Low power LED driver power supply circuit design

A Resistor- Voltage Buckle 1. Principle of Resistor-Voltage Buckling and Application Capacitor Buck is actually using capacitive reactance current limiting, and the capacitor actually acts as a limiting current and dynamically distributing the voltage across the capacitor and the load.
2. When using capacitor step-down, pay attention to the following points. According to the current of the load and the working frequency of the AC, select the appropriate capacitor instead of the voltage and power of the load. The current-limiting capacitor must use a non-polar capacitor and cannot use an electrolytic capacitor. Moreover, the withstand voltage of the capacitor must be above 400V, and the most ideal capacitor is a polypropylene metal film capacitor. Capacitor buck can not be used for high power conditions, generally used for low power applications below 5W. Capacitor buck is not suitable for dynamic load conditions. Capacitor buck is not suitable for capacitive and inductive loads. It is suitable for driving in LED Power Supply. Voltage application.
3. The basic circuit of the RC capacitor is as shown in (Figure 1).

C1 is a step-down capacitor, VD1, 2, 3, 4 are bridge rectifier diodes, VD5 is a Zener diode, and R1 is the charge bleeder of C1 after the power is turned off.
4. When selecting the circuit design of the device, first determine the exact value of the load current, then refer to the example to select the capacity of the step-down capacitor. Since the current Io supplied to the load through the step-down capacitor C1 is actually the larger the capacity of the charge and discharge current Ic.C1 flowing through C1, the smaller the capacitive reactance Xc is, the larger the charge and discharge current flowing through C1 is. When the load current Io is less than the charge and discharge current of C1, the excess current will flow through the Zener diode. If the maximum allowable current Idmax of the Zener diode is less than Ic-Io, the regulator tube will burn out. In order to ensure reliable operation of C1, the withstand voltage selection should be greater than twice the supply voltage. The bleeder resistor R1 must be selected to vent the charge on C1 for a specified period of time.
5. The actual parameter calculation method is known as C1 is 0.33μF, and the AC input is 220V/50Hz. Find the maximum current that the circuit can supply to the load.
The capacitive reactance Xc of C1 in the circuit is:
Xc=1 /(2 πf C)= 1/(2*3.14*50*0.33*10-6)= 9.65K
The charging current (Ic) flowing through capacitor C1 is:
Ic = U / Xc = 220 / 9.65 = 22mA.
Two linear drive circuit 1. Typical circuit (Figure 2)

2. Working principle R3 is a constant current resistor. The voltage drop of R3 is used to control the switch of TL432. The switch of 432 is used to control the conduction of Q1 to achieve the output constant current. The purpose of selecting 432 is to use the 432 reference as 1.21. V to reduce the loss on R3. The current constant current value is 1.21/R3, and the R1 selection is selected according to the amplification factor of Q1.
3. Application Precautions This circuit is recommended for single-voltage input, output current, low-current LED power supply, such as bulb, T-tube, etc. It is generally recommended that the output current be at 100mA. At the same time, the closer the output voltage is to the input, the better, so that the voltage drop of Q1 is too large, resulting in excessive loss and low efficiency. Therefore, the use of LEDs is also preferably used in series.
Three constant current diode drive circuit 1. Typical circuit (Figure 3, Figure 4)

2. Working principle The ideal constant current source is a device with an internal resistance of infinity. The current flowing through it will never change regardless of the voltage across it. Of course this device is impossible to exist. The actual constant current diode is equivalent to a current within a certain operating voltage range, for example 25-100V, whose current is constant to a certain value, for example 20 mA. The equivalent circuit is shown in FIG.

Its internal resistance is Z, and the capacitance in parallel is about 4-10pF. Its typical volt-ampere characteristics are shown in Figure 6.

