In the article, the author describes the power supply circuit of the model PA -1650-66 of an ASUS laptop , with an output voltage of +19 V and a maximum allowable current of 3.42 A, and also describes its repair. The author compiled the circuit from the inspection of the wiring diagram.This power supply (IP), made in China, is made as a separate unit. Its appearance is shown in Fig. 1. It is designed to power laptops with a voltage of +19 V (maximum load current - 3.42 A). It is powered from the mains with a nominal voltage of -230 V, with a permissible range of -100 V ... -240 V.
Externally, the IP enclosure consists of 2 plastic covers glued together (Fig. 1), inside the enclosure there is a C300 94VO XD-103 mounting plate with a size of 100 × 40 mm (Fig. 2 a, b), with two-sided installation of radio elements.
All 20 pcs. mounted elements located on the upper side of the circuit board are designated by the manufacturers, but SMD elements (51 pcs., this is about 70% of all) located on the lower side of the board, between the conductive tracks, are not marked. This greatly complicates the repair of the IP and the preparation of its circuit diagram, because anonymized elements are not known where to put in the circuit.Therefore, I found on the Internet a similar diagram of an earlier model of IP (PA-1650-56) on which all elements of IP are indicated, but their ratings are not indicated. I used part of the notation from it, and those elements that were absent, I designated myself, and so the diagram in Fig. 3 turned out. Pulse IP is assembled according to a single-cycle flyback circuit.
In their products, Chinese manufacturers often do not designate SMD elements to make it difficult to repair these products and force their owners, or repairmen, to discard damaged products and buy new ones. In addition, if you look at the IP mounting plate (Fig. 2, b), you can see that the manufacturer did not install many of the elements that were provided by the developers. The reason for this is the desire to save money to the detriment of IP quality.
To facilitate the reading of the circuit diagram of IP PA-1650-66 (Fig. 3), the purpose of its elements is given below:
RT001 (4 Ohms) - thermistor limiting the starting current of the transmitter after supplying the supply voltage;
RV001 (07471) - a varistor protecting the IP circuit from power surges in the supply network;
С001, L001, L002 - filter;
D050 (2 A, 400 V) - rectifier bridge;
С051 (120 microfarads, 400 V) - smoothing capacitor;
C3, C5, C7, C9 - high-voltage SMD capacitors;
ІС100 (LTA809FA) - The main IC IP, in the SOP-8 package;
Q050 (2SK3569, 600 V, 10 A) - N-channel field-effect key transistor of the converter, it is mounted on an aluminum radiator;
R100, R100-1, D100 - power supply circuit of the microcircuit at the initial moment of switching on the power supply;
R051 (0.27 Ohm, 1 W) - current sensor, the voltage on which is proportional to the current of the transistor 0050;
Winding 4-6 T050, D151 (JSW), R151 (5.7 Ohms), C054-1 (22 μF 50 V), С054 - IC100 power supply circuit in operating mode;
D052, R050, R050-1 (41 kOhm), C053 - a damping chain that suppresses voltage surges on the key 0050, at the time of its closing. These emissions can exceed the network voltage by 3 ... 4 times;
R102-1, D102, R102 - a chain for supplying control pulses from IC100 (pin 8) to the gate of the key Q050 and quick discharge of the gate capacitance Q050;
C101, B103, RT100 - a chain that sets the frequency of operation of the converter of the microcircuit and performs its thermal stabilization;
T050 - pulse ferrite transformer with 3 windings;
D200 (V30112OC0926G) - a powerful Schottky diode in the rectifier circuit +19 V, it is mounted on an aluminum radiator;
C200, P200 (100 Ohms), R200-1 (100 Ohms) - IC circuit connected in parallel to the Schottky diode D200;
C202 (680 μF 25 V), C203 (330 μF 25 V) - smoothing electrolytic capacitors in the +19 V circuit;
С201, С204 - capacitors that suppress high-frequency noise in the +19 V circuit;
LED - green LED indicating the presence of +19 V at the output; R102 (10 kOhm) - a quenching resistor in the power supply circuit of this LED;
R005 (0.05 Ohm) - a powerful SMD resistor, a load current sensor IP.
