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 .

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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.

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. As for output connectors and pinouts, except for some industries, such as PC and compact PCI, in general, they are not standardized and are left up to the manufacturer.

And like each of those gadgets, it has its own advantages and disadvantages. Very high-efficiency levels are achieved as very little energy we dissipated as heat.

As a result of the high efficiency and low levels of heat dissipation, the switch mode power supplies can be compact. I Voltage-Mode Control 60 5. I Hardware Implementations to Address Overvoltage 8. I Line Regulation 9. I Single-Pole Conipcnsation 1 0. Nonetheless, many engineers will be assigned design projects outside their primary field of expertise, among which are switching power supplies.

This is done priniarily because the engineer has a unique ability to learn technical subjects relatively quickly. Unfortunately, the literature available today on the subject of switching power supplies tries to convey an understanding through lengthy derivations of applied mathematics. This does not work since only an intuitive sense of the subject matter creates an understanding of the fundamental re 1at ion sh ips.

This book is written for just this purpose. It contains mathematical derivations. The design examples are written in a clear step-by-step fashion in order to show the reader the steps necessary in a typical switching regulator design.

They were also chosen because of their utility in a wide range of typical applications. They can be easily modified and scaled to fit many more applications. The topics contained in the book range from considerations in capacitor and semiconductor selection to quasi-resonant converter design.

This book has been written as a result of many years of learning about switching power supplies from experience and equally many years of X Preface frustration with the available technical resources.

The material is organized specifically to answer those questions that I and the many engineers with whom I have conversed have had when faced with a switching power supply design.

In short, this material is written for a working engineer by a working engineer. Why Use Switching Pdwer Supplies? This requirement for multiple voltages once again drives up the system cost. Another major disadvantage is the average efficiency of linear regulators. In normal applications, linear regulators exhibit efficiencies of 30 to 60 percent. This means that for every watt delivered to the load, more than one watt is lost within the supply.

This loss, called the headroom loss, occurs in the pass transistor and is, unfortunately, necessary to develop the needed biases within the supply required for operation and varies greatly when the input voltage varies between its high- and low-line specifications.

This makes it necessary to add heatsinking to the pass transistor that will be sufficient to handle the lost power at the highest specified input voltage and the highest specified 1oad. Most of the time the supply will not be operating under these circumstances, which means that the heatsink will be oversized during most of its operating life.

This once again is an added system cost. The point where the heatsink cost begins to become prohibitive is about 10 W of output power. Up to this point, any convenient metal structural member can adequately dissipate the heat.

These shortcomings greatly escalate at higher output power levels and quickly make the switching regulator a better choice. First, the switching supply exhibits efficiencies of 68 to 90 percent regardless of the input voltage, thus drastically reducing the size requirement of the heatsink and hence its cost. The power transistors within the switching supply operate at their most efficient points of operation: This means that the power transistors can deliver many times their power rating to the load and the less expensive, lower-power packages can be used.

Since the input voltage is chopped into an AC waveform and placed into a magnetic element, additional windings can be added to provide for more than one output voltage.

The incremental additional cost of each added output is very small compared to the entire supply cost-and in the case of transformer-isolated switching supplies, the output voltages are independent of the input voltage. The last major advantages are its size and cost at the higher output power levels.

Since their frequency of operation is very much greater than the Hz line frequency, the magnetic and capacitive elements used for energy storage are much smaller and the cost to build the switching supply becomes less than the linear supply at the higher power levels. Why Use Switching Power Supplies? The disadvantages of the switching supply are minor and usually can be overcome by the designer.

First, the switching supply is more complicated than the comparable linear supply. If a switching supply cannot be bought off-the-shelf to suit the needs of the product, then it must be designed. The experienced power supply designer will need a minimum of three worker-months, depending on its complexity, to design. Obviously this design effort comes at a cost, and this must be considered during the product planning stage of the program.

Second, considerable noise from the switching supply is generated on its outputs and input and radiated into the environment. This can be difficult to control and certainly cannot be ignored during the design phase. A little knowledge of radio-frequency RF behavior and design can go a long way in aiding the engineer during the design phase. There can be simple solutions to this problem, but generally additional filtering and shielding will have to be added to the supply to limit the effects of the noise on the load and the environment.

This, of course, adds cost to the supply. Third, since the switching supply chops the input voltage into time-limited pulses of energy. This is called transient response time. To compensate for this sluggishness, the output filter capacitors usually must be increased in value to store the energy needed by the load during the time the switching supply is adjusting its power throughput.

