Monday, 5 December 2011

Schemes flyback transformer BSH

Schemes flyback transformer BSH


Flyback transformer function and design

The low cost, simplicity of design and intrinsic efficiency of flyback transformers have made them a popular solution for power supply designs of below 100W to 150W. Other advantages of the flyback transformer over circuits with similar topology include isolation between primary and secondary and the ability to provide multiple outputs and a choice of positive or negative voltage for the output.

This article discusses design parameters for the flyback or swinging choke type of transformer used in flyback converters. The latter has been used for many years and its topology is unique within the transformer-isolated family of regulators.

Flyback transformer function

When the switch is turned on, energy is stored in the primary (within the core material). As shown in Figure 1, the polarity dots on the transformer and the diode are arranged such that there is no energy transferred to the load when the switch is on. When the switch is turned off, the polarity of the transformer winding reverses due to the collapsing magnetic field, the output rectifier conducts and the energy stored in the core material is transferred to the load. This activity continues until the core is depleted of energy or the power switch is once again turned on.

Figure 1. Typical flyback transformer circuit

The flyback regulator can operate in either discontinuous or continuous mode. In the discontinuous mode (see Figure 2), the energy stored in the core when the FET is on/off is completely emptied from the core during the flyback period. In the continuous mode, (see Figure 3) the FET is turned on before the core empties of flyback energy. A typical flyback transformer may operate in both modes depending on the load and input voltage.

Designers should consider the maximum load at low voltage, including all conditions within the operating range of the flyback, as it will simply shut down (discontinuous mode) between cycles and wait for the load demand to catch up with the power-delivery capability. This is one of the most dynamic characteristics of the flyback, regulated over a wide range of input voltage and load.

Figure 2. Flyback transformer in discontinuous mode

Figure 3. Flyback transformer in continuous mode

Monitor Flyback Transformer pin identification

Understanding the Monitor flyback transformer, pin out would give you many advantages when comes to troubleshooting and repairing Computer Monitors. Many Monitor troubleshooter faced problems dealing with identifying the flyback transformer pin out. Here are the common Monitor flyback pinouts B+ pin, collector pulse, ground, x-ray protect, automatic frequency control (AFC), automatic blanking limiter (ABL), heater (for old monitors), G1 (brightness), G2 (depends on Monitor designs), VCC and also horizontal centering.

If possible, get some free Monitor free schematic diagrams and begin to analyse the flyback circuits pin outs and once you know each functions of these pin outs, repairing Monitor would become easier especially problems that are related to Monitor flyback transformer.

What is a Flyback Transformer?

Flyback transformer, or, line output transformers are a part of the power supplies in cathode ray tubes. The flyback transformer generates a high voltage, as needed by the CRT display or similar devices (e.g. plasma lamps). A flyback transformer generates a voltage between a few kilovolts to 50 kilovolts and uses high frequency switched currents between 17 kHz and 50 kHz.

The chief difference between a flyback transformer and main/audio transformer is that flybacks transfer as well as store energy, for a just a fraction of an entire switching period. The secret behind that is the coil winding on a ferrite core that has an air gap; it increases the magnetic circuit reluctance for storing the energy.

The reason it is called a flyback transformer is because the primary winding uses a relatively low-voltage saw-tooth wave. The wave gets strengthened first and then gets switched off abruptly; this causes the beam to fly back from right to left on the display.

Cathode ray tube.


Plasma Lamps

Any display requiring high voltage to operate.

