|27th July 2005, 06:43 PM||#12|
Join Date: Nov 2004
Ok, now I've been sketching, calculating and burnt a few braincells at the same time.
This is actually the first SMPS I'm doing all the way, the other 12V PSU:s have been made from ready-made schematics with a few modifications only, and performance haven't been an issue.
SO, I've discovered that the questions are many. Any SMPS pro that can guide me through the dimensioning and construction of this?
Here's what I've planned so far:
Output capable of driving 2*100W@4Ohms ( lowered the specs a little bit... )
Input voltage 230Vac
The topology I've chosen is half-bridge
Maybe PFC ( I'll look at that later )
So, here are the questions:
1. In half-bridge, there are two caps in series with the
transformer, how do i dimension these?
2. As i asked before - is there a possibility to skip these caps by
using the CC3810 current control?
3. As primary switch, my only choice is MOSFETs. What specs
should i consider? input voltage will be 280-400V ( 420V maybe
with the PFC later on ) Are 500V mosfets sufficient?
4. How big current should these be capable of handling?
2*100 watts, maybe 75% efficency (???) in the PSU, 85% in
the amp ( class T ). Should be around 310 watts? then how big
current peaks should the primary sw. be capable to handle?
5. I'm thinking about the switching frequency - is 60-70 kHz a
good choice? i want to make the transformer as small as
possible, but on the other hand higher frequency means a lot
more switching losses... is there any "magic" frequency where
both transformer size and switching losses are kept as low as
possible, and still affordable for DIY?
6. Running at 60kHz, my transformer core choice is ETD44(F44)
100N1 / 2*25N2 secondary. Is this core large enough?
That's it for now... :-)
|28th July 2005, 04:47 AM||#13|
Join Date: Jul 2005
Location: 65N 25E
1. By allowed input ripple voltage and rms current rating.
2. I dont see any possibility in half-bridge to skip these caps.
3. Low gate charge(as usually in switchers), enough low rds-on
Actually its compromise in mosfet size, switching losses increase when you choose bigger mosfet and on-state losses degrease.
for 420V operation i might choose 560 or 600V mosfets to be on safe side.
4. Dont choose them according what specsheets say about current, they are "speculative ratings" Instead of that calculate rds-on losses and switching losses and check if result is something what you can cool and tolerate. And remember that your rds-on is function of temperature. Multiply by 1.7 to get rds-on at 100C. This is bit of iterative process, repeat until satisfactory. rds-on losses are maximum at maximum load and high line when smps is running at low duty cycle and peak currents are high at swicthes. Switching losses are tricky to predict, but at 60khz they should be around same as your rds-on losses quite easily. IR has some nice appnotes about mosfets and TI power seminars are worth of solid gold.
5. 60-70 khz sounds fine, easier to source ferrites that are low-loss in this frequency than some of more modern ferrites meant for high-freq operation (ferroxcube has a lot of them to choose)
With increasing frequency transformer desing gets also more tricky and with higher power levels even more so. Magic frequency depends on power output, it might be easy to desing 200W smps to run at 400khz but extremely difficult at 2kW.
"Old style" to dimension power supply is to divide mosfets losses equally between switching losses and rds-on losses and divide transformer losses 50-50% between copper and core losses.
Transformer loss should be small partition of total losses, as it is usually desinged for 20-40C temperature rise above ambient.
6. Calculate flux swing in your core and look at core manufacturer datashit how big core losses you end up. Less than 100mW/cm3 is a good starting point. After that calculate winding losses, again for minimum duty cycle and maximum load. If you can do with something like 3Amps/mm2 you should be on safe side. Dont oversize your wires, you just end up with more proximity and eddy losses. Keep wire thickess below 2x skin depth, split primary to 2 layers and make your secondaries between primary layers. INSULATE properly between primary and secondary, 4kV insulation is required also in sweden. 2 layers of OHP film will do this.
|25th August 2005, 06:59 PM||#14|
Join Date: Nov 2004
Ok, now I'm on my way...
The ferrite core will be an ETD34, with neosids material F44.
The only thing that worries me, is as before, the primary switchers. I bought a bunch of BUK445's, really cheap ( 25 pcs for 12$ ), but I'm not quite sure they will do the job... I'm gonna use them later anyway, for other projects... what do you think?
perhaps two in parallell? or a fullbridge topology instead ( sharing current load )?
Rdson = 1.5 Ohms
I = 2,9 A
U = 500 V
Ptot = 30W
Gate charge = 30 nC ))
I've got my hands on some IRFB9N60A:s
They are a lot more powerful, maybe It's a waste of resources :-)
And I've only got 6 of them...
Rdson = 0,75 Ohm
I = 9,2 A
U = 600V
Ptot = 170W
Gate Charge = 49nC
|26th August 2005, 05:53 AM||#15|
Join Date: Nov 2004
You can also try these:
IRFB11n50 (11A, 500V, 0.52ohm Rds(on) ).
2sk2141 (6A, 600V, 1.1 ohm Rds(on) )
Please follow the thread I started "Assymmetric outputs in half-bridge", as I am basically in the same boat as you.
I am using an ETD44, pretty similar (a bit bigger) to your core. I am trying to get more than 500W out of it, but by the moment I am having problems with mosfets exploding, perhaps due to gate drive issues.
What gate-drive scheme are you thinking of?
|26th August 2005, 07:27 AM||#16|
Join Date: Nov 2004
I've looked in to your thread to, interesting.
I'm planning to drive the mosfets from two small gatedrive transformers I've found here at work, really neat little devices capable of 50W:s :-)
I'm thinking using the TL494 ( got some... )
or maybe the UCC3810/UC2801
The thing with these two transistors i mentioned, is that I've got them, purchased them for a low price, figured that if i cant use them here, they'll fit other experiments...
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