Once again added cost is incurred, but note that all of these disadvantages are under the control of the designer and their impact on the supply and the system can be minimized. Generally, the industry has settled into areas where linear and switching power supplies are applied.

Linear supplies are chosen for lowpower, board-level regulation where the power distribution system 4 1. They are also used in circuits where a quiet supply voltage is necessary, such as analog, audio, or interface circuits. They are also used where a low overhead cost is required and heat generation is not a problem.

Switching power supplies are used in situations where a high supply efficiency is necessary and the dissipation of heat presents a problem, such as battery-powered and handheld applications where battery life and internal and external temperatures are important.

Off-line supplies are also typically switchers because of their efficiency in generating all the voltages needed within the product, especially in veryhigh-power applications, up to many kilowatts. In summary, because of its versatility, efficiency, size, and cost, the switching power supply is preferred in most applications.

The advances in component technology and novel topological design approaches will only add to the desirability of the switching power supply in most applications. Conceptually, switching regulators are not difficult to understand. When viewed as a blackbox with input and output terminals, the behavior of a switching regulator is identical to that of a linear regulator. The fundamental difference is that a linear regulator regulates a continuous flow of current from the input to the load in order to maintain a constant load voltage.

The switching regulator regulates this same current flow by chopping up the input voltage and controlling the average current by means of the duty cycle. When a higher load current is required by the load, the percentage of on-time is increased to accommodate the change.

Two basic types of switching regulators constitute the foundation of all of the pulsewidth-modulated PWM switching regulators. These types are the forward-mode regulators and the flyback-mode regulators. The name of each type is derived from the way the magnetic elements are used within the regulator. Although they may resemble each other schematically, they operate in quite different fashions.

The power switch may be a power transistor or a metal oxide semiconductor field-effect transistor MOSFET placed directly between the input voltage and the filter section. In between the power switch and the filter section there may be a transformer for stepping up or down the input voltage as in transformer-isolated forward regulators.

The shunt diode, series inductor, and shunt capacitor form an energy storage res5 2. Now the inductor is placed directly between the input source and the power switch. The anode lead of the rectifier i s placed on the node where the power switch and inductor are connected, and the capacitor is placed between the rectifier output cathode and ground return.

The flyback's operation can be broken up into two periods. When the power switch is on, current is being drawn through the inductor, which causes energy to be stored within its core material. The power switch then turns off. Since the current through an inductor cannot change instantaneously, the inductor voltage reverses or flies back.


This causes the rectifier to turn on, thus dumping the inductor's energy into the capacitor. This continues until all the energy stored in the inductor during the previous half-cycle is emptied. Since the inductor voltage flies back above the input voltage, the voltage that appears on the output capacitor is higher than the input voltage. Note that the only storage for the load is the output filter capacitor. This makes the output ripple voltage of flyback converters worse than their forward-mode counterparts.

The duty cycle in an elementary flyback-mode supply is 0 to 50 percent. This restriction is due to the time required to empty the inductor's Rux into the output capacitor.

Duty cycles within transformer-isolated flyback regulators can sometimes be larger because of the effects of the turns ratio and the inductances of the primary and the secondary. The relationship of the output voltage to the input voltage is slightly more difficult to describe.

During the power switch's off-time, the inFigure 2. How a Switching Power Supply Works ductor will empty itself before the start of the next power switch conduction cycle. The subsections discussed represent a typical minimum system. Additional functionality may be added to the supply by adding to these basic subsections. The supply discussed is a single output, push-pull regulator. The circuit sections and waveforms are shown in Figures 3.

It serves a dual purpose.

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First, C , and L, act as a high frequency radio-frequency interference RFI filter, which reduces the conducted high frequency noise components leaving the switching suppty back into the input line. These noise currents would then radiate from the input power lines as in an antenna. The lowpass cutoff frequency of this filter should be no higher than 2 to 3 times the supply's operating frequency.

The second purpose of this stage is to add a small impedance L , between the input line and the bulk input capacitor. It basically reduces any lethal transient voltage and allows the bulk input filter capacitor and any surge protector to absorb the destructive energies from the input line spikes or surges with little chance of exceeding any of the components' voltage ratings.

It has the responsibility of storing the high- and low-frequency energy required by the supply dur9 10 3. I A walk through a representative switching regulator circuit.

