What Is an LED Driver?

At present, the light emitting diode driving chips can be divided into: constant voltage driving chips, constant current driving chips, and pulse driving chips. Among them, the constant voltage driver chip is generally our common DC / DC boost chip. The advantage of this solution is that the chip is cheap and does not have complicated peripheral circuits. But only driving the LED with a constant voltage will cause the circuit current to be uncontrollable when driving the output. The consistency of LED brightness cannot be guaranteed.

LED driver

Currently

LED driver (LED Driver) refers to driving LED light or

According to the electricity rules of the grid and

Drive method can be classified

(1) Constant current type: a. Constant current

From

The reasons for the lack of LED driving power are: (1) The technical staff of the company that produces LED lighting and related products does not know enough about the switching power supply, and the power supply can work normally, but some key evaluations and electromagnetic compatibility considerations are not enough There are still some hidden dangers; (2) Most LED power supply manufacturers have transformed from ordinary switching power supplies to make LED power supplies, and they do not know enough about the characteristics and use of LEDs; (3) There are almost no standards for LEDs. Some are based on standards for switching power supplies and electronic rectifiers. (4) Most LED power supplies are not uniform, so most of them are relatively small. If the purchase amount is small, the price is high, and the component suppliers are not very cooperative; (5) The stability of the LED power supply: wide voltage input, high temperature and low temperature work, over temperature, over voltage protection and other issues have not been solved one by one The first is the overall life of the driving circuit, especially the life of key components such as: the life of the capacitor at high temperatures directly affects the life of the power supply; the second is that LED drivers should challenge higher conversion efficiency, especially when driving high-power LEDs , Because all the power that is not used as light output is dissipated as heat, the power conversion efficiency is too low, which affects the performance of LED energy saving effects; in applications with low power (1-5W), the cost of constant current drive power The proportion has been close to 1/3, which is close to the cost of the light source, which has affected the market promotion to a certain extent.

14W High Efficiency LED Driver Power Design

I. Design Features

1.High ambient temperature (75 degrees)

2.High energy efficiency

3, in line with EU CoC / CEC 2008 / Energy Star 2.0 requirements, high efficiency in load mode (up to 86%, 79.6% required); no-load input power at 250 VAC input <250 mW, 300 mW required

4. Hysteresis thermal shutdown protection

5.Load disconnect protection

6. Full EN55015B conducted EMI limit, EMI margin> 8 dB microvolt

Working principle

The figure shows a typical 20 V, 14 W constant voltage (CV), constant current (CV) output power circuit. The light output of the LED array is proportional to the amount of current flowing through it. Therefore, the LED driver should have a constant current output instead of a constant voltage output. In this design, the DC output is not isolated from the AC input, so the LED array and enclosure should be safely isolated from the user.

title

The AC input is rectified and filtered by BR1, C1, and C2. Inductor L1, together with C1 and C2, forms a filter and provides EMI filtering. Fuse F1 provides protection in the event of a severe fault. In order to make the power supply work normally under no load without damage, the Zener diode VR2 is used for constant voltage adjustment and the voltage is maintained at about 21 V.

The constant current characteristic is achieved by detecting the voltage drop across the current detection resistor R7. The shunt regulator IC U3, together with R9, R8, and R8A, generates an accurate voltage reference of 0.07 V at the inverting input of the operational amplifier U2. When the set current is reached, the voltage on R7 will exceed the reference voltage, which will increase the output of the operational amplifier. At this time, D4 will be forward biased to drive the base of Q1, and then draw the current from the EN / UV pin of U1. Capacitor C7 and resistor R11 provide loop compensation. The current limiting method using an operational amplifier minimizes the current sampling voltage, thereby reducing losses and maximizing efficiency.

As long as the current drawn by the EN / UV pin exceeds 115 A, the MOSFET in U1 is disabled on a cycle-by-cycle basis (on / off control). By adjusting the ratio of enabled to disabled switching cycles, the feedback loop can adjust the output voltage or current. The on / off control mode optimizes the converter efficiency under different load conditions at the same time, making it meet the energy efficiency standards.

