INTRODUCTION
This source is characterized by its simplicity compared with the sources of the classic LCD with CCFL tubes. Indeed consumption 32 "is only 80W and that consumption and sophistication not preacondicionadoras such as stages and multiple sources (with two or three pulse transformers) are required.
However, it is common that most modern LED TV, have a pre-conditioning stage. not the problem of the capacitive load on the network, but for another reason: to further reduce the depth of the cabinet, as the pre conditioner works with major 1uF capacitors or less.
Our source is armed FAN7602C using an integrated control circuit only 8 feet, which has no internal power key. Only provides excitation for an external MOSFET and of course the regulation of the output voltage and protections.
3.2 REQUIREMENTS PULSED SOURCE
The main plate requires very little voltage source; 5V and 12V only and exciter chain only LEDs on this apparatus and requires 145V.
As for the required power is only 80W which invited the manufacturer to perform a source of very simple fly-back, with main capacitors low profile to not increase the depth of the cabinet.
The source does not require more instruments for repair. Of course it never hurts an oscilloscope, of whatever type, analog or digital from 10 MHz onwards. Perhaps the biggest difference between the two types of oscilloscopes, is the ease of memorizing oscillograms that has the digital, allowing you to form a good library of waveforms for when you need to repair another source. But if you have no oscilloscope do not worry. It can be effectively replaced by a RF probe and a detector for horizontal output stage, which can be found on this page in the workshop section from where you can download for free.
The most important thing to repair this stage is to know how it works and apply the method we are going to explain here. . If you are thinking: I will change the IC and ready, we notify you that it is not the prime suspect flaws in this source. The main suspect is the Indian classical Tocapotee because failure to restart, the first thing is to start resolder source components and damage what was wrong. In a word we can find any damaged component and need to know how to prove it. It is not uncommon to repair the source is a problem in the parallel flash memory. Tighten the client to confess if before bringing the device took it to another repairer and ask what was the original fault before a budget in the air.
3.3 SOURCE CIRCUIT AND TEST RESISTIVE LOADS
In Figure 3.3.1 we show the original circuit properly processed to read the values of the components. This circuit is not in the manual. If someone asks tell him step off the page Picerno and helps me to increase visitor traffic.
Fig.3.3.1 main source circuit (Note: RM885 is really 100 mOhms)
This circuit, which called main, is not controlled by the microprocessor, ie when the TV to the mains supply immediately the three output voltages are generated without being affected by the safe rear circuit there is a short circuit is connected or overconsumption on some of the loads.
This particular TV has a mode of operation that can exploit our work perfectly restful. You can disconnect the main source plate.In that case the source starts igniting the back ligth.
Then you can measure voltages A5V, A13V and + 145V if you need to connect any resistive load because the LEDs are already consuming. This is not so in all LED TV and pull a fly-back source without any resistive load can damage the MOSFET QM803S key.The problem is if the string of LEDs is open and the main is disconnected. At that time the source starts with a minimum load produced by the controlled section of the source and can damage the MOSFET.
Anyway if the source does not have adequate load signal forms and continuing tensions do not have much value. It is therefore desirable to have a load of 1A for both 5V supply to the 13V. This implies a power consumption of 13W (13Vx1A) for A13V source and consumption of 5W (5Vx1A) for A5V source, which puts us safe from all trouble and allows us to accurately measure voltages.
We recommend then disconnect the main source removing the connector, plug loads of 5 and 12 ohms. With the back ligth connected, connect the power to the network and observe if you turn the screen and if there are 5 and 13V; above and we verify that there are 310 / 155V unregulated source. See Figure 3.3.2.
Fig.3.3.2 Connection platelet repair to remove the source of the main
In a catastrophic failure does not exist any of the three DC voltages and the screen is off. This means that the whole key power and integrated control has a problem. Therefore we will analyze the operation of both.
3.4 KEY POWER MOSFETs
MOSFETs used in power supplies are without any doubt "electronic keys". So they have some characteristic parameters of the keys as being an internal resistance when closed, an ability to withstand voltage when open and current carrying capacity when closed. Leaving excitation conditions then, these features are those that determine the MOSFET to use.