This capacitance must represent a low impedance from direct current DC to many times the switching frequency of the supply. Another factor that necessitates the use of the bulk input capacitor is that the input line may have long lengths of wire or printed circuit board trace, which adds series resistance and inductance between the power source and the supply. The input line at high frequencies actually resembles a current-limited current source and cannot deliver the high-frequency current demands of the supply necessary for the fast voltage and current transitions within the supply.

The input capacitor charges at a low frequency and sources current over a much higher frequency range. Without both a low-frequency electrolyte- 3. The transformer does not store energy in this configuration. Additional outputs may be added by simply adding another winding on the secondary. This allows one switching supply to provide all the voltages required by most product designs. The transformer is also the backbone of the switching power supply.

If the trans- 12 3.

A Walk through o RepresentativeSwitching Power Supply former is improperly designed, it would adversely affect the supply operation and the reliability of the semiconductors.

The power switches also represent the least reliable components within the supply. If any components are to fail during an adverse operating condition, these would be the first ones to fail. So great care should be taken during the design and selection phase to ensure their reliable performance.

The secondary voltage waveforms in isolated configurations such as this have an average DC value of zero centered about 0 V , but during the on-time of the power switches the secondary voltage reaches peak values of the turns ratio times the input voltage. The rectifiers convert this bipolar waveform into a unipolar pulse train.

16 Ways to Design a Switch-Mode Power Supply

Although the rectifier conducts an average current equal to the load current, the peak value of the current will be higher than the average. So during the rectifier selection process the designer should consider any additional losses incurred during these high peak currents and add a margin to the current specification. This filter is called a choke inputfilter or LC filter and is a 3. Its purpose is to store energy for the load during the times when the power switches are not conducting.

It basically operates like an electrical equivalent of a mechanical flywheel. The on-time of the power switches serves only to replenish the energy lost by the inductor during their off-time. Typically, approximately 50 percent more energy is stored in the inductor and capacitor than is needed by the load over the entire period. This reserve can be drawn on by sudden increases in load demand until the control loop can provide more energy by increasing the on-time of the power switches.

Essentially, the purpose is to develop a voltage that is proportional to the output load current. This voltage is then amplified, and if it becomes too high an overcurrent condition , it overrides the voltage regulator control loop and forces a reduction in the output voltage.

Depending on the way the output current is sensed, what other parameters are summed in, and the gain of the current-sensing amplifier, one can either achieve a constant power limiting, a constant current limiting, or a current foldback limiting.

The type of limiting chosen depends on how much power the load can withstand during an overcurrent or short-circuit failure. In voltage-mode regulators this feature remains completely inactive until an abnormal overcurrent condition is entered. The voltage error amplifier amplifies the difference between the ideal level-dictated by the reference voltageand the actual output voltage as presented by the feedback elements and controls the on-time of the power switches accordingly.

It performs the functions of DC output voltage sensing and correction, voltage-to-pulsewidth conversion, a stable reference voltage, an oscillator, overcurrent detection and override, and the power switch driver s.

It may also include a soft-start circuit, deadtime limiting, and a remote shutdown. The oscillator sets the frequency of operation of the supply and generates a sawtooth waveform for the DC-to-pulsewidth converter. This error signal is then presented to the DC-to-pulsewidth converter, which produces a pulsetrain whose duty cycle represents this error signal. This pulsetrain is then presented to the power switch driver s. If the supply is single-ended, that is, has only one power switch, the waveform is used to drive the output driver directly.

If it is a double-ended supply two power switches , this pulsetrain is first placed into a digital flip-flop that steers the pulses alternately between two output drivers. The output drivers themselves usually take one of two forms. First is the uncommitted transistor, which is where both the emitter and collector of the output transistor are brought out of the integrated circuit IC and are better suited for driving bipolar power transistor power switches.

The second type is the push-pull driver. These control functions represent the minimum functionality of a control IC. Added functionality, which varies from IC to IC, should be considered carefully, keeping in mind the system design. This might include soft-start, remote shutdown, and synchronization. Soft-start reduces the inrush current into the supply during startup by overriding the error amplifier and hard-limiting the initial maximum pulsewidths until the supply has reached its desired output.

Remote shutdown is a circuit that inhibits supply operation electrically by shutting down the control functions without removing power to the power sections of the supply. Synchronization is needed for those systems having sections where the fixed frequency output ripple of the power supply would interfere with a critical system circuit such as a 3.

It also may be necessary to synchronize more than one switching power supply. The designer must study each control IC carefully in order to select the most appropriate IC for the application. These basic functional subsections represent the minimum functionality that a typical switching power supply should possess.

