Designing an LED driver scheme, if the current is greater than 500 mA, a switching regulator is usually used to drive the LED ; if the current is between 200 mA and 500 mA, either a switching regulator or a linear regulator can be used; Currents below 200 mA can be used with linear regulators or discrete device schemes such as resistive drivers.
For low-current LED lighting applications with less than 200 mA, linear regulators or resistor-driven circuits have their own advantages and disadvantages. For example, the linear regulator structure is relatively simple, easy to design, provides steady current and overcurrent protection, has an external current set point, and has no electromagnetic compatibility (EMC) problems, but has high power dissipation and cost, and energy efficiency. low. Resistive drivers use a simple discrete device such as a resistor to limit the LED string current. This is the lowest LED driver solution. It is also easy to design and has no EMC problems. However, the outstanding problem of this scheme is the change of current when the input voltage changes. , causing a change in brightness and not maintaining a constant brightness.
From the customer's point of view, many customers in the market need to be more economical than linear regulators, but in terms of performance, they are much higher than the resistor drive circuit. In this context, ON Semiconductor has adopted a patent-pending self-biased transistor (SBT) technology, combined with its own superior process control capabilities, to introduce a new LED driver solution - NSI45 series double-ended linear constant current Voltage regulator (CCR).
CCR characteristics and application advantages
ON Semiconductor's CCR can be characterized as a variable resistor. As the voltage across CCR rises, the internal impedance also rises, maintaining a current close to the steady current (Ireg) specification. CCR also has a negative temperature coefficient, so when CCR dissipates power (temperature rises), the internal impedance increases, causing the current to decrease.
Figure 1: CCR combined with a resistor driver and a linear regulator.
Compared with the resistive drive scheme, ON Semiconductor's NSI45 series CCR has outstanding advantages, such as maintaining constant current when the AC voltage increases, LED no-delay after reaching the LED threshold voltage, keeping the LED bright at low voltage, and keeping LEDs are protected from voltage surges. In addition, thanks to the constant current characteristics of CCR, customers can reduce or eliminate the extra cost of different LEDs from different suppliers and reduce the total system cost.
In a variety of low-current applications, such as decorative lighting, work lights, outdoor lighting , landscape lighting, counter lighting, and LED panel lighting, and in general lighting, AC power can also be used directly. After the AC mains input is bridge rectified, it is only necessary to ensure that the input voltage minus the total voltage of the LED string does not exceed the maximum anode-to-cathode voltage (VAK) of the CCR, indicating that the CCR is very easy to use for design. See Figure 2 for a typical circuit diagram.
Figure 2: Schematic diagram of the basic LED AC application.
CCR application design example
We can either use CCR to open a new design or use CCR to modify an existing design. In this paper, the new design is taken as an example to discuss the application of CCR devices in SOT-223 package in AC lighting under 110 Vac and 220 Vac root mean square (RMS) input conditions. Due to the pulsed operation of full-wave bridge rectification, the CCR with a steady-state current of 30 mA has a rms current of 22 mA in AC applications. The CCR operates from an AC voltage range of 1.8 V to 45 V. The LED turn-on time depends on the forward voltage drop (VF) of the LED string. In the reference circuit of this application, the CCR conduction time is approximately half of the peak voltage conduction time. Therefore, the LED is on for about 50% of the time. Therefore, the LED rms current is about 50% of the steady current.
We need to select the number of LEDs based on the design parameters. The design parameters are 110 Vac RMS, ±10%, and HB LED (VV is 3.3 V at 20 mA current). The analysis is first performed for Vin=+10% (maximum).
Maximum input voltage Vin (max) = 1010 V rms +10% = 120 V rms
Rectification peak voltage Vpeak=120 V rms x 1.414 =170 V
LED string total forward voltage drop = 170 V (peak Vin) - 45 V (Vak max) = 125 V (LED string VF)
Therefore, the number of LEDs is: 125 V / 3.3 V = 38 LED
Then analyze the first for Vin=-10% (minimum).
Test minimum input voltage Vin (min)=110 Vrms-10%=100 Vrms
Rectification peak voltage Vpeak=100 Vrmsx1.414=141 V
CCR anode to cathode voltage Vak=141 V-125 V=16 V
The Vak range will vary with the number of LEDs in the LED string. Adding another 3 LEDs will set the Vak range from 6 V to 35 V. The added HB LED provides higher lumen output and reduces CCR heat dissipation.
Figure 3: Circuit diagram of CCR for direct AC powered LED lighting applications.
The AC rms voltage is converted to a DC pulse with a frequency of 120 Hz after full-wave rectification. When this voltage exceeds the sum of the forward voltage drop of the LED string and the bridge rectifier, the CCR turns on, controls the current, and isolates the LED from the peak rectified voltage.
According to Figure 3, the power dissipation calculation formula for CCR is as follows:
P=(Vak rms)x(IREG*duty cycle)=(120 Vbr rms-(38x3.3 V LEDx0.707))x(30 mAx50%)
=31 V rms x 15 mA=465 mW
The SOT-223 package with a 100 mm2, 1 oz. copper heatsink can operate up to 85 °C. We used an oscilloscope to test 38 series LED application boards under 110 V±10% AC RMS input conditions. The steady current is measured with a sense resistor of 100 Ω and 1% accuracy. See the reference design for related trace diagrams. The application of double-ended linear constant current regulator CCR in AC-powered low-current LED illumination). It is worth mentioning that CCR can also work in parallel to increase the steady current supplied to the circuit. Of course, the larger the CCR current, the more the power dissipation of the circuit increases.
The above design explores the 110 Vac design conditions. Of course, CCR can also be used for 220 Vac designs, all you have to do is increase the number of LEDs. The calculation process of the number of LEDs is performed with reference to the above calculation.
In addition to using CCR in new designs, it is also very simple to use CCR in existing designs. We assume that this design uses 24 LEDs (VV is 3.3 V at 22 mA current) with the same parameters, 110 Vac RMS, ±10%. In order to keep the CCR within its operating voltage limit (maximum VAK of 45 V), we need to add a series step-down resistor (Rs) to the circuit and calculate its resistance.
Similarly, the maximum input voltage is 120 V rms, the rectified peak voltage is 170 V, and the LED string forward voltage drop is 24x3.3 V = 79.2 V. Therefore, the voltage drop required for the series step-down resistor can be calculated by the following formula:
Vdrop, RS=Vpeak-(VF LEDs PK+VAK CCR pk+VRsense pk)
=170 V-(79.2 V+45 V+4V)=41.8 V
The CCR peak current is 34 mA; therefore, Rs = 41.8 V / 0.034 A = 1229 Ω. The actual test circuit selected Rs of 1200 Ω. Therefore, the power dissipation is VxI = 1.42 W pk or 1.0 W RMS.
Use a 1200 Ω series step-down resistor to test at minimum input voltage. At this time, the rectified peak voltage is 100 Vrms x 1.414 = 142 V, so the CCR Vak is 141 V-(79.2 + 41.8 +4) = 16 V, which is also kept within the operating voltage limit.
to sum up:
In low-current LED lighting applications where AC power can be used in buildings and general lighting, ON Semiconductor's NSI45 series of dual-ended linear constant current regulators are less expensive than ordinary linear regulators, but performance is better than The common resistor drive circuit solution is much higher, it is a cost-effective and reliable LED drive solution, and the design process is very simple, can be used for new designs, and can be easily used to modify existing designs.
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