Testing Flyback Transformer

Nowadays, more and more monitor comes in with flyback transformers problems.
Testing flyback transformer are not difficult if you carefully follow the
instruction. In many cases, the flyback transformer can become short
circuit after using not more than 2 years. This is partly due to bad design
and low quality materials used during manufactures flyback transformer.
The question is what kind of problems can be found in a flyback transformer
and how to test and when to replace it. Here is an explanation that will help
you to identify many flyback transformer problems.
There are nine common problems can be found in a flyback transformer.
a) A shorted turned in the primary winding.
b) An open or shorted internal capacitor in secondary section.
c) Flyback Transformer becomes bulged or cracked.
d) External arcing to ground.
e) Internal arcing between windings.
f) Shorted internal high voltage diode in secondary winding.
g) Breakdown in focus / screen voltage divider causing blur display.
h) Flyback Transformer breakdown at full operating voltage (breakdown when under load).
i) Short circuit between primary and secondary winding.

Testing flyback transformer will be base on (a) and (b) since problem
(c) is visible while problem (d) and (e) can be detected by hearing the arcing
sound generated by the flyback transformer. Problem (f) can be checked with multimeter
set to the highest range measured from anode to ABL pin while (g) can be solved by
adding a new monitor blur buster (For 14' & 15' monitor only.) Problem (h) can only be
tested by substituting a known good similar Flyback Transformer. Different monitor have
different type of flyback transformer design. Problem (i) can be checked using an
ohm meter measuring between primary and secondary winding. A shorted turned or open
in secondary winding is very uncommon.

What type of symptoms will appear if there is a shorted turned in primary winding?
a) No display (No high voltage).
b) Power blink.
c) B+ voltage drop.
d) Horizontal output transistor will get very hot and later become shorted.
e) Along B+ line components will spoilt. Example:- secondary diode UF5404 and B+ FET IRF630.
f) Sometimes it will cause the power section to blow.

What type of symptoms will appear if a capacitor is open or shorted in a flyback transformer?

Capacitor shorted

a. No display (No high voltage).
b. B+ voltage drop.
c. Secondary diode (UF5404) will burned or shorted.
d. Horizontal output transistor will get shorted.
e. Power blink.
f. Sometimes power section will blow, for example: Raffles 15 inch monitor.
g. Power section shut down for example: Compaq V55, Samtron 4bi monitor.
h. Sometimes the automatic brightness limiter (ABL) circuitry components will get burned.
This circuit is usually located beside the flyback transformer. For example: LG520si

Capacitor open

a. High voltage shut down.
b. Monitor will have ‘tic - tic’ sound. Sometimes the capacitor may measure O.K. but
break down when under full operating voltage.
c. Horizontal output transistor will blow in a few hours or days after you have replaced it.
d. Sometimes it will cause intermittent "no display".
e. Distorted display i.e., the display will go in and out.
f. It will cause horizontal output transistor to become shorted and blow the power section.

How to check if a primary winding is good or bad in a Flyback Transformer?
a) By using a flyback/LOPT tester, this instrument identifies faults in primary winding by
doing a ‘ring’ test.
b) It can test the winding even with only one shorted turned.
c) This meter is handy and easy to use.
d) Just simply connect the probe to primary winding.
e) The readout is a clear ‘bar graph’ display which show you if the flyback transformer
primary winding is good or shorted.
f) The LOPT Tester also can be used to check the CRT YOKE coil, B+ coil and switch mode power transformer winding.

NOTE: Measuring the resistance winding of a flyback transformer, yoke coil, B+ coil and
SMPS winding using a multimeter can MISLEAD a technician into believing that a shorted
winding is good. This can waste his precious time and time is money.

How to diagnose if the internal capacitor is open or shorted?
By using a normal analog multimeter and a digital capacitance meter. A good capacitor have the range from 1.5 nanofarad to 3 nanofarad.*
1) First set your multimeter to X10K range.
2) Place your probe to anode and cold ground.
3) You must remove the anode cap in order to get a precise reading.
4) Cold ground means the monitor chassis ground.
5) If the needle of the multimeter shows a low ohms reading, this mean the internal capacitor
is shorted.
6) If the needle does not move at all, this doesn’t mean that the capacitor is O.K.
7) You have to confirm this by using a digital capacitance meter which you can easily get one
from local distributor.
8) If the reading from the digital capacitance meter shows 2.7nf, this mean the capacitor is
within range (O.K.).
9) And if the reading showed 0.3nf, this mean the capacitor is open.
10) You have three options if the capacitor is open or shorted.
- Install a new flyback transformer or
- Send the flyback transformer for refurbishing or
- Send the monitor back to customers after spending many hours and much effort on it.