Additional functions that may or should be added are input transient protection, undervoltage lockout, output overvoltage protection, and any power sequencing that the supply may need to provide to the system. Many items need to be considered at the system design specification stage of a system development program and should be discussed as early in the program as possible.

This will aid the designer in outlining the best possible design approach to the switching power supply and avoid any last minute design changes downstream in the program.

Switching power supplies gained popularity in the early s, coinciding with the introduction of the bipolar power transistor. The basic theory of the switching power supply has been known since the s. Since the s, many evolutionary changes have occurred to make the switching power supply meet the needs of many diverse applications. For this reason, many variations have evolved, each with merits that make it better suited for particular applications. Some topologies work better at high input voltages, some at higher output power levels, and some are targeted for the lowest cost.

Keep in mind that many topologies can work for each particular application, but one topology usually has the right combination of features that makes it the best choice. Five primary factors differentiate the various topologies from one another: The peak primary current. This is an indication of how much stress the power semiconductors must withstand and tends to limit a particular configuration in the output power it can deliver and the input voltage over which it can operate.

How much of the input voltage can be placed across the primary winding of the transformer. This indicates how effectively power can be derived from the input line. Switching power supplies are constant-power circuits, so the more voltage supplied to the trans17 18 4.

Switching Power Supply Topologies former or inductor, the less the average and peak currents needed in order to develop the output power. How much of the B-H characteristic can be used within the transformer during each cycle. This indicates which configurations have physically smaller transformers for a rated output power.

DC isolation of the input from the load. This provides DC isolation of the output from the input and allows the designer to add multiple outputs with ease. Transformer isolation may also be necessary in order to meet the safety requirements dictated by the marketplace. Cost and reliability. The designer wishes to select a configuration that requires the minimum parts without subjecting the components to undue overstress.

At the beginning of each power supply design effort the designer should perform a little predesign estimation exercise. This is done by making a reasonable assumption about the supply efficiency and working with the general equations involving the peak currents and voltages.

From this exercise, one can select the best switching power supply topology, select the preliminary choices for the semiconductors, and even estimate the amount of losses within the components. It may also guide the designer in an approach to packaging the power supply and provide some idea as to the final cost of the supply.

This effort can act as an early roadmap during the design phase and also saves time because the designer can order the semiconductor components before the power supply is even designed. The industry has settled into several primary topologies for a majority of the appIications. Figure 4. The boundaries to these areas are determined primarily by the amount of stress the power switches power transistors or MOSFETs must endure and still provide reliable performance.

The boundaries delineated in Figure 4. Higher peak currents can be used but the power switches would begin to exhibit unusual failure modes, and items such as board layout and lead lengths would become even more critical.

It is also no coincidence that these topologies are transformer-isolated topologies. The non-transformer-isolated topologies have very predictable and catastrophic failure modes that most experienced switching power supply designers prefer not to risk.

The flyback configuration is used predominantly for low to medium output power 4. Unfortunately, the flyback topology exhibits much higher peak currents than do the forward-mode supplies, so at the higher output powers, it quickly becomes an unsuitable choice.

For medium-power applications to W the half-bridge topology becomes the predominant choice. The half-bridge is more complicated than the flyback and therefore costs more, but its peak currents are about one-third to one-half those exhibited by the flyback.

Above W, the peak currents once again become very high and it becomes unsuitable. This is because the half-bridge does not effectively utilize the full power capacity o f the input source. Above W the dominant topology is the full-bridge topology, which offers the most effective utilization of the full capacity of the input power source.

It also is the most expensive to build, but for those power levels the additional cost becomes a trivial matter. Another topology that is sometimes used above W is the push-pull topology, which exhibits some fundamental shortcomings that make it tricky to use.

By using Figure 4. I and estimating the major power supply parameters as a preliminary guide at the beginning of a switching power supply 20 4. Switching Power Supplv Topologies design effort, one can be reasonably sure that the final choice of topology will provide a reliable and cost-effective design. These external components are usually Hz transformers or isolated bulk power supplies.

Their typical area of application is in local board-level voltage regulation, The non-transformer-isolated supplies are also easy to understand and thus are used as design examples by various manufacturers and subsequently overused by novice power supply designers. Nonisolatedtype configurations seldom are used by seasoned power supply designers simply because of the severity of the failure modes caused by the lack of the DC isolation.

There are three basic non-transformer-isolated topologies: Each topology generates and regulates an output voltage that is above or below the input voltage.

