Digital vs Analog Isolators in Smps Design

I've seen a few schematics of opto's being used in linear mode for smps designs.. But what about those digital isolators? Are they just for digital circuits?

Right now I'm working on applying a digital isolator for a smps design..

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Note that the datasheet doesn't mention smps applications. hmmm :(

Anybody know some design examples of smps's using digital isolators (opto digital, galvanic, GMR or RF)?

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

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D from BC
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The recent ARRL handbooks have a design for an off-line switcher that puts the controller on the secondary side, driving the primary-side transistor bases through pulse transformers. For start up, the primary- side circuit forms a relaxation oscillator that is triggered by the core saturation; this gets the secondary side voltage ramping up so the controller can start doing it's job.

It uses the venerable LM3524 as the controller, so it's not like it needs anything special.

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Tim Wescott

Sorry -- it's not in the 2009 Handbook, but it is in the 2002 edition.

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Tim Wescott

I don't have the book but the cct description is good.

Yup... Just about everything and the kitchen sink goes on the secondary side when a digital isolator is used.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

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D from BC

I use both in switching power supplies. The analog opto for voltage feedback, the digital isolator (Analog Devices ADuM1200 series) for setting power supply voltage.

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Mark
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qrk

Three bucks for an isolator in a switcher? Yikes.

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Joerg

Spricht so "El Cheapo" Joerg ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Jim Thompson

Relative to sluggish optos, the galvanic ISO721 is not cheap. Probably the only reason I'm using it is to dodge modelling a fast opto in ltspice. Another reason I'm dodging a linear opto is that my smps design is experimental and a poorly applied opto may cause loop stability problems. If anything goes wrong, I can't blame an opto if it's not there.

It's a tradeoff.. So the parts are expensive, but I get the design done earlier.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

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D from BC

:On Fri, 27 Feb 2009 00:44:57 -0600, Tim Wescott wrote: : :> On Thu, 26 Feb 2009 19:40:45 -0800, D from BC wrote: :> :>> I've seen a few schematics of opto's being used in linear mode for smps :>> designs.. :>> But what about those digital isolators? Are they just for digital :>> circuits? :>> :>> Right now I'm working on applying a digital isolator for a smps :>> design..

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Note that the :>> datasheet doesn't mention smps applications. hmmm :( :>> :>> Anybody know some design examples of smps's using digital isolators :>> (opto digital, galvanic, GMR or RF)? :>> :>> :>> D from BC :>> myrealaddress(at)comic(dot)com :>> BC, Canada :>> Posted to usenet sci.electronics.design :> :> The recent ARRL handbooks have a design for an off-line switcher that :> puts the controller on the secondary side, driving the primary-side :> transistor bases through pulse transformers. For start up, the primary- :> side circuit forms a relaxation oscillator that is triggered by the core :> saturation; this gets the secondary side voltage ramping up so the :> controller can start doing it's job. :> :> It uses the venerable LM3524 as the controller, so it's not like it :> needs anything special. : :Sorry -- it's not in the 2009 Handbook, but it is in the 2002 edition.

I pulled a psu from a defunct Epson inkjet printer and it uses a similar setup to your description. The mains input primary side switcher uses a mosfet controlled by bjt's and the secondary side uses a L4962EA 1.5A smps controller to produce a 42V output.

As for the OP's query, my initial reading is that the digital isolator cannot be applied to smps feedback control. The output control of an smps relies on sampling an analog voltage against a reference and producing an analog feedback signal which is applied to the primary side switching control usually via an opto-isolator. Using a digital isolator (which is quite complex in itself) would likely entail conversion from analog to digital on the input side of the isolator and then digital to analog on the secondary side in order to control the primary side switching device. Also, the opto-isolator must have a 5kV minimum galvanic isolation for mains inputs of 240Vac. Even if the digital isolator could work it only has 4kV isolation.

Reply to
Ross Herbert

I don't think so. Once the analog error signal gets to a PWM (or other), the error signal crosses into digital land. This works when the PWM is on the secondary side.

