Common Requirements An Overview

The direct-off-line switchmode supply is so called because it takes its power input directly from the ac power lines, without using the rather large low-frequency (60 to 50 Hz) isolation transformer normally found in linear power supplies. Although the various switchmode conversion techniques are often very different in terms of circuit design, they have, over many years, developed very similar basic functional characteristics which have become generally accepted industry standards. Further,...

Cross Conduction

The term cross conduction is used to describe a potentially damaging condition that can arise in half-bridge and full-bridge push-pull converters. The problem is best explained with reference to the circuit shown in Fig. 1.19.1. It can be clearly seen that in this half-bridge configuration, if Q1 and Q2 are both turned on at the same time, they will provide a direct short circuit across the supply lines transformers T1 and T2 are current transformers and have little resistance . This will often...

Jrj u g

Bus and feeder systems in industrial plants c. Heavy appliance outlets with short connections to the service entrance d. Lighting systems in commercial buildings Note Category B locations are closer to the service entrance. The stress voltages may be similar to those for category A, but currents up to 3000 A may be expected. 3. Category C, Outside and Service Entrance. This location is outside the building. Very high stress conditions can occur, since the line and insulator spacing is large...

52 Fuse Parameters

Let Through Energy Arcing Pre Arcing I2r

From an electrical standpoint, fuses are categorized by three major parameters current rating, voltage.rating, and, most important, let-through current, or Z2t rating. It is common knowledge that a fuse has a current rating and that this must exceed the maximum DC or rms current demanded by the protected circuit. However, there are two other ratings that are equally importantfor the selection of the correct fuse. The voltage rating of a fuse is not necessarily linked to the supply voltage....

2411 Step 10 Check Core Flux Density and Saturation Margin

It is now necessary to check the maximum flux density in the core, to ensure that an adequate margin between the maximum working value and saturation is provided. It is essential to prevent core saturation under any conditions, including transient load and high temperature. This may be checked in two ways by measurement in the converter, or by calculation. Core Saturation Margin by Measurement Note It is recommended that this check be carried out no matter what design approach was used, as it...

723 Basic Principles

Figure 1,12.3a shows the method of energy storage and delivery, hen SW1 is open, capacitors CI and C2 are charged in parallel from the supply lines via resistors R1 and R2. They will eventually charge to the supply voltage vs. If this circuit is now removed from the supply lines and SW1 is closed, CI and C2 will be connected in series, and a voltage of 2 X Vs will be provided at the terminals. In Figure 1.12.36, this circuit in its charged state is shown connected to a linear regulator circuit...

62 Typical Dualvoltage Capacitor Input Filter Circuit

Figure 1.6.1 shows a typical dual-voltage rectifier capacitor input filter circuit. A link option LK1 is provided which allows the rectifier capacitor circuit to be configured as a voltage doubler for 120-V operation or as a bridge rectifier for 240-V operation. The basic rectifier capacitor input filter and energy storage circuit C5, C6, and D1 through D4 has been supplemented with an input fuse FS1, an inrush-limiting thermistor NTC1, and a high-frequency noise filter LI, L2, L3, CI, C2, C3,...

22 Core Parameters And The Effect Of An Air

Hysteresis Loop With Air Gap

Figure 2.2.1a shows a typical BIH hysteresis loop for a transformer-grade ferrite core, with and without an air gap. It should be noted that although the permeability slope of the BIH loop changes with the length of the air gap, the saturation flux density at the combined core and gap remains the same. Further, the magnetic field intensity H is much larger, and the residual flux density Br much lower, in the gapped case. These changes are vety useful for flyback transformers, which use only the...

613 Example Of Rectifier Capacitor Input Filter Dc Output Voltage Calculation

Consider the previous example for a 250-W unit. The input power is 357 ind a voltage doubler circuit is to be used at 110 V input. The total series re - ce S . UtpUt V0ltaBe 0f a ful -wave bridge-rectified capacitor input filter as a function of greater 6 S0UrCC res,stance as a P meter. Valid for capacitor values of 1.5 VV or FIG. 1.6.8 Mean DC output voltage of a voltage doubler capacitor input filter as a function of load power, with effective source resistance as a parameter. Valid for...

127 Snubber Components Halfbridge

Components C3, C4, and Rl are often referred to as snubber components they assist the turn-off action of the high-voltage transistors Q1 and Q2 so as to reduce secondary breakdown stress. As the transistors turn off, the transformer inductance maintains a current flow, and the snubber components provide an alternative path for this current preventing excessive voltage stress during the turn-off action. The conventional diode, capacitor, snubber circuit should not be used in the half-bridge...

57 Snubber Components

Because power FET devices are not subject to the same secondary breakdown mechanisms that occur with bipolar devices, from a reliability standpoint, it is often considered that snubber components are not essential. However, in most FET applications, a small RC snubber network will still be fitted across the FETs to reduce RF radiation and meet the dv df limitations of the FET. With very high dvldt, some power FETs display a failure mode resulting from conduction of the internal parasitic...

162 Example Of A Proportional Drive Circuit

Figure 1.16.1 shows a typical proportional drive circuit applied to a single-ended forward converter. In this arrangement, a proportion of the collector current is current-transformer-coupled by T1 into the base-emitter junction of the main switching transistor Ql, providing positive proportional feedback. The drive ratio Ijlc is defined by the turns ratio of the drive transformer Pl Sl to suit the gain characteristics of the transistor typically a ratio of between Vio and Vs will be used ....

