## 101 Linear Regulators

In Chapter 4 we saw how the use of linear regulators caused a heat dissipation problem because of low efficiency. A linear LED driver is generally less efficient than a switching driver. Sometimes a linear driver can be more efficient. For example, if you have a 12 V power source and three LEDs each having a 3.5 V forward drop, by connecting them in series the total drop is 10.5 V. The efficiency of a linear driver, dropping only 1.5 V will be 87.5 . It would be difficult for a switching LED...

## 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...

## 1021 Buck Regulator Considerations

In Chapter 5 we first looked at the simplest switching regulator, the buck converter. In a buck circuit the load voltage must be less than 85 of the supply voltage, otherwise the output becomes difficult to control. Buck circuits are used for mains powered LED drivers, when driving a long string of LEDs. Buck circuits are also used where the input supply voltage is relatively low, say in a 12 V DC automotive application, but where just one LED is being driven. Buck regulators can be very...

## 1022 Boost Regulator Considerations

The output voltage in a boost circuit must always be higher than the input voltage by about 20 or more, and this was discussed in Chapter 6. Ignoring PFC applications, a boost converter driving LEDs will always be powered from a low voltage DC supply. For example, the backlight in a cell phone with a color LCD display usually employs low cost white light LEDs. A boost regulator is used in this application to drive a string of 20 mA LEDs from a 3-4 V battery. As another example, in flat-screen...

## 1024 Circuits with Power Factor Correction

Power factor is an indication of the relative phase of the power line voltage and the power line current. A power factor of 1 indicates that the voltage and current are in-phase and have low harmonic content. A power factor of 0 indicates that the voltage and current are 90 degrees out-of-phase. In semiconductor circuits powered from the AC mains, a bridge rectifier converts the AC power into DC. The current through the bridge rectifier tends to occur close to the peak voltage, as shown in...

## 1025 Fly Back Converter Considerations

Transformer coupled switching regulators can be designed for a very wide range of supply and output voltages. The most common is a fly-back converter, although forward converters are also popular in higher power applications. Fly-back converters were described in Chapter 9. Fly-back converters allow an isolated LED driver design with about 90 efficiency, but have added cost and complexity. If a wide tolerance can be accepted for the current regulation, a simpler and cheaper circuit can be...

## 11 Objectives and General Approach

The approach of this book will be very practical, although some theory is introduced when necessary for understanding of later chapters. It is important to understand the characteristics of components before they can be used effectively. In most chapters, I will include a section called 'Common Errors'. This section will highlight errors that engineers have made, and how these can be avoided, with the hope that readers will not make the same mistakes. It is said that people learn from their...

## 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...

## 112 Passive Components 1121 Capacitors

In an LED driver, the key function of a capacitor (symbol C) is energy storage. There are two types of storage, slow storage and fast storage. Slow storage is required across the DC terminals of a bridge rectifier, when the LED driver is powered from a low frequency AC supply. The purpose of this storage is to supply energy to the LED driver between the peaks of the AC voltage, which is twice every cycle. The AC frequency is typically 50-60 Hz, although 400 Hz is used in some aircraft, so 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...

## 113 The Printed Circuit Board PCB

Regulations on the use of tin-lead solder have come into force for most applications, for human health reasons. The notable exceptions are military and (ironically) medical applications, although these will be forced to change due to the lack of RoHS lead-free components. Heavy metals and carcinogenic materials will not be allowed in electronic products, including IC packages. This means that soldering profiles have to change - higher temperatures are needed for lead-free solder. The circuit...

## 1131Through Hole PCBs

It is usual for an RF or high speed digital circuit to have an earth plane on the printed circuit board (PCB) component side. In many cases, an LED driver can be considered as a high speed digital circuit. The earth plane serves two purposes it screens the components from tracks passing underneath, and it provides part of a low loss transmission line. By using FR4 board in a standard thickness of 1.6 mm, 50 ohm transmission lines can be created by making the printed circuit tracks 2.5 mm wide....

