(From MONOist) (CCS Compilation)
CYBERNET Application Systems Division
LED lighting design (1) LED lighting basic LED lighting design (2) indispensable "heating solution"
LED lighting design (3) LED electrical characteristics and simple drive circuit LED lighting design (4) pulse modulation PWM circuit detailed
LED lighting has received wide attention as a new generation of lighting. Simply relying on LED packaging does not make good lighting fixtures. This paper mainly explains how to design LED characteristics from electronic circuits, thermal analysis and optics.
In the previous issue of " LED Driver Circuit Design - Basics", the electronic characteristics of the LED and the basic drive circuit were introduced. Unfortunately, the impedance type drive circuit and the constant current source type drive circuit do not have strong performance in a wide range of input voltages and large currents, and sometimes do not exhibit the performance of the LED. Conversely, driving the LED circuit with a pulse modulation method can provide multiple advantages of the LED. This time, the drive circuit using pulse modulation is mainly explained.
What is PWM?
Pulse modulation English representation is Pulse Width Modulation, referred to as PWM. PWM is a way to adjust the duty cycle of a pulse wave. As shown in Figure 1, the duty cycle of the pulse can be expressed in pulse period, On-time, Off-time, as follows:
Duty cycle = On-time (high time of pulse) / one cycle of pulse (On-time + Off-time)
Tsw (one cycle) can be a switching cycle or a switching frequency of Fsw=1/Tsw.
Figure 1 Pulse Width Modulation (PWM)
In the driving circuit using PWM, the average value of one cycle of the pulse can be controlled by increasing or decreasing the duty ratio. Using this principle, the efficiency of the LED current can be adjusted if the turn-on time (closed time) of the switch design (semiconductor tube, MOSFET, IGBT, etc.) on the circuit can be controlled. This is the PWM control to be introduced next.
PWM signal application
A feature of the PWM control circuit is that various outputs can be controlled as long as the pulse amplitude is changed. The buck circuit of Figure 2 helps understand the PWM control principle. In this circuit, converting the input voltage of 24V to 12V requires an increase in load. The load is simply impedance. There are many methods for voltage conversion circuits. What is the effect of using PWM signals?
Figure 2 step-down circuit
Take the PWM control circuit in the step-down circuit of Figure 2, as shown in Figure 3. MOSFEL is used as a switch design. When the switching frequency of the PWM signal is 20 kHz, the conversion period is 50 μs. When the PWM signal is High, the switch is On and current flows from the input through the load. When the PWM signal is in the Low state, the switch is off, there is no input and output, and the current is also broken.
Here, try to fix the duty cycle of the PWM signal at 50% and apply it to the switch.
Current and voltage are applied to the load when the switch is open. When the switch is off, there is no current, so the supply voltage of the load is zero. As shown in Figure 4, the green waveform, V(OUT), sees the output voltage in the load.
Figure 3 Buck circuit using PWM signal
Figure 4 Resolution result duty cycle: 50%
The input voltage is DC, and the output voltage is obtained by the pulse signal. The smoothing circuit is inserted at the front end of the load (the rear end of the switch), and the brown waveform as shown in FIG. 4 can be obtained. When the average value of the output pulse is about 12V, the DC voltage can be supplied to the load.
But what if you don't want 12V but want to get 6V output voltage? The advantages of PWM control are actually here. Just change the pulse amplitude. In fact, you only need to set the duty cycle to 25% to get an average output of 6V. Figures 5 and 6 show the circuit and analysis results in this case.
Figure 5: Step-down circuit using PWM signal
Figure 6 analysis results duty cycle about 25%
The above results indicate that the relationship between the input and output voltages in the step-down circuit can be expressed as:
Output voltage = duty cycle of PWM signal × input voltage
In other words, as long as the duty cycle of the PWM signal is changed, an arbitrary output voltage can be obtained. Next, the method of driving the LED using the buck converter circuit in the actual product design is introduced.
PWM drive circuit example
As shown in FIG. 7, a circuit of a coil, a capacitor, and a diode is added to the step-down circuit described above. The feedback circuit is not considered here. Here we use the LUXEON series of LXM3-PW71 LEDs from Philips Lighting. The coil and the capacitor inserted at the front end of the LED (load) constitute a smoothing circuit, and the pulse output is averaged by conversion. The diode at the front end of the coil continues to supply current to the coil even when the switch is closed. Buck converters are often used as voltage conversion circuits, but when driving LEDs, you need to control the current instead of the voltage.
Figure 7 Example of PWM drive circuit step-down conversion
Recognize the circuit configuration of Figure 7. When the pulse signal is in the On state, that is, when the switch design is in the On state, the current flows in the order of the input signal - switch - coil - load. When the switch design is in the Off state, the current flows in the order of the diode-coil-load. Therefore, controlling the current in the coil is actually equivalent to controlling the current in the LED.
When a voltage of 3.0 V is applied between the positive electrode and the negative electrode, it can be seen from the database that the current of the LXM3-PW71 is about 350 mA. When the input voltage is 12V, the duty ratio of the pulse wave is set to 25% (12V × 0.25 = 3V), and a voltage of 3V can be obtained. When the number of switching frequencies is 100 kHz, the conversion period is 10 μs and the pulse amplitude is 2.5 μs. However, the load is established only in the case of a forward impedance. When the LED is actually used in the load, the load characteristic also changes depending on the current. When the current is about 350 mA, the pulse amplitude modulation is about 3.36 μs. The result of the verification circuit is shown in Figure 8.
Figure 8 verification results of the PWM driver circuit
The current in the LED changes and the current in the coil also changes. By detecting the change in the coil current by the sensing circuit, the current in the LED load can be made constant as long as the opening time of the switch is controlled. Increasing the duty cycle of the PWM increases the current in the LED and increases the brightness. Comparing the impedance-driven circuit and the constant current source-type drive circuit, changing the duty cycle of the PWM is more efficient than changing the impedance value and the circuit constant, and thus the convenience of the PWM control can be understood.
The buck converter introduced this time is used in LED drivers where the voltage is lower than the input voltage. Depending on the lighting fixture and the application, it is sometimes necessary to drive multiple LEDs at the same time, so that the necessary voltage in all LED drivers is higher than the input voltage. In this case, it is necessary to use a boost converter capable of making a voltage higher than the input voltage.
In LED lighting, the use of electric power requires miniaturization. In lighting fixtures, when the input voltage is converted to the LED driving voltage, conversion loss occurs, and the larger the conversion loss, the more likely it is to cause heat. At the same time, if the number of switching frequencies increases, the transformer and the coil will become smaller. Although the entire circuit board can be miniaturized, the high switching frequency is caused by the high switching frequency, and high-order harmonic problems occur. Therefore, in the PWM drive circuit of the LED, strive to achieve high efficiency and few components.
In order to keep the brightness of the lighting fixture stable or to adjust the brightness, it is necessary to detect the load current in the sensor, perform a control calculation, and adjust the duty cycle of the pulse. This paper does not introduce the feedback control circuit, but it is worth noting that the feedback control circuit includes voltage control, hysteresis control, hysteresis control, current control and so on. There are advantages and disadvantages to various control methods. We need to choose the best control method according to the lighting method and the applicable circuit method.
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