Troubleshooting And Repair Power Supply The Easy Way Part 4

Any SMPS that comes across my repair bench, I would not immediately repair it, in fact I will take couples of minutes to analyze the circuit design and see it from all angles before I begin to repair. Troubleshooting SMPS is not limited to only one procedure in fact many electronic repairers have their own unique ways and methods to solve SMPS problems. Some prefer to use light bulb to isolate SMPS faults while others like to use resistors. Troubleshooting SMPS is fun and flexible but in some cases could make you get very frustrated too.

Remember, don’t limit yourself to only one or two sources to get you understand and be able to repair SMPS. If you have the budget, get the books that have related to SMPS repair-study and start doing practical about it. Share your problems with other fellow electronic repairers and the most important thing is don’t give up. There’s lot of mountain in the journey of our live and you yourself have to climb and conquer it. All the best!

Troubleshooting And Repair Power Supply The Easy Way Part 3

Push yourself further by searching the internet for the datasheet of a PWM IC part number. For example, UC3842 PWM IC is mostly used in SMPS. Do you know what the function of pin 5 of this IC is? Do you know which pin the VCC supply enters? Do you know what the actual voltage that flow to the IC is? Do you know which pin that drives the power FET? Can I get a replacement for this IC? And so on………

Let’s take a soldier as an example. Soldiers not only good in handling rifle but also knows all the details about it. They know how to dismantle and assemble back their rifle fast (imagine in the middle of war the rifle jammed-they can repair it fast). They know how much each bullet cost, how far the shooting distance, how big is the diameter of the bullet, how many cm the length of the bullet and so on. Hope you don’t get bored with the soldier’s story, did you get the ideas?

Troubleshooting And Repair Power Supply The Easy Way Part 2

Once the PWM IC received the voltage it will output a signal to drive the transistor (or FET) and produces a changing in magnetic field in the transformer primary winding. The changing magnetic field induces voltage in the secondary windings. Each of these AC voltage produced by the secondary windings is then rectified, filtered, and regulated to produce a clean DC voltage. One of the main DC output voltage is the B+ that supply to flyback transformer (for TV and Monitor Circuit)

The output from the B+ voltage supply is then connected, through a “feedback” loop (which consist of optoisolator ic and an error amplifier TL431 IC), back to the PWM IC. When the voltage from the B+ supply rises or drop a bit, the PWM IC will act to correct the output.

If you still do not understand the above explanation, please do not be discourage because you can always buy technical books and schematic diagrams and read it till you get the whole idea of how a SMPS work. You can ask a repair friend or even surf the internet for a better and easy explanation.

Here I would like you to download a free SMPS article by Sencore and I found it to be a great help for you who are still struggle on how SMPS work and how to troubleshoot when it fails. You must ask your self what is the purpose and its function of the components in the SMPS circuit and how to check them if they fail. Find out on your own the function of these components in SMPS circuit:

Bridge rectifier,

Filter capacitor,

Start up resistors

Chopper/Power FET

Pulse Width Modulation (PWM IC)

Current sense resistor

Switch mode power transformer

Optoisolator/optocoupler

Error Amplifier IC (TL431)

Secondary diodes

Secondary filter capacitors

Troubleshooting And Repair Power Supply The Easy Way Part 1

Troubleshooting linear power supply was quite easy as compare to switch mode power supplies (SMPS). AC voltage enters to the primary side of linear transformer and then converted the AC into a lower or higher AC voltage depending on the secondary winding. The output AC voltage is then rectified and filtered by a diode and capacitors to produce a clean DC voltage. If there is a problem in the linear transformer circuit, I can say that it is very easy to locate the fault. This is somehow different in the case of a switch mode power supply. The designs were complicated and some technicians found it quite hard to fully understand how the switch mode power supplies work.

The working principle of switch mode power supply is different from the linear type. First the AC voltage will flow to a full wave rectifier (bridge rectifier) which produces an uneven DC output and then filtered by a large capacitor (usually 220 micro farad and up to 450 volts). The clean DC voltage will then flows to start up resistors and to the input of switch mode power transformer. Once the voltage passed through the high ohms resistor (start up resistors) the voltage would drop to a value where it then flows to the VCC supply pin of Pulse width modulation IC.

