Flyback Transformers - How and Why.

19,703

38

22

Introduction: Flyback Transformers - How and Why.

This Ible was written to answer the questions of fellow Ibler, Electorials. I hope it helps.

The flyback circuit is a classic method of generating moderately high voltages. It is not, by any means, the ONLY way, but it was once a very common circuit, and was employed in EVERY CRT based TV or monitor.

It is a VERY clever circuit, because it solves two design problems in the system with one circuit.

The scanning electron beam in a CRT needs to be a.) created and b.) scanned.

All pictures bar two are taken directly from Wikipedia.

Step 1: Creating an Electron Beam

The electron beam is created in an "electron gun". A wire filament is heated strongly in a vacuum, and in the presence of a high voltage. Electrons liberated by the heat and voltage are accelerated down the tube and attracted to the screen, where they stop suddenly and cause a phosphor to glow.

Wikipedia

SO, we need a high voltage.

Step 2: Scanning the Beam

How do we scan the beam ?

Well, an electron beam is a current, a current passing through a magnetic field experiences a force at right angles to the direction of both the field and the current.

So if we create a field, at right angles to the beam, we can cause the beam to bend, and scan.
If a current passes through a coil, it creates a field, if the current increases linearly, the field follows, if the field follows, the bending in the electron beam follows linearly too.

....we need a linearly ramping current

Oh, and at the END of the scan, it would be really nice if the trace flew back VERY quickly, so fast you can't see it happen. That's the "Flyback"

Wikipedia: Raster scanning

Step 3: How ? Charging the Primary

Well, we COULD do the two things we need with two completely independent circuits, but back in the dawn of television, a truly brilliant English engineer called Alan D. Blumlein invented a circuit that could do the job, whilst working in electronic television at EMI. It is the flyback circuit.

Here is the topology:

The flyback TRANSFORMER is something of a misnomer. In a classic transformer, changing currents in the primary induce changing currents in the secondary. In a flyback they specifically do NOT

WHY not ?

See the diode in the secondary ? Current can ONLY flow in the secondary....when none is flowing in the primary.....EH ?

So, lets start the circuit, and switch on the transistor under the primary - here its a switch, S, what will happen ?

Current will start to flow in the primary (Is). Since the secondary has a diode in series with it, NO CURRENT FLOWS IN THE SECONDARY....and current ramps.....linearly.....

Put the coils to deflect the electron beam in series with the primary, and that beautiful LINEAR ramping current (see step 7) will cause our beam to scan left to right smoothly.

Score +1 - we have a linear ramping current.

Oh, and STORED ENERGY IN THE MAGNETIC FIELD = 1/2 x L x I^2 Remember that bit.

Step 4: NOT Charging the Primary....

NOW turn OFF the current. What happens ?

WHERE DOES THE ENERGY GO !!

Step 5: MAGIC !

NOW, there is energy stored in the magnetic field in the core. It has to go SOMEWHERE, and it can't go into the primary, because we switched it off by opening S......

The great trick is the collapsing field will now induce a voltage in the secondary - moderated by the turns ratio - usually very high, and aided by the fact that because we are taking energy from the field, it collapses very quickly.......

.....so the current in the primary FALLS very quickly, and the electron beam jerks suddenly to the left.....

Score +2

Step 6: ZAPPED !

The rapidly collapsing field induces a current in the high turns of the secondary.....and the high voltage on the secondary pumps up C is used to drive the electron gun and the tube......

Step 7: Gotchas. Saturation.

MANY flyback circuits on Instructables, or questions asked of them are "why does my switching transistor melt" ? You need to know that a flyback transformer operates at line frequency - how fast does the beam scan ?

TYPICALLY, it would be ~64kHz in a 1024 x 768 monitor.

So, why not slower ?

SATURATION.

???

Just like an electric circuit, where we say that a voltage creates a current: I=V/R, there are magnetic circuits, where we say B=uxH. B is called FLUX DENSITY and its units are Teslas (T) or more likely mT or uT.

H is the field strength, and its related to current by N (number of turns of wire) x I (current)

UNLIKE the electric circuit, there is a maximum limit of H a material can withstand and still give you more B. This is called "Saturation flux density"

The RATE of ramping of the current is fixed by two things: the drive voltage and the INDUCTANCE of the primary.

The inductance is given by the formula L = Al x n2

B supports the inductance of the device. So long as B can increase, I's growth is restriced by L.

V= L di/dt.  - or the voltage ACROSS the coil is restricted by L and the rate of change of current IN the coil

We can also say that V= n d phi/dt - or the rate of change of FLUX in the coil is proportional to the voltage across it, but flux phi is related to B by phi = B x core area. B is a KEY property.

Flyback operation is VERY carefully adjusted to be in the NON-saturated part of the curve.

What happens if the core saturates ?

YOU HAVE NO INDUCTANCE !

