Snubbers

Reeover as mueh of this loss energy as possible and return it to the power flow of the power supply. 

The designer eannot aeeomplish all of the goals eompletely, but improving the eonditions ean add another +3 to +9 pereent in overall power supply effleiency. 

To aeeomplish this, one normally uses additional reaetive elements with diodes or MOSFETk to eontrol the effeets. The types of modifleations to the standard PWM topologies fall into three eategories  

Lossless snubbers and aetive elamps produee PWM waveforms with soft edges. 

For the power switeh ae node, the voltage wants to be delayed on the turnoff transition. This provides loading of the magnetie element during the forward reeovery time of the output reetifler. For the output reetifler ae node, the eurrent wants to be delayed at its turn-off. This limits the refleeted eurrent spike eaused by the reverse reeovery period of the reetifler. These teehniques are shown in the following seetions. 

Another way to draw coils is shown in Fig. 10.6. It is often used in modern publications, because it is easier to draw than complete spirals. 

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The inductance in the d.c. link may cause a reverse voltage spike across the power diodes or thyristors as a residt of the decay of the reverse current (release of its stored energy). A power deviee may be protected against such voltage spikes through tin R-C snubber circuit, as shown in Figure 6.37. (This circuit is discussed later.)... 

The conrbinalion of C-R acts as snubber circuil lo prelect electronic (static) components from high dr/df. 

Snubber circuii More conventional protection from high dvidi is to provide an R-C circuit across each device, as shown in Figure 6.37. The circuit provides a low impedance path to all the harmonic quantities and draws large charging currents and absorbs the energy released, Q, and in turn damps dvIdi within safe limits across each device. Now Q = C idu/di)... 

Pressure Zero shift, air leaks in signal lines. Variable energy consumption under temperature control. Unpredictable transmitter output. Permanent zero shift. Excessive vibration from positive displacement equipment. Change in atmospheric pressure. Wet instrument air. Overpressure. Use independent transmitter mtg., flexible process connection lines. Use liquid filled gauge. Use absolute pressure transmitter. Mount local dryer. Use regulator with sump, slope air line away from transmitter. Install pressure snubber for spikes. 

One ean easily notiee the differenee in the spike energy between the two oseil-lographs. Typieally, this energy is dissipated as heat within a elamp and/or snubber aeross the primary winding. 

Snubbers are passive networks that delay the risetime of the voltage waveform. Historieally, snubbers have been used to keep power deviees within their forward- and reverse-biased safe operating areas (FBSOA and RBSOA) or to eontrol RF emissions from the power supply. They are essentially lossy tank eireuits (L-C eireuits with R). Using them offered more of an advantage than the loss ineurred. Semieonduetor eomponents are more rugged today and the traditional need for the RFC snubber for proteetion has lessened, but oeea-sionally a snubber is still needed. 

The traditional snubber has been the approaeh used to keep bipolar power transistors away from seeond breakdown eonditions. It is also useful in the redue-tion of radiated EMI by eontrolling the dv/dt of reetifiers with abrupt reverse... 

The empirical design process of the traditional snubber is as follows ... 

NOTE Do not use snubber values or snubber elements intended for silicon-controlled rectifier (SCR) circuits in switching power supplies. The impedances and parasitic values of these circuits are much lower than within switching power supplies. They will create far too much loss in switching power supply circuits. 

Figure 4-5 Lossless snubber for a one-transistor forward or flyback converter.

In boost eonverters the lossless snubber shown in Figure 4-6 ean be used. 

The disadvantage of the quasi-resonant converter compared to the newer lossless snubber and active clamp techniques in addition to the basic PWM converters, is the voltage or current stresses placed upon the power components. The peak voltage or current values that exist within quasi-resonant converters can be two to three times higher than in PWM converters. This forces the designer to use higher-rated power switches and rectifiers which may not have as good conduction characteristics. 

The following design examples eontain various teehniques to improve the overall effieieney of eommon designs. Most of the designs of aetive elamps and lossless snubbers are empirieal, so they are not ineluded within these design examples. 

It is desired that the resonanee frequency of the tank circuit be 1 MHz. In ZVS QR converters, the tank circuit is not responsible for storing and passing on energy as it is in ZCS QR converters. The tank circuit can be seen more as an off-time transition shaper similar to a snubber when used in PWM converters. Here a wide range of values for both the inductor and the capacitor will work as long as their combined resonant frequency is 1 MHz or... 

One subtle, but major noise source is the output rectifier. The shape of the reverse recovery characteristic of the rectifiers has a direct affect on the noise generated within the supply. The abruptness or sharpness of the reverse recovery current waveform is often a major source of high-frequency noise. An abrupt recovery diode may need a snubber placed in parallel with it in order to lower its high-frequency spectral characteristics. A snubber will cost the designer in efficiency. Finding a soft recovery rectifier will definitely be an advantage in the design. 

Ben-Yaakov, Sam and Gregoiy Ivensky, Passive Lossless Snubbers for High Frequency PWM Converters, Seminar 12, APEC 99. 

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. 

Pulsation control, 84 Pulsation snubbers, 85 Pulse-width-modulated unit, 278... 

The four primary coolant pumps are connected to the secondary shield wall by three-link snubbers designed to be flexible under static applied loads (thus, allowing thermal expansion) but become stiff under dynamic loads that might occur during an earthquake. Accordingly, the system is coupled to the wall under seismic loading. 

Another hazard with centrifuges is that if they turn the wrong way, the snubber can damage the basket. It is therefore much more important than with pumps to make sure this does not occur. 

Survey and Evaluation of System Interaction Events and Sources Nuclear 4(X) occurrences of snubber failure at U.S. nuclear power plants from event reports Hydraulic and mechanical snubbers 103. 

NUMBER AND TYPE OF RECORDS 400 occurrences of snubber failure at U.S. 

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