Feedback Loop (OS)
It is designed to stabilize the output voltage +19 V, regardless of changes in the supply voltage and the load on the IP.
The main elements of the OS (Fig. 3):
IC2 (LX8357TXA) is an optocoupler, connected between the high-voltage and low-voltage parts of the IP and decouples it with the high-voltage and low-voltage parts. In the high-voltage part, it acts on pin 2 of the IC100 chip, controlling this duty cycle of the pulses of this chip;
IC301 (103A1W) - a specialized chip. It includes a controlled zener diode and two operational amplifiers (Fig. 3), this allows efficiently and with high accuracy to maintain +19 V voltage at the output and protects the switchgear against short circuit in the load.
The operating principle of the OS is to change the duration of the energy pulses supplied by the IC100 chip (through key 0050) to the winding 1-3 of the T050 transformer, depending on the voltage value at the output of the IP. The value of this voltage is monitored by the IC301 microcircuit, together with the elements of its strapping. If the voltage of +19 V sags due to a decrease in the supply voltage or an increase in the load, then the OS immediately increases the energy output to the transformer windings and this will maintain the value of +19 V, within the specified limits. When increasing the supply voltage or reducing the load on the IP, the OS system does the opposite.
Repairs
It starts with providing access to the circuit board, and since it preys inside the plastic case, which consists of 2 covers glued together, they must be disconnected (Fig. 1). It is most convenient to disconnect the covers by moving the knife blade along the perimeter of the bonding site of the halves of the body, slightly striking with a hammer on the outside of the knife blade until the glued halves are disconnected.
Then, having removed the halves of the plastic case, unsolder at 2 points the mounting points of the screen with the mounting plate and remove the screen, and then the insulating gasket.
On the circuit board, above and below, in addition to the radioelements, you will see a white solidified insulating mass that looks like thick sour cream. At the top of the board, it covers the space between the radio elements, and stuck to them, and at the bottom, it isolates the conductive tracks along with the SMD elements.
This white mass has a dual purpose:
first, to protect the radio elements from shaking, and therefore, from tearing their rations from mounting tracks;
the second is to prevent the unit from being repaired, since the flooded mass makes it impossible to access the radio elements, and for repairs it must be removed. Removing this mass takes a lot of time and patience. This makes repairs more expensive, which is why repairmen refuse to repair such IPs, and their owners often throw out failed IPs by buying new ones, which is what Chinese manufacturers require.
But if you decide to repair the IP, then remove (pick out) the white mass is best with the tip of a bent awl. After removing it, it is necessary to inspect the circuit board, and if swollen electrolytic capacitors are found there or other elements that are clearly damaged are damaged, they must be replaced.
And now we will consider the typical external manifestations of the malfunction of this IP (Fig. 3) and how to eliminate them.
The adapter does not work, the green LED, indicating the presence of +19 V voltage, does not light
First of all, it is necessary to check the voltage on the capacitor C051 (Fig. 3), with a supply voltage of ~ 230 V, it should be about + 280 .... + 320 V.
If this voltage is not present, then use an ohmmeter to check the health of the entire circuit, starting from the ~ 230 V input: F001, RT001, L001, L002, D050, C051 (Fig. 3). If an open fuse F001 is detected, then this is usually a consequence of a breakdown of the field effect transistor D050 (2SK3569), or the D050 bridge, they must be checked with an ohmmeter and replaced if necessary. But if the transistor Q050 is broken, then with it the IC100 (LTA809FA) chip and the RQ51 current sensor (027 Ohms) are always damaged, all of them must be replaced.
If there is the required voltage on С051, and the Q050 field-effect transistor is not broken, and the IP does not start, then it is necessary to check the serviceability of the radio elements of the IP start-up chain at the time of inclusion in the power supply: R100, R100-1, D101. If this chain is serviceable, then it is necessary to check the serviceability of the IC100 strapping, the serviceability of the windings 1-3 of the T050 transformer. The malfunction can also be in the secondary circuit +19 V, while it is necessary to check: D200, C202, C203, all elements of the strands of the IC301 chip and the IC2 optocoupler. If all of the above items are working, and the IP does not start, replace the IC100 chip (LTA809FA).