What happens then ?

What limits the current ???

NOTHING LIMITS THE CURRENT, apart from the DC resistance of the switch and the coil !

So, apply a voltage for too long to the core and the flux rises PAST saturation.....and you say goodbye to your switching transistor.

Step 8: Gotchas. That Diode !!

The reverse biased diode on the output is VITAL. It makes the circuit work, and it MUST be connected so that energy ISN'T pumped into the secondary during the charging phase - omit it, and you have a normal  transformer.

46
93 4.0K
266 18K
87 8.4K

22 Discussions

wow~ is that really HV for CRT ?

so... i took a flyback out of an old tube tv, and i'm having a few questions... it's got ten pins, 3 wires, and then one other pin that's wider and flat, i think it might be a ground or sometyhibng. pins 1 and 2 are not connnected to anything as far as i can tell, so i think they're part of the secondary coil(voltage out). pins 3 and five are connected together, about .5 ohms between them, 4 is connected to 6, 7, and 8 and all 4 have about .5 ohms between them. so like... tertiary coils or something? multiple secondaries. probably used to power other things in the television. pins 9 and ten seem to be the primary, have about 1.4 ohms, and are connected. the wire that had the suction cup on it should have been part of the secondary, but my basic multimeter can't read the resistance. it's too high, seems to be connected to nothing. in fact the other 2 wires don't really read as anything either. one of the wires has less insulation surrounding it than the other 2, so i'm assuming it's lower voltage, but the second wire has almost as much as the secondary wire. i think, logically, that the pins 1 and 2 are connected to the secondary wire and the thicker insulated one, and the thin wire is connected to the supposed ground pin. the trouble is, i'm doing a lot of assuming and supposition, which i am NOT willing to rely upon for as high of voltages(and possibly high current if i screw it up) as this can produce. do you have any suggestions upon finding these? ANY AND ALL HELP IS APPRECIATED. thanks everyone.

So how do I limit the power to the flyback when the core does saturate? would running a higher freq. help? or lowering the duty cycle? but all that would decrease the output, and I don’t want that. adding a resistor between the coils and the transistor would fry just about any resistor, would a inductive ballast/and capacitor in a tank circuit config in series with the flyback/mosfet help?

raise the frequency or limit the current going in to the driver

You get NO output when the core is saturated, and, like the formulae say, energy stored in the field = 1/2 L i2

If L = 0, because your saturated, you get no energy storage

Running a higher frequency will help greatly - remember they are DESIGNED for 64kHz typically.
Carefully watching the current in the core, and then cutting it off at the maximum value, before saturation is what is needed. This 'ible has the maths you need to work out the F for a given V and L.

My question is , what is the output which can be produced by a flyback transformer ?
Will i be able to power up anything with the give output ?

Flux can be thought of in non E&M terms too.

Flux is the amount of stuff moving past a point, or more typically through a plane, or through a Gaussian surface.

Guass' Law = Divergence Theorem.

Current , I, is a rate A/s, that is a flow and thus as long as the electrons move throw the surface you have chosen, there is flux.

Flux is positive by convention, if the flow is OUT from the origin and negative if the flow is IN .

Since the flow can be a function of time or some other parameter, we typically integrate to find the total flux. Flux is also a function of space, and we often want to add up all the field lines passing through a plane or sphere.

We almost always use symmetry arguments when integrating flux because fields get too complicated with funny shapes or edges of planes.

good luck !

dang...can't delete or edit my comment....

[:dunce cap:] again

charge not electrons ...I still get this confused obviously...

Is charge carried by the holes ?

I bought a brand-new flyback a couple weeks ago. I'm trying to determine the pins to the primary coil on my flyback.

My flyback has 10 pins. I found out which pins are connected by testing if there was a resistance between two pins with a multimeter.
Pins 1, 5, 9 are connected; pins 2, 8 are connected; pins 3, 4, 6 are connected; pin 7 is by itself; pin 10 is by itself.

This seems problematic because there are only two coils on a flyback, but I have 5 different groups of connected pins. Does this mean that something is disconnected inside my flyback?

Do you have any other ways to determine the primary coils besides this http://lifters.online.fr/lifters/labhvps/tht.htm

Many other coils are frequently wound on a flyback - for other power supplies, for oscillators driven by the coil itself. It can be hard to work out the best pins.

i have a big old transformer dug out from a junked stereo, it isn't specifically a fly-back transformer but could i use the same circuit you showed in step 4 to get a higher voltage?

Not really. The turns ratio won't be anywhere near right, and the core will saturate very quickly.

Steve

Great point about saturation of the core and the resulting conflagration
where the only limit on current is the small DC current path resistance.

A path that includes the switch, the coil and the power source resistance, which will fuse a silicone semiconductor in an instant and heat the wire insulating encapsulation into a chard coil scrap.

A