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...

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,...

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

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...