PROJECT TITLE : Design and Fabrication of a Low Cost DC LED Driver Controller Chip for Solar Home System in Bangladesh

OBJECTIVE : The objective of this project is to develop a simple, low cost, energy-efficient LED driver chip with the ultimate goal to develop a low cost rugged LED light system for the users of the Bangladesh Solar Home Systems (SHS) Program which is the largest national program in the world for off-grid electrification. Bangladesh SHS is the longest, continuously operating off-grid electrification program in the world. About 20 million marginally poor people have obtained electricity services through the SHS Program.

SPECIFIC DESIGN GOAL : The specific design goal of the proposed system is shown below :

BLOCK DIAGRAM :

In figure 1, the block diagram of the controller chip is shown with a solid line, and the off-chip DC-DC converter and LED strip circuit are shown with hatched line.

Fig. 1: Block diagram of the proposed LED driver chip along with off-chip DC-DC converter and LED string.

A Short Description of the circuit:

The objective of the LED driver control chip is to drive a string of ten 277mA, 3.6V rated LEDs. A voltage around 36V is needed across the LEDs. But the battery voltage of a Solar Home System is around 12V. A boost converter is used to get the higher voltage required to drive the LEDs. A feedback resistor is placed in series with LEDs. Current through the LED is proportional to the voltage across the feedback resistance. Error amplifier compares the feedback resistance with the reference voltage and computes the error. The PI controller proportionately integrates the error from the error amplifier. Its output changes until error is zero. Output from the PI controller is compared with a triangular pulse to generate a PWM signal required to control the duty cycle of the boost converter. As the PI controller value changes, the duty cycle of the pulse so the output voltage changes accordingly until feedback voltage is equal to the reference voltage. The pulse converter converts the -1.5V to 1.5V pulse from the output of the comparator to 0V to 5V pulse required to drive the high voltage MOS of the boost converter. The output of the pulse converter controls the duty cycle of the Boost converter hence controls the current through the LEDs. The pulse converter and the boost converter will be external to the chip.

Schematic of Critical Circuit Core:

1. Reference Voltage Source:

The properties of the reference voltage source:

• Constant Voltage Source by series voltage division of V_DD.
• For I_OUT = 280 mA, and R_FEEDBACK =3.33 Ω, V_REFERENCE = 932 mV

                                          Fig. 2: Circuit diagram of Reference Voltage

2. Differential Amplifier Circuit:

The differential amplifier circuit consists of NMOS and PMOS transistors and it will act as a building block for the other circuits.

Fig. 3: Differential Amplifier circuit

3. Error Amplifier Circuit:

A differential configuration amplifier with unity gain works as Error Amplifier. It subtracts the feedback voltage from the reference voltage to calculate the error voltage.

Fig. 4: Error amplifier circuit

4. PI Controller:

The PI controller is developed using an inverting amplifier (P) and an integrator (I) combining with an inverting summer circuit. It generates a DC voltage depending upon the error voltage. P is obtained by the ratio of two resistors and I is obtained by a resistor and an external capacitor.

Fig. 5: PI Controller

5. Square Wave Generator:

A square wave is generated from an Op-amp of comparator configuration by comparing the inverting input with the non-inverting input. The ON and OFF time can be controlled by the value of the capacitor and resistor by changing the charging time and discharging time. The frequency of the square pulse is ~ 500 kHz.

Fig. 6: Square wave generator

6. Triangular Wave Generator:

The triangular wave is generated by integrating the square wave. An integrator using Op-amp is used for the integration of square pulse. The R and C used for the integrator are both on-chip resistance and capacitance.

Fig. 7: Triangular wave generator from square wave

7. Controlled PWM Wave Generator:

The triangular wave is then compared with PI controller output DC voltage to generate controlled PWM. A series of buffers are used to smooth the output square wave.

Fig. 8: PWM wave generator

8. Boost Converter:

There will be a DC to DC boost converter circuit that will be external to the chip.

Fig. 9: Boost converter

Team Members:

(1) Nayeeb Rashid, Undergraduate Student, BUET

      Email: 1606087@eee.buet.ac.bd

(2) Mumtahina Islam Sukanya, Undergraduate Student, BUET

      Email: 1606039@eee.buet.ac.bd

(3) Muhammad Sameen Uddin Mashuk, Undergraduate Student, BUET

      Email: 1606098@eee.buet.ac.bd

Team Mentor: ABM Harun-ur Rashid, Professor, Dept. of EEE, BUET