Each also has only one output since it is not very practical to add additional outputs to them. Non-transformer-isolated supplies also have definite restrictions as to their application in regard to their input voltage with respect to their output voltage. The designer should consider these factors prior to the use of a nonisolated topology.

SMPS: Basics & Working of Switched Mode Power Supply

It is also the easiest to understand and design. The buck regulator is also the most elementary forward-mode regulator and is the basic building block for all the forward-mode topologies. The buck regulator, though, exhibits the most severe destructive failure mode of all the configurations.

For this reason, it should be used only with ext reme disc ret ion.

A steady-state DC current whose average value equals the output load current is always flowing through the inductor. The diode, called a comrnuratirig diode, maintains the flow of the load current through the inductor when the power switch is turned off. There are two current paths inside a buck regulator.

When the power switch is conducting, the current is passed through the input source, the power switch, the inductor, and the load, after which it returns to the input source. Since the input source can provide much more energy than the load wants, the excess is stored in the inductor. When the power switch is off, the load current is passed through the commutation diode to the load and back again. The energy behind the sustained current flow is provided by the excess energy stored in the inductor, which is now being drawn on.

This continues until the power switch is once again turned on and the cycle starts over again. The voltage and current waveforms are shown in Figure 4. Analytically, they are quite easy to describe. They are positive and negative ramps, respectively, riding on a current pedestal. The pedestal is indicative of the residual energy stored within the inductor acting as an energy reservoir. The residual energy is needed to quickly respond to changes in the load current before the control circuit can respond to the change.

The DC average of this current waveform is equal to the DC current being drawn by the load. Regulation of the output voltage is accomplished by varying the duty 4. Conversely, the closer the input voltage gets to the output voltage, the more the duty cycle approaches percent.

The buck regulator topology suffers from some limitations and problems imposed by the physics underlying its operation. The input voltage must always be at least 1 to 2 V higher than the output voltage in order to maintain its regulated output. This can present a problem if the input supply could possibly approach the level of the output.

As a result, the buck regulator can be used only as a step-down regulator. When the power switch turns on, the diode is still conducting the inductor current. A diode takes a finite amount of time to assume a reverse-biased or off state, as specified by the reverse recovery time T J of the diode. While the diode is turning off, current will actually flow from the input line through the power switch and the diode to ground.

This is actually an instantaneous short circuit across the input supply and adds stress to the power switch and diode. There is no way to eliminate this stress. This results in the input being short-circuited to the output load.

Obviously, if there are no other means of protection, the output load circuitry would literally burn up. This is not a good way for a designer or a company to maintain a good reputation. The designer must add an overvoltage crowbar circuit to the output of the supply and a fuse in series with the input. The overvoltage crowbar [a silicon-controlled rectifier SCR driven by a voltage comparator] senses when the output voltage goes above a predetermined threshold, the SCR triggers, thus pulling an enormous current to the input ground return, which subsequently causes a series fuse to blow open.

In reality the crowbar can be activated by spikes that may be asserted by the load or by a sluggish regulator in response to a rapid change in the load current. The regulator in this case enters a current foldback condition. This is an annoyance to the operator of the equipment, who must recycle turn off and then turn on the input power switch.

The designer cannot ignore this failure mode. Component failures during the life of a product are a fact of life, so the designer should always create a design in anticipation of these events. Swltchlng Power Supply Topologies Although this topology is capable of delivering over W to a load in normal operation, it is not a popular choice among seasoned switching power supply engineers because of the above-mentioned shortcomings.

Its output voltage must always be higher than the input voltage. The boost regulator uses the same number of components as the buck regulator, but they have been rearranged as seen in Figure 4. Its operation is also very much different from the forward-mode, buck converter. When the power switch is turned on the input voltage VJ is placed across the inductor. This causes the inductor current to linearly ramp up from 0 A until the power switch is turned off.

During this time energy has been stored within the core material. At the instant the power switch is turned off the inductor voltage flies back above the input voltage. This topology is limited to a 50 percent duty cycle since the core needs sufficient time to empty its energy into the output capacitor. This is the mode in which the vast majority of boost regulators operate. Its waveforms can be seen in Figure 4. The inductor voltage returns to zero or V,, across the power switch when the core has finished emptying its energy.

The current ramp begins from zero. This occurs when the core cannot completely empty itself during the off-time of the power switch and some residual energy remains within the core. Now the inductor voltage does not return to zero and the current ramp rides on a pedestal that has a value proportional to the residual energy remaining in the core. Discontinuous-mode boost regulators can enter the continuous mode at low input voltages since the ontime pulsewidths grow larger in order to bring in the necessary energy required by the load.

