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3.1.2. TROUBLES OF THE POWER SUPPLY UNITS, THEIR SIGNS, CAUSES AND ELIMINATION METHODS

Lecture



The power source is a complex electronic device that needs to be repaired, accurately representing its work and possessing the skills of finding and eliminating defects. When repairing it is recommended to use all available troubleshooting methods in a comprehensive manner.

It must be remembered that the connection to the power supply network should occur only through an isolation transformer.

Repairs should be carried out technically sound devices, using low-voltage soldering irons.

The group stabilization of the output voltages of the UPS is characterized by the fact that with an increase in the load current of one of the secondary rectifiers, the load of the pulse transformer increases, and this affects the values ​​of the output voltages of all the rectifiers connected to it. Therefore, in the repair of BP should use the equivalent load.

For a 200 W power supply unit, load equivalents should be used: for a +5 V power supply, a 4.7 ohm (50 W) load, for a +12 V source, a 12 ohm (12 W) load.

Problems that may occur during a power supply failure can be classified as obvious and non-obvious.

The obvious ones are: the computer does not work at all, the appearance of smoke, the fuse on the switchboard burns out.

Obviously, in order to eliminate errors, determination of a faulty element requires additional diagnostics of the system, however, they may be related to the health of the source:

  • any errors and freezes on power up;
  • spontaneous reboot and periodic freezes during normal operation;
  • chaotic parity errors and other memory errors;
  • simultaneous stop of the hard disk and the fan (no +12 V), computer overheating due to fan failure;
  • restart the computer at the slightest decrease in the network voltage;
  • electric shocks while touching the computer case or connectors
  • small static disruption disrupting the network.

Particular attention should be paid to the “Power OK” signal conditioning circuit; an early on of this signal can lead to CMOS-memory distortion.
When repairing the UPS, you must use the following methods:

Mounting Analysis Method

This method allows using the human sense organs (sight, hearing, touch, smell) to find the location of the defect with the following signs;

  • burnt radio element, poor soldering, crack in the printed conductor, smoke. sparking, etc;
  • various sound effects (squeak, "popping", etc.). the source of which is a pulse transformer UPS;
  • overheating of radioelements;
  • the smell of burnt radio elements

Measurement method

The method is based on the use of measuring instruments when searching for defects, a voltmeter, an ohmmeter, an oscilloscope.

Replacement method
The method is based on the replacement of a dubious radio element with a known good.

Exclusion method

The method is based on a temporary disconnection (with possible leakage or breakdown) or bypassing the conclusions (with a possible break) of doubtful elements.

Impact method

The method is based on the analysis of the reaction of the scheme to various manipulations produced by the technician:

  • changing the positions of the engines of the installation variable resistors (if any);
  • bridging the leads of transistors in DC circuits (emitter with base, emitter with collector);
  • change of the supply voltage (with control on the oscilloscope of the PWM circuit);
  • putting a hot soldering iron tip to the body of a dubious radio element and t p manipulation.

Electric drive method

Allows you to find periodically recurring defects and check the quality of the repair (in the latter case, the run should be at least 4 hours).

Simple method

The method allows to detect installation defects on the switched on power unit by rocking elements, jerking the conductors, tapping the chassis with a rubber mallet, etc.

Equivalents method

The method is based on the temporary detachment of a part of the circuit and its replacement with a set of elements that have the same effect on it. Such parts of the circuit can be pulse generators, auxiliary sources of constant voltage, load equivalents

Moreover, any specific characteristics of the unit, obtained from the documentation for it, or read from its body, can and should be used during its repair.

When troubleshooting a technician must not only apply these methods in a pure form, but also combine them.

Typical faults BP PC
The typical causes of emergency modes in the UPS circuit are:

  • "surge" of the mains voltage, causing an increase in the amplitude of the pulse at the collector of the key transistor:
  • short circuit in the load circuit
  • avalanche-like increase in collector current due to saturation of the magnetic core of the pulse transformer, for example, due to a change in the magnetization characteristic of the magnetic circuit due to overheating or an accidental increase in the duration of the pulse opening the transistor.

ONE OF THE MOST CHARACTERISTIC FAULTS is "breakdown"

rectifier bridge diodes or high-power key transistors leading to short circuit in the primary circuit of the UPS. Breakdown of the diodes of the rectifier bridge can lead to a situation where the alternating voltage of the network will directly fall on the electrolytic smoothing capacitances of the network filter. In this case, electrolytic capacitors standing at the output of the rectifier bridge explode

A fault in the primary circuit of the UPS can occur mainly for two reasons.

  • due to changes in the parameters of the elements of the basic circuits of powerful key transistors (for example, as a result of aging, temperature effects, etc.):
  • due to connecting the computer to the outlet: installed in the network, loaded, in addition to computer equipment, high-current installations (machines, welding machines, dryers, etc.)

As a result, the network may experience impulse noise with an amplitude of up to 1 kV. which lead, as a rule, to "breakdown" over the collector-emitter section of high-power key transistors.

