LED Driver GreenPoint Reference Design Alternative to MR16 LED Applications

In recent years, the market for high-brightness light-emitting diodes (HB-LEDs) has grown rapidly, and LED performance (efficacy, in lumens per watt or lm/W) has more than doubled, making it more suitable for many new applications, such as our handheld flashlights. Many revolutionary new products can be found in applications such as architectural lighting and street lighting. But LEDs are still facing challenges when they are more cost-effective than incandescent and compact fluorescent lamps. In fact, for many applications, the challenge of powering HB-LEDs based on a wide input voltage range is involved. This is especially true for general lighting applications such as track lighting, which use a 12 Vac or 12 Vdc power supply that can be loosely regulated. However, LEDs need to be driven by a current source rather than a voltage source because the forward voltage (rated 3.4 V) of the HB-LED may vary by more than ±20% depending on process tolerances and temperature.

In addition, in terms of the current lumens of a 1 W warm white power LED, typically 3 to 4 LEDs are required to replace the light output of a 20 W incandescent lamp. To achieve predictable and matched brightness and chromaticity, it is also necessary to drive the LED at a constant current. From a architectural point of view, the buck-boost topology meets this requirement, but it is not as common as a standard buck or boost topology. But with a thorough understanding, the buck-boost topology can also provide numerous advantages for cost-effective HB-LED lighting applications where the input voltage (Vin) overlaps the forward voltage (Vf).

Reference design overview

This reference design document describes the carefully constructed and tested GreenPoint® 1 W to 5 W LED driver solution for MR16 LED replacement applications. This reference design circuit is suitable for driving HB-LEDs in a variety of lighting applications, but is sized and configured for MR16 LED replacement applications. This type of configuration is common in 12 Vac/12 Vdc rail lighting applications, automotive applications, low voltage AC landscape lighting applications, and work lighting applications such as cabinet lights and desk lamps that may be powered by standard off-the-shelf AC voltage wall adapters.

A key consideration in this reference design is the flat current stabilization across the input line variation and output voltage variation at 12 Vac input. This reference design circuit is based on ON Semiconductor's NCP3065 and operates at approximately 150 kHz in a non-isolated configuration. The NCP3065 is a monolithic switching regulator that supports a 12 Vdc or 12 Vac power input and is designed to provide constant current to the HB-LED. In addition to the NCP3065, this reference design incorporates an automatic detection circuit. The functional block diagram of this reference design is shown in Figure 1.

Basic power topology

The principle of a buck-boost converter is very simple. In the on state, the input voltage source is directly connected to the inductor (L), accumulating energy in the inductor. At this stage, capacitor C supplies energy to the output load. When off, the inductor is connected to the output load and capacitor through the output diode to transfer energy to the load.

Note that this is an inverting output with a negative output connected to the anode of the LED and a positive output connected to the cathode of the LED. In addition, when measuring with an oscilloscope probe, the ground of the probe is not grounded. The oscilloscope filter will need to be floated (remove the ground connection from the AC wall power supply), otherwise the ground loop/short circuit will cause the device to shut down.

Burst mode control

The basic control loop consists of a 235 mV internal reference, feedback comparator, and two Set-Dominant RS latches. Basically, the NCP3065 supports the power FET in the buck-boost section (switch ON), at which point the feedback voltage drops below the reference voltage. When the Ct drops, the power FET will be forced to turn off unconditionally.

Resistor R8 (see Figure 5) is used to sense the input inductor current and is supplied to the FB pin of the NCP3065. This application produces off-time transient (Ivalley) inductor current control. The switch on-time period can only begin when the off-time inductor current crosses the Vref threshold.

Since the NCP3065 controller does not provide integrated pulse width modulation (PWM) control, only one comparator is used to track the feedback point. Therefore, the peak load current and the average load current are not directly proportional to the buck converter. , but with the following formula:

Among them, Ivalley is the lowest inductor current point. The ratio of the average current (Iave) to the input voltage (Vin) is plotted as a Bode plot to obtain a dynamic curve (see Figure 2a), which can cause a large change in the LED light output.

Therefore, an input voltage feedforward compensation network is used to reduce errors due to the nonlinear response of the Iout vs. Vin curve. A resistor divider network consisting of resistors R3, R5 and summing resistor R4 (see Figure 5) is used to increase the Vin proportional voltage to the FB pin, thereby reducing the load current as Vin increases. This serves to flatten the curve of Figure 2a and reduces the overall current error (see Figure 2b).

