102 Switching Regulators

In Chapters 5 to 9 we looked at switching regulators, which have much higher efficiency, but can generate electro-magnetic interference (EMI) which has to be suppressed by careful circuit board design, screening and filtering. The EMI reducing techniques are described in Chapter 13. Although Supertex's LED driver integrated circuits are used in examples, similar drivers from other manufacturers can also be used. For example, the Linear Technology LTC3783 has similar functions to the Supertex...

111 Discrete Semiconductors

Atoms of materials have a core (nucleus) of positively charged proton and uncharged neutrons. They have negatively charged electrons orbiting around this nucleus, like planets around the Sun. When atoms combine, they share electrons in their outer orbit (the valence band). Lighter atoms, like silicon, are most stable when there are eight electrons in their outer orbit. Semiconductors are (usually) made from silicon, which has four electrons in its outer orbit. The addition of a small amount of...

1112 Bipolar Transistors

Bipolar transistors are used in switching and linear LED driver circuits. They operate by a current magnification effect the collector-emitter current is a multiple of the base-emitter current. The base-emitter voltage is about 0.7 V, being the voltage drop of a forward biased P-N junction. There is some base-emitter resistance, so the forward voltage drop will increase slightly with base current. Matched transistors can be very useful, particularly in current mirror circuits. A current mirror...

1114 Voltage Clamping Devices

Voltage clamping devices are used to limit the voltage across a circuit, as part of a voltage regulator or a transient suppressor. These devices are typically semiconductors Zener diodes, Transorb suppressors or voltage dependent resistors (VDRs). Zener diodes behave like regular diodes in the forward conducting direction, but break down and conduct at a defined voltage in the reverse direction. Low voltage Zener diodes rated below 6 V have a soft knee in their current versus voltage graph the...

1122 Inductors

This section will describe 'off-the-shelf' inductors and transformers. Details of custom-made components will be covered in Chapter 12. Inductors (symbol L) are used to store energy in switching LED driver circuits. A length of wire creates inductance, but winding insulated wire into a coil can magnify this the wire is normally soft copper covered with a thin plastic film. The magnetic field produced by a wire then couples to adjacent wires the inductance is proportional to the number of turns...

114 Operational Amplifiers and Comparators

The operational amplifier has DC characteristics that may change with temperature, but those most affected are the DC offset, bias current, etc. The AC characteristics are less affected by temperature. The greatest problem is that the op-amp is not ideal. The ideal op-amp has infinite input impedance, zero output impedance and a flat frequency response with linear phase. Most practical op-amps have very high input impedance and this does not cause us many problems. The output impedance is not...

12 Description of Contents

In Chapter 2, the description of some LED applications will show the breadth of the LED driving subject and how LEDs' physical characteristics can be used to an advantage. It is also important to understand the characteristics of LEDs in order to understand how to drive them properly. One of the characteristics is colour an LED emits a very narrow band of wavelengths so the colour is fairly pure. The LED color determines the different voltage drop across the LED while it is conducting, and I...

121 Ferrite Cores

Ferrite cores are available in many shapes and material types. These cores are quite brittle and can break if dropped or struck with a hard object. Ferrite is usually a compound made from magnesium and zinc, or from nickel and zinc. Most ferrites have very poor electrical conductivity, which limits any eddy currents in the core. Nickel-zinc ferrites are used in inductors intended for EMI filters, because they have high losses at high frequency - the core absorbs most of the energy above 20 MHz,...

122 Iron Dust Cores

Iron dust cores (also called iron powder cores) are sometimes often made toroidal (doughnut) shaped. The iron dust is ferrous oxide and is mixed with clay-like slurry, which sets when baked. The result is ceramic material with soft-magnetic properties and with high magnetic saturation levels. These cores are good for switching frequencies up to about 400 kHz. From about 10 MHz up to 20 MHz, the core is very lossy. Above 20 MHz the core has little effect and so cannot be used in EMI filtering...