## 1132Surface Mount PCBs

Surface mount components are used extensively in LED driver circuits. Ceramic capacitors are common but can be damaged by stress due to circuit board expansion. One method of minimizing this problem is to use physically small devices devices larger than 1812 (0.18 x 0.12inches) should be avoided. Ceramic capacitors should be protected with a moisture resistant coating. If moisture is absorbed into the ceramic material, the capacitance value will change. Moisture can also be absorbed into...

## 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...

## 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...

## 126 Copper Losses

Copper loss is the term used to describe the energy dissipated by resistance in the wire used to wind a coil. In 99.9 of cases this wire will be made of copper, whose resistivity at 20 C is about 1.73 x 10 8ohm meter. However, coils often have to operate above room temperature and will be heated by the operating losses in any case. The wire resistance at any temperature can be estimated from Table 12.1, developed by Mullard (now Philips). Table 12.1 Wire Resistance Versus Temperature. Table...

## 1311 AC Mains Connected LED Drivers

Any LED driver connected to AC mains supply has to meet the limited specified in harmonic current emissions standard IEC EN 61000-3-2. Within this standard there are several classes and the one related to lighting is Class C. The harmonic emission limits specified in IEC EN 61000-3-2, Ed. 2 2000, up to the 40th harmonic, are listed in Table 13.1. Maximum current, Class C (percentage of fundamental current) Conducted emission limits in the 150 kHz to 30 MHz frequency range are specified in the...

## 1312 General Requirements for all Equipment

All LED drivers have to meet the radiated emissions standards. The standard is IEC EN 61000-6-3, which covers the frequency range 30 MHz to 1 GHz. This standard uses limits previously set by CISPR22 in the USA and by the European Norm EN55022. The limits given in CISPR22 and EN55022 standards were intended for computers and communications related equipment, but these have been adopted as generic limits for all electronic products, including lighting. The emission levels to meet EN55022 CISPR22...

## 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...

## 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...

## 142 Calculating Temperature

The temperature of a device can be calculated using simple 'Ohm's law' type mathematics. Temperature can be seen as being equivalent to a voltage. Thermal resistance can be equated to electrical resistance. Heat flow (watts) can be regarded as the equivalent to electrical current, see Figure 14.2. Figure 14.2 Electrical Equivalent Calculations. Like electrical resistance, thermal resistances can be added when connected in series (see Figure 14.3). Consider a T0-220 package mounted onto an...

## 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,...

## 155 Low Voltage Operation

The UL1310 Class 2 regulations and the European EN60950 safety standard (also known as IEC 60950) are generally applicable to any electronic circuit. The EN60950 standard was originally intended for information technology equipment (i.e. computers and associated hardware) but, since it is one of the few 'harmonized' standards that have been agreed by all of Europe and many other countries in the world, it has been used as a reference for most safety regulations. If equipment complies with...

## 22 Light Measure

The total light flux is measured in units of lumens. The lumen is the photometric equivalent of 1 watt, weighted to match the normal human eye response. At 555 nm, in the green-yellow part of the spectrum where the eye is most responsive, 1W 683 lumen. The term candela is also used. This is the light produced by a lamp, radiating in all directions equally, to produce 1 lumen per steradian. As an equation, 1 cd 1lm sr. A steradian has a projected area of 1 square meter, at a distance of 1 meter...

## 23 Equivalent Circuit to an LED

An LED can be described as a constant voltage load. The voltage drop depends on the internal energy barrier required for the photons of light to be emitted, as described earlier. This energy barrier depends on the color thus the voltage drop depends on the color. Will every red LED have the same voltage drop No, because production variations will mean that the wavelength (color) will not be the same, and thus the voltage drop will have differences. The peak wavelength has typically a 10...

## 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,...

## 311 Passive Current Control

Although the LED voltage drop shifts the curve of the graph to the right, the slope of the graph is just due to the ESR. Low current loads can have a relatively high value resistance added in series, in order to reduce the slope of the current versus voltage graph see curve B in Figure 3.1. With a series resistor added we are able to calculate the variation in current, provided that the variation in supply voltage and load voltage is known. In the equations below, the load voltage includes the...