You sure. Will think you do not need to know anything about the operation of the MOSFET, if I have questions, I'll buy one and change. If you. Aims to achieve the exact failure goes straight to spare, because it is not going to be able to find given the enormous number of variants on the market. In general it is best to form a MOSFET stock not force him to run the shop all the time.
Are the characteristics of the MOSFET are achieved by Internet? Yes, they get but do not look for the first letters of the code because those letters depend on the manufacturer. I searched with the search word "MOSFET 12N65" and found it very quickly. This file is attached so that everyone can download it from the page.
File Specifications 3.4.1 MOSFET
In the forelock of the specification, out indicated the most important features are the maximum current, maximum voltage and resistance saturation. It is a synthetically MOSFET 12A x 650V with Rds = 0.85 Ohms (which is the resistance between the drain and source 10V gate voltage).
With these values you can get an N-channel MOSFET (indicated by the symbol on the circuit) for exceeding 12A current and a voltage less than 650V with an internal resistance equal to or less than 0.85 Ohms.
When you see a MOSFET key in a circuit, you. You should think of it as a key that opens and closes supporting the I and V of the specification and that when closed has a contact resistance Rds. But a key must have a cigarette butt opera control. The equivalent in this case is the gate of the MOSFET closes when the key has more than about 2.5V.
That gate has no electrical connection to the body of the key. It is coupled by a metallization which generates an electric field on the body of the MOSFET. In effect this means that the gate operates as a capacitor to ground and as such must be excited.
A capacitor that value? That depends on the particular transistor we are using. In this case if we look at the table of electrical characteristics is indicated as Ciss (input capacity to source) and is 1480 pF typical. This is the parameter that we needed to find a replacement of a MOSFET. If we find one equal to or less safe serving capacity.
3.5 INTEGRATED CIRCUIT CONTROL
What role does the IC control? Generating a rectangular signal applied to the gate of MOSFET. This rectangular voltage signal is always the order of 12V volts peak and zero at its minimum. As for the frequency can be fixed frequency; a variable frequency in two values depending on the polarization of one leg of the IC or a capacitor that is placed on another leg.
The important thing is that once chosen frequency, it does not change, in the fly-back circuit (almost all TV LED are) and its value is between 50 KHz for older up to 250 KHz or more, for the newer (in our Samsung is 75 KHz).
The CI must perform one of the most important functions, which is controlling the output DC voltage. This is done with the only variable parameter is missing in the rectangular output and which is the period of activity of the gate signal. If the key is closed a long time, a lot of energy input electrolytic transferred to the secondary and the output voltage rises. If the activity period is short, low energy and low output voltage is transferred.
You should also perform control of the current circulating through the key and if it is too high, must cut operation before damage.
An extra feature of the IC is to find some way to boot with voltage 155 / 310v and then feed itself from a coil of the pulse transformer itself.
To understand the operation of the CI we will present first your legs diagram (see Figure 3.4.1) and then its "pin up" See Figure 3.5.2 to know what does each leg concisely.
Specification FAN7602
NO NAME OPERATION
1 LUVP Low - voltage protection network
2 LATCH / Plimit is to cut a leg operation if the network or have a source output voltage too high.
3 CS / FB Pata feedback to regulate the DC output voltage. It also operates to cut the operation when the current exceeds the power key.
4 GND Ground
5 OUT Output for gate
6 Vcc Power source
7 NC No connection
8 Vstr Leg Boot
Fig.3.5.2 Pin up in Spanish
To understand the operation is best to perform a circuit of very simple application. The manufacturer gives us a help with a circuit of one output voltage that can be seen in Figure 3.5.3.
Fig.3.5.3 application circuit for a 13V source
In the circuit we observe C106 and C107 that capacitors are not regulated supply system feeding the transformer pin 1. Pin 3 is the upper connection MOSFET Q101 key (drain). Or the lower connection (source) is virtually connected to ground by resistor R106 0.5 ohms shunt placed on it to measure, a rate proportional to the drain current signal.