124 Core Shapes and Sizes

For custom inductors and transformers, E-cores are popular. An E-core has two halves that look like a capital E. The center segment is designed to pass through the middle of a bobbin on which the windings are wound. This center segment can be machined to create an air gap, as shown in Figure 12.1, to allow high magnetic flux without saturation of the core. Variations on E-cores are EF and EFD cores. The EFD core is shaped so that the center segment is thinner than the main body of the core, so...

132 Good EMI Design Techniques

It is important to look at the circuit diagram and determine where the possible sources of EMI are located. This should happen before the printed circuit board (PCB) is designed. The center point for EMI sources must be the MOSFET switch. This turns on very quickly and so has sharp edges with high frequency content. When looking at the circuit schematic, consider the effect of high frequencies (1-200 MHz). At very high frequencies, an inductor that was thought to block AC signals suddenly...

1321 Buck Circuit Example

Let us take a look at a simple buck circuit to see where the EMI can arise. Figure 13.1 shows a typical buck circuit. The integrated circuit is a PWM controller. Internally, a clock signal triggers a latch, causing the gate drive output to be activated. The MOSFET Q1 turns on and the current increases at a fairly constant rate, due to the inductance of L1. When the CS pin is raised above 250 mV, due to current in R2, the internal latch is reset and the gate drive output is disabled. The MOSFET...

133 EMC Standards

The EMC performance is often automatically assured by the EMI precautions previously described. If radio frequency signals cannot get out of some equipment, they cannot get in either. However, ESD (electro-static discharge) and surge immunity are two areas that are not taken into account in EMI practices. People generate high electrostatic voltages during normal activities, such as walking across a carpet or opening a plastic envelope. A charged person touching electrical equipment can cause...

134 EMC Practices

Equipment connected to AC mains power lines must be surge tested. The surges are applied, which are added to the normal AC voltage, at times to coincide with different phases of the AC line. The source impedance of the surge test pulse generator is a nominal 50 ohms. The energy in surge pulses can be absorbed or reflected to limit its damaging effects in the equipment under test. Absorbing the energy in surge pulses is the most common method of preventing damage. A varistor, which is a voltage...

141 Efficiency and Power Loss

People sometime refer to LEDs as being a cold light source. This is true in the sense that an element is not heated to thousands of degrees Celsius in order to produce light. However, LEDs do indeed generate heat and this has been the cause of failure of several designs. As a first approximation, the heat generated is voltage drop multiplied by current flow. A white LED with a 3.5 V drop at 350mA will produce about 1.225 W of heat. Actually the emission of photons (light) will reduce this power...

143 Handling Heat Cooling Techniques

If there is high thermal resistance from the source, the source temperature will rise until sufficient heat is dissipated (or until components are destroyed). High temperatures will reduce the reliability of components, so temperatures should be reduced somehow. One obvious cooling technique is to reduce the thermal resistance, and thus dissipate heat easier, by using a heatsink. This is fine if the heat is all generated in one place (like in a MOSFET or a...

151 AC Mains Isolation

Safety isolation can only be achieved with a transformer. This transformer can be placed on the AC mains supply, or as part of the switching regulator circuit. Transformer isolation on the AC mains supply is bulky because the AC signal is operating at 50 Hz or 60 Hz. Conversely, a transformer that isolates the output of a switching regulator can be very small because it is operating at the switching regulator frequency of typically 50 kHz or more. If accurate current control is needed,...

152 Circuit Breakers

In the event of an over-current, the most common circuit breaker is a fuse. This is basically a piece of wire that is heated by the current flow. The wire eventually melts, thus breaking the circuit. Sometimes two wires are joined by solder, one wire being a weak spring as the heat softens the solder joint sufficiently the spring wire pulls the joint apart. Fuses tend to be slow and are rated so that a current twice the normal load may be needed to blow the fuse. Electronic circuit breakers are...

