Design and implementation of a high-power LED drive circuit

Since the 1990s, with the rise of gallium nitride-based third-generation semiconductors, the successful development of blue and white light-emitting diodes, solid-state light source LEDs with the advantages of high efficiency, energy saving, environmental protection, and long life have truly ignited the glory of green lighting and are considered to be the most valuable new light source in the 21st century. However, high-power LEDs have large electrical discreteness and are easily affected by temperature. After the light-emitting diode is turned on, a slight increase in the voltage applied to both ends of the LED will cause the current flowing through the LED to rise sharply. In severe cases, the LED will work for a long time at a current exceeding the rated current, which can easily burn out the semiconductor chip of the LED. To control the brightness of high-power LEDs, the driver must provide a constant current. Commonly used constant current drivers for high-power LEDs include resistor current limiting, open-tube converters, and dedicated chips. The resistor current limiting control method is simple, but this method cannot solve the problem of luminous flux output changes caused by power supply voltage fluctuations, and has large energy loss and low efficiency, and cannot achieve the purpose of energy saving. Many semiconductor manufacturers have launched high-power LED dedicated driver chips, which have achieved good results, but the price of such chips is generally expensive. Aiming at the shortcomings of the existing driving circuit, this paper proposes a high-power LED constant current driving circuit based on UC3843 peak current control. The circuit is easy to use, with simple control method, high efficiency and low cost. The peripheral circuit of UC3843 is optimized and designed to realize PWM dimming control, avoiding the color coordinate offset problem caused by analog dimming. The experimental results verify the feasibility of the driving circuit.

1. Driving circuit composition and design

High-power LEDs are current-type devices. The brightness of LEDs increases with the increase of working current. To ensure that the current flowing through each LED is the same and the brightness of each LED is uniform, LEDs are connected in series. The block diagram of the entire circuit is shown in Figure 1. Current detection provides feedback for the PWM control circuit and compares it with the output signal of the error amplifier in the PWM control chip to control the duty cycle of the output pulse, thereby stabilizing the current flowing through the LED. PWMD is the digital dimming pulse signal input. By adjusting its duty cycle, the brightness of the LED can be adjusted to achieve the purpose of dimming.

The IC chip UC3843 used is a high-performance fixed-frequency current mode controller. The chip has 8 pins, simple external circuit wiring, few components, superior performance, and low cost. The chip has a fine-tunable oscillator (capable of precise duty cycle control), a temperature-compensated reference, a high-gain error amplifier, and a current sampling comparator. It has good voltage regulation, good frequency response, and a large stable amplitude; it has low startup current, undervoltage lockout with hysteresis, an operating frequency of up to 500kHz, and a large current totem pole output, which is very suitable for driving MOS field-effect tubes.

Its internal structure block diagram is shown in Figure 2.

Figure 1. Block diagram of the drive circuit

Figure 2 Simplified block diagram of the internal structure of UC3843

1.1 Main driving circuit design

The circuit is mainly composed of UC3843, MOSFET Q1, inductor L1, high-power LED series freewheeling diode D1 and detection resistor R12, which is a BUCK type peak current control mode circuit, as shown in Figure 3. The resistor and capacitor R13 and C9 connected to pin 4 determine the sawtooth wave oscillation frequency of PWM; the voltage reference of the current detection feedback composed of R1, R2, potentiometer R3TL431, R4, R5, C3 and the error amplifier inside UC3843 is compared with the current detection signal through the current detection comparator to control the duty cycle of the PWM signal and limit the current peak value flowing through the LED. The peak current flowing through the LED is:

Ipcak≈Is =(Vpin1- 1. 4V)/3R12(1)

Where Vpin1 is the voltage of pin 1 of UC3843 chip, Ipcak is the peak current flowing through the LED, and Is is the peak value of the detection current.

Figure 3 Driving circuit schematic

1.2 Slope compensation circuit design

The waveform of the current iL in the inductor L1 under CCM is shown in Figure 4, and it can be obtained:

α=△IL/△I0=D/(1 - D)=m2/m1(2)

In the formula, Ue is the output voltage of the error amplifier, m1 and m2 are the rising and falling slopes of iL in the circuit, △I0 is the increment of the initial value of iL caused by a certain periodic disturbance, and △IL is the change of iL at the end of the period.