The third cause of faults in the primary circuit of the UPS is the illiteracy of the maintenance personnel who conduct the measurements with a grounded oscilloscope in the primary circuit of the TRS!

When a short circuit in the primary circuit, the UPS burns out (with an explosion) a current-limiting thermistor with a negative TKS. This occurs after replacing a blown fuse and re-plugging into the network if the main cause of the fault remains unresolved. Since it is sometimes difficult to get these resistors, sometimes the specialists who carry out repair of power supplies simply install the short-circuit jumper to the place where the thermistor should be. Thus, the current protection of the diodes of the rectifier bridge is removed, and the power supply unit will very soon fail again.

When replacing high-power key transistors, it is best to use transistors of the same type and manufacturer. Otherwise, the installation of transistors of another type can lead either to their failure or to the failure of the UPS start-up circuit (in case of using more powerful transistors than previously used in the circuit)

SECOND CHARACTERISTIC PROBLEM UPS is a failure of the control chip type TL494. The health of the chip can be established by evaluating the work of its individual functional units (without watering the UPS from the circuit). For this, the following method may be recommended:

Operation 1 . Generator health check DA6 and reference source DA5

Not including the UPS in the network, apply a voltage of 10-15 V from a separate source to pin 12 of the control chip

The operability of the DA6 generator is estimated by the presence of a sawtooth voltage with an amplitude of 3.2V at pin 5 of the microcircuit (assuming that the frequency setting capacitor and resistor connected to pins 5 and 6 of the microcircuit are in good condition, respectively).

The operability of the reference source DA5 is estimated by the presence of a + 5V dc chip on pin 14, which should not change when the supply voltage on pin 12 changes from + 7V to + 40V.
Operation 2. Check the health of the digital path.

Not including the UPS in the network, apply a voltage of 10-15 V from a separate source to pin 12 of the control chip

The operability of the digital path is estimated by the presence of the chips on the pins 8 and 11 (in the case of switching on the output transistors of the chip according to the circuit with OE) or on the terminals 9 and 10 (in case of their switching on on the circuit with OK) of rectangular sequences of pulses at the moment of power supply.

Check for a phase shift between the output pulse sequences, which should be half the period.

Operations 3 Check Comparator "dead zone" DA1.

Not including the UPS in the network, apply a voltage of 10-15 V from a separate source to pin 12 of the control chip.

Make sure that the output pulses on pins 8 and 11 disappear when the output pin 14 of the chip with pin 4 is closed

Operation 4 Checking the health of the PWM comparator DA2.

Not including the power supply to the network, apply a voltage of 10-15 V from a separate source to pin 12 of the control chip

Make sure that the output pulses on pins 8 and 11 disappear when the output pin 14 of the chip with pin 3 closes.

Step 5 Verifying the health of the error amplifier DA3.

Not including the power supply in the network, apply to the output 12 of the control chip supply voltage 10-158 from a separate source.

Check the voltage level at pin 2, which must be different from zero. Changing the voltage at pin 1, supplied from a separate power source, in the range from 0.3V to 6V: check the voltage change at pin 3 of the chip.

Step 6: Check the error amplifier DA4. Not including the UPS in the network, submit a conclusion 12 control chips supply voltage 10-15V from a separate source.

Check the voltage level at pin 3. after setting the DA3 amplifier to the "hard 0" state at the output. For this, the voltage at pin 2 must exceed the voltage at pin 1. Monitor the appearance of voltage at pin 3 when the potential applied to pin 16 exceeds the potential applied to the pin

THE THIRD SPECIFIC FAULT is a way out of a rectifier diode in the secondary circuits of the UPS (the rule of thumb is breakdown or a decrease in the inverse resistance of the diode).

We draw your attention to the correct choice of the replaceable diode for current, limit switching frequency and reverse voltage "

In the channel of production + 5V are Schottky diodes. and in the remaining channels - the usual silicon diodes.

It is necessary to provide a good heat sink for the rectifier diodes in the + 5V and + 12V output channels.

When controlling rectifier diodes, it is advisable to unsolder them from the circuit, because, as a rule, numerous elements are connected in parallel with them, and the control of the diodes without soldering them from the circuit in this case becomes incorrect

Importantly, the PSU can generate all the output voltages, and the PG signal will be 0V and the processor will be blocked.

The PG signal generation scheme includes quite a lot of elements that can also fail.

The listed malfunctions are major and. usually easy to find.

Sometimes failures that occur in the power supply circuit in the process of measurement, lead to emergency operation of the power transistors. Failures may be caused by an increase in the value of the installation capacitance of the elements of the BP circuit at the point where the probe is connected.

The mains fuse (3-5A) is always located on the power supply circuit board and practically protects the network from short circuits in the power supply, and not the power supply from overloads

Almost always the blown mains fuses indicate the failure of the PSU.

A peculiar indicator of a working UPS can be the rotation of the fan, which is triggered by an output voltage of + 12V (or -12V).

However, to bring the power supply to the nominal mode and correctly monitor all output voltages of the power supply, an external load is needed either on the motherboard or on resistances, which provide the entire range of current loads listed in Table 2. To evaluate the UPS’s performance in the first approximation, you can use a load resistor with nominal about 0.5 ohms and power dissipation of at least 50W through the channel output + 5V.

