Low-power LED driver power supply technology topology solution

As countries around the world pay more and more attention to energy conservation and emission reduction, LED as a new light source is increasingly widely used due to its high efficiency and energy saving. The following mainly introduces the non-isolation technology of LED driver application in the small power range of 1-30W.

One RC voltage reduction:

1. The principle and application of RC voltage reduction:

Capacitor voltage reduction actually uses capacitive reactance to limit current, and the capacitor actually plays a role in limiting current and dynamically distributing the voltage across the capacitor and the load.

2. When using capacitors to reduce voltage, the following points should be noted:

Select the appropriate capacitor according to the load current and the working frequency of the AC power, rather than the load voltage and power. The current limiting capacitor must be a non-polar capacitor, and electrolytic capacitors cannot be used . In addition, the capacitor's withstand voltage must be above 400V. The most ideal capacitor is a polypropylene metal film capacitor . Capacitor voltage reduction cannot be used for high-power conditions. It is generally used for low-power applications below 5W. Capacitor voltage reduction is not suitable for dynamic load conditions. Capacitor voltage reduction is not suitable for capacitive and inductive loads. It is suitable for single voltage applications in the application of LED power supply driving.

3. The basic circuit of the simple RC step-down power supply is as shown in (Figure 1)

Figure 1.

C1 is a step-down capacitor, D1, 2, 3, 4 are bridge rectifier diodes, ZD1 is a voltage regulator diode, and R1 is a charge discharge resistor of C1 after the power supply is turned off .

4. Device Selection

When designing a circuit, the exact value of the load current should be determined first, and then the capacity of the step-down capacitor should be selected with reference to the example. Because the current Io provided to the load by the step-down capacitor C1 is actually the charge and discharge current Ic flowing through C1. The larger the capacity of C1 and the smaller the capacitive reactance Xc, the greater the charge and discharge current flowing through C1. When the load current Io is less than the charge and discharge current of C1, the excess current will flow through the voltage regulator. If the maximum allowable current Idmax of the voltage regulator is less than Ic-Io, it is easy to cause the voltage regulator to burn out. To ensure that C1 works reliably, its withstand voltage should be selected to be greater than twice the power supply voltage. The selection of the discharge resistor R1 must ensure that the charge on C1 is discharged within the specified time.

5. Actual parameter calculation method:

Given that 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 circuits:

1. Typical circuit as shown in (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 purpose of outputting constant current. The purpose of selecting 432 is to use the 432 reference of 1.21V to reduce the loss on R3. The current constant value is 1.21/R3. R1 is selected according to the amplification factor of Q1.

3. Application notes:

This circuit is recommended for single voltage input and low output current LED power driver, such as bulbs, T-tubes, etc. The output current is generally recommended to be below 100mA. At the same time, the closer the output voltage is to the input, the better, so as to avoid excessive loss and low efficiency caused by excessive voltage drop of Q1. Therefore, it is best to use LEDs in series.

Three constant current diode drive circuit

1. Typical circuits are shown in (Figure 3, Figure 4)

Figure 3

Figure 4

2. Working Principle

An ideal constant current source is a device with infinite internal resistance. No matter what the voltage across it is, the current flowing through it never changes. Of course, such a device is impossible to exist. The actual constant current diode is equivalent to a device whose current is constant at a certain value, such as 20mA, within a certain operating voltage range, such as 25-100V. Its equivalent circuit is shown in Figure 5.

Figure 5

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

Figure 6

It has a constant current interval within 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 Notes