The boost supply can be designed to operate in the continuous mode but this presents some stability problems, as described in Chapter IO. An important question that must be answered during the design of the boost regulator is whether or not the inductor can provide enough energy to the load for its steady-state requirements. This can be determined by knowing the basic relationships within the boost regulator. The anioiint 4. Swltching Power Supply Topologies 26 of energy stored within the core during each on period of the power switch is 4.

The P,,,,,determined above should always be greater than the highest power needed by the load. If it is not, then the regulator will operate at light loads but will be unable to maintain regulation at the heavier loads. So the problem is to make the inductance value low enough but not too low as to resemble a short-circuit to be able to accept sufficient energy at the lowest specified input voltage.

This can be seen below. IC In order to maintain this energy, dictated by I,,, at a low input voltage, the on-time must be increased.

Soon a point is reached where the ontime pulsewidth extends into the period when the core is supposed to empty its energy into the output. Beyond this point any increase i n pulsewidth only serves to add to the residual energy remaining in the core and the regulator will cease to regulate the output voltage. The designer's role is to determine the value of the inductance at which this occurs below the minimum specified input voltage.

This topology operates at about three times the peak current of forward-mode regulators. This is due mainly to having a 50 percent duty cycle limit. This high peak current limits its usefulness above W since the stress on the semiconductor power switch beconies too great.

As with all non-transformer-isolated topologies, the ability of the boost regulator to prevent hazardous transients or failures within the supply from reaching the load is quite poor. For instance, if a large positive surge were to enter the regulator, it would exceed the output voltage and conduct directly into the load.

Obviously, one could add transient protection, but many designers use the flyback regulator topology in place of the boost regulator. The transformer isolation vastly improves this condition. It is also known 4. The difference between the boost and the buck-boost regulators, as seen in Figure 4. This stored energy is then released below ground or 26 4. Switchlng Power Supply Topologies the input return lead through the rectifier into the output storage capacitor.

The result is a negative voltage whose level is regulated by the duty cycle of the power switch. The buck-boost regulator is also limited to below a 50 percent power switch duty cycle since it requires time to empty the core of its stored energy.

The equations related to the core and its energy requirements are identical to those of the boost regulator. Once again, the inductor must store enough energy during each cycle of operation in order to sustain the load during that same period. This is determined at the low input voltage, where the voltage across the inductor is at its lowest and hence not able to absorb as much energy per microsecond, and at the maximum rated output load. This is the worst possible point of operation where the duty cycle approaches its physical niaximum of 50 percent.

As in all flyback-mode regulators, to increase the rate of storage within the inductor, if the energy is insufficient as seen by the regulator falling out of regulation at low input voltages, the designer decreases the inductance of the inductor. This helps the flyback-mode regulators operate at lower input voltages, but in this situation the peak currents can become too large to ensure reliable operation of the semiconductors.

The buck-boost regulator can suffer from catastrophic failure modes similar to those of either the buck or the boost regulator separately.

First, if a negative transient is allowed to enter the regulator, a bipolar power transistor power switch may avalanche overvoltage breakdown the reverse-biased base-collector junction, which would cause the transistor to fail. This would then allow the negative transient voltage to enter the output and place an overvoltage stress on the load. Conversely, if a large positive transient enters the regulator, any semiconductor power switch will eventually enter avalanche breakdown and once again fail, short-circuit, and subsequently cause the rectifier to enter avalanche.

This would then cause the positive input voltage to be placed across circuitry that is expecting only a negative voltage.

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Obviously, this will cause the load to fail. The good news is that for the more common source of power switch failure, overdissipation, the rectifier does offer some means of protection by virtue of its reverse-biased condition during the times when positive voltage is on its cathode.

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 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.

Practical Switching Power Supply Design

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.Within microseconds the power switch could fail.

If it is not, then the regulator will operate at light loads but will be unable to maintain regulation at the heavier loads. The duty cycle determines the output voltage. Some control ICs have singleoutput transistors for their output drivers. In determining the number of turns required by the highest power secondary, a few items must now be considered. It is a good idea to design the transformer for slightly lower maximum flux excursions than normal simply for this reason.

When viewed as a blackbox with input and output terminals, the behavior of a switching regulator is identical to that of a linear regulator. So care should be taken during this step.

As a result, the engineer is left to sift through the readily available information, which contains large gaps in content. The transformer does not store energy in this configuration.