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Old 26th April 2012, 06:00 PM   #11
luka is offline luka  Slovenia
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west buys already made parts from east, so your argument is invalid
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Old 26th April 2012, 06:04 PM   #12
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not when small quantity is required
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Old 26th April 2012, 06:09 PM   #13
luka is offline luka  Slovenia
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hundreds of millions of power supplies that are out there in the field, are from east, and so no small quantity is required... for small you can do it by hand and/or use ready made parts yet again

Oh and what is small quantity, even then they can do it just the same as with large quantity
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Old 26th April 2012, 06:14 PM   #14
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sometimes power supply have to be done to fit unusual spec or to fit in given enclosure......and when quantity is also small, its not worth going ot China to get it done.
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Old 26th April 2012, 06:42 PM   #15
gmarsh is offline gmarsh  Canada
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Originally Posted by eem2am View Post
sorry gmarsh, but any half-bridge, with either voltage or current mode needs a current transformer in primary side.

The Chinese do this type of power supply and they do it well....and as we have agreed, they CAN do it well, because they have lots and lots of willing, dextrous Chinese fingers to wind all the windings that the half-bridge needs.....

And that is WHY the Chinese do the half-bridge....because they know that no Western compeny will ever be able to copy their design because in the west, we simply dont have the staff to wind all those current transformers and transformers with balancing windings.

By the way, do you understand that without the balancing winding, all rail splitting caps need to be rated to the full supply DC bus voltage.
You don't need a current transformer for a voltage mode supply. What would you use it for, OCP? In that case it's adequate to just measure the current through the low side switch with a sense resistor - even with a shorted output, it'll take a number of cycles for current in the output inductance to charge up so you don't need cycle by cycle limiting.

You also don't need balancing windings - in voltage mode operation, the converter will naturally balance the voltage across the two caps. If there's a small difference in duty cycle between the upper and lower primary switches, the balance will shift slightly, but it's not hard to keep duty cycle variation to a reasonable amount.

Don't believe me? take the primary schematic of a half bridge supply, replace the transformer primary with an inductor, draw the switches on the left and the capacitors on the right. Call the center node of the two capacitors "Vout". You've just drawn a synchronous buck regulator, but with a capacitor going between Vin and Vout which makes no difference in circuit operation other than precharging Vout to Vin/2 when you ramp up Vin. Provided the on time of upper and lower switches are the same, the circuit will force Vout to be Vin/2. If the on times are different, it's pretty easy to calculate what "Vout" will be - I'll leave derivation of that equation in your capable hands.
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Old 27th April 2012, 06:56 AM   #16
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The great majority of chinese ATX computer power supplies are half bridge. No need for a film cap, they use two electrolytics in series for the input capacitance and connect the midpoint to one side of the primary.
in 2006 i scrapped a cubic meter of used/failed ATX computer power supplies and none of them directly connected the transformer to the 200v electrolytic caps.
All of them had a .47-2.0 uf polyester cap in series.
most were two transistor driven half bridges, current transformer and and base drive transformer using the primary current to provide the base drive power.
about a third were forward or flyback converters.

honestly i have never seen an ATX supply directly connect the primary to the rail splitting caps.
but maybe i live near an expensive city (seattle) and that's why.. idk


anyhow, if OP has to plan around a failure mode and buy 400V caps to split a 400V rail.. then why not throw an extra 5$ at it and go with a full bridge?

some designers just put a 230v MOV across the 200 v caps..
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Old 27th April 2012, 06:47 PM   #17
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thanks gmarsh

-Sorry i was unclear here, i am talking about voltage mode half-bridges which have gone into cycle-bycycle current limiting due to overload or heavy load transient

-when a voltaage mode controller is overloaded, or suffers a large load transient, it goes into cycle-by-cycle current limiting


Please view this

http://www.ti.com/lit/ds/symlink/lm5039.pdf

Its the voltage mode LM5039, which controls half bridges.

It has a special feature whereby it goes into average current mode control when it is overloaded......and this stops the imbalance of the rail-splitting capacitors.

-but this is the only controller in the world that has this feature.

So if you are not using the LM5039 then you need to make your rail-splitting caps rated to the full rail voltage.

the first page of the datasheet tells of the "balancing act"
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Old 27th April 2012, 09:57 PM   #18
gmarsh is offline gmarsh  Canada
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Only matters if you're using cycle by cycle current limiting - which is effectively the same as current mode control when it's active, just with a fixed threshold instead of a threshold controlled by the output voltage.

If you use latched/hiccup overcurrent protection, or an output current regulation loop that overrides the voltage feedback loop like you'd find on a bench supply, you're fine.

Certainly if cycle-by-cycle limiting is part of your design criteria, you're definitely better off with a different topology than a stacked-cap half-bridge.

Last edited by gmarsh; 27th April 2012 at 09:59 PM.
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Old 28th April 2012, 03:32 PM   #19
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Cycle by cycle limiting is the cheapest..so i would use that.

I must admit i dont like the latched/hiccup overcurrent protection, because it may trigger on a heavy load transient and put the supply into hiccup mode unwantedly
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