It can be seen that to make the system stable, △IL < △I0, so D < 0.5; when D > 0.5, the circuit will have subharmonic oscillation, making the circuit unstable. The technology to eliminate harmonic oscillation is to increase slope compensation, that is, to add a slope with a negative slope to Ic. After adding slope compensation, the slope of the new control amount is -ma. The new m′1 = m1 + ma, the new m′2 = m2 - ma, the waveform of the inductor current is shown in Figure 5, and Equation 2 becomes:

α=(m2- ma)/(m1+ ma)(3)

By designing a reasonable ma, |α| < 1 can be achieved, that is, the system is stable. Generally, ma = 0.75m2.

Figure 4 Waveform in CCM mode

Figure 5 Waveform after slope compensation

The designed slope compensation circuit uses the peak-to-peak voltage signal on CT as the input signal of slope compensation. The principle circuit of the slope compensation network in Figure 3 consists of transistor Q4, R8, R9, R10, R12, C5, and C8. C5 is an AC coupling capacitor that isolates the DC component of the oscillation signal output from pin 4. In order to reduce the mutual influence between the timing resistor R13 and the compensation network, an emitter follower is added between the oscillator output and the compensation network input. R8 and R10 form a voltage divider network to obtain the slope compensation signal at pin 3 of UC3843. At the same time, R10 and C8 form a peak current absorber to filter out the peak interference signal. The slope compensation signal and the current detection signal are summed at pin 3 of UC3843 to achieve slope compensation.

1.3 Dimming circuit design

Inside UC3843, the reverse input terminal of the current detection comparator is clamped at 1V by the built-in Zener diode. As long as the voltage on the chip pin 3 reaches 1V, the terminal 6 is closed, and the MOS tube Q1 is immediately turned off. Therefore, the average current flowing through the LED in a cycle can be changed by controlling the input voltage value of the pin 3 to control the dimming of the LED. In Figure 3, it is composed of R14, R15, R16, R17, Q2 and Q3. In order to prevent the human eye from feeling the flicker of the light, the frequency of the PWMD signal is 100-200HZ. When the PWMD signal is high, Q3 is turned off, and the signal of the 3rd pin of UC3843 is the sum of the current detection signal and the slope compensation signal. At this time, the circuit works normally; when the PWMD signal is low, Q3 is turned on, and the voltage applied to the 3rd pin exceeds 1V, and the output terminal 6 of UC3843 immediately turns off the MOS tube. When the duty cycle of the PWMD signal changes, the average current flowing through the LED in a cycle also changes, so the luminous flux output by the LED also changes, achieving the purpose of controlling the brightness of the LED. In practical applications, PWMD can be generated by a simple microcontroller with PWM function.

2 Test results

The designed driving circuit is tested. When the DC input is 9.5V, the output peak current is 300mA, the timing resistor R13 is 10k, and the timing capacitor is 1nF, the experimental results of driving a 1W high-power LED are as follows: When the PWMD pin is connected to a high level, the voltage signal waveform Vg (lower waveform) of the switch gate and the waveform Vs (CH1, upper waveform) of the current detection signal of R12 are shown in Figure 6, and the voltage Ve waveform across the LED is shown in Figure 7. The experimental results are basically consistent with the theoretical values.

When the PWMD pin is connected to a pulse width signal with a frequency of 100Hz and a duty cycle of 50%, it is observed that the brightness of the LED is significantly reduced, and the voltage signal waveform of the switch gate (CH1, upper waveform) and the waveform of the current detection signal flowing through the current limiting resistor R12 (CH2, lower waveform) are shown in Figure 8. It basically conforms to the results of theoretical analysis, but the transient response of the dimming circuit is not ideal.

Figure 6 Gate voltage Vg and current detection signal Vs waveform

Figure 7. Voltage Ve waveform across the LED

Figure 8 Gate voltage and current detection signal waveform

3 Conclusion

The high-power LED drive circuit designed with UC3843 can overcome the change of luminous intensity caused by unstable voltage. The whole circuit structure is simple, the response is fast, and the current stabilization performance is good. Dimming control is achieved through PWM, which basically meets the design requirements. The circuit still has some shortcomings. The transient response of the dimming part is not very ideal and needs further improvement. The next step will focus on this aspect. This circuit can be used in solar lighting systems and also in AC/DC lighting systems.