24 Voltage Drop Versus Color and Current

The graph in Figure 2.5 shows how the forward voltage drop depends on the light color and on the LED current. At the point where conduction begins, the forward Typical Forward Voltage, Vf Red 2 V Blue 3.5 V Figure 2.5 Forward Voltage Drop Versus Color and Current. Typical Forward Voltage, Vf Red 2 V Blue 3.5 V Figure 2.5 Forward Voltage Drop Versus Color and Current. voltage drop, Vf, is about 2 V for a red LED and about 3.5 V for a blue LED. The exact voltage drop depends on the manufacturer,...

314Detecting Failures

If we have a short circuit condition in the LEDs, the voltage across the current limiter will increase. We can use this change to detect a failure. In the circuit shown in Figure 3.6 Shorted Load Indication. Figure 3.6, a 10 V Zener diode is used in series with the base of an NPN transistor. When the voltage at the 'IN' terminal of the LM317 reaches about 11V, the Zener diode conducts and turns on the transistor. This pulls the 'FAILURE' line to 0 V and indicates a short circuit across the...

32 Current Source

Since an LED behaves like a constant voltage load, it can be directly connected to a current source. The voltage across the LED, or string of LEDs, will be determined by the characteristics of the LEDs used. A pure current source will not limit the voltage, so care must be taken to provide some limit this will be covered in more detail in the next subsection. If the current source produces much more current than the LED requires, current-sharing circuits will be required. The simplest of these...

322 Voltage Limiting

In theory, the output voltage of a constant current driver is not limited. The voltage will be the product of the current and load resistance in the case of a linear load. In the case of an LED load, the voltage limit will depend on the number of LEDs in a string. In practice, there will be a maximum output voltage, because components in the current source will break down eventually. Limiting the LED string voltage is necessary to prevent circuit damage and the voltage level will depend on the...

412 Voltage Regulators as Current Source or Sink

In Figure 4.3 are shown two circuits using a voltage regulator as a current limiter, one is configured as a current source and the other as a current sink. Figure 4.3 Constant Current Circuits Using the LM317. Figure 4.3 Constant Current Circuits Using the LM317. As previously described, the LM317 regulates when there is + 1.25 V between the OUT and ADJ pins. In Figure 4.3, a current sense resistor (R1) is connected between the OUT and ADJ pins. Current flowing through R1 will produce a voltage...

413 Constant Current Circuits

There are many constant current circuits some using integrated circuits, some using discrete components, and others using a combination of both ICs and discrete devices. In this subsection, we will examine some examples of each type. A simple constant current sink uses an op-amp with an input voltage range that extends to the negative rail, as shown in Figure 4.4. In order to set the current level, a voltage reference is required. The voltage drop across a current sensing resistor is compared...

42Advantages and Disadvantages

The advantage of linear power supplies is that they produce no EMI radiation. This advantage cannot be overstated. A switching power supply may appear to have few components, but this does not take into account the EMI filtering and screening. These additional circuits can double the overall cost of the LED driver. If the LEDs are distributed, such as in channel lighting where there is no opportunity to shield any EMI, both common mode and differential filtering are required. And common mode...

44Common Errors in Designing Linear LED Drivers

The most common error is to ignore the power dissipation. Power dissipation is simply the voltage drop across the regulator multiplied by the current through it. If the voltage drop is high, the current must be limited to stay within the device package power dissipation limits. A surface mount D-PAK package may be limited to about 1W, even when there is some copper area soldered to the tab terminal. Heatsinks are now available for surface mount packages, which eases the problem. Another error...

51 An Example Buck Converter Control IC

The Supertex HV9910B integrated circuit was designed especially for LED driving. It is a good example of a low cost, low component count solution to implement the continuous mode buck converter (the IC itself needs just three additional components to operate). Linear or PWM dimming can also be easily implemented using the IC. A diagram of the HV9910B is shown in Figure 5.2. The HV9910B has two current sense threshold voltages - an internally set 250 mV and an external voltage at the LD pin. The...