When the IP is connected to the network, a green LED signaling the presence of +19 V lights up for 2 seconds and then goes off and then the IP does not work. If the IP is reconnected to the network, the LED flashes again
The very fact of the short-term appearance of the LED glow indicates the serviceability of the IP start-up system (R100, R100-1, D101) (Fig. 3), together with the IC100 chip and the Q050 field effect transistor. A faulty may be:
feedback loop (POS) stabilizing +19 V within the specified limits: IC301 together with the harness elements (D301, optocoupler IC2, and the most likely damage is a break in resistors R101 or R305);
IP power system in working condition: winding 4-6 T050, D151 (JSW), R151 (5.7 Ohms), C054-1 (loss of capacitance), C054;
significant loss of capacity by capacitors C202, C203.
Therefore, it is necessary to check the serviceability of the above radio elements and replace if necessary.
The green LED indicating the presence of +19 V voltage is on, but there is no voltage on the adapter plug
Malfunctioning can be a break in the conductors inside the +19 V cable (this is the most likely reason), or a violation of the soldering of wires inside the plug, or, for a rare reason, a break in the resistor R005 (0.05 Ohms) (Fig. 3).
In the presence of this malfunction, it is necessary to get to the circuit board, but there is no need to pick out the insulating mass from the circuit board with white. The conductivity of conductive conductors is checked with an ohmmeter, a break is most often observed near the cable exit from the IP or near the plug. To find the place of the cliff, an ohmmeter must be connected in turn to each conductive conductor and, bending the cable, observe the readings of the ohmmeter, when bending at the site of the cliff, its readings will show a break. The detected cable break should be cut out and re-soldered or a new cable should be used.
If the ohmmeter showed that the cable conductors are working, then an open can also be inside the plug (open cable soldering to the plug). The plug must be cut and soldered new, and if it is not, cut the soft plastic on the plug into 2 parts, and soldered the cores, and then, glue the plastic back. As glue, you can use hot melt. If the resistor R005 (0.05 Ohm) breaks, replace it.
The voltage of +19 V on the IP plug is normal, when connected to a laptop, the latter “does not see” the IP voltage, but if you move the plug, then everything returns to normal
The first, most probable reason, the cheeks of the +19 V internal contact plate parted inside the socket of the plug, they need to be squeezed, and this is best done with a thin needle. The second reason, violation of the soldering of the +19 V socket inside the laptop, it must be soldered, but this does not apply to IP repair.
There is +19 V voltage on the plug, but the adapter does not hold the load, i.e. at its load, the voltage of +19 V "sags"
The reason for “subsidence” is the loss of capacity (more than 60%) of the electrolytic capacitors C051, C202, C203 (Fig. 3), so their capacity must be checked and replaced if necessary. In general, if a long-running adapter has been repaired, then it is advisable to replace all electrolytic capacitors in it.
Conclusion
Repair masters should remember:
There is a wide variety of models of +19 V (3.42 A) AC adapters for laptops on the market, and even the same ASUS company has several models of similar adapters, and although almost all of them are made in China and assembled on one-stroke, return-flow principles, but have different schematic diagrams, and therefore their repair requires different approaches. The same article shows the diagram and describes the adapter only models PA-1650-66, laptop ASUS.
After repair, for the subsequent adjustment of the switchgear, its inclusion in the power supply should be carried out through a 40 ... 60 W ~ 230 V incandescent light bulb connected in series with it, it will protect the adapter from damage in case of possible defects in it.
Having repaired the adapter from which you removed the white mass that protected the radio elements from vibrations, its functions can be replaced by a silicone sealant, while not all radio elements should be strengthened, but only individual ones, for example, C051 capacitor (120 μF 400 V).
Author: Nikolay Vlasyuk, Kiev
Source: Electrician №1-2 / 2018