The most likely damage a source MOSFET is in its key and especially in this open or shorted the raceway. The custom takes repairers to desolder and measured with the tester in ohmmeter function. It really is not necessary to completely desolder. To cut the leg drainage is sufficient. This is important if it is a SMD where drainage is still below the transistor and is welded to an island dissipation. In the Samsung the problem is very simple because you just have to cut with wire cutters center leg. A measurement with the tester as ohmmeter between the severed leg (red tip) and ground (black tip) should indicate a very high resistance value (more than 1 MOhms). Then take the controlled variable source, set to 3V and connect to the gate (left leg looking from below) red black tip and ground. The resistance indicated by the tester must fall below 10 Ohms. If the MOSFET is damaged it is very likely that it is also the shunt resistor or at least this debased. With the tester and without removal of the wafer can be measured if it is not cut. A measurement value can only be done with a special ohmmeter built according to the instructions of my book "special instruments".
The gate is connected to the output of integrated by a diode (D103) and resistor (R104) circuit. We explained that the gate of a MOSFET can be considered as a capacitor connected to the source terminal. And that capacitor must be charged and discharged to close and open the key. Now let's add that the key must move from closed to open as quickly as possible so that the key is never in an intermediate state between the opening and closing, because in those moments is when power key is generated and therefore the corresponding heat.
The leg 5 (OUT) toggles between source (13V) and ground. When in power, the equivalent capacitor is charged to 13V gate through resistor R104 to the diode is because in reverse. When this ground the diode D103 discharge the equivalent capacitor. If you open or unsoldering R104 MOSFET remains unexcited and we catastrophic failure. But in this circuit, usually fails diode and the circuit continues to operate, but is not the same download the equivalent capacitor with a diode with a resistor of 150 Ohms; cutting key becomes slow and the key is heated and burned in a few hours.
When the key is closed current begins to flow through the transformer primary. The current can not grow suddenly because the magnetic field generated by the primary produces a signal that opposes the current flow. The current increases with a fixed slope dependent voltage source (155 / 310v) and the inductance of the primary L1-3. When the key opens the primary reacts by generating a voltage peak. For this peak MOSFET not burn to the D101, R103 and C106 components operating as pulse absorbers are added. C106 is charged to a value that depends on their own ability and value of R103 which discharges to the correct value. In a word between C and R are as a variable battery so that the voltage can rise to the specified value and then operates the diode voltage limiting. Needless to say that a network failure pulse limitation drain immediately damages the MOSFET; ie a domino effect.
The current flowing to ground by the key, it does through the resistor R106 0.5 Ohms generating a sawtooth voltage that is representative of the current circulating through the key. That leads to tension leg 3 to cut the operation when it reaches a predetermined value. So we analyze the key corresponding to the power section.
For the excitation CI send key must meet several conditions. The first is that it must be fed by the leg 6. Then explain how to start the system, we now assume that already started and the winding 5-6 generates a voltage that the diode 102 rectifies and sends the source capacitor C109, if the voltage output is correct is charged to 13V more about. 15V diode ZD101 limits the output voltage in the event of a failure of regulation, resulting in a drop in the resistor R109 which the manufacturer indicates as 0 Ohms, but that surely is a mistake and is 10 Ohms.
The leg 8 (Vstr) is the kickstart. When the TV is connected to the network the leg 6 without voltage supplied by the transformer.But by the resistor R114 you get a current leg 8 which internally gives tension momentarily to the leg 6 for the system to boot and then stop drinking.
This system has two weak points. One is that the resistor R114 does not present good characteristics to the voltage pulses if SMD, even if it is not usually common mounting withstand the pulses are applied during a storm. In short it is common to find it open. In this case the circuit is in perfect condition but lacks the start pulse, which is like kick starting a motor bike. In these cases it is best replaced by a series of four resistors 12K 1/8 W. The other is that when the circuit starts no paw consumption and 8 is in the worst case to 310v generating circuit leaks printed if not well designed. For that reason the leg 7 is left unused and directly missing leg. It is common for this track environmental fluff that should be cleaned comes together. The way to test a boot problem is adjusted using a 10V external source but applied to the IC through a 1N4004 diode source with the anode to the source. Thus when the IC starts the diode is reversed and the circuit is self-powered.
