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Dem 9th May 2005 12:07 PM

Self-oscillating SMPS with saturable drive transformer
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Does someone neve an experiense with such topology?
I have designed such supplies for up to 200W , but now I need 500W to 1kW... It should not be stabilized, but simple and cheap...
I'm especcially interesting in current-proportional-drive in this SMPS (I didn't use it in 100-200W designs ) and reliability in this mode.
Does someone have proved (or not so-proved:) ) designs of this topogy? Thank You all.

richwalters 9th May 2005 06:05 PM

Hi there......A well designed high power switch-mode psu should exhibit s/c ; o/v protection amongst a host of other things.

On low power designs the self oscillatory mode or (gain limited switching) is capable of higher o/p currents than a flyback.

Drawbacks (serious).....Core driven to saturation, for high frequency operation, a square loop low loss ferrite is used...however may not always start with extreme squareness ...critical material selection; good Fe material selection can result in poor switching performance.... high loss and low effic.
Primary side switching stress is high........semi's rated for 2x bulk Vin DC....... to low Pout applications.

Personally (sorry) I would put cold water on this idea!


Dem 9th May 2005 11:19 PM

richwalters - thank You for the reply,
Usually I love to put cold water on ideas too...
But this is not just an idea - I have built such supplies and successfully used them for powering 2x90W audio amplifiers, it worked just excellent. I was too young to measure all it paramerers as I plan now :), but as I remember at 180W (+/-35v) it had about 10% rippel @100hz and about 4% @30kHz, output impedance was ~0.6-0.8Ohm - all parameters are comparable with "classic" 50/60Hz PS designs. I didn't measured efficiency, but no parts were hot above 60C.
Now I need simple and cheap PS and I think "to restore" my old design, but I'm not sure that it can be simple forced from 200 to more than 500W by transformers enlarging... I think proportional-drive should be used, but I have never dealed with it...

About "a host of other things" in good PS - this simple design has a lot of hidden things - it hasn't overvoltage problem, it does have overcurrent protection and magnetic loop autobalance etc.

I'm thinking about more modern designs too (like IR2153 + IRFxxx), but this design is also is an option, and not worst option to my opinion. It's very old topolgy , of course, so what ?

Eva 10th May 2005 01:08 AM

I've used proportional drive for more tan 1Kw output with bipolar transistors in a full bridge configuration. It works reliably and outperforms MOSFETs in efficiency at low frequencies ~30Khz, but it requires a control IC in order to provide a stable oscillator and active turn-off

In proportional drive circuits the Volts*Second product applied to the pulse transformer is usually proportional to the load current so saturable reactors are not practical

Dem 10th May 2005 08:44 AM

Eva - it's not exactly: Volts*Second product applied to the pulse transformer depends only of Volts applied to transformer and Seconds that it's applied. The voltage applied to drive transformer is always = Base-Emitter voltage of saturated transistor, and it doesn't depend of how we obtain it - from voltage feedback or proportional current feedback. It more or less stable (about 1V at deep saturation). Off course, with proportional drive base current is not constant, but Vbe changes will be not more than 10%.
Only the problem I see in this mode - with proportional drive we can lose "built-in" current limit - with voltage only FB, base current is limited, so collector current is limited too, when overcurrent occures - power transistors just exits from saturation to active mode, it's very hard mode for transistors, but no problem to stand in this mode couple of milliseconds until protection fuse will blow. With proportional drive power transistors will kept opened and saturated in overcurrent mode, so no current limit in this mode. A possible solution is to minimize proportional drive and prevent keeping power tr-rs be saturated (if for example hfe of tr-rs is about 10, so proportional drive current ratio should be less than 1/20 - only a half of required base current will obtained from the collector current), so it will not enough to keep tr-r saturated due to high output current, but on-off behavior will improved. But it's ony my theoretical ideas and should be tested.

richwalters 10th May 2005 09:28 AM


Originally posted by Eva
but it requires a control IC in order to provide a stable oscillator and active turn-off
Hi there......that's where the component count goes up....although I've been <designing> smps for many years, I've weaned myself towards standard topologies with pfc included....the cost of exotic ferrite that isn't a standard product makes the self oscillating confiuration esp 500W upwards more non viable......Here in the Alps I get billed for poor cos factor ...

The important thing about the magnetics choice is the Br/Bs ratio less than 80%.....with steep squared amorphous ferrites the loop core oscillation may not start as the flyback action on start up is too weak. However for high frequency appls.....square loop ferrites are ideal. As EVA points out......using a control ic solves this problem.


John Hope 10th May 2005 10:36 AM

Self-oscillating supply
Dem: Hi! I tend to agree with Rich on this one, at the power levels you require. And if you have to employ 'add-on' control electronics to get it to work properly, you might as well go for a more conventional topology suited to the 500W-1000W range: If fed from 300-400VDC, half bridge or full bridge is typical. If fed from battery supply for car stereo applications, push-pull is popular.

Whatever you choose: Design in some cycle-by cycle current limit mechanism, preferably with latching shutdown if the overload is sustained. When 1000W goes BANG :eek: right under your nose it's not nice.

Rich: I stand corrected, but is the design posted by Dem not what is called a Royer oscillator? I remember similar circuits being popular in 1970's capacitor discharge ignition system designs, like the Practical Electronics Scorpio. Also in inverters for fluorescent lights and other stuff 100-200W.


John Hope

Dem 10th May 2005 12:35 PM

Billing for poor cos factor ... just nightmare... Did You buy PFC power supply Your PC? :)
Could You decribe more detailed about "squared" B-H ferrites in this application? I used simple ferrites (like 3C85, 3F3) for drive transformer without problem, I paid on this only by not very optimal design of output transformer (lower than need B - less than 0.15T) to be sure that drive tr-r will saturated before even during asymmetric start-up.

John Hope:
You are right - this is so-called Jensen variation of Royer oscillator, and it really great for up to 100-200W.

N-Channel 10th May 2005 03:09 PM

Royer Oscillator

What you're looking for is the classic Royer-driven Oscillator. And, the good news here is........they can be found in every TL494-based computer PSU. By removing the PWM section and keeping the control-driver transformer, you will retain the basic oscillator.

Upon start-up, the Royer Oscillator gets things going, then as voltage builds up on the secondaries, and becomes available to power the '494, the '494 takes over for the proportional-drive operation.

There is a great article from QEX magazine that you can reference for this. It is about 3-4 pages long, and is quite thorough. It centers around a 300-400W power supply, for modding it from the original output voltages (+3.3, 5, 12, -5, and -12V) to +13.8V at something like 25-30A out for powering Ham radios and such.

Anyway, this is a good source of info for doing just what you're looking for.

Best of luck with this project. :cool:


Dem 10th May 2005 04:45 PM

N-Channel: Thank You, I know about this feature of PC power supplies, I plan to use it as my developement platform. But I can't use it "as-is" because it has only proportional-drive feedback that unstable on light loads when magnetizing current is less than reflected output current. I'll add or voltage FB, or minimal load...

But before I will start, I want to collect all available info, and the article that You referenced on, can be very helpful for me. Can You post link to it? (Or to send by e-mail a soft copy, if You have it of course) - I don't know about QEX, and Google didn't help me with this. Do You know exact name of the article? Thank You.

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