Due to the high ambient temperature, U1 will operate in reduced current limit mode. This can improve the overall efficiency of the power supply and improve its heat dissipation performance. Primary clamps (D1, VR1, C3, and R3) control the maximum peak drain voltage internally

The 700 V BVDSS breakdown voltage of the MOSFET . Resistor R23 reduces high-frequency leakage inductance oscillation, thereby reducing EMI. The secondary-side output is rectified and filtered by diodes D2, D3, and C6.

Design points

1. To choose a fast diode instead of an ultrafast diode, improve efficiency by restoring part of the leakage inductance energy.

2. Capacitor C3 is used to improve EMI performance.

3. Select resistor R10, which is used to supply 1 mA to U3 when the minimum output voltage is 6 V.

4. U1 selectable current limit point allows to optimize the current limit point and device size to suit the ambient temperature. For example, to reduce dissipation, the TNY280GN device can be used in the same design by changing C3 from 1 F to 0.1 F. Alternatively, in environments with high thermal performance, the TNY278GN device can be used by changing C3 from 1 F to 10 F.

5. The source can work correctly when the LED string voltage is between 6 V and 20 V. But because the output current is constant, the lower the string voltage, the lower the output power.

Design of LED Lighting Driver Based on Buck Converter

Although the output voltage may be higher or lower than the input voltage, the discontinuous buck-boost converter with peak current mode control is a good choice for LED drivers. However, when using such a buck-boost converter to design a driver, a change in LED voltage will change the LED current, and an open LED will cause an excessively high voltage at the output, which will damage the converter. This article will discuss this converter design for LED in detail, and give a variety of methods to overcome its inherent disadvantages.

Light-emitting diodes (LEDs) have been used for many years, and with the latest technological advances, they are gradually becoming strong competitors in the lighting market. The new high-brightness LED has a long life (about 100,000 hours) and high efficiency (about 30 lumens / watt). Over the past three decades, the light output brightness of LEDs has doubled every 18 to 24 months, and this growth momentum will continue. This trend is called Haitz's law, which is equivalent to Moore's law of LEDs.

Electrically, LEDs are similar to diodes in that they are unidirectionally conductive (although their reverse blocking capability is not very good, high reverse voltages are easily damaged (LED) and have low dynamics similar to conventional diodes Impedance VI characteristics. In addition, LEDs generally have a rated current when they are safely turned on (the rated current of high-brightness LEDs is generally 350mA or 700mA). When the rated current is passed, the difference in LED forward voltage drop may be large, usually 350mA white The voltage drop of LED is between 3 ~ 4V.

Driving LEDs requires a controlled DC current. In order to make the LED have a longer service life, the ripple in the LED current must be very low, because a high ripple current will cause the LED to have a large resistive power consumption and reduce the LED service life. LED driving circuits need higher efficiency, because the overall efficiency depends not only on the LED itself, but also on the driving circuit. The switching converter working in the current control mode is an ideal driving scheme to meet the high power and high efficiency requirements of LED applications.

Driving multiple LEDs also requires careful consideration. Figure 1 is a series-parallel connection circuit for LEDs. Figure 1 (a) shows the parallel connection circuit of LEDs. Figure 1 (h) is a series connection circuit of LEDs. Because the dynamic impedance and forward voltage drop of each LED are different, if there is no external current sharing circuit (such as current mirror), it is impossible to ensure that the current flowing through the LED is the same; In addition, the failure of one LED will make the LED string Turning off causes all LED current to be distributed among the remaining LED strings, which will cause the current on the LED strings to increase, which may damage the LED. Therefore, for the above two reasons, it is generally not necessary to design a parallel LED circuit as shown in Figure 1 (a).

Therefore, it is better to connect the LEDs in series. The disadvantage of this method is that if one LED fails, the entire LED string will stop working. A simple way to keep the remaining LED strings working is to connect a Zener diode (with a rated voltage greater than the maximum voltage of the LED) in parallel with each (or each group) of LEDs, as shown in Figure 1 (b). In this way, after any LED fails, its current will flow to the corresponding Zener diode, and the rest of the LED string can still work normally.

