SMPS电源技术.docx

Switch Mode Power Supply Basics Tutorial- summary or tutorial about the basics of switch mode power supplies, their design, operation and advantages and disadvantages.SMPS TUTORIAL INCLUDES Switch mode power supply Switch mode regulator fundamentals Step down Buck converter Step up Boost converter Buck-Boost converterSwitch mode power supplies are widely used because of the advantages they offer in terms of size, weight, cost, efficiency and overall performance.As a result of their performance, switch mode power supplies are used in all but the most exacting applications to provide an efficient and effective source of power for most forms of electronic systems.Switch mode power supplies have become an accepted part of the electronics scene and are often referred to as switch mode power converters, or just switchers.Switch mode power supply terminologySwitch mode power supply, SMPS, technology may be referred to by a number of similar terms. While they all look at the same basic technology, they refer to different elements of the overall technology: Switch mode power supply, SMPS: The term switch mode power supply is generally used to indicate an item that can be connected to the mains, or other external supply and used to generate the source power. In other words it is a complete power supply. Switch mode regulator: This typically refers just to the electronic circuit that provides the regulation. A switch mode regulator will be part of the overall switch mode power supply. Switch mode regulator controller: Many switch mode regulator integrated circuits do not contain he series switching element. This will be true if the current or voltage levels are high, because an external series switching element would be able to better handle the higher current and voltage levels, as well as the resultant power dissipation.Switch mode power supply basicsThe basic concept behind a switch mode power supply or SMPS is the fact that the regulation is undertaken by using a switching regulator. This uses a series switching element that turns the current supply to a smoothing capacitor on an off.Switching regulator concept used in switch mode power supplyThe time the series element is turned on is controlled by the voltage on the capacitor. If it is higher than required, the series switching element is turned off, if it is lower than required, it is turned on. In this way the voltage on the smoothing or reservoir capacitor is maintained at the required level.Switch mode power supply advantages / disadvantagesThe use of any technology is often a careful balance of several advantages and disadvantages. This is true for switch mode power supplies which offer some distinct advantages, but also have their drawbacks.SMPS advantages High efficiency: The switching action means the series regulator element is either on or off and therefore little energy is dissipated as heat and very high efficiency levels can be achieved. Compact: As a result of the high efficiency and low levels of heat dissipation, the switch mode power supplies can be made more compact. Flexible technology: Switch mode power supply technology can be sued to provide high efficiency voltage conversions in voltage step up or Boost applications or step down Buck applications.SMPS Disadvantages Noise: The transient spikes that occur from the switching action on switch mode power supplies are one of the largest problems. The spikes can migrate into all areas of the circuits that the SMPSs power if the spikes are not properly filtered. Additionally the spikes or transients can cause electromagnetic or RF interference which can affect other nearby items of electronic equipment, particularly if they receive radio signals. External components: While it is possible to design a switch mode regulator using a single integrated circuit, external components are typically required. The most obvious is the reservoir capacitor, but filter components are also needed. In some designs the series switch element may be incorporated within the integrated circuit, but where any current is consumed, the series switch will be an external component. These components all require space, and add to the cost. Expert design required: It is often possible to put together a switch mode power supply that works. To ensure that it performs to the required specification can be more difficult. Ensuring the ripple and interference levels are maintained can be particularly tricky. Costs: Careful consideration of the costs of a switch mode power supply must be made before designing or using one. Beyond the basic power supply, additional filtering may be required and this can add to the cost.Despite the disadvantages, switch mode power supply technology is the major form of power supply technology used for a whole variety of applications especially those included in computers. For applications where very low noise are required, linear regulator technology is still widely used.Switch Mode Regulator Fundamentals- summary of the fundamentals of how a switch mode regulator works and the fundamentals of its operation.SMPS TUTORIAL INCLUDES Switch mode power supply Switch mode regulator fundamentals Step down Buck converter Step up Boost converter Buck-Boost converterA switch mode regulator is at the heart of any switch mode power supply.The switch mode regulator is the circuit that provides the voltage regulation. It can be used on its own or as part of a complete power supply.Switch mode regulators come in a variety of forms, but each one will be able to provide voltage regulation by using a series switching element that charges up a reservoir capacitor when the voltage falls below a predetermined level.Switching regulator