A serviceable UPS should run silently. This follows from the fact that the conversion frequency is beyond the upper threshold of the range of hearing. The only source of acoustic noise is a running fan.

If, in addition to the fan’s buzz, you hear a squeak, “tsykane” or other sounds, then this clearly indicates that the UPS is out of order or is in emergency mode! In this case, immediately turn off the UPS from the mains and eliminate the fault.

For more complex cases of failure of the UPS it is necessary to bury the principles of the UPS. the causal relationship of the individual nodes of the scheme and. Of course, have a schematic diagram of the power supply

3.1.2.  TROUBLES OF THE POWER SUPPLY UNITS, THEIR SIGNS, CAUSES AND ELIMINATION METHODS

Typical power supply problems

Check of radio elements

Detailed testing of radio elements can be performed using both digital multimeters and analog (switch). Consider checking typical power supply elements.

Diodes

Testing of semiconductor diodes with a pointer device should be carried out by turning on the device for measuring resistance, starting from the lowest limit (set the switch to the xl position). In this measure the resistance of the diode in the forward and reverse directions. In the case of a healthy diode, the device will show a small resistance (several hundred ohms) for the forward bias of the diode, in the reverse direction - an infinitely large resistance (break). For a faulty diode, the forward and reverse directions differ little.

When checking with a digital multimetre, the device is transferred to the test mode (otherwise, in the mode of measuring the resistance in the forward and reverse directions, the diode will show a break). If the diode is in good condition, then on the digital display shows the voltage of the pn junction, in the forward direction for silicon diodes this voltage is 0.5 ... 0.8 V, for germanium 0.2 ... 0.4 V, in the opposite direction - gap.

Transistors

Given that the transistor has two pn junctions, when testing transistors, both transitions are tested, otherwise the verification is similar to that of diodes. Testing is conveniently carried out by measuring the resistance of the transitions relative to the base output, placing one of the electrodes of the device to the base of the measured transistor. For low-power transistors when measured with a pointer device, both transitions in the forward direction have fairly close values ​​(on the order of hundreds of ohms) and in the opposite direction - a discontinuity.

The collector-emitter transition, which must also have a discontinuity, is subject to additional verification. When testing powerful transistors, the resistance of the transitions in the forward direction can be several ohms. The digital device shows the voltage for the direct direction of transitions of 0.45 ... 0.9 V.

To determine the structure and outputs of an unknown transistor, it is desirable to use a pointer device. When determining the findings, you must first ensure that the transistor is in good condition. To this end, the base output is determined by approximately the same small base-emitter and base-collector resistances in the forward and larger - in the opposite direction.

The polarity of an instrument probe that biases the transitions in the forward direction will determine the structure of the transistor: if the instrument probe has a polarity of “-”, then the transistor has a pn-p structure, and if a “+” is, then np-p. To determine the emitter and collector pins of the transistor, the probes of the device are connected to the yet unknown pins of the transistor. Found the output of the base through a 1 kΩ resistor is alternately connected to each of the remaining pins. In this case, the collector-emitter transition resistance is measured alternately. The pin to which the resistor is connected, having the smallest resistance value of the junction, will determine the collector of the transistor, the remaining electrode will be an emitter.

Optocouplers

To test the optocouplers, the input part (light emitting) is energized from an external power source. In this case, the transition resistance is controlled, as a rule, the collector-emitter in the receiving part. A healthy optocoupler has a much lower collector-emitter junction resistance when the power is on (several hundred ohms) than when it is turned off. The constant resistance of the collector-emitter junction indicates a fault in the optocoupler.

Capacitors

Faulty capacitors may be detected during an external inspection of a faulty power supply. Attention should be paid to cracks in the housing, electrolyte leakage, corrosion at the terminals, heating of the capacitor body during operation. A good test can be a parallel connection to a previously known good capacitor. The absence of such information indicates the need to water a suspicious condenser. The device included in the resistance measurement mode is set to the upper limit. When testing, check the ability of the capacitor to the processes of charge and recharge. Testing is convenient to carry out the pointer device. In the process of charging, the needle of the device deviates to the zero mark, and then returns to the initial state (infinitely large resistance). The greater the capacitance of the capacitor, the longer the charging process. In the "leak" capacitor, the charge process continues with the discharge process, i.e. the subsequent process of reducing resistance. Digital multimeter when checking capacitors beeps. If there is no signal, the capacitor is defective.

Thermistors

In these resistors, the resistance varies considerably with temperature. Thermistors are tested at normal temperature and at elevated. Increased temperature can be achieved by heating the thermistor case, for example, using a soldering iron. Thermistors are usually used in power supplies, the resistance of which at normal temperature is unity Ohms, with a negative temperature coefficient of resistance, therefore the resistance of a functioning thermistor should decrease when heated.


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Diagnostics, maintenance and repair of electronic and radio equipment

Terms: Diagnostics, maintenance and repair of electronic and radio equipment