Problemas ambientales colombianos

For the off-time the duty cycle definition of (4.26) can also be expressed as:

toff = (1 − D) tc (4.31)

Voltage to Voltage converter From equations (4.25), (4.29) and the duty cycle definition (4.26)

and (4.31) a steady-state voltage to voltage converter formula can be derived:

Uin

Uout

= 1 − D (4.32)

From (4.32) it can be seen that the output voltage can be as low as the input voltage. It can also be observed that a duty cycle D = 1 is not possible, if an input voltage is present.

LED current sensing and control To sense the current flowing through the LED, a resistor R1 is

used as a shunt, referring to Figure 29. The measured current is used for control. The PWM waveform typically is generated by a fixed frequency PWM with variable duty cycle. The duty cycle typically is determined by a PID controller.

Summary The boost converter is a well known topology for increasing the output voltage. The

efficiency of the boost converter highly depends on the increase in voltage. For the most efficient design, the output voltage should only be increased slightly. As the boost converter ratio increases, it efficiency typically decreases. Despite the number of LEDs is not limited theoretically, efficiency wise the maximal number of LEDs is limited.

4.5

High Voltage Topologies without Galvanic Isolation

High voltage power supplies typically are used in grid connected LED luminaires. The metal case of non-galvanic isolated LED drivers should be directly connected to ground for safety reasons.

4.5.1 Capacitive Power Supply

A low cost option for driving LED luminaries is the use of a capacitive power supply. A typical circuit diagram is shown in Figure 31.

Operation principle In Figure 31, the AC power line voltage is reduced by the reactance of C1. R2

is used for to limit the circuits inrush current. The AC input current is rectified by the diode bridge DB1and the double mains frequency ripple is smoothed by C2. D1is used to protect C2against over

4.5 High Voltage Topologies without Galvanic Isolation 4 OVERVIEW DRIVER TOPOLOGIES

C1

R1

-

+

DB1

R2

C2

D1

D2

R3

Figure 31: Capacitive voltage dropper for driving LEDs. The voltage is lowered by the capacitor C1 and R2.

voltages. R3limits the current flowing through the LED D2. R1is required to discharge the capacitor

C1, when the LED luminary is removed from the socket.

Lifetime In this solution the C1capacitor faces surges of the AC grid. As self-healing class X safety

capacitors are used, they will not be destroyed. However, an overvoltage event reduces its capacitance. Thus, the reactance increases, leading to lower LED supply current. Therefore, series capacitor power supplies tend to reduce the LED drive current over time.

Further, as DC link capacitor C2 in most cases an electrolytic capacitor is used. As LED PCBs tend

to get hot, the electrolytic capacitor aging is accelerated and it fails subsequently. The fundamentals of lifetime calculus are discussed in section 6.

Summary As a capacitor is used for voltage reduction, this solution requires a significant amount of

reactive power. Further, the LED current cannot be controlled actively by using semiconductor devices. Thus, a changing grid frequency or grid voltage may change the LED current. When the LEDs forward voltage decreases, e.g. one LED of the string fails to short circuit, the LED current will increase.

If more output current is required a physically larger capacitor must be used. Typically this limits the feasible power density.

4.5.2 Buck Converter

The AC buck converter shown in Figure 32 is similar to the DC buck converter shown in Figure 28 on page 56. The circuit is extended with an AC rectifier and a larger DC link capacitor C2. Except from

that, the operation and control principle is equal to the conventional buck converter.

Compared to the capacitive power supply, the LED current is directly controlled. However, its construction is more complex.

4.5 High Voltage Topologies without Galvanic Isolation 4 OVERVIEW DRIVER TOPOLOGIES LED L2 C1 Q1 D1 C2 D2 D3 D4 D5 PWM

Figure 32: An AC buck converter is a classic LED driver topology. The upstream full bridge rectifier is connected to the AC grid.

the overall converter efficiency typically is higher. One drawback of this solution is the still low power factor of typically 0.55 due to reactive power that is caused by the AC/DC bridge rectifier: Power from the grid is only taken at peak values, as it can be seen in Figure 33. It is known as displacement power. To obtain an unity power factor, an additional power factor correction circuit must be added upstream. This will be discussed in the subsequent subsection.

4.5.3 Boost Converter

Instead of a buck converter, a boost converter with the previous full-bridge rectifier could be used as well. As the boost converter always increases the output voltage, referring to section 4.4.2, this leads to a very high output voltage. Assuming a 250V grid, the minimal feasible output voltage would be 353V. Not only this requires a significant number of series LEDs, an additional risk of electric shock is present at the LEDs. For that reason, a voltage increasing topology is rarely used in grid connected LED lighting.

4.5.4 PFC/Buck

To achieve a power factor close to one, a power factor correction (PFC) is added upfront. The LED driver now consists out of two stages.

The PFC converter shown in Figure 34 starts by rectifying the AC using the full bridge (D2-D5)

which is connected to a boost converter, consisting out of L3, Q2 and D6. The converter operates in

4.6 Topologies with galvanic isolation 4 OVERVIEW DRIVER TOPOLOGIES