SMPS troubleshooting

I'm relatively new at electronics repair with little experience, and have a difficult problem. Posted at a couple other sites and have not had much help.

I have an Akia 200watt SMPS that I have partially repaired. The supply powers a 27 inch lcd tv display. Originally it had 5v standby power only and would not turn on. Almost all of the low voltage output capacitors were domed and leaking. I replaced all of the output caps with new low esr, high frequency caps of the same voltage and capacitance. Both main filter caps were replaced also as a precaution.

The display now functions properly. 5v, 12v and 24v outputs appear good.

Unfortunately, the supply has a noticeable buzz coming from both PQ3220-1000C transformers when turned on. Annoying to the point that the set is not usable. I do not know if the supply buzzed prior to the cap failure since the unit was given to me dead.

Perhaps the bad caps caused damage to other components?? I see no other visible damage. I have checked almost all other components in circuit (to the best of my ability) with a digital VOM and cannot find any bad components.

The basics of SMPS troubleshooting.

Careless troubleshooting of a line-powered switchmode power supply can result in severe electrical shock or electrocution. This is potentially more lethal than the high-voltage section of a TV or monitor due to the availability of high current. Even the charge on the main filter capacitors with the unit unplugged can kill. This warning includes those innocent-looking laptop and Zip-drive power packs as well.

Neither the author or publisher will be responsible for damage to equipment, your ego, countywide power outages, spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury or worse that may result from the use of this material.

Secrets Of SMPS Repair And Troubleshooting Switch Mode Power Supply

Smps repair sometimes can be easy and sometimes are quite difficult to solve the problem. The power supply fault could be an open start up resistor only or could be even more than ten components burnt due to heavy lightning strike. In this article, I’m going to explain to you the power supply problems that caused by defective components at secondary side. Sometimes no power, power blink and low output power symptoms are due to problems in the main circuit board. It is not necessary the fault must be in the primary power section and shorted secondary output diodes. The fault can be further down the secondary output lines which are in the main board. If you had measured all the components in the power supply section and could not find the defective component, then try troubleshoot components beyond the power supply section. A shorted small ceramic capacitor in the main board can cause the whole power supply to malfunction

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A Switch Mode Power Supply

All smps has outputs at the secondary section. Some designed have four outputs, some have five and so on. Each of the output is connected to an individual circuit. For instance, a 6.3 volt output supply is send to the picture tube so that it could light up and heat the cathode gun. A 5 volt supply will usually entered into microprocessor and EEPROM IC as vcc supply, 60-100 volt to B+ pin of flyback transformers and so forth. If one of the components or circuit developed a short circuit, the voltage (vcc supply) line will drawn a lot of current and thus the power will shut down, fluctuate, low output power or even no power at all. You must know how to isolate or repairs the problem otherwise you will be wasting your time troubleshooting good components in the power supply section.

Set your analog meter to times 1 ohm range and check all the secondary output (after diodes) between of ground and all the output voltage line. Put your red probe to the cold ground and the black probe to each of the outputs of secondary diodes. It should measured only one reading and sometimes the pointer will kick up a little bit. This is a normal reading for a good output voltage line. This happened due to the main board resistance and some undischarged voltage from the secondary output filter capacitor.

If you are checking the negative output then you have to reverse the meter’s probe. If any output voltage lines showed two same reading (low ohms readout), then suspect that there is problem in that output line. Carefully follow the suspected output voltage line and gently remove components one by one at a time to isolate the short circuit. From my experienced it is usually caused by a shorted IC, a transistor, a diode or even a resin type ceramic capacitor. If you repair Monitor smps, if one of the output voltage lines shows two readings, please do not think that the lines is having a problem. The real caused as to why do you get two readings are due to that you are checking the filament (heater) circuit. The line goes to the picture tube’s filament and direct to cold ground, that’s why you have two readings. Please don’t set your analog meter to times 10 Kilo ohm otherwise all the reading that you measured will show two readings which is not accurate.

