Mains to 15VDC/20mA non isolated - diyAudio
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Old 4th August 2009, 02:59 PM   #1
Hurtig is offline Hurtig  Denmark
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Default Mains to 15VDC/20mA non isolated

I am in the middle of a High Power SMPS design, and need a 15VDC non isolated supply, to feed my onboard PFC circuit.

The PFC is build around the UCC28019 from TI, that needs this auxilary supply of approx 15VDC drawing max 10mA.
I could use a small power transformer, but it does not seem that elegant.

As far as I remember, I saw some integrated swithing regulators, that only need a few external components to do this, like an inductor and some capacitors. But I cannot remember where
Please help.
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Old 4th August 2009, 03:04 PM   #2
luka is offline luka  Slovenia
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you can use transformes to power that IC all the time, or you can make power zener, right of 330Vdc rails... for that see my smps schematic how I did it
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Old 4th August 2009, 03:46 PM   #3
star882 is offline star882  United States
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Add an auxiliary winding.
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Old 4th August 2009, 05:08 PM   #4
Hurtig is offline Hurtig  Denmark
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@ Luka:
I am aware of the zener version, directly from the mains voltage. But since the PSU is designed for a wide input voltage range, 85-265 VAC, the DC voltage after the rectifier will be between 120-375 volt.
At 120 VDC, I need a resistor of 5,25kohms to get 20mA. Power dissipation in the resistor will be 20mA*105V = 2,1watt.
At 275 VDC, the current will be 50mA, and the power dissipated will be almost 13 watt.
I could go for a CCS to feed the zener, and keeping the current at 20mA. This wil need a few "no cost" components, but will still give a power dissipation of min 5 watt.

The PSU will be rated at 300 watt. And 5 watt dissipated will lower the efficiency by 1,66% at full load. At lower loads, the efficiency is even worse.

Therefor I will not go for the zener version, even though it is very easy.

@ Star882:
I could use an auxiliary winding. But I need the 15VDC supply to be always ON, even when the PFC/PSU is in power down mode. And that will not be possible with the auxiliary winding.

So what I need, is a simple switcher, that can run directly on the mains voltage, and give me 15 VDC. As mentioned, I do not need it to be isolated.
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Old 4th August 2009, 05:19 PM   #5
luka is offline luka  Slovenia
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It doesn't matter what is your input voltage, it works from say few volts above set voltage and up to voltage of transistor rating

I have missed at first read, that this is PFC, so you have no trouble starting ic(right?), you need supply for it, and as it was said, put winding on inductor, and that will power IC

You can check my PFC too, also on my page... All I can say, number of turns will depend on:
When minimal input voltage, you still need voltage that will IC need to function, and at high input voltage, you'll need to check that voltage will be below max IC rating, or somehow limit it to some value below that
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Old 4th August 2009, 06:14 PM   #6
Hurtig is offline Hurtig  Denmark
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@ Luka:
I still need this 15VDC when the PFC and DC-DC is in power down. Therefor I can not use an extra winding for the 15VDC.

That's why I'm looking for a simple switcher, that can give the 15VDC directly from the mains.
I have seen such an switcher a few years ago, but can't remember where
I did some "googling" but did not find it.
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Old 4th August 2009, 07:45 PM   #7
luka is offline luka  Slovenia
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You could always make one using TOPfet like IC and make flyback, or use normal trafo anything ready made will probably very expensive
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Old 4th August 2009, 10:03 PM   #8
star882 is offline star882  United States
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Quote:
Originally posted by Hurtig
I could use an auxiliary winding. But I need the 15VDC supply to be always ON, even when the PFC/PSU is in power down mode. And that will not be possible with the auxiliary winding.

So what I need, is a simple switcher, that can run directly on the mains voltage, and give me 15 VDC. As mentioned, I do not need it to be isolated.
You can use a simple buck converter based on a transistor driven by a feedback winding on the inductor. I'm not sure how to design one, however.
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Old 4th August 2009, 10:41 PM   #9
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So you have a DC voltage after the rectifier? How does the PFC work this way?
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Old 5th August 2009, 03:51 AM   #10
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Non dissipative capacitor + a surge resistor + a 15 Volt Zener + a cap filter
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