## 313Short Circuit Protection

The current limiting circuits described in the previous subsection will provide automatic short circuit protection. If the LED goes short circuit, a higher voltage will be placed across the current limiter. Power dissipation is the main issue that needs to be addressed. If the power dissipation cannot be tolerated when the load goes short circuit, a voltage monitoring circuit will be needed. When a higher than expected voltage is placed across the current limiter, the current must be reduced to...

## 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...

## 324Detecting LED Failures

In a constant current circuit, a failure of an LED can mean that either a whole string is off (open circuit LED) or a single LED is off (short circuit LED). In the case of an open circuit LED, the load is removed and so the output voltage from the current source rises. This rise in voltage can be detected and used to signal a failure. In circuits where over-voltage protection is fitted, this can be used to indicate a failure. If a current mirror is used to drive an array of LEDs with a number...

## 33 Testing LED Drivers

Although testing an LED driver with the actual LED load is necessary, it is wise to use a dummy load first. There are two main reasons for this (1) cost of an LED, especially high power devices, can be greater than the driver circuit and (2) operating high brightness LEDs for a long time under test conditions can cause eye strain and temporary sight impairment (if LEDs viewed at close range). A further reason is that some dummy loads can be set to limit the current and so enable fault-finding...

## 411 Voltage Regulators

Many voltage regulators are based on the LM317 originally from National Semiconductor, but which is now made by a number of manufacturers. Inside the LM317 are (1) a power switch, which is an NPN transistor (2) a voltage reference set to produce 1.25 V and (3) an operational amplifier (op-amp) to control the power switch, as shown in Figure 4.1. The op-amp tries to keep the voltage at the output equal to the voltage at the adjust (ADJ) pin minus the reference voltage. To produce a certain...

## 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...

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...

## 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...

## 523Choosing the Input Capacitor C1

An electrolytic capacitor is good to hold the voltage, but the large ESR of these capacitors makes it unsuitable to absorb the high frequency ripple current generated by the buck converter. Thus, metallized polypropylene capacitors or ceramic capacitors in parallel are needed to absorb the high frequency ripple current. The required high frequency capacitance can be computed as In this design example, the high frequency capacitance required is about 4.7 mF 50 V. This capacitor should be located...

## 524 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 Lx 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 Io,max and dt is the off-time. Then, the inductor Ll can be computed at the...

## 525 Choosing the Mosfet Q1 and Diode D2

The peak voltage seen by the MOSFET is equal to the maximum input voltage. Using a 50 safety rating. The maximum RMS current through the MOSFET depends on the maximum duty cycle, which is 80 in our example. Hence, the current rating of the MOSFET is Ifet Io,max x 0.8 0.28 A. Typically a MOSFET with about three times the current is chosen to minimize the resistive losses in the switch. For this application, choose a 50 V, > 1 A MOSFET a suitable device is a Supertex part, VN3205N8, rated at 50...

## 527 Common Errors in Low Voltage Buck Design

Using an inductor that has too high inductance. Although increasing the inductor value may seem to be the answer to reduce current ripple, it actually causes problems because the current does not fall enough between switching cycles for proper control by the controller IC. The voltage seen across the current sense resistor at switch-on will be almost at the current sense comparator reference voltage. At switch-on there will be a current surge, caused by the flywheel diode reverse current and...

## 53 Buck Circuits for AC Input

I will now discuss the design of a buck-based LED driver using the HV9910B with the help of an AC mains input application example. The same procedure can be used to design LED drivers with other input voltage ranges. The schematic is shown in Figure 5.4. Figure 5.4 Universal Mains Input Buck Circuit. Figure 5.4 Universal Mains Input Buck Circuit. Designs for an AC input have two problem areas to address. In addition to considering the LED driving aspects, we must also consider the low frequency...

## 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...

## 536 Choosing the Mosfet Q1 and Diode D2

The peak voltage seen by the MOSFET is equal to the maximum input voltage. Using a 50 safety rating, The maximum RMS current through the MOSFET depends on the maximum duty cycle, which is 50 by design. Hence, the current rating of the MOSFET is Typically a MOSFET with about three times the current is chosen to minimize the resistive losses in the switch. For this application, choose a 600 V, > 1 A MOSFET a suitable device is an ST part, STD2NM60, rated at 600 V 2 A. This MOSFET has 2.8 U...

