High brightness LED drive circuit

Preface

High-brightness LEDs are highly efficient and reliable, and are a very promising low-energy power source. Due to the large range of forward voltage variations of LEDs and the steep V/I relationship curve, constant current drive is required. Different applications have their own power supply requirements. For example, landscape lighting often uses low-voltage AC power supplies, while automotive lighting uses low-voltage DC power supplies. In addition, efficient drive circuits should also be able to operate in a wider input voltage range to drive one or more series-connected LEDs. This article introduces an LED drive circuit designed using NCP/NCV3065. The device is suitable for AC/DC power supplies and can be configured into different circuit structures to provide a variety of output currents. NCP is designed for commercial/industrial temperature environments, while NCV has passed automotive application certification.

Buck drive circuit

Figure 1 is a DC power supply drive circuit, and Figure 2 is an AC power supply drive circuit. LED brightness or light intensity is measured in lumens, which is proportional to the current flowing through the LED. The light efficiency varies with the LED current, and the selection of circuit components is critical.

Inductor·Selecting an inductor is a trade-off between inductance and peak current. In conventional applications, the ripple current is between 15% and 100%. To reduce the ripple current, the inductance value must be increased. The advantage is that the output current of the switching regulator is maximized.

Working with output capacitors, for conventional buck structures, an output capacitor is added after the inductor, which is connected in parallel with the LED or LED series to reduce the ripple current. Using an output capacitor can reduce the inductance value. In addition, the circuit can operate at a lower frequency to improve efficiency and expand the output voltage range. The output capacitor can be calculated based on the current ripple D:

COUT = ΔI/Δv × 8 × f

=VIN×(1-D)×D/8×L×f2×ΔVOUT

Works without output capacitor. Constant current buck regulator mainly controls the current flowing through the load, rather than the voltage across the load. The switching frequency of NCP3065 is between 100KHz and 300KHz, which is much higher than the flicker that can be detected by the human eye. This creates conditions for relaxing the requirements for ripple current, allowing a higher peak-to-peak value, that is, configuring NCP3065 in a continuously conducting buck structure to eliminate the output capacitor. The important design parameter is to keep the peak current below the maximum current allowed by the LED. The ripple value of 15% is the best compromise value. For the commonly used 350mA, 700mA and 1,000mA LED current values, the ripple is selected as ±52.5mA, ±105mA and ±150mA respectively. At this time, the inductor value that can be selected is:

L = (VIN-VOUT) × TON / ΔIMAX

Current Feedback Loop To keep the LED in constant current mode, the feedback of the regulator is obtained from the voltage on the detection resistor at the COMP pin of the device. The RC circuit between the detection resistor and the COMP pin can improve the transient response of the converter. The reference feedback voltage is selected as 235mV.

IOUT=VREF/Rsense=0.235V/Rsense(A)

Brightness adjustment. The light intensity emitted by the LED is proportional to the average output current. For brightness adjustment, a variable duty cycle PWM signal is commonly used to manage the output current value. The COMP or IPK pin of the NCP3065 is used to provide brightness adjustment function. In digital input mode, the PWM input signal causes the regulator to work intermittently, thereby reducing the average current flowing through the LED. In analog input mode, the PWM input signal is filtered by the RC circuit, and the generated signal is superimposed on the feedback voltage to reduce the LED current. The RC component value depends on the PWM frequency. The PWM frequency is basically between 200Hz and 1KHz. A low frequency is beneficial to reduce EMI, but a frequency below 200Hz will cause the human eye to feel flickering.

Pulse feedback circuit NCP3065 works in a pulse train mode, and the output switching frequency is related to the input and output conditions. The device oscillator generates a constant frequency signal set by an external capacitor, which is gated by a peak current comparator. When the output current is higher than the threshold, the switch is turned off; when it is lower than the threshold, the switch is turned on and the oscillator is gated. This mode of operation may produce overshoot on the output waveform. Using a pulse feedback circuit can reduce overshoot, thereby stabilizing the switching frequency and reducing output ripple. The pulse feedback circuit is implemented by adding a resistor R8 (see Figure 1) between the CT pin and the inductor, which is generally between 3KΩ and 200KΩ. Table 1 lists typical applications and recommended R8 values.

Boost drive circuit

The boost converter circuit is shown in Figure 3. When the power switch is turned on, current flows through the inductor at the input end, and Iton rises. When the switch is turned off, the current Itoff flows through the diode D to the capacitor and the load. At the same time, the voltage on the inductor is superimposed on the input voltage. As long as the voltage is higher than the output voltage, the current will flow through the diode continuously. If the current flowing through the inductor is always positive, the converter operates in continuous conduction mode (CCM). In the next switching cycle, the above process is repeated.

If the converter operates in CCP mode, the output voltage is:

VOUT=VIN×1/(1-D)

The duty cycle D is:

D = ton/(ton+toFF) = ton/T

The input ripple current is defined as:

ΔI=VIN×D/(f×L)

The load voltage is always greater than the input voltage, where

Vload=Vsense+n×Vf

In the above formula, Vf is the forward voltage drop of the LED, Vsense is the reference voltage of the converter, and n is the number of LEDs in series. Since the converter needs to stabilize the current so that it is independent of the change of the load voltage, a detection resistor is placed at the feedback voltage:

Vsense=Icoad×Rsense

Vsense is equivalent to the internal reference voltage or feedback comparator threshold.

The NCP3065 boost converter is very suitable for automotive and industrial LED driver applications with limited circuit board space, high voltage and high temperature environments. The device has peak current protection and thermal shutdown functions. In constant current operation, protection is also required when an LED open circuit fault occurs, otherwise the output current will continuously charge the output capacitor, causing the voltage to continue to rise. At this time, an external Zener diode is used to clamp the output voltage. In addition, although the NCP3065 has been designed to work at 40V, it is best to add an external transient voltage protection circuit in automotive applications with inductive loads.

The main operating frequency of the circuit is determined by the external capacitor C4. The Ton time is controlled by the internal feedback comparator, the peak current comparator and the main oscillator. The output current is determined by the internal feedback comparator with a negative feedback input. The positive input is connected to the 0.235V internal voltage reference tube. The feedback resistor sets the normal LED current:

IOUT=0.235/Rsense

The device also has a second comparator with a threshold voltage of 200mV. Resistor R1 is used to limit the maximum current value:

Ipk=0.2/R1

R1 can be a single 150mΩ resistor or a combination of three 1Ω resistors in parallel. The maximum output voltage is clamped by an external Zener diode D2 and is usually set at 36V.

Conclusion

LEDs are gradually replacing traditional incandescent bulbs and are expanding their application in lighting for buildings, industries, residential buildings, and automobiles. Monolithic integrated converters have been able to solve the constant current drive problem of power LEDs. With the advancement of technology, they can further improve functions, increase efficiency, and meet the characteristics required by different application environments.

Figure 1 DC power supply step-down converter

Figure 2 AC power supply step-down converter

Figure 3 Boost converter

Table 1 Recommended resistor R8 values ​​for typical applications