522Choosing the Switching Frequency and Resistor R1

The switching frequency determines the size of the inductor L1. A larger switching frequency will result in a smaller inductor, but will increase the switching losses in the circuit. A typical switching frequency for low input voltage applications is fs 150 kHz, which is a good compromise. From the HV9910B datasheet, the timing resistor between the RT pin and ground that is needed to achieve this frequency is 150 k . However, in this case, the minimum input voltage is only 80 of the maximum...

533Choosing the Input Diode Bridge D1 and the Thermistor NTC

The voltage rating of the diode bridge will depend on the maximum value of the input voltage. A 1.5 multiplication factor gives a 50 safety margin. Vbridge 1-5 X (V2 X Vmax,ac) 562 V The current rating will depend on the highest average current drawn by the converter, which is at minimum input voltage (DC level, allowing for a 'droop' across the input capacitor between the AC line voltage peaks) and at maximum output power. The minimum input voltage must be more than half the maximum LED string...

534 Choosing the Input Capacitors C1 and C2

The first design criterion to meet is that the maximum LED string voltage must be less than half the minimum input voltage. This is to satisfy the stability requirements when operating at a constant switching frequency. As we have already seen, the minimum rectified voltage should be The hold-up capacitor required at the output of the diode bridge will have to be calculated at the minimum AC input voltage. The capacitor can be calculated as The voltage rating of the capacitor should be more...

535 Choosing the Inductor L1

The inductor value we use depends on the allowed level of ripple current in the LEDs. Assume that 15 ripple (a total of 30 ) is acceptable in the LED current. The familiar equation for an inductor is E L x d. Considering the time when the MOSFET switch is off, so that the inductor is supplying energy to the LEDs, E Vled Vo,max L x d. Another way of writing this is L Vo,max x d. Here, di is the ripple current 0.3 x o,max and dt is the off-time dt v xVac.n m Note, a buck circuit duty cycle is...

54 Buck Circuits Powered by an AC Phase Dimmer

An LED driver powered by an AC phase dimmer needs special additional circuits. These additional circuits are required because of the phase dimmer circuit. Phase dimmers usually use a triac activated by a passive phase shift circuit. Because of switching transients, which would otherwise cause serious EMI problems, the triac is bypassed by a capacitor (typically 10 nF) and has an inductor in series with its output. The phase dimmer circuit is shown in Figure 5.5. The input of an inactive LED...

61 Boost Converter Operating Modes

A boost converter can be operated in two modes - either continuous conduction mode (CCM) or discontinuous conduction mode (DCM). The mode of operation of the boost converter is determined by the waveform of the inductor current. Figure 6.2(a) is the inductor current waveform for a CCM boost converter whereas Figure 6.2(b) is the inductor current waveform for a DCM boost converter. The CCM boost converter is used when the maximum step-up ratio (ratio of output voltage to input voltage) is less...

721 Basic Sepic Equations

The boost or step-up topology, as shown in Figure 7.13, is the basis for the SEPIC converter. The boost-converter principle is well understood first, switch Q1 conducts during the on-period, TON, which increases the current in L1 and thus increases the magnetic energy stored there. Second, the switch stops conducting during the off-period, TOFF, but the current through L1 cannot change abruptly - it continues to flow, but now through diode D1 and into Cout. The current through L1 decreases...

91 Two Winding Fly Back

A schematic of a typical fly-back circuit for driving LEDs is shown in Figure 9.2. The dot alongside the transformer winding indicates the start of the winding. In this case the start is connected to the MOSFET drain, which alternates between a ground connection and open circuit. The voltage at the drain, and hence the winding start point, varies considerably during switching. Conversely, the outer layer (end of the winding) is at a fixed high voltage and tends to screen the inner layers, which...

911 Fly Back Example

Let us make an isolated 3 W lamp by connecting three white power LEDs in series. Suppose we have a primary voltage of 48 V, an on-time of 5 microseconds, and the primary to secondary turns ratio is 1 0.1. If we are driving a 10 V LED load, the off-time will be 240 100 microseconds (2.4 ms). Thus the switching period must be greater than 12.4 ms in order to allow complete removal of the magnetic energy in the transformer core. A switching frequency of below 65 kHz will be satisfactory, say 60...

