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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
HR Diemen - the Spanish manufacturer of after-market LOPT spare parts - publish a vast compendium of data on LOPT's called "The Book" which you can download chapter by chapter from various sites around the web.
"The Book" is generally reckoned to be the Bible of all things LOPT.
Data sheets (but not "The Book") for many hundreds of LOPTs can be downloaded free from the HR Diemen website here:
There is an English language version button marked with a Union Flag:
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
GeordieBoy wrote ...
Also, remember that the stack of series connected diodes in a TV flyback transformer only needs to be rated for slightly more than the DC voltage that it puts on the picture tube's anode. The diodes used in this application ARE NOT required to withstand a reverse voltage of TWICE the DC output voltage like you would find with most normal half-wave rectifiers. Can anyone explain why?
Because the flyback pulse is magnitudes bigger than the forward one...
I'm actually running this one in flyback mode, maybe the reason why it still lives...
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
> Because the flyback pulse is magnitudes bigger than the forward one...
BINGO - Correct Answer! I thought that if anyone got that right it would be you!
Yes, the voltage waveform in a typical flyback converter is highly asymmetric. That is the forward voltage is relatively low in magnitude and applied for a long time, and the flyback voltage is very high in magnitude and exists for only a short time. The volt x second balance is maintained across all the windings still.
This property is one of the many reasons that the flyback converter is so popular in cheep PSUs and works well in the LOPT stage of TVs. In designs with HV output half-wave rectification can be used whilst minimising the troublesome PIV requirement of the rectifier diode. Conversely in LV output designs (shaver chargers etc) the duty ratio can be pushed to the other extreme - A high PIV output rectifier can be used on the low voltage output side in order to benefit from minimising the troublesome reflected voltage across the primary-side switching transistor. Here endeth today's lesson of flyback SMPSU design!
> I'm actually running this one in flyback mode...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
The diodes used in this application ARE NOT required to withstand a reverse voltage of TWICE the DC output voltage like you would find with most normal half-wave rectifiers. Can anyone explain why?
How many igbt's do I get for the answer? :P
It's because the negative half-cycle output voltage of the transformer is much lower, being only forward-mode voltage of primary supply voltage*turns ratio. So the diodes only need to withstand sum of this voltage and DC output voltage.
In order to satisfy volt-second requirement, this off-period also needs to last proportionally longer than the high voltage pulse does. Hence, even the maximum duty cycle in flyback mode is limited by the diode ratings (although, in most flybacks, diode ratings seem to be fairly generous for some reason).
Running the transformer in forward mode with symmetric square wave on primary is, of course, much harder on the diodes for same output voltage.
PS. Now I'm beaten by Jan because of holding my reply up for 20 minutes :P
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Flybacks- Big & Small; Performance Analysis