* However certain monitors may have the value of 4.5nf, 6nf and 7.2nf.
Note: Sometimes the internal capacitor pin is connected to circuits (feedback) instead of ground.
Tv rca flyback transformer circuits usually do not have a internal capacitor in it.
If you have a flyback diagram and circuits which you can get it from the net, that would be an advantage to easily understand how to check them.

Simple High Voltage Generator Low Voltage DC In, up to 30 KV Out


The basic circuit described in this document is capable of generating up to
30 kilovolts or more from a low voltage DC source using the flyback (LOPT)
transformer salvaged from a B/W or color TV or computer monitor. Typical
output with a 12 VDC 2 A power supply or battery will be 12,000 V. Maximum
output current at full voltage is typically around 1 to 2 mA. Higher currents
are available but the output voltage will drop. At 2 KV, more than 10 mA may
be possible depending on your particular flyback transformer input voltage
and current.

As you can see from the schematic below, it doesn't get much simpler than this!

+Vcc Q1 +----------------+ |:|
o | )|:|
| B |/ C )|:|
| +------| 2N3055 )|:|
| | |\ E 5 T )|:| +------|>|----------o +HV
| | | )|:|( HV Diode, usually
| | -_- )|:|( built in.
| | )|:|(
+--|-------------------------+ |:|(
| | Q2 _-_ )|:|(
| | | )|:|( Secondary (HV) winding,
| | B |/ E 5 T )|:|( intact.
| | ----| 2N3055 )|:|(
| | | |\ C )|:|(
| | | | )|:|(
| | | +----------------+ |:|(
| | | |:|(
| | -----------------------+ |:| +------------------o -HV
| | 2 T )|:|
| | +---------+ |:|
| | | 2 T )|:| T1 - Flyback transformer from B/W or
| +-------------------------+ |:| color TV or computer monitor.
| |
| R1 | R2
110 27 _|_
5 W 5 W -

This design is derived from a circuit found in: "Build your own working
Fiberoptic, Infrared, and Laser Space-Age Projects", Robert E. Iannini,
TAB books, 1987, ISBN 0-8306-2724-3.


CAUTION: See the document: "Safety Guidelines for High Voltage and/or Line
Powered Equipment" before firing up this circuit!

Read the following in its entirety!

1. Obtain flyback transformer with known good HV secondary winding. primary
may be left intact if it is known to be in good condition - non shorted.
A flyback removed due to failure may be used if it was the primary that
failed and the primary turns can be removed without damaging the HV
secondary or losing the secondary return connection! Flybacks fail
in both ways (primary and secondary).

2. Locate the return for the high voltage winding. This may be a different
color wire than the low voltage winding or may exit from the potted part
of the flyback in a different place. It is not possible to use an
ohmmeter to locate the return for the high voltage winding if your
flyback has a built-in HV rectifier or multiplier as the forward voltage
drop of the rectifier diodes is much greater than the battery voltage
used in your multimeter. However, a winding connection that has
infinite resistance to every other terminal is likely to be the HV
return. On flybacks with no HV rectifier or multiplier, the return
is easily located by measuring resistance between the HV output and all
other terminals. The HV winding will have a resistance of 100s-1000s
of ohms compared to single digit readings or less for all the other

3. Wind 10 turn center tapped drive winding and 4 turn centertapped feedback
winding using #16 to 20 gauge insulated wire. Make sure both halves of each
coil are wound in same direction. Connect centertap in each case at the
winding - do not bring out a loop. Insulate well with electrical tape.

