by: Abraham Pressman. Abstract: Fully updated to reflect the latest technologies and materials. This bestselling tutorial shows you how to designstate-of-the-art. This paper gives a practical example of the design of an off-line switching power supply. Factors governing the choice of a discontinuous flyback topology are. Faced with the complexity of implementing a DC to DC switching power supply design under these circumstances, designers can be put under stress and.
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PDF | 23+ hours read | Marty Brown and others published Practical Switching Power Supply Design / Marty Brown. supplies allows the designer to complete this portion of the system design quickly and easily. . Driving MOSFETs in Switching Power Supply Applications. Switching Power Supply Design. Third Edition. Abraham I. Pressman. Keith Billings. Taylor Morey. New York Chicago San Francisco Lisbon London Madrid .
It is then switched at a huge rate of speed and fed to the primary side of the step-down transformer. The step-down transformer is only a fraction of the size of a comparable 50 Hz unit thus relieving the size and weight problems. We have the filtered and rectified output at the secondary side of the transformer. It is now sent to the output of the power supply. A sample of this output is sent back to the switch to control the output voltage. The diode carries the current during the OFF period of the transistor. Therefore, energy flows into the load during both the periods.
The operating frequency of an unloaded SMPS is sometimes in the audible human range, and may sound subjectively quite loud for people whose hearing is very sensitive to the relevant frequency range.
Power factor Low for a regulated supply because current is drawn from the mains at the peaks of the voltage sinusoid , unless a choke-input or resistor-input circuit follows the rectifier now rare. The internal resistance of low-power transformers in linear power supplies usually limits the peak current each cycle and thus gives a better power factor than many switched-mode power supplies that directly rectify the mains with little series resistance.
Inrush current Large current when mains-powered linear power supply equipment is switched on until magnetic flux of transformer stabilises and capacitors charge completely, unless a slow-start circuit is used. Extremely large peak "in-rush" surge current limited only by the impedance of the input supply and any series resistance to the filter capacitors.
Empty filter capacitors initially draw large amounts of current as they charge up, with larger capacitors drawing larger amounts of peak current.
Being many times above the normal operating current, this greatly stresses components subject to the surge, complicates fuse selection to avoid nuisance blowing and may cause problems with equipment employing overcurrent protection such as uninterruptible power supplies. Mitigated by use of a suitable soft-start circuit or series resistor. Risk of electric shock Supplies with transformers isolate the incoming power supply from the powered device and so allow metalwork of the enclosure to be grounded safely.
Transformerless mains-operated supply dangerous. In both linear and switch-mode the mains, and possibly the output voltages, are hazardous and must be well-isolated. Two capacitors are connected in series with the Live and Neutral rails with the Earth connection in between the two capacitors. This forms a capacitive divider that energizes the common rail at half mains voltage.
Its high impedance current source can provide a tingling or a 'bite' to the operator or can be exploited to light an Earth Fault LED. However, this current may cause nuisance tripping on the most sensitive residual-current devices. It can also provide some very mild tingling sensation but it's safe to the user .
Risk of equipment damage Very low, unless a short occurs between the primary and secondary windings or the regulator fails by shorting internally. Can fail so as to make output voltage very high. Stress on capacitors may cause them to explode.
Can in some cases destroy input stages in amplifiers if floating voltage exceeds transistor base-emitter breakdown voltage, causing the transistor's gain to drop and noise levels to increase. The floating voltage is caused by capacitors bridging the primary and secondary sides of the power supply.
Connection to earthed equipment will cause a momentary and potentially destructive spike in current at the connector as the voltage at the secondary side of the capacitor equalizes to earth potential. This is called rectification.
In some power supplies mostly computer ATX power supplies , the rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically. 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. To correct for this, many newer SMPS will use a special PFC circuit to make the input current follow the sinusoidal shape of the AC input voltage, correcting the power factor.
This type of use may be harmful to the rectifier stage, however, as it will only use half of diodes in the rectifier for the full load. This could possibly result in overheating of these components, causing them to fail prematurely. The diodes in this type of power supply will handle the DC current just fine because they are rated to handle double the nominal input current when operated in the V mode, due to the operation of the voltage doubler.
This is because the doubler, when in operation, uses only half of the bridge rectifier and runs twice as much current through it. 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.
Voltage converter and output rectifier[ edit ] 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 buck—boost converters.
These belong to the simplest class of single input, single output converters which use 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. 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. 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[ edit ] This charger for a small device such as a mobile phone is a simple off-line switching power supply with a European plug. The simple circuit has just two transistors, an opto-coupler and rectifier diodes as active components. A feedback circuit monitors the output voltage and compares it with a reference voltage, as shown in the block diagram above.
Depending on design and safety requirements, the controller may contain an isolation mechanism such as an opto-coupler 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 impedance 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[ edit ] Any switched-mode power supply that gets its power from an AC power line called an "off-line" converter  requires a transformer for galvanic isolation.
Some DC-to-DC converters may also include a transformer, although isolation may not be critical in these cases. SMPS transformers run at high frequency. There are additional design tradeoffs. The terminal voltage of a transformer is proportional to the product of the core area, magnetic flux, and frequency. By using a much higher frequency, the core area and so the mass of the core can be greatly reduced.
However, core losses increase at higher frequencies. Cores generally use ferrite material which has a low loss at the high frequencies and high flux densities used. Also, more energy is lost during transitions of the switching semiconductor at higher frequencies. Furthermore, more attention to the physical layout of the circuit board is required as parasitics become more significant, and the amount of electromagnetic interference will be more pronounced.
For these frequencies, the skin effect is only significant when the conductors are large, more than 0.
A sample of this output is sent back to the switch to control the output voltage. The diode carries the current during the OFF period of the transistor. Therefore, energy flows into the load during both the periods. The choke stores energy during the ON period and also passes some energy into the output load.
Flyback converter In a flyback converter, the magnetic field of the inductor stores energy during the ON period of the switch. The energy is emptied into the output voltage circuit when the switch is in the open state.
The duty cycle determines the output voltage. Self-Oscillating Flyback Converter Advertisement This is the most simple and basic converter based on the flyback principle.
The voltage induced in the secondary winding and the feedback winding make the fast recovery rectifier reverse biased and hold the conducting transistor ON. When the primary current reaches a peak value Ip, where the core begins to saturate, the current tends to rise very sharply. This sharp rise in current cannot be supported by the fixed base drive provided by the feedback winding.
As a result, the switching begins to come out of saturation. A series switching element turns the current supply to a smoothing capacitor on and off. The voltage on the capacitor controls the time the series element is turned. Design basics AC power first passes through fuses and a line filter. Then it is rectified by a full-wave bridge rectifier.