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OfflinetrendalM
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Switch-mode power supply questions
    #4495654 - 08/04/05 04:50 PM (12 years, 3 months ago)

Just wondering if anyone has a link (or explanation) to how switch-mode power supplies function.

Specifically, how is the voltage regulated to a specific value?

Say the line voltage is 110v during initial operation, and the regulated output voltage is +5v DC. If the line voltage were to drop down to 105v or 100v, how does the PS maintain the +5v output?

I'm guessing it has something to do with how the chopper transistor "chops" the incoming waveform up?


--------------------
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OfflineBigGameHunter
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Re: Switch-mode power supply questions [Re: trendal]
    #4495945 - 08/04/05 06:26 PM (12 years, 3 months ago)

From my understanding of it, the output voltage is regulated by the demand of load on that line through an opto isolater connected back to the primary side of the power supply. The switching frequency of the switching transistors is changed to compensate for the change in the output. If the line voltage is lowered, theoretically the stress on the output voltage is going to change & then the switching frequency of the switching transistors will change accordingly. I'm not 100% sure of this, but I think the higher demand on the output, the slower the frequency of the switching transistors. This allows the transistors to stay on longer.


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OfflinetrendalM
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Re: Switch-mode power supply questions [Re: BigGameHunter]
    #4495988 - 08/04/05 06:43 PM (12 years, 3 months ago)

But how does it step up the voltage?

I can easily understand how you get from 110v AC line current to +5v DC output via transformer....but how does a switchmode power supply stay at +5v DC output independant of the input voltage?


--------------------
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OfflineShdwstr
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Re: Switch-mode power supply questions [Re: trendal]
    #4496037 - 08/04/05 06:59 PM (12 years, 3 months ago)

The 110 AC is converted to DC in the Primary of the circuit. The regulation drops the voltage to 5vdc.
There would be no reason to step up the voltage unless the input voltage drops below the regulated 5vdc. Regulation us usually done with a voltage regulator IC or a series of chopper cuircuts and zeners.

Hope this explains it a little.

Shdwstr


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OfflinetrendalM
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Re: Switch-mode power supply questions [Re: trendal]
    #4496043 - 08/04/05 07:03 PM (12 years, 3 months ago)

I worded that badly :frown:

What I'm trying to figure out is: how does the power supply vary the voltage ratio of line:output?

Say if it starts at 110:5 (22:1) but then the line voltage is lowered to 100, how does the supply avoid a similar reduction of the output voltage?

If it was just a transformer with a 22:1 ratio, 100v would only give you 4.54v output. Obviously a switch mode supply isn't just a transformer...but how can it change this ratio on the fly?


--------------------
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Logic is a threat
Reason searched and seized


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OfflineBigGameHunter
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Re: Switch-mode power supply questions [Re: trendal]
    #4496068 - 08/04/05 07:14 PM (12 years, 3 months ago)

The output stays at the +5vdc by having a feedback line connected from the +5vdc line through the opto isolator device back to the primary side of the transformer to usually an IC that controls the frequency of the switching transistors. As the output voltage starts to change a change is also noticed through the feedback line back to the primary side of the transformer. The switching IC compensates for the change by increasing or decreasing the switching frequency of the switching transistors & thus regulates the output(+5vdc). This happens very fast so your not going see the change on a digital voltmeter unless the output has a sudden severe current demand on it.


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OfflinetrendalM
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Re: Switch-mode power supply questions [Re: BigGameHunter]
    #4496101 - 08/04/05 07:31 PM (12 years, 3 months ago)

How does switching on and off (it's around 20hz, right?) determine the output voltage?


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OfflineBigGameHunter
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Re: Switch-mode power supply questions [Re: trendal]
    #4496172 - 08/04/05 08:02 PM (12 years, 3 months ago)

The switching doesn't determine the value of the output voltage mainly the regulation. The value of the output would be determined by the number of windings in the step down transformer and/or zener regulators, IC regulators, transistor regulators. on the rectified output. I thought the frequency would be higher than 20khz out of the audible range of most humans' hearing.


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OfflinetrendalM
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Re: Switch-mode power supply questions [Re: BigGameHunter]
    #4496272 - 08/04/05 08:24 PM (12 years, 3 months ago)

Well if the line voltage changed, wouldn't the output voltage of the transformer change?