The Back Portion Of A Switch Mode Power Supply Section

True case study- A Samtron Monitor 4Bn sent for repair with the complaint of no power. After troubleshooting the smps section for sometime, I’ve concluded that the power supply is working and suspect something shorted after the secondary output diodes and this made me to trace beyond the power supply unit. By using the explanation above, I managed to traced and found a shorted small resin ceramic capacitor located at the 80 volt line in the Monitor color board. The capacitor shorted to ground and made the power supply to stop operating.

The value of the shorted capacitor was .22 microfarad 100 volt. Replacing only the small capacitor brought the power supply back to life again. In smps repair, we as an electronic repairer must know how to distinguish as which section is giving problem. Spending and concentrating your whole time to check on a working section is just a waste of time, you should only concentrate on section that have problems so that you could locate the fault and make the equipment work again.

MOSFET Switching: Part I - Turn-ON, Hard Switching by MAYA CREATION

This article is trying to make sense out of confusing information regarding the behavior of a MOSFET during switching sequences, in numerous technical articles.

We are not attempting to explain the physics behind a MOSFET structure. For those interested to find more about a MOSFET structure, we recommend the SGS-Thomson technical articles mentioned in the references. The purpose of the article is to present a power supply design engineer with facts that will help design a MOSFET driving circuit, calculate the estimated losses for critical events, predict the efficiency of a power supply, estimate the junction temperature for critical components and various stresses, and ultimately, helping make decision to optimize a design.

The MOSFET switching events are analyzed for an inductive load, diode clamping circuit, the only one that applies to a switching power supply. The data sheet information or technical articles regarding resistive loads have little or no relevance to switching a MOSFET in a switch mode power supply. Also the article is considering only 500V/600V MOSFETs, most relevant and for switch mode power supplies.

Capacitors reference designators are the same as in SGS-Thomson articles. Check the references if a more detailed explanation of their significance is necessary.

switching mode power supply

This site is dedicated to switching-mode power supply circuit designers. Here you will find solutions to the most difficult problems facing you as a power supply designer, a tutorial that places power supply design problems in context, and resources that aid in your power supply design tasks. If you are a first-time visitor you will want to read this page as an orientation. If you have been here before (thank you for coming back), use the menu on the right to find the power supply design information you need.

Power supply design problems and solutions are discussed in a unique problem-solving format, where each problem is identified and discussed, then a test is provided to determine the relevance to your design followed by a general discussion of its solvability. Finally, pointers are provided to where you can find a solution. These pointers are usually in the form of applicable key papers, application notes, websites, and book chapters. The Switching-Mode Power Supply Design Problem List gets you to this, the heart of the website. The emphasis is on the design of switching-mode power supplies, also called switching power supplies or switching regulators. However, much is of interest to those in any electronic design discipline, since topics on simulation with SPICE and other simulators, circuit layout, measurement, testing, and trouble-shooting are included.

A power supply design tutorial ties the problems together and serves as an introduction to switching-mode power supply design.

For years we have been providing answers to power supply design questions. You can find some of these recorded in a power supply design blog along with other topics.

Resources useful to the power supply circuit designer form an integral part of the website. The resource menu gets you to these resources which include lists of books, a recommended power supply design personal library, websites, vendors, seminars, full-text papers, and design aids related to switching-mode power supply design.

A personal anecdote is included in each problem discussion to personalize the information. There is also information about SMPS Technology, the origins of the website, and its author/editor, about Jerrold Foutz.

Transactional Analysis theory provides the unique problem solving approach used throughout the website.

A source of inspiration for us to keep working on the website has been what others say about our website.

A powerful search engine capability has been added to the website that allows you to search for the topics of most interest to you.

Email Policy. Through August, 2001, I had a policy of answering all email. Due to work load I have had to change that policy. I now read all email but until further notice can not answer most of it, including all design questions asking for help. You will find some standard answers at Power Supply Design Frequently Asked Questions (FAQ).

