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4hv.org :: Forums :: High Voltage
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Driving 'Fiddy's Flybacks' in flyback mode.

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Ash Small
Wed Nov 05 2014, 08:22PM Print
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
EDIT: While it's obviously possible to use any flyback driver to drive these (single transistor, etc), this thread is aimed at getting the best results from it, rather than just 'getting it to work'.

While waiting for my AC flyback to arrive (See thread here: Link2 ), I've started thinking about the best way to drive it in 'flyback mode' (as opposed to push-pull, or ZVS mode).

As I see it, until we know more about them, we need to be able to vary the 'on' time and 'off time' independantly, regardless of volts per turn, etc. in order to satisfy the 'volt seconds' requirements, etc.

I'm currently planning on using two NE555 chips (or similar), one in astable mode and one in monostable mode, but both variable, so that the astable one determines the frequency and the monostable determines the 'on' time.

The monostable will be triggered by the Astable turning on, but will turn off after it's own predefined period. The monostable will then drive a MOSFET or similar switch.

I'm using this approach because most situations require unequal 'on' and 'off' times.

Using this approach should enable the optimum frequency and 'on' time to be determined for any number of primary turns/input voltage.

I'm wondering what thoughts others have on this subject, or how they plan to run theirs in flyback mode?
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Patrick
Wed Nov 05 2014, 09:56PM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
look at my high voltage bench top HV supply.
I used one 555 timer with the "diode trick" to vary the duty cycle beyond the common 555 circuit. As you suspect, the out put varies as you drive it through a duty-cycle.


EDIT: Found it ! Link2 look through all 3 pages I learned a lot as I went forward.






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dexter
Wed Nov 05 2014, 10:05PM
dexter Registered Member #42796 Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195
varying the frequency at the first 555 will limit the flexibility of the second 555. aka at some point the ON duration of the monostable will either be to low or higher than the period of first 555 giving weird output signal

so why not the good old TL494 with the CTRL pin connected to ground and output transistors paralleled?
using a pot on the timing resistor and another on the input comparators will allow frequency and 0 to 95%
duty cycle selection

here's a example:

1415224855 42796 FT167108 Tl494n

just chose R3 R4 R5 carefuly so that pin 1 and 16 see a voltage between 0.7 to 3.3V from the 5V reference output
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Patrick
Wed Nov 05 2014, 10:09PM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Sigh, here we go with the TL494 again....
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Ash Small
Wed Nov 05 2014, 10:50PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
@Patrick: Can you please elaborate on the 'diode trick'? I had a look through all three pages, but couldn't find details of this.

@ Dexter: I did consider that, but how do you vary the frequency without varying the 'on' time? (I think it's important to be able to do this).
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Patrick
Wed Nov 05 2014, 11:09PM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Your right, I thought I put the full schematic for my etched board up, but I must have forgot.

Anyway, look at the second one down here : Link2 this is the one I mostly use, and used on the planar thread recently.

this one ive used long ago, cant remember why : Link2
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Ash Small
Wed Nov 05 2014, 11:58PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...

Your right, I thought I put the full schematic for my etched board up, but I must have forgot.

Anyway, look at the second one down here : Link2 this is the one I mostly use, and used on the planar thread recently.

this one ive used long ago, cant remember why : Link2

Thanks for posting this Patrick. My browser won't open the second link, but the first link explains the 'diode trick'. I don't think it's precise enough for my plans, though. wink
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Sulaiman
Thu Nov 06 2014, 07:17AM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Ash, when driving a monostable 555 the output will be high;
1) until the threshold voltage (>= 2/3 Vcc) is reached by the timing capacitor
2) while the trigger pin is low (<= 1/3 Vcc)

so you need to either
3) have a very short LOW output from the astable = low value discharge resistor
or
4) a.c. couple the astable output to the monostable input
e.g. 1nF astable output to monostable trigger and 3k3 astable trigger to +Vcc
............................................. .................................................. ...............
EDIT: as pointed out below, the 3k3 goes from MONOSTABLE trigger input to +Vcc (brain fart!)
Thanks for spotting it Ash.
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Steve Conner
Thu Nov 06 2014, 10:18AM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
My Dee-lux interrupter circuit is an example of how to make independently adjustable pulse repetition rate and width using two 555s.
Link2

Jan Martis' "super flyback driver" is the best circuit I know of for driving a flyback in flyback mode.

Finally, remember that the flyback needs an air gap to work in flyback mode.
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Ash Small
Thu Nov 06 2014, 11:35AM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
@ Sulaiman: Thanks for that info. I'll look into this some more while waiting for the flyback to arrive.

@ Steve: I've just had a look at Jan's schematic Link2 and, from what I can tell from a 'first glance', it uses the UC3844 chip which (according to the datasheet Link2 ) only allows 50-70% deadtime, and therefore doesn't allow indepandant adjustment of 'on' time and frequency. Your 'Dee-Lux' interruptor circuit is more along the lines of what I have in mind. 'Fiddy's Flybacks' are shipped with an airgap, although I may well try varying it wink

I guess it's time to dig out a couple of NE555's and a suitable MOSFET and start on a schematic before the flyback arrives.
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