1 Introduction

According to the characteristics of LED lighting load, the current topology of non isolated constant current driving power supply is basically buck buck buck structure. The mainstream scheme is to fix the peak current through fixed off time, so as to achieve the control strategy of fixed output current. This paper will discuss the principle of this control strategy to achieve constant current, analyze the advantages and disadvantages of this open-loop control strategy, and the peripheral compensation required to apply this control strategy. At the same time, based on the new product du8608 chip of duty cycle semiconductor company, this new closed-loop current control strategy will be introduced in detail, and how this control strategy can breakthrough to improve the accuracy of LED output current, From open-loop to closed-loop is its essential breakthrough.

2 principle and design

2.1 at present, the mainstream control strategy in the field of LED non isolated constant current drive current

As shown in Figure 1, the circuit is buck buck buck structure, and the chip controls the source of MOSFET, which is an open-loop constant current control mode. The control principle is as follows:

When the MOSFET is turned on, the current flows from the dcbus through the LED load, through the inductor and into the ground.

Vi-Vo=Ldi/dt=L*Ir/DT （1）

When the MOSFET is turned off, the inductive current freewheeling from D1. The following formula is obtained:

Vo=Ldi/dt=L*Ir/（1-D）T （2）

As shown in Figure 2, IO (average) = ipk-1 / 2 * IR (3)

By (2) and (3)

Io=Vref/Rs-Vo*（1-D）T/（2*L） （4）

VREF and RCS are set fixed values. Since the current flows through the LED load, if the current is fixed, it can be considered that the voltage Vo of the LED is fixed. Therefore, from equation (4), as long as the inductance value L is fixed and the off time (1-D) t is fixed, IO is fixed. Figure 1: schematic diagram of non isolated step-down constant current LED driving power supply Figure 2: inductance current waveform in open loop control strategy

Therefore, this open-loop control strategy is to set the off time of MOSFET by the resistance connected to RT. At the beginning of each cycle, the MOSFET is turned on until the inductance current rises to the peak VREF / rs. at this time, the MOSFET is turned off, and the off time is determined by RT. After the set off time, the MOSFET is turned on again, so it works again and again. The off time controls the average current of ripple current LED. According to the current value to be output, adjust the RS value of CS pin and RT value, and fix the off time of each switching cycle as a value, so as to realize the constant current of output current.

This is a simple and effective control strategy, but because it is an open-loop control mode, it can only detect the peak current on the inductor and cannot detect the output current. The output current accuracy is prone to deviation in three cases:

1. Input voltage fluctuation（ Open loop control, unable to feedback, caused by system delay)

2. Inductance deviation in mass production（ In equation 4, the change of L causes the change of IO)

3. LED load voltage is different (VO)

For the first deviation, voltage compensation can only be adopted, that is, detect the input voltage and adjust the internal CS reference level value according to the input voltage, but the effect is general. For the second and third deviations, it is difficult to solve.

A new full closed-loop control strategy can completely avoid the above deviations and fundamentally realize the real LED constant current.

2.2 how does du8608 realize real full closed-loop constant current control Figure 3: schematic diagram of full closed loop non isolated step-down constant current LED driving power supply

The so-called closed loop is to really detect the output current value and take it as the standard to send PWM signal. The so-called open-loop does not use the detected output current value as a reference for sending PWM signal. From the circuit topology, there is no difference between the two. However, in the chip, the detected inductance current signal of CS pin as shown in Figure 3 is processed with patented technology, as shown in part truec2 of Figure 4. In this way, the average value of the inductance current, that is, the average value of the output current, is detected. According to the detected value, the chip controls the output duty cycle and realizes the closed-loop control. This control structure also makes the circuit very simple. Compared with Fig. 1, Fig. 3 eliminates the input voltage compensation circuit required for open-loop control and the RT resistance required to control the fixed off time. It is worth mentioning that due to the closed-loop control current, compared with the open-loop mode, this line has the function of inductive short-circuit protection and the function of open circuit and short-circuit protection of RCS sampling resistance. This is very important in production application, which greatly reduces the risk of system failure. This makes this scheme not only have a qualitative leap in performance, but also greatly improve the reliability. Figure 4: internal function diagram of du8608 Figure 5: working diagram of truec2

3 experimental verification

We selected a typical LED fluorescent lamp application for IC function verification. The basic electrical parameters are as follows:

Input voltage range: 180 ~ 305vac / 50Hz power factor: 0.9 efficiency: 90%

Output voltage range: 3 ~ 80VDC output current: 240mA Figure 6: application schematic diagram of du8608 fluorescent lamp

When the input voltage and load led change, we get the following cross adjustment rate results: Figure 7: system linear adjustment rate (within the error range of ammeter, regarded as constant) As can be seen in Figures 7 and 8, due to the closed-loop control, the output current maintains a fixed value within the designed normal working range, and a single system can be considered as a constant output current, that is, the linear theoretical value is 0 and the load adjustment rate is ± 0.5%. During mass production, due to the consistent distribution of parameters, a large number of data show that the constant current accuracy is less than ± 0.9%.

For the change of output inductance, we test the following results:  Figure 9: output current accuracy vs. inductance variation (difference between closed-loop control and open-loop control)

Figure 9 shows the change of output current when the inductance changes in a wide range (design nominal value 1.5mH), as shown in the left figure. If this test is used for open-loop system chips in the current market, the current will fluctuate linearly in a large range according to formula (4) derived above, as shown in the right figure. The closed-loop mode keeps the output current within ± 8600.8%. The importance of closed-loop system for improving the constant current accuracy of the whole system is fully explained. Figure 10: LISN test results Figure 11: CDN test results

The three current changes caused by the open-loop mode mentioned above have been fundamentally solved in principle:

1. Input voltage fluctuation（ Cycle by cycle closed-loop control, immediate response, no change in output current)

2. Inductance deviation in mass production（ Inductance value insensitive, even inductive short circuit protection)

3. The LED load voltage is different and the output remains unchanged. This feature is of great significance to the production of power plant, because it means that different numbers of lamp bead loads can be designed with the same set of power supply, which reduces material preparation and cost.

4 Conclusion

Full closed loop control, which detects the output current to send PWM signal, is a real constant current power drive control technology. Experiments show that compared with other non closed-loop schemes, this unique closed-loop constant current control technology makes a qualitative leap in the output current accuracy, and makes the current accuracy of the whole machine power supply in the range of full voltage, full load and inductance reach the highest ± 0.9% in the industry. At the same time, due to the closed-loop control, the line has inductive short circuit Compared with the open-loop scheme, the circuit reliability is greatly improved, and multiple sets of lamp loads can use one set of power supply, which is of great value in production.

Responsible editor; zl