Resistor R9 and capacitor C6 are used to limit the gate-to-source voltage of the high input voltage external switch. A resistor divider network consisting of R9 and R2 is used to set the maximum gate-to-source voltage (Vgs):

Therefore, an input voltage feedforward compensation network is used to reduce errors due to the nonlinear response of the Iout vs. Vin curve. A resistor divider network consisting of resistors R3, R5 and summing resistor R4 (see Figure 5) is used to increase the Vin proportional voltage to the FB pin, thereby reducing the load current as Vin increases. This serves to flatten the curve of Figure 2a and reduces the overall current error (see Figure 2b).

Resistor R9 and capacitor C6 are used to limit the gate-to-source voltage of the high input voltage external switch. A resistor divider network consisting of R9 and R2 is used to set the maximum gate-to-source voltage (Vgs):

Therefore, an input voltage feedforward compensation network is used to reduce errors due to the nonlinear response of the Iout vs. Vin curve. A resistor divider network consisting of resistors R3, R5 and summing resistor R4 (see Figure 5) is used to increase the Vin proportional voltage to the FB pin, thereby reducing the load current as Vin increases. This serves to flatten the curve of Figure 2a and reduces the overall current error (see Figure 2b).

Resistor R9 and capacitor C6 are used to limit the gate-to-source voltage of the high input voltage external switch. A resistor divider network consisting of R9 and R2 is used to set the maximum gate-to-source voltage (Vgs):

Pulse feedback resistor

Resistor R7 and diode D5 are used to reduce the likelihood of pulse skipping. Since burst mode control involves only one feedback voltage and cross-cycle detection per cycle, it does not include the use of a window comparator. It is possible to generate a skipped pulse. This skipped pulse does not affect the DC regulation, but if the pulse There are low frequency components that may flicker in LED applications.

R7 and D5 increase the current flowing to the Ct timing capacitor C2. This effectively limits the maximum duty cycle that the NCP3065 can provide. When the condition allows a low duty cycle, R7 and D5 will cause the duty cycle above the desired value to not occur. During shutdown, D7 is required to block the voltage because this is a buck-boost topology. More information on pulse feedback compensation can be found in ON Semiconductor's NCP3065 data sheet.

AC work Vs. DC

Since there is a half sine wave input to the buck-boost section, the operating point will be different compared to a pure DC input. Since small size is a goal of this design, very small input capacitors are used after the full bridge rectifier.

Therefore, the line voltage can be reduced as low as 3 V depending on the selected input capacitance. Therefore, the input of the converter is a full-wave rectified sine wave. Since the voltage regulator is non-functional at voltages below about 4 V, there is a dead zone. Therefore, we are finally regulated by a limited portion of approximately 80% of the 120 Hz line cycle, with the remaining approximately 20% being unregulated. This reduces the average current by approximately 20% when operating with AC input.

Thermal issues should be considered when operating with voltages greater than 12 Vac. In most applications, this module will increase heat dissipation. Input voltage compensation adds an additional AC compensation network to handle different operating points.

protection

Zener diode Z1 and resistor R1, as well as the current limit function of NCP3065, are used for open circuit protection. In the event of a load open event, the loop will attempt to increase the output voltage to meet the current demand for zero current feedback. When (Vin+Vout) exceeds the voltage of Z1, current will flow through R1, triggering the current limit function of NCP3065.

The short circuit protection is handled by the fuse F1 at the input. Surge protection for inductive loads must also be carefully considered, especially in transformer feed systems, which carry large amounts of source inductance, as is the case with magnetic transformers in landscape lighting applications. A surge protection device that selects the appropriate voltage must not exceed the power FET gate-to-source voltage with a reasonable voltage margin. This may require selection by trial and error because the clamp voltage may expand depending on the amount of energy absorbed.

Increase output current

The configuration of this reference design is for an average LED current of 350 mA. Increasing the current regulation point of this reference board is as simple as halving the value of the current sense resistor R8 from 250 mΩ to 125 mΩ. In addition, the input fuse must also be increased to accommodate increased input current consumption. When moving to a higher power design, a heat sink may be required depending on the housing environment parameters.

Test Results

The reference output design of the output current under different AC input voltage conditions and the energy efficiency test results under different DC voltage conditions are shown in Figures 3a and 3b, respectively. Among them, as shown in Figure 3b, this reference design has an energy efficiency of more than 0.75 in the 11 to 17 Vdc range, and this energy efficiency data is outstanding in such low power applications.