92 Three Winding Fly Back

Some fly-back power supplies use a third winding, called a bootstrap or auxiliary winding, as shown if Figure 9.3. This is used to power the control IC, once the circuit is operating. The bootstrap winding has the same orientation as the secondary winding and the voltage is simply determined by the turns ratio of the bootstrap compared to the secondary. In our example of a 10 V output from the secondary, the bootstrap could have the same number of turns and thus give (approximately) 10V for the...

921 Design Rules for a Fly Back Converter

This section gives design rules for a fly-back converter based on either turns ratio selection determined by the maximum duty cycle allowed (case 1), or by the optimum turns ratio based on the maximum working voltage of the MOSFET switch (case 2). In case 1, a design based on the maximum duty cycle (at the lowest input voltage) Figure 9.3 Fly-Back Using a Three-Winding Transformer. Figure 9.3 Fly-Back Using a Three-Winding Transformer. allows the widest input voltage range. In case 2, a design...

And Transformers

Standard off-the-shelf transformers and inductors were described in Chapter 11. This chapter will describe magnetic materials and techniques for constructing custom transformers and inductors. The primary design requirement is to minimise losses, but to do this we have to consider copper losses, core losses, magnetic saturation, size and construction. Since this book is about designing LED drivers, only the basics of magnetic materials will be given here. For more detail, the reader should...

Buck Based LED Drivers

The first switching LED driver that we will study is the buck converter. The buck converter is the simplest of the switching drivers, and is a step-down converter for applications where the load voltage is never more than about 85 of the supply voltage. The limit of about 85 is due to switching delays in the control system. In a buck converter circuit, a power MOSFET is usually used to switch the supply voltage across an inductor and LED load connected in series. The inductor is used to store...

Fly Back Converters

A traditional fly-back converter uses an inductor with at least two windings (really, this is a transformer). Consider two windings one is the primary, which is connected to the input power supply and a switch to ground the other is the secondary, which is connected to the load. The circuit is arranged so that magnetic energy is stored in the inductor during the time that the switch is on, when current increases in the primary winding. When the switch is off, the magnetic energy is released by...

Index

AC Input, 46-60, 167-68 Active current control, 20-4, 172-73 Backlight, 12, 168 Bi-bred, 142-44 Bifilar, 137, 196-97 Bipolartransistor, 119, 131, 179-80 Blanking time, 166 Bobbin, 187 Boost, 61-98, 168 Boost-buck, 169 Buck, 39-60, 163-68 Buck-boost, 139, 169 Buck-Boost & Buck (BBB), 144-46 182-87, 226-27 Carbon film, 190 Ceramic, 183-86, 227 Channel lighting, 10 Circuit breaker, 226 Clamping circuit, 83 Color, 9, 14-5 Common-mode choke, 169, 212-13 Comparator, 193-94 Compensation network,...

Introduction

As a field applications engineer for one of the pioneering developers of integrated circuits for driving power LEDs, I meet many potential customers who have little or no idea of how to drive an LED properly. The older type of LED requiring a 20 mA supply can be abused to some extent. However, power requirements have been increasing current ratings of 30 mA, 50 mA, 100 mA, 350 mA and higher are becoming common. There are several manufacturers that produce power levels up to 20 W, and more these...

J

In case of an output over-voltage condition or an output short circuit condition, the FAULT pin is pulled low and an external MOSFET switched off to disconnect the LEDs. The FAULT pin is also controlled by the PWM dimming signal, so that the pin is high when the PWM dimming signal is high and vice versa. This disconnects the LEDs and makes sure that the output capacitor does not have to be charged discharged every PWM dimming cycle. The PWM dimming input to the...

Power Supplies for LED Driving

AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA Linacre House, Jordan Hill, Oxford OX2 8DP, UK Copyright 2008 by Elsevier Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher....