## 537 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. A lower voltage could be applied to the LD pin to enable a convenient value of R2 to be used, as described earlier. For this design, R2 0.625 U. The nearest standard value is R2 0.62 U. Note that capacitor C3 is a bypass capacitor for holding up the HV9910B internal supply VDD during MOSFET...

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...

## 55 Common Errors in AC Input Buck Circuits

The most common error is trying to drive a single LED from the AC mains supply. The duty cycle is Vout Vin, so for universal AC input 90 V to 265 V AC, the rectified voltage is about 100 V to 375 V. The worst case is the higher voltage consider driving a white LED with 3.5 V forward voltage. The duty cycle will be 3.5 375 0.9333 duty cycle. If the switching frequency is 50 kHz, with 0.02 ms second period, the MOSFET on-time will be just 186 ns. This time is too short for the current sense...

## 57 Hysteretic Buck

As an alternative to the peak current control buck, hysteretic control can be used. This uses a fast comparator to drive the MOSFET switch. The input to the comparator is a high side current sense circuit, where the voltage across a resistor in the positive power feed to the LED load is monitored. This is shown in Figure 5.10. Figure 5.10 Hysteretic Current Control Circuit. Figure 5.10 Hysteretic Current Control Circuit. The MOSFET is turned on when the current level is at or below a minimum...

## 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...

## 65Common Mistakes

The most common mistake is not having adequate over-voltage protection at the output. If the LEDs are disconnected while the circuit is operating, the output voltage will rise until components start to break down. The over-voltage limit set at the output of the boost converter should be lower than the breakdown voltage of any component connected across it. 2. Testing the circuit with a short string of LEDs. The forward voltage drop may be lower than the supply voltage, and in this case there is...

## 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...

## 74Common Mistakes in Boost Buck Circuits

Boost-buck circuits operate with both inductors in continuous conduction mode. Hence the inductor should be chosen with a value higher than that calculated, to allow for tolerances and for saturation effects (the inductance falls with increasing current). Calculate the value, add 20 , and then pick the next highest standard value. Current ratings of inductors are given for a certain temperature rise in the core, typically 40 C. So if temperature rise is an issue, pick a component with a higher...

## 82 BiBred

The Bi-Bred is very similar to the Cuk boost-buck that we described in the previous chapter, see Figure 8.2. The main difference between the Cuk and the Bi-Bred is that, in a Bi-Bred, the input inductor is in discontinuous conduction mode (DCM) and operation of the output stage is in continuous conduction mode (CCM). The energy stored in each inductor is proportional to the inductance value. This means that in the design, the input inductor L1 must have a small enough energy stored to ensure...

## 83 Buck BoostBuck BBB

The Buck-Boost-Buck (BBB) is a proprietary circuit, patented by Supertex, and is illustrated in Figure 8.3. It resembles the Bi-Bred in some respects, except for two current steering diodes D1 and D2. RT GATE GND VIN HV9931 CS2 CS1 VDD PWM RT GATE GND VIN HV9931 CS2 CS1 VDD PWM Figure 8.3 Buck-Boost-Buck Circuit. Like the Bi-Bred, the input inductor is in discontinuous conduction mode (DCM) and operation of the output stage is in continuous conduction mode (CCM). The energy stored in each...

## 84Common Mistakes with PFC Circuits

The most common mistake is to use a standard inductor for L1. Inductors are sized for their magnetic saturation level and for resistive heating. Thus an inductor may be specified as I(av) 500 mA, I(sat) 400 mA. This inductor can pass 500 mA with a temperature rise of, say, 40 C. It can pass 400 mA before the inductance is reduced by, say, 10 . If this inductor were used in a PFC stage with a peak current of 400 mA it would overheat. Using an inductor with a much higher saturation current rating...

