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"Flyback" Mode Flyback Driver Design

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Mr.Warwickshire

Thu Jul 06 2017, 10:25PM

Registered Member #61674 Joined: Mon Jul 03 2017, 04:01AM
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Posts: 22

Hello everyone! This is my first post on the forum here. Anyways, after researching the archives on this forum and other websites I believe I have designed the most stable 555 "flyback" mode driver. Here's a picture of my block diagram hybrid schematic. Any issues with it?
Picture:

Hazmatt_(The Underdog)

Fri Jul 07 2017, 04:22AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Your flyback will work better in the 10-20% duty cycle range. You're assuming flybacks work well with 50% duty and that just causes a lot of heat buildup in the switches (Mosfets).

You really don't need 2 mosfets either, one is ample, but it's really up to you how hard you want to push the flyback.

I would also get rid of the capacitor across the flyback winding, it will probably smoke or arc over during tuning.

Mr.Warwickshire

Fri Jul 07 2017, 05:59AM

Registered Member #61674 Joined: Mon Jul 03 2017, 04:01AM
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I should be able to run the circuit at 10 to 20 percent duty cycle with a 555. Doesn't the capacity across the primary increase efficiency? I thought that's what made it quasi resonant.

Sulaiman

Fri Jul 07 2017, 06:53AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

What will be the main use of the circuit ?

1 constant voltage and current/power
2 capacitor charging
3 arcs

Each has different requirements of the flyback inverter.

In my opinion, a capacitor across the primary with the frequency chosen for resonance is only useful for #3

Regarding frequency, duty eyc.

. ON time determines peak current in the primary (Ip = Vdc x Ton/Lpri)
hence the energy available for each cycle (0.5 x Lp x Ip x Ip)
and (output power) = (energy per cycle) x (cycles per second)

.OFF time is required to allow the energy stored in the flyback magnetic field to discharge into the load via the secondary winding,
one way to determine the minimum time required is Toff >= Ton x (dc supply voltage) / (primary flyback voltage)
and (primary flyback voltage) = (secondary output voltage) / (flyback transformer sec : pri turns ratio)

The off time can be increased above this minimum to reduce output power and frequency,

So it is common to have a short ON time and long OFF time for low power
and a long ON time with short OFF time for high power

If the ON time is too long then something will die due to over-current
If the OFF time is too short then something will die due to over-current

If you use a resonant capacitor on the primary then the frequency should be fixed at the resonant frequency and the duty cycle adjusted for power control.
(use pin3 instead of pin7 to drive the timing section, e.g.

(copied from the above link without permission)

When a flyback is used to charge a high voltage capacitor starting from near zero voltage across the capacitor
the capacitor will typically charge over many 10's of flyback cycles
so during each cycle of charging and discharging the high voltage capacitor
the OFF time should be long at the beginning of the charging cycle and short at the end.

This is the main weakness of a 555 based flyback inverter
great for fixed voltage and current output, not so good for capacitor charging.

I do not like to see all three terminals of transistors paralleled as it OFTEN causes r.f. oscillations that heat up the transistors
the frequency of oscillation is often above the capabilities of my 20 MHz 'scope !
So even if only switching at low frequencies, use separate gate/base drive resistors/networks as a habit.

One big problem with 555-flyback inverters is
what happens when energy is pumped into the primary every cycle but there is no load on the eht output ?
(this is important to understand and design for, in my opinion)

Also, notice that for a given Ton and Toff, the POWER is fixed
output power is volts x amps
so if either is changed, the other will change to maintain constant POWER.

Despite all of these considerations, I still like the 555 flyback for its simplicity
and because unlike most smps ICs it will be available in dil packages for a long time
and a 0.1" pitch dil IC is a lot easier to play with than a micro.mini.nano-scopic smd IC

P.S. I always try to have the lowest operating frequency of the inverter well above the highest frequency that I can hear.
(hence I rarely use automotive ignition coils even though I like their ruggedness)
Your crt flyback transformer secondary will have an intrinsic self-resonant frequency well above audio,
a large resonance capacitor could easily reduce the resonant frequency into the audio range,
which is cool for a while - then it becomes VERY irritating.

P.P.S. For most mosfet or igbt transistors a separate gate drive circuit is unneccessary as the gate loading is mainly capacitive which is no problem for the 555.
If available, use one suitable transistor rather than two 1/2 -suitable ones in parallel.

For the snubber I prefer the DCR type, have a look at Figure 2 here

as an added bonus, if the capacitor has sufficient capacitance you get a (very dangerous and/or painful) high voltage supply
with voltage of Vdc + (primary flyback voltage)

Mr.Warwickshire

Fri Jul 07 2017, 03:49PM

Registered Member #61674 Joined: Mon Jul 03 2017, 04:01AM
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Posts: 22

The main purpose of this driver is to achieve the highest voltage possible.current can come second. I have a very nice half bridge driver but I get all current with decent voltages. Oh and I plan on using this to drive both ac and DC flyback.
EDIT: My 555 circuit has both variable frequency and duty cycle. So I don't think I need to use that other method although it seems a little more stable/professional.

Sulaiman

Fri Jul 07 2017, 04:04PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

For the highest seondary voltage possible, you will be limited by one of these constraints;

. insulation of the flyback transformer secondary
. load on secondary
. voltage rating of flyback transistor must be more than (dc supply voltage) + (peak output voltage / transformer turns ratio)

Mr.Warwickshire

Fri Jul 07 2017, 04:14PM

Registered Member #61674 Joined: Mon Jul 03 2017, 04:01AM
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Posts: 22

I have purchased both 600v misfits and igbts along with 200v ones. So I should be fine. And shouldn't the TVS's in my design protect the transistors. I bought some that breakdown at 170 volts. As for the flyback insulation, I have 3d printed bobbins for ac flyback which look like your typical inverter stype XRT. They will be immersed in oil.

Mr.Warwickshire

Fri Jul 07 2017, 04:31PM

Registered Member #61674 Joined: Mon Jul 03 2017, 04:01AM
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Posts: 22

Here it the new schematic.

Sulaiman

Fri Jul 07 2017, 04:55PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

looks like a plan ... good luck and have fun.

Before testing the whole circuit, replace the flyback transformer primary with a suitable filament lamp,
you will be able to see the effect of duty cycle by lamp brightness if all is well.

P.S. probably not useful but this arrangement needs only three wires to the transformer for easier insulation / mounting

Mr.Warwickshire

Fri Jul 07 2017, 05:01PM

Registered Member #61674 Joined: Mon Jul 03 2017, 04:01AM
Location:
Posts: 22

Alright, thanks for the help. I will post pictures if working.

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