Preface

The LED has been available for many years now, initially as a red colored indicator. Later, yellow amber, green and finally blue colored LEDs became available, which triggered an explosion in applications. Applications included traffic lights, vehicle lights and wall-washes (mood lighting). Recently blue colored LEDs have been combined with yellow phosphor to create white light. The amount of light available from LEDs has also increased steadily, and now power levels of 1W, 3 W and 5 W are...

Table of Contents

Preface Chapter 1 1.1 Objectives and General 1.2 Description of Chapter 2 Characteristics of 2.1 Applications for 2.2 Light 2.3 Equivalent Circuit to an 2.4 Voltage Drop Versus Color and 2.5 Common Chapter 3 Driving 3.1 Voltage 3.2 Current 3.3 Testing LED 3.4 Common Chapter 4 Linear Power 4.4 Common Errors in Designing Linear LED 37 Chapter 5 Buck-Based LED 5.1 An Example Buck Converter Control 40 5.3 Buck Circuits for AC 5.4 Buck Circuits Powered by an AC Phase 5.5 Common Errors in AC Input...

0821

The input current level at the minimum input voltage should be calculated first, because this gives the highest current level. The value obtained will be used to work out the current ratings of the various components. The first step is to compute the off-time. The off-time of the converter can be calculated as Assuming a 25 peak-to-peak ripple in the output current (Aio 87.5 mA), and accounting for the diode drop in the input voltage by substituting V n min Vd in place of Vin, yields 598 ns...

62 HV9912 Boost Controller

Supertex's HV9912 integrated circuit is a closed-loop, peak current controlled, switch-mode converter LED driver. The HV9912 has built-in features to overcome the disadvantages of the boost converter. In particular, it features a disconnect MOSFET driver output. The external MOSFET driven from this output can be used to disconnect the LED strings during short circuit, or input over-voltage, conditions. This disconnect MOSFET is also used by the HV9912 to dramatically improve the PWM dimming...

81 Power Factor Correction

Power factor correction, or PFC, is a term used with AC mains powered circuits. A good power factor is when the AC current is in phase with the AC voltage. A pure resistive load has a power factor of 1, but active loads tend to have power factors closer to 0.5, unless special measures are taken to 'correct' this. The most common power factor correction circuit is a boost converter. The AC line voltage is boosted to about 400 V and the amplitude of the current pulses into a storage capacitor is...

312 Active Current Control

Since a series resistor is not a good current control method, especially when the supply voltage has a wide tolerance, we will now look at active current control. Active current control uses transistors and feedback to regulate the current. Here we will only consider limiting LED current when the energy is supplied from a voltage source driving LEDs using energy from current sources will be discussed in Section 3.2. A current limiter has certain functional elements a regulating device such as a...

64 Design of a Discontinuous Conduction Mode Boost LED Driver

As a reminder, discontinuous mode is used when the output voltage is more than six times the input voltage. Input voltage range 9-16 V LED string voltage range 30-70 V (Note, with 9 V input and 70 V output, the Vo Vin ratio is approximately 7.8) A typical circuit for a discontinuous mode boost converter, using the HV9912 IC identical to the continuous mode circuit shown in Figure 6.5, but repeated here for convenience, in Figure 6.9. 6.4.3 Selecting the Switching Frequency (fs) For low voltage...

1023 Boost Buck Regulator Considerations

To operate in an environment where the input voltage could be higher or lower than the output voltage, a buck-boost (or boost-buck) circuit is necessary. Boost-buck circuits were described in Chapter 7. The situation of having a load voltage range that overlaps the supply voltage range is commonly found in automotive applications. The battery voltage rises and falls with a large variation, as the engine speed and battery conditions change. The two types of converters often found in boost-buck...