1811 Ideal Drive Circuits For Highvoltage Bipolar Transistors

Figure 1.18.4 shows a combination of the snubber diode and Baker clamp circuits, with a push-pull base drive to Ql. This arrangement is particularly suitable for high-voltage flyback converters where the collector voltage may be of the order of 800 V or more during the flyback period. It operates as follows. When the drive voltage goes high, Q2 is turned on and Q3 off. Current flows via R3, Q2, C2, and D7 to the base of the power transistor Ql. The overdrive provided by the low-impedance R3, C2...

173 Type 7 Scr Crowbar Overvoltage Protection

As the name implies, crowbar overvoltage protection requires the short-circuiting of the offending power supply output in response to an overvoltage condition on that output. The short-circuiting device, usually an SCR, is activated when the overvoltage stress exceeds a preset limit for a defined time period. When the SCR is activated, it short-circuits the output of the power supply to the common return line, thus collapsing the output voltage. A typical simple SCR crowbar overvoltage...

74 Active Limiting Circuits Triac Start Circuit

For high-power converters, the limiting device is better shorted out to reduce losses when the unit is fully operating. EES. 1.7.2 Resistive inrush-limiting circuit with triac bypass for improved efficiency. Note Higher inrush current for bridge operation. EES. 1.7.2 Resistive inrush-limiting circuit with triac bypass for improved efficiency. Note Higher inrush current for bridge operation. Position R1 will normally be selected for the start resistor so that a single triac or relay may be used....

Lowpower Selfoscillating Auxiliary Converters

Many of the larger power converters require a small amount of auxiliary power for the supply of the control and drive circuits. Often the auxiliary requirements are derived from 60-Hz line transformers. This is not always very efficient, as the size of the transformer will often be determined by the need to meet VDE and UL creepage distance specifications rather than by the power needs. As a result, the transformer will often be larger in size than is required to meet the power requirements...

102 Typical Causes Of Turnon Voltage Overshoot In Switchmode Supplies

In most switchmode power supplies, a controlled start-up sequence is initiated at switch-on. Shoiild the turn-on be from a line input switch, the first action will be inrush limiting, where a resistive element in series with the line input reduces the peak inrush currents for a few cycles while the input capacitors are charged Following this inrush limiting, there will be a soft-start action. For soft start, the pulse width to the power switching devices is progressively increased to establish...

52 Operating Principle

In the circuit shown in Fig. 2.5.1, the high-voltage DC line is switched to the primary of a transformer by two power FET transistors, FT1 and FT2. These switches are driven by the control circuitry such that they will both be either on or off' together. Flyback action takes place during the off state, as in the previous flyback examples. The control, isolation, and drive circuitry will be very similar to that previously used for single-ended flyback converters. A small drive transformer is...

60hz To 50hz Converter

60hz 50hz Converter Schematic

ELECTROMAGNETIC INTERFERENCE EMI IN SWITCHMODE POWER SUPPLIES Electromagnetic interference EMI , otherwise referred to as radio-frequency interference RFI , the unintentional generation of conducted or radiated energy, is indefatigable in all switchmode power supplies. The fast rectangular switching action required for good efficiency also produces a wide interference spectrum which can be a major problem. Further, for proper operation of any electronic system, it is imjftlrtant that all the...

43 Transformer Faraday Screens And Safety Screens

To prevent circulation of RF currents between the primary and secondary windings or between the primary and the grounded safety screen, the main switching transformer will usually have at least one RFI Faraday screen in the primary winding. In some applications, an additional safety screen will be required between the primary and secondary windings. There are major differences in construction, location, and connection between the Faraday RFI screens and the safety screens. Safety regulations...

45 Reducing Radiated Emi In Gapped Transformer Cores

Femte flyback transformers and high-frequency inductors will usually have a relatively large air gap in the magnetic path, to define the inductance or to prevent saturation. Considerable energy can be stored in the magnetic field associated with this air gap. Unless the transformer or choke is screened, an electromagnetic field EMI will be radiated from the gap, and this can cause interference to the supply itself or to local equipment. Further, this radiated field may exceed the radiated EMI...

38 Line Impedance Stabilization Network Lisn

Line Impedance Stabilization Network

Figure 1.3.6 shows the standard LISN, used for the measurement of line-conducted interference, as specified by CSA C108.8-M1983 Amendment 5, 1983. Similar networks are specified by the FCC and VDE. In principle the wideband line chokes Ll and L2 divert any interference noise currents from the supply into the 50-R test receiver via the 0.1-p.F capacitors C3 or C4. The line not under test is terminated in 0.1 jiF and 50 I. It is normal to test both supply lines independently for common-mode...

Acknowledgements

We progress not only by our own efforts, but also by utilizing the work of those around us and by building on the foundations of those who went before. The reference section is an attempt to acknowledge this, and I have no doubt that many more works should have been mentioned. I sincerely apologize for any omissions it is often difficult to remember the original source. I am grateful to the many who have contributed to this work, but worthy of special mention is my former...

Power Supply Problems Tektronix 2213

Units, Symbols, Dimensions, and Abbreviations Used in this Book xix PART 1 FUNCTIONS AND REQUIREMENTS COMMON TO MOST DIRECT-OFF-LINE SWITCHMODE POWER SUPPLIES 1. COMMON REQUIREMENTS AN OVERVIEW 1.3 LI Introduction. 12 Input Transient Voltage Protection. 1.3 Electromagnetic Compatibility. 1A Differential-ModeNoise. L5 Common-Mode Noise. 1.6 Faraday Screens. L7 Input Fuse Selection. L8 Line Rectification and Capacitor Input Filters. 1.9 Inrush Limiting. 1.10 Start-Up Methods. 1.1 1 Soft Start....