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Registered Member #2614
Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26
Antonio wrote ...
This system drives the flyback transformer as a regular transformer, not in "flyback" mode.
Um, all other types of switching converters use diodes somewhere in the primary. The flyback does not, and the approach i described doesn't mention any diodes.
Care to show me a "flyback" driver while you're at it?
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Th3_uN1Qu3 wrote ...
Antonio wrote ...
This system drives the flyback transformer as a regular transformer, not in "flyback" mode.
Care to show me a "flyback" driver while you're at it?
A flyback driver is a single ended driver using one switching device, which can drive the primary at a relatively low voltage, and when the switch turns off, when the voltage swings negative, the voltage rises to many times that which you are driving it at.
The first two are flyback, the third is royer, and the fourth is full bridge, which i forgot what class that would make it. Here:
Th3_uN1Qu3 wrote ... Um, all other types of switching converters use diodes somewhere in the primary. The flyback does not, and the approach i described doesn't mention any diodes.
Well, push-pull has no diodes, full bridge, half bridge, and phase shifted full bridge have no diodes, and active clamp forward has no diodes.
Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
I found Arcstarter's page is very valuable so I made it into a PDF to drop on a non-internet machine or as a reference; if anyone wants it. It gave me diagrams of the topologies that could be used in comparison via a hand-held, Kindle, laptop, etc. Worthy of printing out. ]
smps_switching_power_supply_topologies__comparison
_and_selection.pdf[/file]
Registered Member #2614
Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26
Arcstarter wrote ... A flyback driver is a single ended driver using one switching device, which can drive the primary at a relatively low voltage, and when the switch turns off, when the voltage swings negative, the voltage rises to many times that which you are driving it at.
Okay. So what exactly makes my driver "forward mode"? The lack of HV diode at the output? Good point but i'm not interested in generating HVDC. I want AC for a plasma speaker.
Arcstarter wrote ... Well, push-pull has no diodes, full bridge, half bridge, and phase shifted full bridge have no diodes, and active clamp forward has no diodes.
In theory. In practice only push-pull has no diodes, the half and full bridges use diodes across C-E (or D-S) of switching transistors (or MOSFETs) as voltage clamps.
Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
While it will work, you can get a lot more power if you have control over the frequency and duty cycle so you can tune it to resonance. My favorite basic flyback driver so far is one using a 555 driving a mosfet. While I don't see a lot of people doing this, add an RCD snubber across the primary to protect the switch from spikes. Even if your mosfet is avalanche rated, a snubber will allow it to run much cooler, and a power resistor is cheaper than a gigantic oversized mosfet.
I've had great luck using one of the original low voltage secondaries on the flyback as a primary. You get MUCH higher output from a low voltage input than you will by driving the original primary.
One of the interesting things about a single ended flyback circuit is that the output of all the windings is pretty closely tied together, so if you want to regulate it, you can add a small (1-3 turn) winding or use the existing filament winding if present and rectify that to feed back into a voltage divider to your PWM IC.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Here's a little history about flyback transformers. The word "flyback" refers to the portion of the horizontal scanning lines on a television Cathode Ray Tube (CRT) when the electron beam is cut-off (not producing light on the viewing screen) as it "flies back" to the left hand side of the screen (Yup, that's where the word "flyback" comes from!). The same sawtooth waveform that is used to scan the electron beam on the CRT is also used to generate the high voltage for the 2nd anode of the CRT by driving the flyback transformer. The flyback transformer produces its very high voltage output during the portion of the sawtooth waveform when it suddenly collapses to zero volts (during the "flyback" portion of the scan lines). This happens at a fequecy of approximately 15.7KHz.
So, I guess techinically, if you want to operate a flyback transformer in "flyback mode" you need to drive it with a linear sawtooth waveform. And if it's a flyback that you ripped out of a TV, then it would work best at 15.7KHz. Flybacks from computer monitors may operate differently, and at a much higher frequency than what you find in a TV. In most models of computer monitors, I believe that the high voltage power supply is self-contained and is driven separately, not from the horizontal scanning system.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Xray wrote ...
Here's a little history about flyback transformers. The word "flyback" refers to the portion of the horizontal scanning lines on a television Cathode Ray Tube (CRT) when the electron beam is cut-off (not producing light on the viewing screen) as it "flies back" to the left hand side of the screen (Yup, that's where the word "flyback" comes from!). The same sawtooth waveform that is used to scan the electron beam on the CRT is also used to generate the high voltage for the 2nd anode of the CRT by driving the flyback transformer. The flyback transformer produces its very high voltage output during the portion of the sawtooth waveform when it suddenly collapses to zero volts (during the "flyback" portion of the scan lines). This happens at a fequecy of approximately 15.7KHz.
So, I guess techinically, if you want to operate a flyback transformer in "flyback mode" you need to drive it with a linear sawtooth waveform. And if it's a flyback that you ripped out of a TV, then it would work best at 15.7KHz. Flybacks from computer monitors may operate differently, and at a much higher frequency than what you find in a TV. In most models of computer monitors, I believe that the high voltage power supply is self-contained and is driven separately, not from the horizontal scanning system.
Now I read 10 years ago that the word 'flyback' actually comes from the saw-tooth waveform. The waveform rises gradually, then the power is generated as the waveform 'flies back' to zero, hence 'flyback transformer'.
(I'm not saying I'm right and you are wrong, though.)
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Ash Small wrote ...
Now I read 10 years ago that the word 'flyback' actually comes from the saw-tooth waveform. The waveform rises gradually, then the power is generated as the waveform 'flies back' to zero, hence 'flyback transformer'.
(I'm not saying I'm right and you are wrong, though.)
Read this:
Here's a brief quote from that page: "The primary winding of the LOPT is driven by a relatively low voltage sawtooth wave, which is ramped up (and sweeping the beam across the screen to draw a line) and then abruptly switched off (and causing the beam to quickly fly back from the right to the left of the display) by the horizontal output stage."
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Xray wrote ...
.Read this:
Here's a brief quote from that page: "The primary winding of the LOPT is driven by a relatively low voltage sawtooth wave, which is ramped up (and sweeping the beam across the screen to draw a line) and then abruptly switched off (and causing the beam to quickly fly back from the right to the left of the display) by the horizontal output stage."
It also says this:
" The flyback (vertical portion of the sawtooth wave) is extremely useful to the flyback transformer: the faster a magnetic field collapses, the greater the induced voltage. Furthermore, the high frequency used permits the use of a much smaller transformer"
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Simplest way to drive a flyback. Really.