72 Sepic Buck Boost Converters

The abbreviation SEPIC comes from the description Single Ended Primary Inductance Converter. A SEPIC is a boost-buck converter, like a Cuk, so its input voltage range can overlap the output voltage. SEPIC circuits can be designed for constant voltage or constant current output. The SEPIC topology has been known for some time, but only recently has there been a revival in its application because (a) it needs low ESR capacitors and these are now widely available and (b) it can be used to create...

21 Applications for LEDs

Soon new semiconductor materials were developed and gallium arsenide phosphide (GaAsP) was used to make LEDs. The energy gap in GaAsP material is higher than GaAs, so the light wavelength is shorter. These LEDs produced a red color light and were first just used as indicators. The most typical application was to show that equipment was powered, or that some feature such as 'stereo' was active in a radio. In fact it was mainly consumer products like radios, tape recorders and music systems that...

63 Design of a Continuous Conduction Mode Boost LED Driver

As a reminder, continuous conduction mode is valid when the output voltage is between 1.5 and 6 times the input voltage. LED string dynamic impedance 18 ohms A typical boost converter circuit is shown in Figure 6.5. Figure 6.5 Continuous Mode Boost Converter. Figure 6.5 Continuous Mode Boost Converter. 6.3.3 Selecting the Switching Frequency (fs) For low voltage applications (output voltage < 100 V), and moderate power levels (< 30W), a switching frequency offs 200 kHz is a good compromise...

71 The Cuk Converter

In spite of the many advantages of the Cuk converter, a couple of significant disadvantages exist which prevent its widespread use. The converter is difficult to stabilize. Complex compensation circuitry is often needed to make the converter operate properly. This compensation also tends to slow down the response of the converter, which inhibits the PWM dimming capability of the converter (essential for LEDs). An output current controlled boost-buck converter tends to have an uncontrolled and...

331 Zener Diodes as a Dummy Load

Figure 3.9 shows how Zener diodes can be used as a dummy load. This is the simplest and cheapest load. The 1N5334B is a 3.6 V, 5 W Zener diode 3.6 V typical at 350mA . This is not the perfect dummy load. This reverse voltage is slightly higher than the typical forward voltage of 3.42 V of a Lumileds 'Luxeon Star' 1 W LED. The 1N5334B has a dynamic impedance of 2.5 ohms, which is higher than the Luxeon Star's 1 ohm impedance. The impedance will have an effect on some switching LED drivers that...

153 Creepage Distance

In most electrical circuits connected to the AC mains supply, creepage distance is a concern. The concern is two-fold electrocution or fire - for example, a loose piece of solder could short out a pin carrying high voltage to another low voltage point in the circuit or moisture and dust could bridge the gap and allow a current to flow. In either example, the current may not be high enough to blow the fuse, but could be lethal to the user through electrocution or toxic smoke inhalation. The...

1027 Soft Start Techniques

Poor Sampling Technique

Some applications need the input current to be controlled, to prevent high current spikes when power is first applied. This could be to reduce damage to switch contacts by the risk of sparking. Clearly the inrush techniques just described could be used, but sometimes it is necessary to control the output power instead. For example, a circuit for driving one or two power LEDs from the AC mains could use a double-buck topology. But typical applications for this circuit are inside lamp housings,...

56 Double Buck

Single Switch Buck Boost

The double buck is an unusual design, as shown in Figure 5.9. It uses one MOSFET switch, but two inductors L2 and L3 in series. Diodes steer the current in L2, which must operate in discontinuous conduction mode DCM for correct operation. The double buck is used when the output voltage is very low and the input voltage is high. An example is driving a single power LED from an AC supply line. A single buck stage cannot work easily because the on-time of the buck converter is too small, unless a...

526 Choosing the Sense Resistor R2

The sense resistor value is given by This is true if the internal voltage threshold of 0.25 V is being used. Otherwise, substitute the voltage at the LD pin instead of the 0.25 V into the equation. Note that the current limit is set to 15 above the maximum required current, due to the total 30 ripple specified. For this design, R2 0.625 U. The nearest standard value is R2 0.62 U. If a standard value is not close to the value calculated, or if a lower power dissipation in the sense resistor is...