Problems driving HV flyback - Page 1
Forum section: electronics / beginners

author: Chryseus
author_rank: Regular Contributor
timestamp: October 27, 2012, 06:11:59 pm
content: I recently took apart a CRT monitor and looted all the stuff inside including the flyback which is a MCF-56674, datasheet is impossible to find of course so I checked the PCB which they have very kindly printed the pinout on, I've also measured the resistance between pins to make sure, the pinout is as follows as marked on the board:1. 125V2. G23. unmarked (connected to GND)4. 250V5. ABL (seems to be the HV return)6. -126V7. 24V (Flyback)8. GND9. B+10. VcpThere is also 3 pins off to one side, the 2 outer pins go to GND and the middle goes to a transformer on the board, an auxiliary power supply would be my guess.Pins 10 to 9 measure 1.1 Ohm, the main primary winding I assume.Pins 7 to 3 measure 0.4 Ohm, this I assume is the feedback winding.Pins 1, 2, 4, 6, 8 are the lower voltage secondary windings.Now the problem is I originally tried a simple resonant one transistor driver and while this indeed worked it also rapidly killed transistor, so I decided to use a simple free running driver with a reverse parallel diode to protect the transistor, however I'm not seeing any arcs but I can hear it running.Poorly drawn schematic of my circuit as it currently is (not including supply filtering).Input is a variable duty / frequency DC square wave 5-400kHz 40ns rise 80ns fall time.I can provide waveforms if needed.

author: SeanB
author_rank: Super Contributor
timestamp: October 27, 2012, 08:44:51 pm
content: Transformer and diode are both wrong. You will have the BU508D that came with the monitor, and the efficiency diode which was there in parallel as well. The diode is in parallel with the transistor, and there are a few 400V caps as well connected across this lot. You need to drive the system as it came on the monitor ( but can short out the relays or mosfets that connected one end of the capacitor to ground) to get the flyback to work. You will also need the driver transformer and the small transistor driver as well as the base resistors and other components off the board, to give the right drive waveform to the power switch. Otherwise it has a very short and unhappy life.

author: Chryseus
author_rank: Regular Contributor
timestamp: October 27, 2012, 10:34:43 pm
content: I'm afraid that is not possible as the board it came from is in the trash, the drive transistor was a 2SC5404 1500V NPN.As I have already said before it produces arcs fine in the self resonant configuration but the kickback keeps killing transistors.What I'm interested in is why this particular configuration does not work, it's almost identical to several 555 based drivers floating around the net that apparently work just fine.

author: Psi
author_rank: Super Contributor
timestamp: October 28, 2012, 12:18:03 am
content: How much current is it drawing?Normally with flybacks from a CRT you have to wind your own primary in the exposed core area because the internal primary isn't intended for 12V

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2012, 05:54:40 am
content: Use the HV transistor then, but you need extra capacitance to lower the self resonant frequency as well.Go to eserviceinfo.com and download a few monitor/TV manuals and see how they do the driving and the values they use as well. Samsung, Panasonic, LG and such, or look at consumer and TV.

author: Chryseus
author_rank: Regular Contributor
timestamp: October 28, 2012, 02:06:54 pm
content: Thanks I'll give winding my own primary a try and see how it goes.EditThat seems to have done the job, 5 turns around the core and it's putting out some decent sparks.Thanks guys!

author: PA4TIM
author_rank: Super Contributor
timestamp: October 31, 2012, 12:06:49 am
content: Adding a snubber and neonlight makes a big difference. First my 2n3055 got real hot and one killed, after i placed the snubber circuit all kept working and much cooler.I also made a new primairy winding. Output was over 40KV and i first isolated the unused legs with teflon but this sparks, then I cut them, teflon over the short stubs and shielding in between. Then i got very nice plasma streamers.

author: Chryseus
author_rank: Regular Contributor
timestamp: October 31, 2012, 01:44:51 am
content: Funny you should mention that I've tried it with the snubber diode and the output is really really poor, without the diode it gives decent output with about 20V or so kickback, it might have something to do with the slow turn off of all them transistors so I'm going to redesign the base drive and maybe try out some high speed diodes.

author: PA4TIM
author_rank: Super Contributor
timestamp: October 31, 2012, 08:40:14 am
content: Not a flyback diode, that will off cause not work, a classic snubber network ( RC and neon in series)

author: Chryseus
author_rank: Regular Contributor
timestamp: October 31, 2012, 02:42:09 pm
content: I thought it might be something like, back to the joys of testing.