basicsThe basis of switch mode regulator revolves around the ability of inductors and capacitors to store energy. The capacitors and inductors are integral elements of the switch mode regulator technology. Capacitance If a current is applied to a capacitor, the capacitor gradually charges up and the voltage across it rises linearly at a rate equal to I/C where is the applied current and C is the capacitance. In this case the voltage across the capacitor cannot change instantly.When an instantaneous change in current occurs, the voltage changes linearly. This assumes a current source with an infinite voltage capability is used. Inductor: For an inductor, it is not possible for there to be an instantaneous change in current. Instead, when a voltage is applied, the current builds up linearly over time at a rate equal to V/L where V is the applied voltage and L is the inductance.Using the standard equations it is possible to determine the current and voltage profiles:The energy from the rising current is stored in the magnetic field associated with the inductor. If the current flowing through the inductor is suddenly interrupted, the magnetic field reacts against this and produces a very high back emf to counteract the change.Having seen the fundamental or basic concepts behind switching voltages and currents to capacitors and inductors, these basic concepts can be applied to switch mode regulator solutions to provide a variety scenarios for voltage step up and step down circuits.As the technology uses switching techniques where the series element is on or off, this approach provides much better levels of efficiency than a linear where power is dissipated.Capacitor based switch mode regulationThe basic concept of the capacitor switched mode regulator is shown in the diagram. When the switch is closed, current is able to flow into the reservoir capacitor and provide charge. When the voltage on the capacitor is at is required level, the switch opens and the load will draw current from the capacitor.As the voltage falls, this will be sensed by the control circuitry and the series switch will be turned on again to bring the capacitor voltage up to the required level.This circuit is not as effective as may be thought at first sight. Although the only resistive element in the theoretical circuit is the load, this is not the only way in which energy is lost because charging a capacitor directly from a voltage source or a capacitor dissipates as much energy as is transferred to the capacitor. As a result of this, switching mode regulators cannot use capacitor switching techniques alone.Inductor based switch mode regulationIt is also possible to use inductors as an element in switch mode regulators.The inductor can be used to transfer energy from one voltage source to another. While a simple resistor can be used as a dropper to drop voltage when transferring from one voltage source at a higher voltage to one at a lower voltage, this is very wasteful in terms of power. If an inductor is used, then all the energy is transferred, assuming a perfect inductor.The use of an inductor has the advantages that energy can be transferred from one source to another regardless of the respective values of voltage and their polarities. To achieve this the proper configuration is obviously required.When the switches are in the positions shown above, the voltage V1 is applied across the inductor and the current i1 builds up at a rate equal to V1/L. Therefore the peak value obtained will be proportional to the time the switches are in this position, i.e. (V1/L) x tWhen the switches are reversed, the current will continue to flow at a rate i2 which is equal to -V2/L.As an ideal inductor dissipates no energy, there is no power loss in an ideal system using an inductor in this fashion. As a result, it is this method of energy transfer that forms the basis for all switching regulators.Step Down Buck Regulator / Converter- summary or tutorial about the circuit and operation of a step down or buck regulator using switch mode power supply techniques.SMPS TUTORIAL INCLUDES Switch mode power supply Switch mode regulator fundamentals Step down Buck converter Step up Boost converter Buck-Boost converterWith power being a key parameter in many designs, step down or buck regulators are widely used.Although a resistor would enable voltage to be dropped, power is lost, and in applications such as the many battery powered items used today, power consumption is a crucial element.As a result step down switch mode converters or as they are more commonly termed, buck regulators are widely used.Linear step downThe most basic form of step down transition is to use a resistor as a potential divider or voltage dropper. In some cases a zener diode may also be used to stabilise the voltage.The issue with this form of voltage dropper or step down converter is that it is very wasteful in terms of power. Any voltage dropped across the resistor will be dissipated as heat, and any current flowing through the zener diode will also dissipate heat. Both of these elements result on the loss of valuable energy.Basic buck converter or regulatorThe fundamental circuit for a step down converter or buck converter consists of an inductor, diode, capacitor, switch and error amplifier with switch control circuitry.The circuit for the buck regulator operates by varying the amount of time in which inductor receives energy from the source.In the basic block diagram the operation of the buck converter or buck regulator can be seen that the output voltage appearing across the load is sensed by the sense / error amplifier and an error voltage is generated that controls the switch.Typically the switch is