Switching power supply efficiency

Waveshaping Techniques to Improve Switching Power Supply Efficiency... 

High efficiency resonant technology switching power supplies... 

First, the designer should choose the type of rectification technology that is most appropriate for the application. The choice is whether to use passive rectification in which semiconductor rectifiers are used or synchronous recification in which power MOSFE B are placed in parallel with a smaller passive rectifier. Synchronous rectifiers are typically used in battery operated portable products where the added efficiency, usually an added two to eight percent, is important to extend the operating life of the battery or in applications where heat is important. In today s switching power supplies, passive rectifiers can dissipate 40 to 60 percent of the total losses within the power supply. Synchronous rectifiers affect only the conduction loss, which can be reduced by as much as 90 percent. 

To improve the efficiency of a switching power supply, one must be able to identify and roughly quantify the various losses. Tosses within a switching power supply roughly fall into four categories switching, conduction, quiescent, and resistive losses. These losses usually occur in combination within any lossy component and are treated separately. 

Section 3.4, Table 3-3, provides some insight as to where the major losses occur and to what degree. The losses highlighted in that table are for the basic PWM switching power supply without much effort placed into making them more efficient. These efficiency numbers, therefore, can be seen as the baseline... 

A PWM switching power supply that is designed with no extraordinary loss-control methods will exliibit efficiencies as seen in Table 3-3. For switching power supplies that have no problem in getting rid of the heat, such as some off-line applications, the aforementioned efficiencies may be satisfactory. For portable applications and equipment that must be small in size much better efficiencies must be sought. To improve the overall efficiency of a power supply, several techniques can be used. 

Only a few controller ICs available at the time of this publication directly support an active clamp drive (for example, Texas Instruments UCC3580). There will be more since its function seems to increase the efficiency of a switching power supply by several percentages. 

Power Supply Cookbook, Second Edition has been updated with the latest advances in the field of efficient power conversion. Efficiencies of between 80 to 95 percent are now possible using these new techniques. The major losses within the switching power supply and the modern techniques to reduce them are discussed at length. These include synchronous rectification, lossless snubbers, and active clamps. The information on methods of control, noise control, and optimum printed circuit board layout has also been updated. 

What happens if Vsw < VD In fact that is the situation in most commercial Flybacks. But note that to do a proper comparison, you have to reflect the diode drop to the primary side. And for that we have to multiply the diode drop by the turns ratio (see the equivalent Buck-Boost models of a Flyback section in my book, Switching Power Supply Design Optimization). So, for example, if the turns ratio is 20 and the diode drop is 0.6V, the effective VD we need to compare with Vsw for our time-sharing analysis is 0.6 x 20 = 12V. And that is usually greater than the (average) drop across the switch. Therefore, we tend to say that in a Flyback, decreasing D (increasing input) will worsen the total conduction loss and decrease the efficiency. But of course that never happens, because as we increase the... 

We have seen that one reason why switching regulators have such a high efficiency is because they use a switch (rather than a transistor that thinks it is a resistor, as in an LDO). Another root cause of the high efficiency of modern switching power supplies is their effective use of both capacitors and inductors. 

These devices advantage is that their internal dynamic resistance is very low and therefore the voltage can be regulated very quickly. The losses in the series control are comparatively high and therefore the efficiency is low. Primary-chopped switching power supplies will more and more substitute these chargers (Figure 13.9). A more detailed description is therefore not made here. 

As seen in Section 4.1, the major types of losses are the conduction and switching losses. Conduction losses are addressed by selecting a better power switch or rectifier with a lower conduction voltage. The synchronous rectifier can be used to reduce the conduction loss of a rectifier, but it can only be used for forward-mode topologies, and excludes the discontinuous boost-mode converters. The synchronous rectifier will improve the efficiency of a power supply about one to six percent depending upon the average operating duty cycle of the supply. For further improvements, other techniques must be pursued. 

The next question is, is the efficiency being lost in the switch If so, there could be many reasons for that. A switch could be lossy simply because its drive is inadequate. Early self-oscillating converters (ringing choke oscillators) were extremely lossy because the drive would slowly droop to the point where it just couldn t sustain itself and then the switch would turn OFF. Modern self-oscillating converters have improved tremendously on this, and you can even find full-fledged multi-output PC power supplies that don t have a single... 

A maximum 10% of auctioning would make around 210 Mt C02/year available through auctions, out of a total allocated of 2100 Mt C02. The overall shortfall - to be met through emissions abatement (e.g. fuel switching end-use efficiency in the power sector) and international purchase - would then still be only around 100 Mt C02/year, close to the revealed surplus in 2005 and much smaller than the lowest estimates of the total supply of Kyoto project credits. 

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