Basic single-stage switching converters fall into three categories: buck converters, boost converters, and buck-boost converters. When the voltage of the LED string is lower than the input voltage, the buck converter Figure 2 (a) is the ideal choice; when the input voltage is always lower than the string output voltage, it is more appropriate to use a boost converter. Figure 2 (b) ; When the output voltage may be higher or lower than the input voltage (caused by output or input changes), it is more appropriate to use a buck-boost converter as shown in Figure 2 (c). The disadvantage of the boost converter is that any transient in the input voltage (which can increase the input voltage and exceed the output voltage) will cause a large current to flow in the LED (due to the low dynamic impedance of the load), thereby damaging the LED. Buck-boost converters can also replace boost converters because transients in the input voltage do not affect the LED current.

How buck-boost converters work

For LED drivers in low voltage applications, a buck-boost converter is a good choice. The reason is that they can drive LED strings (boost and buck) with voltages higher and lower than the input voltage, high efficiency (easy to reach more than 85%), and discontinuous operation mode can suppress changes in input voltage (provided Excellent line voltage adjustment), peak current control mode allows the converter to adjust the LED current without complicated compensation (simplified design), it is easy to achieve linear and PWM LED brightness adjustment, switch transistor failure will not damage the LED and so on. Figure 2 shows the connection circuit of the buck, boost and buck-boost converter to the LED string.

However, this method still has disadvantages: First, the peak current is controlled, because the step-up and step-down converter using discontinuous current mode is a converter with constant power. Therefore, any change in the LED string voltage will cause a corresponding change in the LED current; another problem is that the open-circuit state of the LED will cause high voltage in the circuit that will damage the converter; in addition, an additional circuit is required to convert the constant power converter to constant Current converter and need to protect the converter under no load conditions.

Figure 3 shows a specific buck-boost converter application circuit. The controller has an built-in oscillator for setting the switching frequency. At the beginning of the switching cycle, Q1 is turned on. As the input voltage VIN is added to the inductor, the inductor current (iL (t)) starts to rise from zero (initial steady state). When the induced current rises to a preset current value (ipk), Q1 turns off. The switch on time (ton) is determined by the following formula:

ton = ipkL / VIN

At this time, the total energy (J) stored in the inductor is:

J = Li2pk / 2

In this way, although the switch is closed at this time, the current flowing through the inductor will not be interrupted. This will turn on the diode D1 and generate an output voltage (-Vo) across the inductor. This negative voltage will cause the inductor current to drop rapidly. After a certain time tOFF, the inductor current approaches zero. This time can be calculated by the following formula:

tOFF = ipkL / VO

In order for the converter to work in discontinuous conduction mode, the sum of the switch on time and the inductor current fall time must be less than or equal to the switching period TS, so as to ensure that the inductor current can start from zero in the next switching period.

In fact, (tON + tOFF) can reach the maximum value with the minimum input voltage and the maximum output voltage. Therefore, ensuring that the converter operates in discontinuous conduction mode at these voltages can guarantee that the conditions listed in the following formula can be met under any conditions: tON + tOFFTs

The product of the power (Pin) inductance obtained by the converter from the input and the switching frequency f:

Pin = fsLi2pk / 2

Assuming the voltage (VO) of the LED string is constant and the efficiency is 100%, then the LED current (iLED) is:

iLED = PIN / VLED = Li2pkfs / 2V

In peak current control mode, ipk is usually a fixed value. Therefore, the LED current is completely independent (in theory) from the input voltage. At a fixed IPK, the rise (fall) of the input voltage will cause the transistor's on-time to decrease (increas) in inverse proportion, which will provide good line voltage regulation. In practical applications, the delay between the detection of the current peak by the control IC and the actual shutdown of the GATE pin will cause the input power to vary. Shorter on-time results in more errors due to the delay time because the delay time will account for a significant portion of the on-time.

In fact, the LED current is inversely proportional to the voltage of the LED string. A circuit with a nominal output of 20 V and 350 mA will generate 700 mA at a 10 V output voltage, which is obviously not the desired result. However, by making the switching frequency proportional to the output voltage, the above formula provides a way to convert a constant power converter into a constant voltage converter.