controlled by a pulse width modulator, the switch remaining on of longer as more current is drawn by the load and the voltage tends to drop and often there is a fixed frequency oscillator to drive the switching.Buck converter operationWhen the switch in the buck regulator is on, the voltage that appears across the inductor is Vin - Vout. Using the inductor equations, the current in the inductor will rise at a rate of (Vin-Vout)/L. At this time the diode D is reverse biased and does not conduct.When the switch opens, current must still flow as the inductor works to keep the same current flowing. As a result current still flows through the inductor and into the load. The diode, D then forms the return path with a current Idiode equal to Iout flowing through it.With the switch open, the polarity of the voltage across the inductor has reversed and therefore the current through the inductor decreases with a slope equal to -Vout/L.The step down, buck converter circuit can be further explained by examining the current waveforms at different times during the overall cycle.In the diagram of the current waveforms for the buck converter / switching regulator, it can be seen that the inductor current is the sum of the diode and input / switch current. Current either flows through the switch or the diode.It is also worth noting that the average input current is less than the average output current. This is to be expected because the buck converter circuit is very efficient and the input voltage is greater than the output voltage. Assuming a perfect circuit, then power in would equal power out, i.e. Vin In = Vout Iout. While in a real circuit there will be some losses, efficiency levels greater than 85% are to be expected for a well-designed circuit.It will also be seen that there is a smoothing capacitor placed on the output. This serves to ensure that the voltage does not vary appreciable, especially during and switch transition times. It will also be required to smooth any switching spikes that occur.Regulator input and output filteringA key aspect of switch mode power supply regulators is the input and output filtering. This is a particular issue because of the switching that occurs at the input.In reality ripple voltage on the output is dependent not only on the output smoothing, but more importantly on an input filter capacitor.Step Up Boost Regulator or Converter- summary or tutorial about the circuit and operation of a step up or boost regulator using switch mode power supply techniques.SMPS TUTORIAL INCLUDES Switch mode power supply Switch mode regulator fundamentals Step down Buck converter Step up Boost converter Buck-Boost converterOne of the advantages of switch mode power supply technology is that it can be used to create a step up or boost converter / regulator.Boost converters or regulators are used in many instances from providing small supplies where higher voltages may be needed to much higher power requirements.Often there are requirements for voltages higher than those provided by the available power supply - voltages for RF power amplifiers within mobile phones is just one example.Step-up boost converter basicsThe boost converter circuit has many similarities to the buck converter. However the circuit topology for the boost converter is slightly different. The fundamental circuit for a boost converter or step up converter consists of an inductor, diode, capacitor, switch and error amplifier with switch control circuitry.The circuit for the step-up boost converter operates by varying the amount of time in which inductor receives energy from the source.In the basic block diagram the operation of the boost converter can be seen that the output voltage appearing across the load is sensed by the sense / error amplifier and an error voltage is generated that controls the switch.Typically the boost converter switch is controlled by a pulse width modulator, the switch remaining on of longer as more current is drawn by the load and the voltage tends to drop and often there is a fixed frequency oscillator to drive the switching.Boost converter operationThe operation of the boost converter is relatively straightforward.When the switch is in the ON position, the inductor output is connected to ground and the voltage Vin is placed across it. The inductor current increases at a rate equal to Vin/L.When the switch is placed in the OFF position, the voltage across the inductor changes and is equal to Vout-Vin. Current that was flowing in the inductor decays at a rate equal to (Vout-Vin)/L.Referring to the boost converter circuit diagram, the current waveforms for the different areas of the circuit can be seen as below.It can be seen from the waveform diagrams that the input current to the boost converter is higher than the output current. Assuming a perfectly efficient, i.e. lossless, boost converter, the power out must equal the power in, i.e. Vin Iin = Vout Iout. From this it can be seen if the output voltage is higher than the input voltage, then the input current must be higher than the output current.In reality no boost converter will be lossless, but efficiency levels of around 85% and more are achievable in most supplies.Buck-Boost Converter or DC-DC Regulator- summary or tutorial about the circuit and operation of a buck-boost converter, a dc-dc converter able to provide voltages lower or higher than the input voltage.SMPS TUTORIAL INCLUDES Switch mode power supply Switch mode regulator fundamentals Step down Buck converter Step up Boost converter Buck-Boost converterA simple buck converter can only produce voltages lower than the input voltage, and a boost converter, only voltages higher than the input. To provide voltages over the com