4. Vcc should typically be in the range 12 to 24 volts at a couple of amps.
Circuit should start oscillating at around a Vcc of 5 V or so. If you do
not get any HV out, interchange the connections to the transistor bases.
Heat sinks are advised for the transistors. Be aware of the capability of
your flyback (B/W monitors up to 15 KV, color up to 30 KV). You risk
destroying the secondary windings and/or HV rectifier if you get carried
away. Running this on 24 volts will probably cause an internal arc-over
in a small flyback, at which point you start over with more caution and a
new flyback.

5. Actual output will depend on turns ratio of the flyback you have.

* For a typical small B/W TV, monochrome computer monitor, or video display
terminal, you should be able to get around 12,000 volts with 12 VDC input.

I built one from a dead Mac-Plus flyback from which I removed the (dead)
primary windings.

* With a large color TV or color monitor flyback, 30,000 V or more will be
possible using a 24 VDC power supply.

6. The frequency of operation will be in the KHz to 10s of KHz range depending
on Vcc, load, and specific flyback characteristics.

7. You can experiment with the number of turns, resistor values, etc. to
optimize operation and power output for your needs.

8. CAUTION: contact with output will be painful, though probably not
particularly dangerous due to low (a few mA) current availability.

HOWEVER, if you add a high voltage capacitor to store the charge,
don't even think about going near the HV!

Inverter parts list (excluding low voltage power supply):

None of the component values are critical. It is quite likely that everything
needed is already patiently waiting in your junkbox. If not, except for the
flyback, most if not all of the parts should be available from Radio Shack.
See the section: "Low voltage power supply" for a simple design to use with
this inverter.

Some experimenting with different value resistors and even the number of turns
on each winding may improve performance for your particular flyback.

Q1, Q2 - 2N3055 or similar NPN power transistors (reverse polarity of Vcc if
using PNP transistors.) Maximum stress on transistors are about 2
to 3 times VCC. Heat sinks will be needed for continuous operation.

R1 - 110 ohms, 2 W resistor (5 W for Vcc of 24 V). This provides base
current to get circuit started.

R2 - 27 ohms, 5W resistor. This provides return path for base feedback
during operation.

T1 - Flyback transformer from/for B/W TV, video display terminal, color
TV, computer monitor, etc., modified according to text above.

Most modern flybacks include built-in HV rectifier diode(s) and/or
voltage multiplier (tripler) so output without additional components
will be high voltage positive or somewhat smoothed HV DC.

Note: this kind of flyback transformer drives the CRT directly and
uses its glass envelope as the main high voltage filter capacitor.
(A foot square piece of 1/8 inch Plexiglas with Aluminum foil plates
makes a filter capacitor.)

Wire - a couple of feet of #16-#20 hookup wire, magnet wire, or any other
insulated wire for home made primaries. Use electrical tape to
fix windings to core. Wind feedback winding on top of drive winding.

Low voltage power supply:

The power supply (12 to 24 V) doesn't need to be anything fancy. Regulation
is not needed so a simple power transformer-bridge rectifier-filter capacitor
design will be fine. The circuit described below will provide about 15 VDC at
up to 3 A. Unless you are going for maximum output, this should be adequate.

During initial testing at least, a Variac on the input (or variable voltage
power supply) is highly desirable to avoid blowing anything should your wiring
or parts not be quite right and to gain a feel for the capabilities of your
circuit before it is too late! If neither of these is available, use a 10 ohm
25 W power resistor or 100 W light bulb in series with the load (inverter) to
limit current to a safe value - one that won't fry too many things too quickly.

A typical circuit is shown below:

_ T1
H o-----o/ o---- _------+ 5 A diodes
S1 Power F1 Fuse )|| or bridge
1 A )|| +---------+----|>|-------+-------+-----o +Vcc
)||( ~| D1 |+ |
)||( +----|<|----+ | +_|_ C1
115 VAC )||( 12 VAC D2 | | ___ 20,000 uF
)||( +----|>|----|--+ - | 25 V
)||( | D3 | |
)|| +---------+----|<|----+----------+--+--o Gnd
)|| ~ D4 - _|_
N o---------------------+ -

Low voltage power supply parts list:

All of these are readily available.