Pin 1 (LUVP) takes the low voltage sensing network using the voltage divider R112 and R113. When the voltage of the leg reaches less than 2 volt IC stops generating output. Therefore it is necessary to measure this voltage as the first point of the method. The capacitor C110 operates as a pulse filter that can come from the network.
The leg 2 (Latch / Plimit) is a leg protection that operates in the opposite direction to 1. If the voltage exceeds 4V IQ fails to deliver excitement to the MOSFET key. As we have both a reference to the rectified voltage network and the output voltage of the source. The network is applied directly through R102 form an attenuator with which R108 and OP2 optocoupler output is low resistance. The source output can not be applied directly, because the primary and secondary masses are different. Therefore you should use an optocoupler. The LED output opto through R207 connects to limit current and ZD201 (13V) to generate the reference cutting operation in this case is the zener voltage of more inner barrier LED; approximately 14V.
Paw Paw 3 is a dual use. It serves both cutting operation when the current through the shunt resistor is too high, to receive feedback from the output voltage regulating operation. The separation of the two signals is easy to perform within the IC that one (the current sample, is alternated) and the other, the output feedback is continuous. The output feedback should include a voltage comparator with a stable reference. This reference is known IC201 programmable zener which is a 431 used as a high gain amplifier comparator. The output is coupled through R204 / R205 divider is compared with the internal voltage is programmable zener 2.5V.The resistor R202 serves to properly polarize the programmable zener, while R201 is the current limiter LED operational. C203 and R203 is a network that limits the frequency response of the programmable zener not to oscillate at high frequency.
The programmable Zener operational and tested simultaneously with a variable voltage regulated supply. Only Paw 4 opto have to lift and place the tester in ohmmeter function on legs 3 (black tip) and pin 4 (red tip). adjustable external source on the output is then applied with a low voltage. The ohmmeter should mark a very high value, say above 1 MOhm. Now the tension starts to rise noting that no changes in resistance, up to 13V, when the resistance can drop to less than 1K values. If this does not happen, you must measure all resistors section and if you are good, you have to directly disconnect the opto and place a red LED, where pins 1 and 2. Re were testing but now observing that when arriving at 13V LED lights. If this occurs it is damaged the opto and if not, means that damaged the programmable zener.
Now we have to control the section of the secondary that is very simple to verify, since it only contains a pair of diodes D202 and D204 are fast diodes 100V 20A and pi filter with its electrolytic capacitors C201, C202 and L201 inductor.
If all primary section works must test the secondary section. But we can prove that the primary section works. First you have to perform all the tests mentioned here, to make sure there are no problems in the feedback circuit in the primary crop in the transistor, etc. In a word that being a source a strong feedback circuit repair is complex because a failure in the secondary can affect the primary circuit and lose track of which component failed. So if you. Observe the explanations, all tests were performed without running the entire circuit, but by parties or external sources and it does not desoldamos any component. This is very important not to this particular source is armed with conventional components, but for the newest having only SMD components.
The secondary section is virtually explanation because it is a simple rectifier with filter in pi, where all components can be measured with the tester without unsolder, if we care desoldering the circuit load.
If you performed these tests we can not find the damaged component, we only have to prove that a component is the pulse transformer, also called choper. To test the generator we have been promising for delivering 2 of this course and that we will have in the next article is required.
In Figure 3.5.4 we give a help to locate the MOSFET and opto.
Fig.3.5.4 Help to locate important components
3.6 CONCLUSIONS
This article practically finished explaining the operation of the source section dedicated to the generation of tensions regulated by a source with a single output voltage.
In the next installment we will build our signal generator for testing sources and if we have enough space we will explain the changes introduced by Samsung source with respect to which we explain here.
