开关电源的基础-

1、Switching Power Supply Fundamentals1 To start, we will review the basic operation of switching power supplies as a basis for understanding theselection criteria for the external components needed for optimum performance of the power supply. Despite the increasing popularity of monolithic switching r

2、egulators, linear regulators still dominate the market. Inexpensive and easy to use, these regulators can be found almost anywhere. Even so, understanding the differences between different types of linear regulators can be key to making the best use of this ubiquitous component. Buck Calculations fo

3、r Inductance Need to know V Output CapacitorSynchronous Buck Converters OfferVQ1 INVINFPWMENPGOODVDDFREQ10 msec/div1 msec/div©A reduction in start-up current is useful in situations where the input power source is limited in the amount of current it can deliver. In some applications Soft Start

4、can be used to replace under voltage lockout or The example of Soft Start above is from the LM2597 datasheet. The soft start feature reduces the startup down to 160 mA, and delays and slows down the output voltage rise time.Another situation where Soft Start is useful is in allowing the use of an in

5、ductor with a much lower saturation current than the current limit of the switcher. Soft Start prevents the high currents which wouldDeriving a Step-Up (Boost RegulatorInductor CurrentThe output capacitor in a boost converter is the reservoir that provides all the output current when theor a boost c

6、onverter a much lower capacitor ESR is required to get the same output ripple voltage as a buck regulator at the same current TYPICAL xDSLDUTY CYCLE V INside of the winding to the ground side of the winding. We have also changed because of the presence of leakage inductance. The leakage inductance s

7、tores energy, and delays the transfer of current to the secondary. Since the current must continue to flow, a path must be provided or very high voltage spikes will snubber across the primary provides a place to clamp the voltage, dissipating power from the energy stored in the leakage inductance.Op

8、eration on the output looks just like a boost converter and the input capacitor also must support large AC currents. This means big capacitors at both the input and output. The input capacitor is needed to bypass AC ©2006 National Semiconductor Corporation Deriving Positive Output Flyback2006 N

9、ational Semiconductor Corporation Another advantage of flyback converters is the ability to provide multiple outputs from a single regulator. Here is an example of a SIMPLE SWITCHER®regulator being used in a dual output flybackThe voltage on the regulated winding (the one with the output couple

10、d back to the F Need to know reverse voltage ratingTransorbsLow RDS(ONElectrical CharacteristicsCapacitance at rated voltage (ceramics!Voltage Rating, RMS current ratingAluminum ElectrolyticSpecialty Polymer1uF, 6.3V, 10051uF, 10V,16081uF, 16V, 16081uF, 25V,20121uF, 16V,20121uF, 40V, 32161uF, 25V,32

11、161uF, 50V, 3225Capacitor Data Provided By:Its a 1 uF Capacitorbut what capacitance are you getting?!?All curves are from high quality ceramic capacitors with good X5R or X7RSpecification ParameterSwitching Regulator Control Architectures31 Up to this point we have looked at switching converters as

12、being defined by the way in which similar external components are connected, and we have established selection criteria for those external components based on the operating conditions of each configuration. We have not shown how the feedback is achieved or the effects that a given topology will have

13、 on the stability of that feedback loop. Next we will address how the feedback loop controls the regulator and the precautions that are necessary for stable operation. First we will look at feedback in general,and how it applies to linear regulators, to form a basis for analysis of the more complex

14、switching regulator architectures.V INSOURCE d i v 100Voltage Mode BuckRegulators37 As noted earlier, identification of the poles (and zeros for a switching converter are not as easy as that for a linear regulator. Some understanding of the switcher architecture is required to ensure that stability requirements are met.In the beginning we looked at a similar block diagram