Semiconductor laser driver power supply based on NCP5662

1 Circuit structure and principle:

The semiconductor laser driving power supply consists of four parts: pulse circuit, control circuit, current stabilization circuit and protection circuit. The system block diagram is shown in Figure 1.

1.1 Current stabilization circuit

In the voltage or current stabilization power supply, the commonly used ones are switching power supply and linear power supply. Since the transient response of the switching power supply is poor and the ripple coefficient is large [2], it is more reasonable to use a linear power supply for semiconductor laser driving power supply with high requirements on transient characteristics and temperature. In order to achieve high current stability, most driving circuits adopt the negative feedback control method. The schematic diagram is shown in Figure 2. During operation, the driving current is actively controlled by the resistance current sampling feedback. The method is to connect a sampling resistor RS in series with the source of the power transistor for sampling feedback. After the sampling voltage is converted by I/U, it is used as the feedback voltage to compare with the set voltage, and then the output current If is adjusted by adjusting the resistance of the power transistor. The entire closed-loop feedback system is in dynamic balance to achieve the purpose of stabilizing the current. The relationship between the output current If and the set reference voltage Vref can be obtained by the negative feedback principle. The above formula is only an approximate expression. With the change of load and input voltage, the output current still has a slight change, but because the preamplifier has a high amplification factor, the output current changes very little, and the stability can generally reach the order of 10-5.

In fact, the structural principles of linear voltage regulator and current regulator are basically the same, but the output mode is different, that is, the load loading mode is different. For example, in Figure 2, if the load is also connected in parallel with the sampling resistor, Figure 2 becomes a voltage regulator with a constant output voltage of Vref. Similarly, the voltage regulator based on this method can also be used as a constant current source with a slight adjustment. At present, various adjustable voltage regulator integrated chip technologies are mature and the products are rich, so the functions of this chip can be expanded to meet our design requirements.

Considering the actual application, such as the size of the power supply, the output current, and especially the transient response, we selected the low voltage difference and high current integrated adjustable voltage regulator chip NCP5662 from ONSEMI. Its transient response is faster than similar voltage regulators, with a settling time of 1-3us, a current value of 2A, and internal current limiting and thermal protection functions. Its functional block diagram is shown in Figure 3. Figure 3 shows the voltage regulation operation. According to the principle described above, the integrated circuit is expanded to be designed as a constant current source with high stability. Among the several expansion methods, the more reasonable working mode is shown in Figure 4. Ignoring the circuit in the dotted box in the figure, when the power is turned on, the circuit starts to work and enters the steady state. Due to the feedback effect inside the integrated circuit, the voltage across R11 is always maintained at 0.9V, so the voltage across R7 is always maintained at , so the current Is flowing through R7 is determined by the following formula:

Therefore, changing R3, R11, and R7 can flexibly adjust the output current value. According to the values ​​of each component in Figure 4, Is=2A is calculated. This constant current will flow through the laser diode to the ground. Since the comparator inside NCP5662 has a very high amplification factor, the current stability is very good. In addition, the current flowing from R11 and the GND end of NCP5662 to the load is less than 4mA, which has little effect compared to 2A. The role of C8 in the figure is to improve the transient response characteristics of the power supply, which will be described in detail in the experimental analysis section.

1.2 Pulse control circuit

The pulse control circuit is shown in the virtual box of Figure 4. When the pulse control signal Vpulse is at a low level, the transistor Q1 is cut off, and its collector is controlled at a high level by the power supply voltage, and the diode D5 is forward-conducted, so the ADJ terminal of NCP5662 is forced to be at a high level. This level value must be higher than the level value of the ADJ terminal when a constant current flows through the load, so that the voltage across R11 is much higher than 0.9 volts, so that the power transistor inside NCP5662 is cut off, so that the current flowing through LD is approximately zero. Therefore, the role of the 1K resistor R11 connected in parallel with the laser diode is to prevent the high resistance characteristics of the laser diode in the cut-off state from causing the level of the ADJ terminal to be unstable. When the load presents a general resistance characteristic, R11 can be omitted. When Vpulse jumps to a high level, the collector potential of transistor Q1 turns to a low level, which instantly makes the potential of the ADJ terminal very low, making the voltage across R11 far below 0.9 volts, so that the power transistor inside NCP5662 is turned on, the potential of ADJ begins to rise, and finally enters a stable working stage, and a constant current flows through the laser diode. Diode D5 also enters a reverse bias state and acts as an isolation until the next low-level Vpulse control signal arrives. This is repeated to achieve fast switching of large currents, which is mainly due to the fast response characteristics of NCP5662, which is essentially because it integrates a high-speed amplifier and a high-speed power amplifier transistor.

