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Old 29th December 2005, 07:48 PM   #1
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Default Synchronizing SG3524

Hi,
I want to build power supply with 3 output voltage.
and I use SG3524 for each output.

Is it okay to sync. all of SG3524 together? They run at DIFFERENT DUTY CYCLE.

The osc & CT pin connected together, RT leave open.
Only the master have RT resistor and CT capacitor.

Thanks
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Old 29th December 2005, 10:47 PM   #2
Eva is offline Eva  Spain
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If I remember properly, this is the right way to synchronize them (I've suffered a hard-drive crash in the computer where I stored all my datasheets, schematics and electronics stuff ). However, in case not all the outputs require tight voltage regulation you may consider a single control circuit and a single switching stage instead, with multiple transformer secondaries and/or a coupled output inductor to achieve several output voltages.
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Old 30th December 2005, 04:37 PM   #3
Onra is offline Onra  Germany
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Hi,
what is the advantage of synchronized regulators?
Is`nt it better to spread the distortions (by switching) on the supply rails?
Onra
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Old 30th December 2005, 04:56 PM   #4
Danko is offline Danko  Hungary
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For example, you build 2 regulators. They are unsynchronized. One SG3524 is switching at 199khz, and the other is switching at 200khz. Then the difference between theese frequencies is 1khz, which is audible. and this can disturb some things. For example, if you are running an amplifier from common rails of theese SG3524s, then the amplifier will "subtract" 200khz-199khz, and this will be on the speaker.

As far as I know... :-)

If I'm wrong, please, somebody correct me! :-)
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Old 30th December 2005, 05:25 PM   #5
gmarsh is offline gmarsh  Canada
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Quote:
Originally posted by Onra
Hi,
what is the advantage of synchronized regulators?
Is`nt it better to spread the distortions (by switching) on the supply rails?
Onra
I've worked on designs which have multiple switching regulators, synchronized at the same frequency but also running at different phase angles relative to each other.

Synchronization will reduce low frequency "beat" ripple on the input side, and phasing will also reduce the peak primary side current making life easier on your input capacitors, etc.
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Old 30th December 2005, 07:14 PM   #6
Eva is offline Eva  Spain
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Danko is absolutely right concerning beat frequencies (intermodulation). Also, delaying some regulators half cycle respect to the others is useful for current ripple and EMI reduction.
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Old 30th December 2005, 10:26 PM   #7
Onra is offline Onra  Germany
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Hi,
Thanks a lot to all.
My question was triggered by an appnote, i read a while ago. They added a bit of jitter to their SMPS- oscillator to reduce EMI by spreading the noise spectrum.
Onra
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Old 30th December 2005, 10:50 PM   #8
Eva is offline Eva  Spain
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Adding jitter noise to the oscillator spreads EMI better across its frequency range, thus producing less EMI in average. However, note that jitter noise is not the same as intermodulation between two oscillators closely tuned. The same EMI-reduction principle could be employed in synchronized regulators by adding a random delay to the sync pulse or a random component to the master oscillator.
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Old 30th December 2005, 10:58 PM   #9
Onra is offline Onra  Germany
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Hi,
That is understood.
I was fixed on the distortions produced by switching on several devices at the same time and didŽnt realize the problem of intermodulation at all.
Onra
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