## 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...

## 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...

## 93 Single Winding Fly Back Buck Boost

In the buck-boost converter, a single inductor winding is used for the primary and secondary. This is shown in Figure 9.4. Current is forced through the inductor by a MOSFET connecting the inductor across the power supply rail. The current level rises almost linearly with time. At a predetermined current level, the MOSFET is turned off and the current is forced to flow through a diode to charge the output capacitor and drive the load. The current in the inductor falls back to zero and so...

## Aio

To find the value of L2 using the time delay equations above results in a cubic equation. This cubic has one real root and two complex roots. The inductor value is the real root of the cubic raised to the third power. The actual off-time Toff,ac can be computed by substituting the chosen inductor value back into the equations for Tr, Tf and Tf2, to get Tr ac, Tf ac and Tf2,ac. TT Tr,ac Tf2,ac Tf,a V o The actual ripple in the inductor current Aio,ac is 7.1.4 Stability of the Boost-Buck...

## 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...

## Bibliography

Power Supply Cookbook. Woburn MA Newnes. 2. Pressman, Abraham I. 1998. Switching Power Supply Design. New York McGraw-Hill. 3. Billings, Keith. 1999. Switch-Mode Power Supply Handbook. New York McGraw-Hill. 4. Harrison, Linden T. 2005. Current Sources & Voltage References. Burlington MA Newnes. 5. Zukauskas, Arturas, Shur, Michael S. and Gaska, Remis. 2002. Introduction to Solid State Lighting. New York Wiley Interscience. 6. Kervill, Gregg. 1998. Practical Guide to the...

## Boost Converters

Boost converters (see Figure 6.1) are ideal for LED driver applications where the LED string voltage is greater than the input voltage. Normally, a boost converter would only be used when the output voltage minimum is about 1.5 times the input voltage. The converter can easily be designed to operate at efficiencies greater than 90 . Both the MOSFET and LED string are connected to a common ground. This simplifies sensing of the LED current, unlike the buck converter where we have to choose...

## Boost Buck Converter

A boost-buck converter is a single-switch converter, which consists of a cascade of a boost converter followed by a buck converter. The power train of typical boost-buck circuit topology (used as an LED driver) is shown in Figure 7.1. Figure 7.1 Boost-Buck (Cuk) Power Train. Figure 7.1 Boost-Buck (Cuk) Power Train. The converter can both boost and buck the input voltage. Thus, it is ideal for cases where the output LED string voltage can be either above or below the input voltage during...

## 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...

## Characteristics of LEDs

Most semiconductors are made by doping silicon with a material that creates free negative charge (N-type), or free positive charge (P-type). The fixed atoms have positive and negative charge, respectively. At the junction of these two materials, the free charges combine and this creates a narrow region devoid of free charge. This 'intrinsic region' now has the positive and negative charge of the fixed atoms, which opposes any further free charge combination. In effect, there is an energy...

## EMI and EMC Issues

The first two questions regarding EMI and EMC are what is the difference between EMI and EMC And which standards apply Subsequent questions relate to how equipment can be made to meet the standards. Of course, meeting the standards often costs money (filter components, screening and suppressors) so the aim is to just meet the standards with a small safety margin. EMI is electro-magnetic interference. This is the amount of radiation emitted by some equipment when it is operating. EMI is caused...

## 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...

## Gpssc 2kHz Aps 040996

To get a phase margin of about 45 (the recommended phase margin range is 45 -60 ), the phase boost required will be Based on the value of the phase boost required, the type of compensation can be determined. boost < 0 ) Type I controller 0 < boost < 90 ) Type II controller 90 < boost < 180 ) Type III controller Type-III controllers are usually not required to compensate an HV9912-based boost LED driver and thus will not be discussed here. The implementations for the Type-I and Type-II...

## 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 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...

## 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...

## 1027 Soft Start Techniques

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,...

## 1026 Inrush Limiters

Because almost all circuits have decoupling capacitors, when a power source is connected there will be an inrush current. This current can be very high, causing temporary heating in the capacitor and possible damage to switch contacts or components connected in series. Inrush current limiting using passive or active components can be provided to reduce this risk. For AC mains applications, an NTC thermistor designed to carry high current is often used. In the active state, the flowing current...

## 56 Double Buck

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...