Alternative to LED 1 W to W 5 LED driver GreenPoint reference design for MR16 applications

In recent years, high brightness light-emitting diode (HB-LED) the rapid development of the market, the performance of the LED (efficacy, unit for LM / W or lm/W) more than doubled, the use of more suitable for many new applications, such as we can find many revolutionary new products in handheld flashlight, architectural lighting and street lighting applications in. However, LED is still a challenge to be more cost-effective than incandescent and compact fluorescent lamps. In fact, for many applications, it involves a wide range of input voltage based power supply to the HB-LED power supply challenges. Among them, the application of general lighting such as track lighting is particularly true, these applications may be very loose voltage 12 Vac or a 12 Vdc power supply. However, the LED needs to be driven by a current source rather than a voltage source, because the variation in the forward voltage of the HB-LED (3.4 V) may exceed + +, depending on the process tolerance and temperature.

In terms of the current volume of 1 lumens W warm white LED light output power, usually takes 3 to 4 LED instead of 1 20 W incandescent lamp. In order to obtain the desired brightness and chroma, it is necessary to drive LED with constant current. From the architecture point of view, the buck boost topology meets this requirement, but it is not as common as standard buck or boost topology. But to understand thoroughly, buck boost topology can also provide a number of advantages for the input voltage (Vin) and the forward voltage (Vf) overlap high performance HB-LED lighting applications.

Reference design overview

The reference design document describes carefully build and test GreenPoint? 1 W to 5 W LED drive scheme for MR16 LED replacement applications. This reference design circuit is suitable for driving a variety of lighting applications in the HB-LED, but its size and configuration for MR16 LED alternative applications. This configuration is common in 12 Vac/12 Vdc track lighting applications, automotive applications, low-voltage AC landscape lighting applications, and lighting applications, such as cabinet lamp and lamp may adopt the standard off the wall type AC voltage power supply adapter.

A key to this reference design consideration is at 12 Vac input conditions across the input line change and change of output voltage, realize the current flat. This reference design circuit based on the A Morimi semiconductor NCP3065 construction, operating frequency is about 150 kHz, using non isolated configuration. NCP3065 is a monolithic switching regulator that supports 12 Vdc or 12 Vac power input and is designed to provide constant current for the HB-LED. In addition to NCP3065, this reference design also incorporates an automatic detection circuit. The functional block diagram of the reference design is shown in figure 1.

Figure 1: A Morimi semiconductors for MR16 LED replacement applications with 1 W to 5 LED GreenPoint driver reference design block diagram (W).

Basic power topology

The principle of buck boost converter is very simple. The input voltage source is directly connected to the inductor (L) in the conduction state, so as to accumulate energy in the inductor. At this stage, the capacitor C provides energy for the output load. When the state is turned off, the inductance is connected to the output load and the capacitance through the output diode, thereby transmitting energy to the load.

To note that this is a reverse (inverting) output, anode negative output connected to the LED, and the output is connected to the LED cathode. In addition, when the probe is measured with an oscilloscope probe, the ground end of the probe is not grounded. The oscilloscope filter will need to be floated from the AC wall power supply to remove the ground connection. Otherwise, the ground loop / short circuit will cause the device to turn off.

Burst mode control

Basic control loop contains 235 mV internal reference and feedback comparator and 2 set priority (Set-Dominant) RS latch. Basically, the NCP3065 supports power FET in the buck boost segment (switch ON), where the feedback voltage drops below the reference voltage. While the Ct drops, the power FET will be forced to shut down unconditionally.

Resistor R8 (see Figure 5) is used for sensing the input inductor current, and provide it to the NCP3065 FB pin. This application generates closed time transient (Ivalley) inductor current control. The turn-on time cycle can only be started when the inductor current passes through the Vref threshold.

Because the NCP3065 controller does not provide integrated pulse width modulation (PWM) control, only using a comparator to track feedback (trip), therefore, the peak load current and average load current is not as direct as buck converter is proportional to, but in accordance with the following formula:

Among them, Ivalley is the lowest point of the inductor current. The average current (Iave) and the input voltage (Vin) dynamic curve chart can be drawn than Potter (see Figure 2a), may lead to substantial changes in LED light output. Figure 2: average LED current vs. Vin curve without Vin compensation and Vin compensation.

Therefore, to reduce the error response leads to the nonlinear Vin curve using vs. Iout input voltage feed-forward compensation network. A resistor divider network consisting of a resistor R3, R5, and a resistor R4 (see Figure 5) is used to increase the Vin proportional voltage to the FB pin, thereby reducing the load current when the Vin is increased. This results in a flattening of the 2A curve and reduces the overall current error (see Figure 2B). Resistor R9 and capacitor C6 for high input voltage limits the external switch gate source voltage limit. The resistance is composed of R9 and R2 divider network is used to set the maximum limit gate source voltage (Vgs): pulse feedback resistor

Resistance R7 and the diode D5 to reduce the jump (pulse skipping) the possibility of pulse. Since burst mode control involves only one feedback voltage and cross detection per cycle, it does not include the use of windows