Wednesday, August 12, 2009

Switched-mode power supply




Input rectifier stage

AC, half-wave and full wave rectified signals
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.

Inverter stage


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.

Voltage converter and output rectifier


If the output is required to be isolated from the input, as is usually the case in mains power supplies, the inverted AC is used to drive the primary winding of a high-frequency transformer. This converts the voltage up or down to the required output level on its secondary winding. The output transformer in the block diagram serves this purpose.
If a DC output is required, the AC output from the transformer is rectified. For output voltages above ten volts or so, ordinary silicon diodes are commonly used. For lower voltages,
Schottky diodes are commonly used as the rectifier elements; they have the advantages of faster recovery times than silicon diodes (allowing low-loss operation at higher frequencies) and a lower voltage drop when conducting. For even lower output voltages, MOSFETs may be used as synchronous rectifiers; compared to Schottky diodes, these have even lower conducting state voltage drops.
The rectified output is then smoothed by a filter consisting of
inductors and capacitors. For higher switching frequencies, components with lower capacitance and inductance are needed.
Simpler, non-isolated power supplies contain an inductor instead of a transformer. This type includes
boost converters, buck converters, and the so called buck-boost converters. These belong to the simplest class of single input, single output converters which utilize one inductor and one active switch. The buck converter reduces the input voltage in direct proportion to the ratio of conductive time to the total switching period, called the duty cycle. For example an ideal buck converter with a 10 V input operating at a 50% duty cycle will produce an average output voltage of 5 V. A feedback control loop is employed to regulate the output voltage by varying the duty cycle to compensate for variations in input voltage. The output voltage of a boost converter is always greater than the input voltage and the buck-boost output voltage is inverted but can be greater than, equal to, or less than the magnitude of its input voltage. There are many variations and extensions to this class of converters but these three form the basis of almost all isolated and non-isolated DC to DC converters. By adding a second inductor the Ćuk and SEPIC converters can be implemented, or, by adding additional active switches, various bridge converters can be realised.
Other types of SMPSs use a
capacitor-diode voltage multiplier instead of inductors and transformers. These are mostly used for generating high voltages at low currents (Cockcroft-Walton generator). The low voltage variant is called charge pump.

Regulation


A feedback circuit monitors the output voltage and compares it with a reference voltage, which is set manually or electronically to the desired output. If there is an error in the output voltage, the feedback circuit compensates by adjusting the timing with which the MOSFETs are switched on and off. This part of the power supply is called the switching regulator. The Chopper controller shown in the block diagram serves this purpose. Depending on design/safety requirements, the controller may or may not contain an isolation mechanism (such as opto-couplers) to isolate it from the DC output. Switching supplies in computers, TVs and VCRs have these opto-couplers to tightly control the output voltage.
Open-loop regulators do not have a feedback circuit. Instead, they rely on feeding a constant voltage to the input of the transformer or inductor, and assume that the output will be correct. Regulated designs compensate for the
parasitic capacitance of the transformer or coil. Monopolar designs also compensate for the magnetic hysteresis of the core.
The feedback circuit needs power to run before it can generate power, so an additional non-switching power-supply for stand-by is added.

Transformer design


SMPS transformers run at high frequency. Most of the cost savings (and space savings) in off-line power supplies come from the fact that a high frequency transformer is much smaller than the 50/60 Hz transformers formerly used.
There are several differences in the design of transformers for 50 Hz vs 500 kHz. Firstly a low frequency transformer usually transfers energy through its core (soft iron), while the (usually ferrite) core of a high frequency transformer limits leakage. Since the waveforms in a SMPS are generally high speed (PWM square waves), the wiring must be capable of supporting high harmonics of the base frequency due to the
skin effect, which is a major source of power loss.

Power factor


Simple off-line switched mode power supplies incorporate a simple full wave rectifier connected to a large energy storing capacitor. Such SMPSs draw current from the AC line in short pulses when the mains instantaneous voltage exceeds the voltage across this capacitor. During the remaining portion of the AC cycle the capacitor provides energy to the power supply.
As a result, the input current of such basic switched mode power supplies has high
harmonic content and relatively low power factor. This creates extra load on utility lines, increases heating of the utility transformers and standard AC electric motors, and may cause stability problems in some applications such as in emergency generator systems or aircraft generators. Harmonics can be removed through the use of filter banks but the filtering is expensive, and the power utility may require a business with a very low power factor to purchase and install the filtering onsite.
In 2001 the European Union put into effect the standard IEC/EN61000-3-2 to set limits on the harmonics of the AC input current up to the 40th harmonic for equipment above 75 W. The standard defines four classes of equipment depending on its type and current waveform. The most rigorous limits (class D) are established for personal computers, computer monitors, and TV receivers. In order to comply with these requirements modern switched-mode power supplies normally include an additional
power factor correction (PFC) stage.
Putting a current regulated boost chopper stage after the off-line rectifier (to charge the storage capacitor) can help correct the power factor, but increases the complexity (and cost).