Digital isolators are easy to apply and are easy to model. My digital isolator model uses a buffer with only 5 parameters Td Tr Tf Vh Vl.. I tried modelling a 6N135 linear opto, what a pita.

If a mosfet gate is getting pulses, that's digital.. The mosfet driver can get its signal instead from a digital isolator which gets pulses from a secondary side PWM.

4kV is fine for my prototype. I'm using 120Vac..

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

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D from BC

:On Sat, 28 Feb 2009 03:06:38 GMT, Ross Herbert : wrote: : :>

:>I pulled a psu from a defunct Epson inkjet printer and it uses a similar setup :>to your description. The mains input primary side switcher uses a mosfet :>controlled by bjt's and the secondary side uses a L4962EA 1.5A smps controller :>to produce a 42V output. :>

:>As for the OP's query, my initial reading is that the digital isolator cannot :>be applied to smps feedback control. The output control of an smps relies on :>

: :I don't think so.

:Once the analog error signal gets to a PWM (or other), the error :signal crosses into digital land. :This works when the PWM is on the secondary side.

Take a look at the data sheet for a typical controller IC commonly used in SMPS, eg. UC3842.

"Pin 2 - Voltage feedback - This is the inverting input of the Error Amplifier. It is normally connected to the switching power supply output through a resistor divider."

That says the feedback signal is analog, not digital.

In SMPS which don't use a PWM controller IC, the feedback signal is always analog.

:>sampling an analog voltage against a reference and producing an analog :>feedback signal which is applied to the primary side switching control usually :>via an opto-isolator. Using a digital isolator (which is quite complex in :>itself) would : :Digital isolators are easy to apply and are easy to model. :My digital isolator model uses a buffer with only 5 parameters Td Tr :Tf Vh Vl.. :I tried modelling a 6N135 linear opto, what a pita.

6N135 is not a good choice for SMPS feedback control imo. Why not use a device which is designed and recognised for this application?
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Another commonly used OC for f/b control in SMPS is the Sharp PC817 series. There are others as well.

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Reply to
Ross Herbert

Si, puedo construir una placa de circuito para todo con eso :-)

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Cordialmente, Joerg

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Joerg

Sluggish? How fast do you need it to be? I haven't seen loops much past a couple hundred kHz in switcher.

I had used optos up to 10Mb/sec, no problem. The only reason I kicked some out and replaced them with signal transformers was because the signal transformers were multi-sourceable and 5c cheaper back then. That has changed though.

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Joerg

I think the UC3842 is meant to be used on the primary side. I don't think a digital isolator can be used with a current mode controller (UC3842) because it uses 2 feedback loops. One loop is the current ramp from the primary side and other loop is the voltage monitoring (via linear opto) from the secondary side. To use a digital isolator, there can only be linear feedback loops on secondary side. (The UC3842 uses a primary (I mode) and secondary (V mode) feedback loops.) A digital isolator can work with a voltage mode controller used on the secondary side. (The UC3842 is a current mode controller.) With a digital isolator, it's the mosfet gate signal that is isocoupled not a sample of the output voltage.

Maybe it's due to being a newbie at smps design, but it was taking too much time for me to model an opto. For example, I'd like to see a bode plot of those optos in spice.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

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D from BC

Con "queso" ?:-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

A linear opto in the feedback loop is plenty fast enough. I'm using a PS2801-1, about $0.40 in quantity, in a 50W, 600 kHz switcher. It's almost trival to use an opto. Programmable shunt regulators like the LMV431 are about $0.20.

-- Mark

Reply to
qrk

Price isn't an issue since this is a low volume item and the sensor it goes into sells for >$100k. Plus, the size of the solution is small so it fits nicely on our board. BTW, the ADuM1200 is under $2.

Reply to
qrk

Now that's slightly under the belt.

Jon

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Jon Kirwan

Ok, low volume apps are a different game. On many of my designs even $2 would be a small fortune.

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Joerg

My wife makes a marvelous chili con queso dip, definitely under the belt ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Jim Thompson

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