Power supply control, switching

Short Overview of Switching Power Supply Control... 

Selecting the SMPS controller IC. The important factors within this application that affect the choice of switching power supply controller IC are MOSFET driver needed (totem-pole driver), single-ended output, 50 percent duty cycle limit desired, and current-mode control desired. The popular industry choice that meets these needs is the UC3845B. 

Sanjaya Maniktala is back with a new installment of his popular power supply design column. The question he asks is how to make up for the erratic quality of the 3842 and 3844 switching power supply controllers now on the market. An adjustment of the RC components in the hysteresis loop allowed a power supply company to safely utilize the batch lot of jittery components they were stuck with — some 50,000 of them. 

Power Supplies and Controls. Induction heating furnace loads rarely can be connected directiy to the user s electric power distribution system. If the load is to operate at the supply frequency, a transformer is used to provide the proper load voltage as weU as isolation from the supply system. Adjustment of the load voltage can be achieved by means of a tapped transformer or by use of a solid-state switch. The low power factor of an induction load can be corrected by installing a capacitor bank in the primary or secondary circuit. 

This design example purposely avoids the use of a highly integrated buck controller IC. It would be instructional to show the selection and design process involved in a switching power supply. Refer to Figure 3-65... 

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. 

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. 

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. 

Quasi-resonant converters are a separate class of switching power supplies that tune the ac power waveforms to reduce or eliminate the switching loss within the supply. This is done by placing resonant tank circuits within the ac current paths to create pseudo-sinusoidal voltage or current waveforms. Because the tank circuits have one resonant frequency, the method of control needs to be modified to a variable frequency control where the resonant period is fixed and the control varies the period of the non-resonant period. The quasi-resonant converters usually operate in the 300 kHz to 2 MHz frequency range. 

B.2 Defining the Open Loop Response of the Switching Power Supply—The Control-to-Output Characteristics... 

The dc gain exhibited by the power section of the switching power supply of a current- or voltage controlled flyback converter is approximately... 

Controlling high frequency noise generation and radiation is the blackest of the black box art in switching power supply and product-system design. It is a subject that warrants a book all to itself and it is the final area that will interfere with the release of your product into the market. This appendix cannot adequately cover the subject, but will overview the major considerations involved with product design. 

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. 

Four different types of tasks are performed by automation. Two involve the sequencing of valves and pumps Involved 1n the setup and completion of the designed experiment through the operation of the test and hydraulic fluid systems. The other tasks involve the control of the temperature bath and data collection. To perform these tasks, a1r-actuated solenoids and optically coupled sol Id-state relays are used. These devices are controlled by an electrical circuit consisting of the device connected 1n series with a power supply and a channel on the actuator card In the HP 3497. The power supply 1s either 24 VDC for use with the solenoids or 5 VDC for the solid-state relays. The actuator output channel acts as a simple on/off switch which allows power to be supplied to the solenoid or relay when closed. The logic of the circuit 1s controlled by application programs running on the local HP 1000. 

So where does soft-start really help Mainly in bringing up the output voltage rail smoothly, maybe to avoid jerking the system connected at the output of our power supply. Yes, perhaps within a certain range of output C and L, it can also help control the stresses on the switch at power-up. But that support is hardly unconditional. Maybe it helps somewhat in lowering the overshoot of the rail at startup. But as mentioned in Question 2, it can itself be a reason for the overshoot too. Therefore, in all the AC-DC Flybacks we... 

Even conventional hotplates can be fitted with a continuously variable temperature control thanks to platinum thin-film sensors. The encapsulated platinum sensor is positioned so that it is in contact with the cast iron plate from below and thus able to register the temperature of the hotplate. Normally the power supplied to the plate is regulated by a stage switch. The increased control sensitivity is made possible by incorporating electronic circuitry capable of interpreting and acting upon the sensor signals. 

Accident Mitigation 28 Detection of leaks/ruptures 29 Emergency shutdown switch locations 30 Accessibility of isolation valves 31 Potential for fire/explosion in unit affecting other equipment 32 Critical controls, mitigation, communication, and fire protection sy stems functional and accessible after initial explosion or release 33 Back-up power supply/redundant feeds for critical electrical systems 34 Water supply for fire fighting 35 Routing of utilities... 

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