-compensator resistor, but that didn't work. I used instead of that an emitter follower. I have changed the current sense resistors also. Now there are 2 paralelled 0.1R/2W resistors.
I will post an updated schematic, when I have time to see what I have changed.

[Danko]

Eva: There's one, but that haven't burned out. Maybe it's too high rated As far as I remember, it's a 6.3A, "T", or normal.

[luka]

How did you winded your main trafo?

[Danko]

On the pictures, there's an ETD49, which I re-re-re-rewinded many many times, this is why it looks like that...
http://sziget.mine.nu/~danko/aramkor...6/dscn8740.jpg
http://sziget.mine.nu/~danko/aramkor...6/dscn8741.jpg
http://sziget.mine.nu/~danko/aramkor...6/dscn8743.jpg
http://sziget.mine.nu/~danko/aramkor...6/dscn8744.jpg
http://sziget.mine.nu/~danko/aramkor...6/dscn8745.jpg
http://sziget.mine.nu/~danko/aramkor...6/dscn8746.jpg
http://sziget.mine.nu/~danko/aramkor...6/dscn8747.jpg

After theese steps I solder the end of the copper wires to the pins, put in the cores, and I solder the transformer onto the PCB.


Edit: I don't remember the number of the turns, and the number of wires I used to wind. One wire's diameter was 0.4mm.

[luka]

you should winded your trafo like this (Fig. 17)
First you place 1/2 of primary then all secondery's and the 2/2 primary. This helps to minimaze a lot of things as can Eva tell you, making trafo perform best

[Danko]

oh, damnit! I forgot to check, how I wound the trafo. thanks! Actually, I wound that trafo quite a long ago, becouse the shop sent me only an ETD44 bobbin, becouse they haven't got the cores, and I had to wait about 3 weeks to get the cores. I forgot how I wound the trafo... :-(

[chas1]

You mentioned you have an ETD44 core, according to Pressman
at a freq of 100kHz this core set can provide over a Kw. I am working on HalfBridge using voltage mode control @75kHz an using 90 Volts as a low input and a bmax of 1600 the turns calculate as: Using SG3525A PWM chip

90VAC * 1.4 - 20V(for ripple and drops)
gives abot 107VDC.

Np = 107*E8/ (4*1600*75*E3*1.74) = 12 Turns

Since this will be a center tap FWB output I use this formula to calculate the secondary and I want a +/- 55 Volts @ 8A
out (aprox: 900 watts) Ipri = 10A . I plan to use IGBT's as switch

Ns = Np*Vs/Vp = 12*110/110 = 12 T

Formulas are from George Chryssis "High Frequency Switching Power Supplies Theory and Design" dtd 1984 I also have Marty Brown and Pressman as a reference.

Before winding the primary I plan on placing a shield that will be grounded to the output center tap, and then wind 1/2 primary then wind all of secondary and place another shield grounded as before, wind remainder of primary and place another shield grounded as before. A winding of three turns for aux power and another shield grounded as before. the primary will be 4 strands of AWG #19 bifilar wound, secondary will be two #17 AWG. I know wire size might be large but more than 50% of area will be used?????? I hope this will be ok. Inductors will be 140uH place in each output but wound together on a common core. Two 1000 uF caps as output filters on each rail. A type 3 compensation will be used. I was thinking about using the TL431 for isolation but the specs are for 36 volts max and I don't know how a zener in series would effect it yet (still a thought) All windinding's will be insulated with 3m yellow tape.

After reading Brown and comparing his calculations for xfmr

on page 123 the calculation for the primary he recommends the highest expected input voltage .

therefore my xfmr primary turns would be

Npri = 368 V *E8/(4 *75*E3* 2800G *1.74) = 26 T
(2 times the copper) and the secondary would be

Nsec = 1.1(111*25)/252*.95 = 12 T ok


and Pressman uses 259VDC as Vmin therefore
Npri = 130E8/(4*1600*75E3*1.74) = 16T
Nsec = Np*VS/130 = 16*110/130 = 14 T

and I have noticed that a value of 160 is used quite often in this forum therefore:
Npri = 160E8/(4*1600*75E3*1.74) = 20 T
Nsec = 20*110/160 = 14 T

Can you help with the correct numbers?? I like Browns book but has quite a few error's from the printer I think (math values ) in examples......and then again maybe I need to read it again.

[N-Channel]

Chas1-

I have read all of the books you mentioned. Indeed, all together (Chryssis, Pressman, and Brown) they form the "bible" I refer to whenever doing SMPSs. I have not done a flyback yet (almost always half- & full-bridges & push-pulls) but after re-reading the transformers section of Chryssis' 2nd edition (1989), your numbers seem correct.

Also, you mentioned you are using SG3525AN as your control. How are you driving IGBTs? IR2110/2113? Driver xfmr? If you were to use the MC33025 (the '3525's current-mode cousin), you could just follow the design example in Brown's book for a 280W (28V@10A) Half-bridge supply. Just scale the magnetics up to the ETD44, plug in your desired output voltages in place of the 28V given, and let 'er rip.. Also follow the section that covers feedback compensation, using a TL431 and Optocoupler for isolation. I have used this example as boilerplate for many AC-DC and DC-DC converters I've done over the years, with very good results.

Danko-

If you can get any one, or better yet, all three of these books, you will have some very powerful "tools" in your design toolbox.

Just my humble opinion.

Steve

[chas1]

hey Steve
thanks for conformation, below is what I have so far.

I am using gate drive transformer wound on a ferroxcube toroid
846t250-3c8 has an AL 1650mh/1000T Npri = 16T (inductance of 420uh) and sec of 12 T, I pri works out 12*6.6us/422uh = 185ma

energy = 1/2 * 422uh(185)^2 = 7uJ and this core can store 16uJ

I put a 2.5 ohm resistor in series with pri of transformer, I might increase this to 4.5 ohm. My large problem is closing feedback loop using the TL431, can't seem to find decent app note for this and after crunching numbers in Browns examples on pg 180 - 183
simulating them along with my bode plots I can't get close to what he has in fig 4-24 (note: using his exact numbers as an example for proof) therefore there must be a cockpit problem or I
don't understand bode plots.