The pulse control circuit is the most important part of the entire design. Although there are other ways to achieve this function, such as connecting a power MOS switch in series with the load, or connecting a high-side MOS switch in series with the power supply, both theory and experiment have proved that these two methods have problems. For example, the stability and response characteristics of the power supply are not as good as the working method shown in Figure 4.

1.3 Pulse generation circuit

The pulse control signal Vpulse in FIG4 comes from a pulse generating circuit [5], and the pulse generating circuit is shown in FIG5a and FIG5b.

Figure 5a is the internal oscillation circuit of the power supply, which is composed of 7555. The four NAND gates are used to select whether to accept internal control signals or external control signals to be output to Vtrigger. The Vtrigger signal controls the frequency of the required Vtrigger signal. Figure 5b is a falling edge triggered monostable circuit composed of a 555 timer. The characteristic of this monostable circuit is that the pulse width has a very good linear relationship with the voltage at the 5th terminal of the 7555 timer. This is mainly due to the use of a bootstrap circuit composed of an amplifier LM358 and a capacitor C6 [6]. Therefore, this realizes the function of independently controlling the pulse frequency and pulse width of the pulse constant current source. The pulse width can be controlled by an external voltage signal, such as the voltage signal from the thermistor of the temperature sensor.

1.4 Protection Circuit

Since semiconductor lasers have a poor tolerance for electrical shocks, during use, the most common electrical shocks are voltage and current surges generated during the power on or off process. Therefore, protection measures must be taken in the power supply. There are many traditional protection circuit methods, such as slow start circuits and short circuit protection switches. In this application, the power supply is powered by a battery, the supply voltage fluctuation is small, and the selected integrated chip has slow start, thermal protection, and peak current limiting functions. Therefore, only an ordinary diode is reversely connected in parallel at both ends of the semiconductor laser diode to prevent reverse surges.

2 Test results and analysis

All the above circuit modules were simulated and optimized on PSpice A/D in advance, and finally the physical circuit was made. The test results met the expected assumptions. When the load is a pure resistance of 1Ω, the pulse control signal cycle is 1ms, the pulse width is about 40us, and there is no parallel capacitor at both ends of R3, the voltage waveform at both ends of R7 is shown in Figure 6. As can be seen from the previous, the voltage at both ends of R7 is completely corresponding to the current flowing through the load, with only a proportional coefficient. From the oscilloscope, it can be seen that the rise time of the constant current pulse is about 2us and the fall time is about 1us. Under the same conditions, the voltage waveform at both ends of R7 when the load is a 808nm high-power semiconductor laser is shown in Figure 7. The reason for selecting a pure resistance of 1Ω for the previous load is: when 2A current flows through the laser diode, the equivalent load resistance of the laser in steady state is about 1Ω, so that their working conditions can be better compared. From the oscilloscope, it can be seen that in the rising stage, there is a period of attenuation oscillation lasting about 5us. This is mainly because the impedance characteristics of the laser diode change greatly before reaching steady state, such as parasitic inductance and capacitance. From the circuit principle analysis, connecting a capacitor C8 in parallel at both ends of R3 can eliminate this performance deterioration. Through experimental measurement, the capacitance value is more suitable between 1-2nf. If the capacitance value is too small, the oscillation cannot be completely eliminated. If the value is too large, the pulse will slowly rise along the slope and the response will be slow. Figure 8 shows the voltage waveform at both ends of R7 when the capacitance is 1nf. It can be seen that the output characteristics have improved a lot. This also shows that for constant current sources, the impedance characteristics of the load, such as parallel inductance and series capacitance, have a great influence on the transient characteristics of the power supply output! Pay special attention when using a constant current source to drive a semiconductor laser.

Most resistors and capacitors in the power supply use SMD components, two-layer wiring, and double-sided arrangement of components. The entire power supply can be made very small, up to 4cm×4cm×1.5cm, which is very convenient for applications such as laser ranging that require a small power supply volume. In addition, under the power supply of 6.5v battery, the power supply is used to drive 808nm high-power semiconductor diodes, and the switch test is repeatedly carried out. The laser works well!

3 Results

Based on the integrated voltage regulator chip NCP5662, a low-voltage, high-current pulse semiconductor laser driver power supply is designed with the least number of devices. The power supply is stable, reliable, small in size, simple to control, and the pulse width and frequency are independently adjustable. When the driving current is 2A, the pulse rise time is less than 4us, the fall time is less than 2us, the response is fast, there is no overshoot or recoil, and it meets the power supply requirements for semiconductor lasers in airborne missile ranging. This also shows that a reasonable choice of mature voltage regulator chips can design a feature-rich constant current source. The design ideas in this power supply can be applied to the power supply design of other pulse constant current sources.