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Join Date: Jan 2017
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Switching Regulator Controller

01/19/2017 1:43 PM

Can someone please explain what the following means? Thanks

"The LT3748 is a switching regulator controller specifically designed for the isolated flyback topology and capable of high power. It drives a low side external N-channel power MOSFET from an internally regulated 7V supply. No third winding or opto-isolator is required for regulation as the part senses the isolated output voltage directly from the primary-side flyback waveform."

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#1

Re: switching regulator controller

01/19/2017 2:30 PM

What exactly are you having problems with?

Have you had a good look through the datasheet and the block diagram of the internals inside the controller chip along with the explanation of all the inputs and outputs?

Datasheet

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#6
In reply to #1

Re: switching regulator controller

01/19/2017 10:40 PM

If you an electronics geek, all this is mostly obvious. If you aren't, it will (probably) always be incomprehensable.

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#2

Re: switching regulator controller

01/19/2017 2:30 PM

I'll try...

http://www.linear.com/product/LT3748

A switching regulator turns the output on-and-off very rapidly. In the "on" condition or "off" condition, very little power is dissipated. This is in contrast to a linear regulator which acts as a variable resistor to throttle the output and thus generates heat.

The regulator switches the external transistor on and off (at GATE). It senses the current (SENSE) and uses that as feedback to control the switching duty cycle. This is on the primary side of the transformer. The secondary side has an AC waveform which is rectified and filtered by the diode and capacitor. The transformer (flyback transformer) isolates the output circuit from the regulator.

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#3

Re: switching regulator controller

01/19/2017 2:40 PM

This looks very straight forward and easy to understand for me. Therefore I don't know what to explain.

Why don't we start from the beginning. What part of this do you think you understand and what is your understanding?

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#4

Re: switching regulator controller

01/19/2017 2:43 PM

Here is what I found using Google and the term:

"primary side sensing feedback controllers"

A separate winding on the primary side of the transformer provides the waveform for the controller to infer the output side. Application Note AN-8033 from Fairchild is an excellent reference.

Or you could look at TI's TPS92315

VSNS (Voltage-Sense): The VSNS pin is connected to a resistor divider from the auxiliary winding to ground. The output-voltage feedback information is sampled at the end of the transformer secondary current demagnetization time to provide an accurate representation of the output voltage. Timing information to achieve valley-switching and to control the duty cycle of the secondary transformer current is determined by the waveform on the VSNS pin. Avoid placing a filter capacitor on this input which would interfere with accurate sensing of this waveform.

The VSNS pin also senses the bulk capacitor voltage to provide for AC-input run and stop thresholds, and to compensate the current-sense threshold across the AC-input range. This information is sensed during the MOSFET on-time. For the AC-input run/stop function, the run threshold on VSNS is 220 μA and the stop threshold is 80 μA. The values for the auxilliary voltage divider upper-resistor RAUX1 and lower-resistor RAUX2 can be determined by the equations below.

From Linear Tech:

Some Linear controllers use the primary winding directly.

www.linear.com/docs/29630

Of course if you looked at page 8 of the data sheet for the LT3748 you would find:

After the external MOSFET is turned off, the voltage on the drain of the MOSFET rises to the output voltage multiplied by the primary-to-secondary transformer turns ratio plus the input voltage.

This is where the magic happens. When the switch is turned off, the regulator "looks" at the waveform on the secondary side through the turns ratio.

Good luck with your quest.

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#5

Re: switching regulator controller

01/19/2017 4:06 PM

That is electrical engineer goobledygook for "this voltage regulator is really expensive, and you will like it."

Think of it as a new kind of H-drive.

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#7

Re: switching regulator controller

01/20/2017 2:52 AM

Of course. The equipment manufacturer can: .

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#8

Re: switching regulator controller

01/20/2017 4:35 AM

Lots of good answers already here, but maybe I can ask, what do you want to achieve?

There are many ways to skin a cat!!

Then we could maybe help you FAR better....

I know some power supply chips, also switching, that are far less dangerous to an amateur.....as they use an input transformer to drop the AC voltage for you first!

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#9
In reply to #8

Re: switching regulator controller

01/20/2017 5:17 AM

The datasheet states that the input voltage range is 5 to 100V(dc).

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#10
In reply to #9

Re: switching regulator controller

01/20/2017 5:21 AM

....and what is he planning to use?

Anything over 50 volts is considered to be possibly dangerous nowadays....so that is a 50-50 chance......

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#11
In reply to #10

Re: switching regulator controller

01/20/2017 11:19 AM

Arc Flash does not care whether someone is an expert or an amateur. Agreed, 50 Volts is a good place to leave it alone if not knowing the safety protocols.

Money is the only thing apparently standing between the OP and getting a 12 V supply with good characteristics that can deliver 19 Amps without much droop at all. ~$70 at Radio Shack, not to mention what is on Ebay or Amazon, etc.

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#14
In reply to #10

Re: switching regulator controller

01/28/2017 2:23 AM

Extra Low Voltage (ELV) is generally considered to be non-lethal, and that is <50vAC or <120v DC, so there should be little shock risk associated with an input voltage up to 100v DC.

The problem can occur where the PWM controller converts that DC into a square wave pulse which now has a frequency. This effectively converts the DC into AC, and the safe voltage level falls dramatically.

Even with ELV DC, other dangers can exist dependent on the supply and source impedance and independent of the source voltage, ie. a short circuited 12v A23 55mAh alkaline battery won't give you much of a fright, but a short circuited 12v 160Ah LiFePo4 battery which can supply 1000 amps for over 5 minutes will certainly make you take notice

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#12

Re: Switching Regulator Controller

01/22/2017 3:41 PM

So that we may concentrate on what you do not understand, next time you post a similar question....

List the whole text, as you did. Highlight with mouse, then copy [ctrl-C keys] a part of that text you do not understand.

Type "Please explain meaning of following words" then press Enter key for new line.

Insert [ctrl-V keys] the words you copied, then press Enter key for new line.

Repeat for other parts of text you wish explained.

Even better, use the 1 2 3 "numbered list" option at top of CR4 message window inserting each of your text parts following the number written automatically.

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#13

Re: Switching Regulator Controller

01/26/2017 7:30 AM

isolated flyback topology -- src. : Topologies Compared

low side external N-channel power MOSFET -- user can choose

an internally regulated 7V supply -- usually the 7V is the Gate to Source voltage where the MOSFET reaches it's saturation current - though - to set 7V to gate fast the suply had to provide some extent to it ? ...

the part senses the isolated output voltage directly from the primary-side flyback waveform. -- . . . good if so

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