Is the voltage regulated on the line or output side of the transformer?


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Logic is a threat
Reason searched and seized


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OfflineBigGameHunter
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Re: Switch-mode power supply questions [Re: trendal]
    #4496486 - 08/04/05 09:16 PM (12 years, 3 months ago)

If the line voltage changes the output of the transformer will change, but I think the change in the output will be very little unless there is a significant change in the line voltage. The DC output voltage (if there are zeners, IC, or transistor regulators on the output) should not change unless there is a significant change in the line voltage.


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OfflinetrendalM
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Re: Switch-mode power supply questions [Re: BigGameHunter]
    #4496644 - 08/04/05 09:40 PM (12 years, 3 months ago)

So the voltage isn't really regulated at all, then, it's still dependant on line voltage?

What is it that is being regulated in a switchmode supply?


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Logic is a threat
Reason searched and seized


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OfflineChuangTzu
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Re: Switch-mode power supply questions [Re: BigGameHunter]
    #4498909 - 08/05/05 07:19 AM (12 years, 3 months ago)



Edited by ChuangTzu (08/05/05 07:22 AM)


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OfflinetrendalM
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Re: Switch-mode power supply questions [Re: ChuangTzu]
    #4499018 - 08/05/05 09:13 AM (12 years, 3 months ago)

I've read both of those and they still don't answer my question :frown:


--------------------
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Logic is a threat
Reason searched and seized


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OfflinetrendalM
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Re: Switch-mode power supply questions [Re: trendal]
    #4499271 - 08/05/05 11:40 AM (12 years, 3 months ago)

What I'm looking for is technical details as to how the voltage is regulated to maintain the +5v output independant of line voltage (that IS what a switchmode supply is for, isn't it?).

I don't want an answer like "there's an IC regulator to keep the voltage constant" because that doesn't tell me how the voltage is maintained...just what components maintain the voltage.

Well I want to know how those components maintain the voltage. So far I can't find any info on it :frown:


--------------------
The story book's been read
And every line believed
Curriculum's been set
Logic is a threat
Reason searched and seized


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OfflineBigGameHunter
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Re: Switch-mode power supply questions [Re: trendal]
    #4501040 - 08/05/05 07:50 PM (12 years, 3 months ago)

http://users.pandora.be/educypedia/electronics/powerelectronicssmps.htm

http://www.epanorama.net/links/psu_switching.html

I don't know if these links will help you, but if you can't find the answers you want on the web then you may need to go to a library or purchase a good book or speak to an electronics engineer.


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OfflineChuangTzu
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Re: Switch-mode power supply questions [Re: trendal]
    #4503362 - 08/06/05 10:48 AM (12 years, 3 months ago)

From the first link:
Quote:

"A feedback control loop is usually employed to maintain (regulate) the output voltage by varing the duty ratio to compensate for variations in input voltage....

Regulation

A feedback circuit monitors the output voltage and compares it with a reference voltage, which is set manually or electronically to the desired output. If there is an error in the output voltage, the feedback circuit compensates by adjusting the timing with which the MOSFETs are switched on and off. This part of the power supply is called the switching regulator. The "Chopper controller" shown in the block diagram serves this purpose."




You have a voltmeter measuring the output voltage. When the voltage falls, it sends a signal to the chopper which then increases the amount of time the current is switched on compared to the amount it is switched off (the duty cycle is increased). Increasing the duty cycle directly increases the RMS voltage. This gets fed through a transformer and is then rectified. The chopper itself is just a MOSFET switch with an IC controller. It outputs high frequency square waves, usually of fixed frequency; the IC adjusts the peak width to control the voltage. Since the circuit is only taking the current that it needs, it's easy to see how it maintains 5Vdc when the mains drop.

http://www.onsemi.com/pub/Collateral/HB206-D.PDF p.69


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OfflinetrendalM
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Re: Switch-mode power supply questions [Re: ChuangTzu]
    #4503655 - 08/06/05 01:33 PM (12 years, 3 months ago)

Ahh ok, that explans it perfectly! Thanks eh :smile:


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And every line believed
Curriculum's been set
Logic is a threat
Reason searched and seized


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