T1 - 12 V, 3 A power transformer.
S1 - SPST toggle switch.
F1 - Fuse, 1 A.
D1-4 - Silicon rectifier diodes, 5 A minimum. Or, 5 A bridge rectifier.
C1 - Electrolytic filter capacitor, 20,000 uF or more, 25 V minimum.

Typical flyback schematic:

This diagram shows a typical flyback that might be found in a direct
view color television or computer monitor. Resistances are included for
illustrative purposes only and may be quite different on your flyback!

The high voltage section on the right may actually be constructed as a
voltage multiplier rather than a single winding with multiple HV diodes.
The rectifiers or multiplier, and/or focus/screen divider may be external
to the flyback transformer in some models.

Flyback transformers used in black-and-white TVs and monochrome computer
monitors do not have a focus and screen divider network.

The ferrite core of a flyback transformer is constructed with a precision
gap usually formed by some plastic spacers or pieces of tape. Don't lose
them if you need to disassemble the core. The ferrite core is also
relatively fragile, so take care.

The focus and screen divider network uses potentiometers and resistors
(not shown) with values in the 10s to 100s of M ohms so they may not
register at all on your multimeter. The high voltage rectifiers (CR1
to CR3 on this diagram) are composed of many silicon diodes in series
and will read open on a typical VOM or DMM.

Note that there is no standardization to the color code. However, the fat
wire to the CRT is most often red but could also be black. Of course, you
cannot miss it with the suction cup-like insulator at the CRT anode end.

|:| +--|>|-----------o HV to CRT
_ 1 |:|( CR1 (25 to 30 KV,
| B+ o-------------+ |:|( suction cup on
Drive | )|:|( fat red wire)
winding < )|:| +-------+
| 1.32 )|:| |
| 2 )|:| +--|>|--+
|_ HOT o-------------+ |:|( CR2
_ 3 |:|(
| 50 o-------------+ |:|(
| )|:| +-------+
| .11 4 )|:| |
| 35 o-------------+ |:| +--|>|--+
Various | )|:|( CR3 |
auxiliary < .28 )|:|( /
windings | 5 )|:|( \<-------o Focus
| 16 o-------------+ |:|( / (3 to 10 KV,
| )|:|( \ orange wire)
| .12 6 )|:|( |
|_ 0 o----------+--+ |:|( 9 |
_ | 7 |:| +--+ /
| H1 o----------)--+ |:| | \<-------o Screen
CRT Heater < .08 | 8 )|:| | / (200 to 800 V,
|_ H2 o----------+--+ |:| | \ brown wire)
| |:| | |
| |:| +----|--------o To CRT DAG
| | ground

-- end V1.20 --

Flyback Transformer

A flyback transformer (FBT), also called a line output transformer (LOPT), is a special transformer, which is used for conversion of energy (current and voltage) in electronic circuits. It was initially designed to generate high current sawtooth signals at a relatively high frequency. In modern applications is used extensively in switched-mode power supplies for both low (3V) and high voltage (over 10 kV) supplies.
Flyback Transformer

It was invented as a means to control the horizontal movement of the electron beam in a cathode ray tube (CRT). Unlike conventional transformers, a flyback transformer is not fed with a signal of the same waveshape as the intended output current. A convenient side effect of such a transformer is the considerable energy that is available in its magnetic circuit. This can be exploited using extra windings that can be used to provide power to operate other parts of the equipment. In particular, very high voltages are easily obtained using relatively few turns of winding which, once rectified, can provide the very high accelerating voltage for a CRT. Many more recent applications of such a transformer dispense with the need to produce high currents and just use the device as a relatively efficient means of producing a wide range of lower voltages using a transformer much smaller than a conventional mains transformer would be.