Tuesday, August 11, 2009

Testing Resistor




Testing Resistor - How to accurately check resistors on board



Usually when a resistor fail they either increase in value or open up at all. You can check the resistance of a resistor with an ohmmeter. If the resistor is in circuit, you will generally have to isolate the resistor so you are measuring only the resistor, not other components in the circuit. Always be aware of possible back( parallel) circuits when performing in-circuit resistance measurements.

As a repairer, most of the times we want to troubleshoot and solve problems as fast as possible thus removing all resistors from the board and check the resistors one by one will take up a lot of our precious time. There have to be a simple way to check resistor on board.

Using analog meter to check resistor on board often produced a wrong reading. This is due to the reason that the output from the analog meter is from 3 volt to 12 volt. The voltages are quite high and it can trigger the semiconductor devices around the resistors such as diode, transistor and ICs. Do you know that semiconductors only need voltage of 0.6v in order to conduct. Since the output voltage from the analog meter is higher than the semiconductors, checking the resistor in circuit won't give you an accurate reading!

In order to measure resistors while it still in circuit, you need to get a digital multimeter that have the output of less than 0.6v. This is to avoid conducting the semiconductor devices around the circuit that you want to check. Currently i' m using the Greenlee digital meter that have output around 0.2volt. Though it cannot give me a 100% accurate result at least it can help me to speed up my troubleshooting job. Why not 100%? This is due to that some circuit have resistors that is directly parallel to each other.

Testing time- If you connect your digital meter leads across a resistor in a circuit and it measures higher than it should, then you know the resistor is either open or has gone up in value. Other circuit components cannot possibly increase the value of a resistor; any parallel circuit could only make the resistance reading lower. In rare cases, sometimes an undischarge capacitor can cause the measurement higher than it should be. Only through more practice will make you know when you should remove the resistor and check it off board.

How To Test Mosfet


Mosfet Testing Tips-Test Fet with Analog Multimeter

The right way of testing mosfet transistor is to use an analog multimeter. Mosfet stand for Metal oxide semiconductor field effect transistor or we just called it fet. Switch mode power supply and many other circuits uses fet transistors as part of a circuit. Mosfet failure and leakage are quite high in a circuit and you need to know how to accurately test it.

Measuring component's that have two leads such as the resistors, capacitors and diodes are much easier than measuring transistor and fet which have three legs. Many electronic repairers have difficulty especially checking the three leads components. First, find out the gate, drain and source pinout from semiconductor replacement book or search its datasheet from search engine.

Once you have the cross reference or diagram for each pin of the mosfet, then use your analogue multimeter set to times 10K ohm range to check it. Assuming you are testing the n channel mosfet then put the black probe to the drain pin.

Touch the gate pin with the red probe to discharge any internal capacitance in the mosfet. Now move the red probe to source pin while the black probe still touching the drain pin. Use your right finger and touch the gate and drain pin together and you will notice the analogue multimeter pointer will move forward to center range of the meter's scale.


Use your finger to touch on the gate and drain pin.

Lifting the red probe from the source pin and putting it back again to the source pin, the pointer will still remain at the middle of the meter's scale. To discharge it you have to lift the red probe and touch just one time on the gate pin. This will eventually discharge the internal capacitance again.

At this time, use the red probe to touch on the source pin again, the pointer would not kick at all because you have already discharge it by touching the gate pin. These are the good mosfet characteristic.You need to practice more by taking some fet from your bench or from your component’s compartment. Once you know the secrets, testing other mosfet is as simple as testing diode.
If you notice that all the result that you measured kicked towards zero ohms and will not discharge, then the fet is considered shorted and need replacement. Testing the P channel fet field effect transistor is just the same way as when you check N channel fet. What you do is to switch the probe polarity when checking the P channel. Some analog multimeter have the times 100k Ohm range, this type of meter can’t really test fet due to the absent of 9 Volt battery inside the multimeter. This type of meter will not have enough power to trigger the mosfet. Make sure you use a meter that have the times 10k ohm range selector.