Since the constant current diode needs a certain voltage Vk to enter the constant current, it cannot work with a power supply voltage that is too low. Usually this Vk is about 5-10V, so most battery-powered LEDs cannot work. The maximum current is limited by the power consumption of the constant current diode, so too large a current is also inappropriate. For example, a 1W LED usually requires 350mA, which is difficult for a constant current diode to provide. At present, the more suitable use occasion is that the LED lamps powered by AC mains use many low-power LEDs in series, that is, the situation of high voltage and low current is the most suitable; but since the withstand voltage of the constant current diode is limited, the power supply voltage change it can absorb is also limited. Take the 100V withstand voltage CRD as an example. It can only deal with limited voltage changes when used in a 220V mains power supply. After the 220V is bridge rectified, its output DC voltage is about 264V. If the mains changes by +10% ~-15%, it is equivalent to 290~187V after rectification, and the voltage change is 103V. It has exceeded its withstand voltage. Because the LED volt-ampere characteristic is nonlinear, it is difficult to express it with a formula. In short, when the mains voltage decreases, the current in the LED will decrease with the decrease of the mains voltage. Its brightness will also change. Figure 3 is a typical application circuit in the typical application circuit, and Figure 4 is an application circuit with a resistor-capacitor voltage reduction to cope with low voltage output occasions.

Four Buck circuits using unipolar PFC

With the current regulations and energy efficiency requirements, LED applications require high PF and reliable operation over the entire voltage range, and are moving towards miniaturization. Therefore, the previous valley-filling PFC circuit also needs to add two high-voltage capacitors . Due to the size limitation, it is not suitable for application. In view of this, many manufacturers at home and abroad have launched non-isolated power application drive solutions for bulbs and T-tubes. The following is a typical introduction of LD7832 from Tongjia Technology.

1. LD7832 Introduction

LD7832 is a high PF LED driver control chip that uses TM mode control in Buck circuits. It uses fewer peripheral components to minimize the PCB size, has complete protection functions, and meets the requirements of various functional tests and reliability application tests. The design and debugging are quite simple, and it can meet customer requirements for rapid design and mass production and meet regulatory requirements. It is suitable for products such as bulbs below 30W, T-tubes, etc. In order to meet different needs, LD7832 has different versions of external MOS and internal MOS (2A) to choose from.

2. Features Built-in 600V high voltage starting circuit

High PFC Function Controller

Efficient Transition Mode Control

Low-cost design with minimal peripheral components

High current adjustment accuracy

Wide range UVLO (17V on, 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. Working Principle

LD7832 is a PFC controller with fixed on-time that works under boundary conditions using voltage mode control. It compares the IC's Comp voltage with the IC's internal Ramp signal to determine the MOS on-time. The working principle waveform is shown in Figure 7.

Figure 7

In half of the input voltage cycle, TON is controlled to be fixed, so that the inductor current peak follows the input voltage peak with the same phase, achieving a high power factor PF, with the following equation:

LT）t（V）t（IONIN）peak（L= （1）

4. Typical application circuit:

Figure 8

5. Key parts parameter design

5.1 Buck Inductor Design

First determine the maximum duty cycle, then calculate the Buck inductance from 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 LD7832 is 0.2V, so:

ILED=0.2/Rs （4）

5.3 Zcd parameter design:

Fig. 9

The internal voltage of LD7832 ZCD is clamped at 0.3--5V. IC controls Gate on/off by detecting the voltage of ZCD pin and ensures that IC works in TM mode. At the same time, this pin also has OVP protection function. If IZCD>200uA, ZCD OVP function is activated. The purpose of adding Rz2 is to reduce the interference to ZCD pin when high voltage input occurs, and to avoid false triggering of ZCD OVP. The recommended Rzcd (RZ1) resistance value is as follows. The Rzcd resistance value 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

Refer to Figure 8, Zenor value is designed according to VOUT voltage, generally Vcc value is set at about 16V, Zenor=Vout-Vcc, Vcc capacitor is set at 10-22μF.

5.5 Comp parameter selection

The recommended Comp capacitor value range is around 0.22-1μF.

5.6 Application example (output 24V300mA):

5.6.1 Actual application circuit diagram

Fig.10

5.6.2 Actual test of 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

5. Conclusion

This article briefly explains the non-isolated circuit

Some applications on LED driver power Compared with other applications, the Buck PFC method can work at full voltage. The LED constant current accuracy is basically not affected by the input voltage. The LED can work at a larger current and achieve higher efficiency in the full voltage range. The operation is more stable and reliable. Of course, because it is a high-frequency switching working mode, it will also bring some EMC problems, which is unavoidable.