Text. Y. Fang/SH Wong/HS Ling
Compared with incandescent lamps, high-brightness light-emitting diodes (HB LEDs ) provide better performance and better stability. Especially in the shadow of the energy crisis, humans are paying more and more attention to high-brightness LEDs. High-brightness LEDs are different from incandescent bulbs in that the brightness and color temperature are related to the forward current of the LED. Therefore, high-brightness LEDs require accurate and stable current drive to maintain a stable light output. This basic requirement is still a challenge for engineers who are designing.
In addition, high-brightness LEDs have a wide range of drive currents from 0.35 amps to a maximum of 10 amps, so overall power efficiency must be improved, otherwise high-brightness LEDs are difficult to use. This article will gradually analyze the design steps of such a power converter and demonstrate how to use this circuit to drive a 0.35 amp high-brightness LED string.
From the perspective of power conversion efficiency, the Switched Power Supply (SMPS) is definitely more advantageous than the linear regulator. Among the many methods available for the SMPS topology, we choose the most appropriate topology based on the available input power supply and the number of high-brightness LEDs that need to be driven. This article will discuss the breadth and simplicity of buck converters for non-isolated buck switching power supplies.
First, the most affordable topology for a built-in buck converter was chosen for this application, and then modulated for this converter to improve system robustness and power efficiency. Finally, a new pulse length modulation (PLM) control method with an enhanced converter is expected to re-improve the originally sacrificed current regulation accuracy. The last step is to actually test the circuit to verify its effectiveness.
Floating Buck Topology Helps Simple Gate Drive Circuit Design
Figure 1 shows the various non-isolated buck converter topologies used to drive high-brightness LEDs. Figure 1(a) and (b) show two typical buck topology methods, and Figure 1(c) And (d) is a floating buck topology. In general, since the Rds|on of the N-MOSFET (N-FET) is lower than that of the P-MOSFET (P-FET), the buck converter system in Figures 1(a) and (c) is generally considered Have better power performance. When designing a buck converter, design engineers tend to use the buck converter system of Figures 1(a) and (c) instead of the circuits in Figures 1(b) and (d). Figure 1(a) is more complicated in driving a high-order N-FET than the low-order N-FET in Figure 1(c) because Figure 1(a) uses the self-bootstrapping gate drive technique, except as shown in the figure. In addition to the gate drive voltage supply Vcc, the circuit also includes a rectifier diode and a flywheel capacitor. The same situation can also be applied to Figure 1 (d) and Figure 1 (b). For a simple gate drive circuit design, the buck converter topology of Figure 1(c) is better than Figure 1(a), which is also a floating buck converter with a low-side N-FET. the reason.
Figure 1 is suitable for buck converters that drive high-brightness LEDs. The power-switching technology uses (a) high-order N-FETs, (b) high-order P-FETs, (c) low-order N-FETs, and (d) Low order P-FET
Improve LED efficiency with accuracy tradeoffs
The floating buck converter in Figure 2 is used to drive a multi-light string of long-range high-brightness LED arrays, in existing systems, whether based on heat dissipation, unfavorable operating conditions, ease of maintenance, or module replacement issues. Most of the controllers are separate from the LEDs. Typical examples include large outdoor commercial electronic billboards and building exterior lighting. Placing the current sense resistor RISNS under the main power switch makes it easier to use low-side current sensing, which reduces the number of lines by nearly half. More importantly, the shorter current sensing line prevents the current regulation of the LED from being electromagnetically disturbed.
