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Old 15th September 2008, 09:50 PM   #1
mag is offline mag  Switzerland
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Default 500W offline SMPS

Hi All,
I have just finished the first draft of the schematics of a 500W offline SMPS. The chosen topology is the double switch forward.

Please take a look at that and let me know what do you think.

This is my first design of an offline SMPS (some experience with low voltage but never tried offline), so any comments or suggestions are highly appreciated.

Thank you

ciao

-marco
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Old 15th September 2008, 10:41 PM   #2
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Do you have any other documentation?

w
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Old 16th September 2008, 03:12 AM   #3
star882 is offline star882  United States
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There's a runaway problem if the 12v output is overloaded.
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Old 16th September 2008, 05:03 AM   #4
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what abt current limiting ?
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Old 16th September 2008, 09:37 AM   #5
mag is offline mag  Switzerland
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By now I have only the schematic, I have not yet started the PCB layout.

Current limit is included in the SMPS chip. The IS input is used for current mode feedback but also as an ever current protection.

When the voltage on is is >2V the OCP trips. I have set it to 10A, maybe a bit too high.

@start882

Why are you sayin this? It is because the two ouptut inductors are separated?
the +/-12V is used to power op amps and other low power stuff, the maximum current on that output will be under 100mA.
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Old 16th September 2008, 11:39 AM   #6
m-tech is offline m-tech  Serbia
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Be careful with feedback loop.

I think half bridge topology is much simpler, and you can find lots of half bridge designs here, on forum.
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Old 16th September 2008, 02:07 PM   #7
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Star is right, if there is no 12V becuse of overload, the feedback loop is broken, this is dangerous with forward converter.
It may not be a big problem if you are sure that 12V supply is never going to be overloaded.
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Old 16th September 2008, 02:41 PM   #8
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isn't the current limiting omly watching the one direction of current flow?? Wouldn't you need to put current sensing in the other "leg" too?

I personly would use a Half Bridge design myself.
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Old 16th September 2008, 05:25 PM   #9
mag is offline mag  Switzerland
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Thank you darkfenriz,
of course if I short the +12V there will be no current suppling the
optocoupler. This will open the feddback loop and make the PWM go at maximum duty cycle (and thus at maximum output voltage).

I will change the schematic deriving the optocoupler power supply directly from the +85V rail trough a resistive divider.

@ifrythings

In a double switch forward topology the energy goes from the bus capacitor to the load only when both switches are closed.
When the switches are open the two diodes will reset the transformer core.

The current sense resistor is therefore places in series with the two mosfet because it the real measure of the current supplied to the load

ciao
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Old 16th September 2008, 05:56 PM   #10
star882 is offline star882  United States
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A better solution to that runaway problem is to add a crowbar circuit. That way, it would also prevent runaway from faulty components. Running the feedback circuit from a higher voltage would decrease efficiency.
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