It has a constant current interval in a certain voltage range. In this interval, the current flowing through it is almost constant. VL is the voltage value reaching IL, and IL is about 0.8Ip.
3. Application Precautions Since the constant current diode requires a certain voltage Vk to enter the constant current, the too low power supply voltage cannot work. Usually this Vk is about 5-10V, so most battery-powered LEDs can't work. The maximum current is limited due to the power consumption of the constant current diode, so too much current is not suitable. For example, 1W LED usually needs 350mA, and constant current diode is difficult to provide. At present, it is more suitable to use AC power supply LED lamps with many small power LEDs in series, that is, high voltage and small current is the most suitable; but because of constant The withstand voltage of the flow diode has a certain limit, so the variation of the power supply voltage that it can absorb is also limited. Take the 100V withstand voltage CRD for 220V mains power supply can only deal with limited voltage changes. After 220V is bridge rectified, its output DC voltage is about 264V. If the mains change +10% ~ -15%, it is equivalent to 290~187V after rectification, and the voltage change is 103V. It has exceeded its withstand voltage. Because of the nonlinearity of the volt-ampere characteristics of LEDs, it is difficult to express them by formula. In short, when the mains voltage is lowered, the current in the LED will decrease as the mains voltage decreases. Its brightness will also change. Figure 3 shows a typical application circuit in a typical application circuit. Figure 4 shows an application circuit with a resistor-capacitor step-down to handle low-voltage output.
Four Buck circuits with unipolar PFC
With the current regulations and energy efficiency requirements, high PF for LED applications and reliable operation for full voltage range, and development towards miniaturization, so the previous valley filling PFC circuits also need to add two high-voltage capacitors that are less suitable for applications due to volume limitations. In view of this, many manufacturers at home and abroad have introduced non-isolated power supply application solutions for bulbs and T-tubes. The following is a typical introduction of Tongjia Technology's LD7832.
1. Introduction to LD7832 LD7832 is a high-PF LED Driver control chip that is controlled by TM mode in Buck circuit. The application of external components minimizes PCB size and protects functions. It satisfies various functional tests and reliability. Application testing requirements, design and debugging is quite simple, to meet customer requirements to quickly design on-line mass production and meet regulatory requirements, suitable for applications below 30W bulbs, T-tubes, etc., in order to adapt to different needs, LD7832 has external Different versions of MOS and built-in MOS (2A) are available.
2. Features Built-in 600V high voltage start-up circuit High PFC function controller Efficient transition mode control Low-cost design Application peripheral parts Minimum current adjustment accuracy Wide range UVLO (17V open, 8V off)
Vcc overvoltage protection function ZCD undervoltage protection function Cs short circuit protection function loop open circuit protection function IC internal OTP protection function (for integrated MOS IC)
250mA/-500mA drive capability 3. Operating principle The LD7832 is a PFC controller with a fixed turn-on time that operates in a voltage mode to control the boundary conditions. The CMOS turn-on time is determined by comparing the IC's Comp voltage with the IC's internal Ramp signal. The working principle waveform is shown in Figure 7.

During half of the input voltage period, the control TON is fixed, then the peak value of the inductor current follows the peak value of the input voltage, and the phase is the same, achieving the high power factor PF, which has the following equation:
LT)t(V)t(IONIN)peak(L= (1)
4. Typical application line

5. Key component parameter design 5.1 Buck inductor design first determines the maximum duty cycle, and then calculates the Buck inductor by the output LED voltage and current:
D= VLED/VINDC (2)
L=[(1-D)*VLED]/(2*FSW*ILED) (3)
5.2 Iled Current Setting The built-in constant current voltage level of the LD7832 is 0.2V.
ILED=0.2/Rs (4)
5.3 Zcd parameter design

The internal voltage of the LD7832 ZCD is clamped at 0.3--5V. The IC controls the Gate on/off by detecting the ZCD pin voltage and ensures that the IC operates in TM mode. The Pin also has OVP protection. If IZCD>200uA, ZCD OVP function Startup, plus the purpose of Rz2 is to reduce the interference of ZCD pin at high voltage input, OVP of false trigger ZCD. The recommended Rzcd (RZ1) resistance is as follows, Rzcd resistance is recommended to be at least greater than 100k:
1.3*uA2005V)(VR)(ZCDOUTZCD(Rz1)_OVP?> (5)---If there is no Rz2
1.3*}uA200{5/Rz25V)(VR)(ZCDOUTZCD(Rz1)_OVP+?> (6)-----If Rz2 is added
5.4 Vcc design reference Figure 8, Zenor value according to VOUT voltage design, the general Vcc value is set at about 16V, Zenor = Vout-Vcc, Vcc capacitance is set at 10-22μF.
5.5 Comp parameter selection The recommended Comp capacitor value ranges from 0.22-1μF.
5.6 Application example (output 24V300mA)
5.6.1 Actual application circuit diagram

5.6.2 Actual test output current accuracy and efficiency

Test Conditions:
Input: AC90/110/220/264 (60HZ)
Output: CV mode: 20.4-27.6V
Current accuracy (%):
5.6.3 PF and THD