In the system of Fig. 2(b), since the position of the RISNS is redesigned, its power efficiency is improved a lot compared to Fig. 2(a). In addition, the low-order N-FET and RISNS in Figure 2(b) only conduct the inductor current in the upper ramp portion of the cycle, while the RISNS in Figure 2(a) covers the entire period of the inductor current, so the graph 2(b) The power loss of RISNS is the power loss of Figure 2(a) RISNS multiplied by the switching duty cycle, and the value of this duty cycle is usually less than 1. Therefore, the RISNS power loss in Figure 2(b) reduces one switching period factor, and the saved power P can be expressed by the following equation:
Figure 2 is a buck converter for driving multiple long-distance high-brightness LED strings. The technologies used are (a) high-side current sensing resistors (2N for long-distance lines) and (b) high-end current sense. Measuring resistor (the number of long-distance lines is N+1)
The RISNS in the equation is the current sense resistor, D is the duty cycle, Ipeak is the peak value of the inductor current, L is the inductance value, T is the switching cycle time, and Vout is the output voltage. Figure 2(b) uses a conventional control method to adjust the peak current, although a larger inductor value allows the adjusted peak current to be closer to the system's actual average current, but this approach lacks complete considerations and It is easily affected by changes in line voltage and component values.
PLM solves the average current regulation of LED string
In conjunction with reconfiguring the position of the sense resistor RISNS, the floating buck converter is the simplest architecture for driving high-brightness LEDs and is the most durable and power-efficient system solution. However, the traditional control method can only adjust for the peak current, but can not provide the actual average current regulation for the LED string. In order to solve this problem, a new control method - pulse length modulation control came into being.
Figure 3(a) shows the PLM architecture applied to the floating buck converter, while 3(b) shows the main waveform of the PLM circuit. The traditional SMPS control method incorporates an error amplifier that minimizes the adjustment error relative to a fixed reference voltage. As for the PLM aspect, it applies an error amplifier to the time integral VISNS(t) of the sensed waveform, wherein the adjustment is made with respect to the time integral VRP(t) of the reference waveform. The waveform (v) in Fig. 3(a) indicates that the PLM is adjusting the trapezoidal pulse train of the sensing signal, and the adjustment is performed with respect to a square wave pulse chain having a reference level. Since both have the same duty cycle, the midpoint of the trapezoidal wave slope is the same as the reference level. Based on the linear nature of the intermediate point above, the average inductor current or average LED current is adjusted to be equal to the reference current. Figure 4 shows the adjustment process in the closed loop operation of Figure 3(a).
Figure 3 (a) is a schematic diagram of the PLM floating buck converter; (b) is the PLM main waveform
Figure 4 VISNS and VRP waveforms in closed loop operation
From the experimental results, the available components constitute a floating buck converter, and the test circuit board of FIG. 5 is used to demonstrate the performance of the buck converter. Figures 6 and 7 respectively indicate that the intermediate slope voltages (labeled V1 and V2) do not change due to changes in inductance. The main reason is the PLM monitoring characteristics, which make the PLM immune to inductance and input voltage. For this reason, it can be clearly seen from Figure 8 that the floating buck converter can provide very accurate output current regulation with an error of only ±0.5%. Figure 9 is a comparison of the efficiency of the input voltage. It can be clearly seen that the efficiency of the PLM floating buck converter is significantly improved compared to the conventional average current sensing control method.
Figure 5 (a) is a recommended driver verification board circuit diagram (b) is a physical picture of the printed circuit board
Figure 6 Waveform measured at L1=22μH (average voltage flowing through RISNS = 200mV)
Figure 7 Waveform measured at L1 = 33μH (average voltage flowing through RISNS = 200mV)
Figure 8 Relationship between efficiency and input voltage when driving two high-brightness LEDs
Figure 9 Relationship between output current and input voltage when using PLM control (Vin min is the minimum input voltage required)
The new PLM control method combined with the traditional SMPS power stage provides a high power performance, simple, durable and accurate current regulator solution for driving high brightness LED applications. This article begins with the selection of a floating buck converter as a starting point and improves the performance of the circuit to achieve better durability and power efficiency. Finally, the new PLM control technology is applied to the improved power converter to re-improve the previously sacrificed current regulation accuracy. This paper introduces the design of the new PLM and verifies it through actual testing, and also affirms the value of this new technology design concept.
(The author of this article works for National Semiconductor)
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