SMPS

A switched-mode power supply (also switching-mode power supply, SMPS, or simply switcher) is an electronic power supply unit (PSU) that incorporates a switching regulator in order to provide the required dc output voltage.
Although the term "power supply" has been in use since radios were first powered from the line/mains, that does not mean that it is a source of power, in the sense that a battery provides power. It is simply a device that (usually) accepts commercial AC power and provides one or more DC outputs. It would be more correctly referred to as a power converter, but long usage has established the term.

Whilst a linear regulator maintains the desired output voltage by dissipating excess power in a pass power transistor, the switched-mode power supply switches a power transistor between saturation (full on) and cutoff (completely off) with a variable duty cycle whose average is the desired output voltage. It switches at a much higher frequency (tens to hundreds of kHz) than that of the AC line (mains). This means that the transformer that it feeds can be much smaller than one connected directly to the line/mains. Switching creates a rectangular waveform that typically goes to the primary of the transformer.

Usually several secondaries feed rectifiers, series inductors, and filter capacitors to provide various DC outputs with low ripple.

THe main advantage of this method is greater efficiency because the switching transistor dissipates little power when it is outside of its active region (i.e., when the transistor acts like a switch and either has a negligible voltage drop across it or a negligible current through it). Other advantages include smaller size and lighter weight (from the elimination of low frequency transformers which have a high weight) and lower heat generation due to higher efficiency. Disadvantages include greater complexity, the generation of high-amplitude, high-frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), and a ripple voltage at the switching frequency and the harmonic frequencies thereof.

Very low cost SMPS may couple electrical switching noise back onto the mains power line, causing interference with A/V equipment connected to the same phase. Non power-factor-corrected SMPSs also cause harmonic distortion.

If the SMPS has an AC input, then the first stage is to convert the input to DC. This is called rectification. The rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically. This is a feature of larger supplies to permit operation from nominally 120 volt or 240 volt supplies. The rectifier produces an unregulated DC voltage which is then sent to a large filter capacitor. The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks. These pulses have significant high frequency energy which reduces the power factor. Special control techniques can be employed by the following SMPS to force the average input current to follow the sinusoidal shape of the AC input voltage thus the designer should try correcting the power factor. An SMPS with a DC input does not require this stage. An SMPS designed for AC input can often be run from a DC supply (for 230V AC this would be 330V DC), as the DC passes through the rectifier stage unchanged. It's however advisable to consult the manual before trying this, though most supplies are quite capable of such operation even though nothing is mentioned in the documentation. However, this type of use may be harmful to the rectifier stage as it will only utilize half of diodes in the rectifier for the full load. This may result in overheating of these components, and cause them to fail prematurely. [3]

If an input range switch is used, the rectifier stage is usually configured to operate as a voltage doubler when operating on the low voltage (~120 VAC) range and as a straight rectifier when operating on the high voltage (~240 VAC) range. If an input range switch is not used, then a full-wave rectifier is usually used and the downstream inverter stage is simply designed to be flexible enough to accept the wide range of dc voltages that will be produced by the rectifier stage. In higher-power SMPSs, some form of automatic range switching may be used.

The inverter stage converts DC, whether directly from the input or from the rectifier stage described above, to AC by running it through a power oscillator, whose output transformer is very small with few windings at a frequency of tens or hundreds of kilohertz (kHz). The frequency is usually chosen to be above 20 kHz, to make it inaudible to humans. The output voltage is optically coupled to the input and thus very tightly controlled. The switching is implemented as a multistage (to achieve high gain) MOSFET amplifier. MOSFETs are a type of transistor with a low on-resistance and a high current-handling capacity. Since only the last stage has a large duty cycle, previous stages can be implemented by bipolar transistors leading to roughly the same efficiency. The second last stage needs to be of a complementary design, where one transistor charges the last MOSFET and another one discharges the MOSFET. A design using a resistor would run idle most of the time and reduce efficiency. All earlier stages do not weight into efficiency because power decreases by a factor of 10 for every stage (going backwards) and thus the earlier stages are responsible for at most 1% of the efficiency. This section refers to the block marked Chopper in the block diagram.