Assuming fs = KVO, where K is constant, then:

iLED = kLi2pk / 2

In this way, iLED will be independent of the input and output voltages.

Another disadvantage of the flyback converter is that it is susceptible to the open-circuit state of the output. When the LED is open, the energy stored in the inductor will be transferred to the output capacitor at the end of each switch on time. In this way, a load lacking a capacitor discharge will cause the voltage across the capacitor to rise gradually, eventually exceeding the nominal value of the device and damaging the power stage. Therefore, additional circuits can be added to provide output voltage feedback and overvoltage protection.

Output voltage feedback

Figure 4 is an additional circuit that can implement overvoltage protection and open LED protection. In fact, many peak current mode controller ICs have dedicated RT pins. The resistor connected to this pin can be used to set the internal current, which is used to charge the oscillator capacitor (which can be internal or external). The ramp voltage on the oscillator capacitor controls the switching frequency so that the switching frequency is proportional to the output current at the RT pin. The smaller the resistance (larger), the larger the current (smaller) and the higher the switching frequency (lower). Based on this principle, the output frequency feedback can be used to adjust the switching frequency.

In the circuit shown in Figure 4, resistors R3 and R4 form a voltage divider. The voltage across R4 minus the voltage drop (Vbe) between the base and emitter of transistor Q2 is the voltage across R5. Therefore, the current flowing through R5 (IR5) is:

This current is obtained from pin RT of the control IC using a matched transistor pair.

Resistor R2 in Figure 4 is used to start the converter. In the startup state, the output voltage is zero, so IR5 is also zero. The converter cannot start because there is no current from the controller's RT pin. Increasing the resistance R2 can get a small part of the current in the startup state, and the size of R2 satisfies:

IR5 >> V (RT) / R2

Where V (RT) is the voltage on the RT pin of the controller. Meeting this condition ensures startup of the converter and minimizes the error introduced by R2. If you choose R3 = R4, you have:

IR5 >> VO / 2R5

It is assumed here that the output voltage is much larger than the base-emitter voltage drop of Q2.

In this way, according to the above formulas, the output LED current can be obtained as:

iLED = KICLi2pk / (2 × 2R5)

In this way, the LED current will no longer be determined by the input or output voltage. The use of resistor R6, transistor Q3 and Zener diode D2 can increase the overvoltage protection function. In the open LED state, when the switch is on, the inductor stores energy, and when the switch is off, this energy is transferred to the output capacitor. Because there is not enough load to discharge the capacitor, the output voltage will gradually increase every cycle. When the voltage rises above the Zener diode's turn-on voltage, the Zener diode branch circuit consisting of D2 and R6 begins to conduct. This also provides a path through the base current of Q3, turning Q3 on. At this time, the resistor R4 is actually shorted. Therefore, the PN junction of the base-emitter of Q2 will be closed, resulting in zero current on R5. This will stop the internal oscillation of the controller until the output voltage drops below the Zener diode voltage, and the above process continues. This burst mode minimizes the average power of the LED in an open circuit state. This overvoltage protection method will force the control IC into a low-frequency, low-power operating mode.

The current in the Zener diode's resistance branch circuit must be able to generate enough voltage on R6 to bias the PN junction between the base and emitter of the transistor.

Concluding remarks

In switching LED drivers with output current feedback, feedback compensation is generally required to stabilize the converter and adjust the current to achieve the desired current value. The transient response performance of these feedback schemes is limited and cannot meet the fast on / off transient response required by the PWM brightness adjustment of LEDs. However, the converter described herein does not require any feedback compensation. The only feedback used in this control scheme is to obtain the peak current through the MOSFET through the sense resistor. Because the converter stores the required energy every cycle, it can respond instantly to transients. So it can easily work with the PWM brightness adjustment scheme.

The buck-boost converter is an effective solution for low DC voltage input LED drivers. It can drive LED strings regardless of whether the output voltage is higher or lower than the input voltage. In addition, small and inexpensive additional circuits can be added to the converter to overcome problems with load regulation and no-load conditions. The converter is easy to implement and does not require feedback compensation during peak current mode control. Its open-loop nature also makes it ideal for applications that require PWM dimming. ^[1]