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Design of BUCK Constant Current Source Circuit Based on RCC

by: Oct 30,2020 1402 Views 0 Comments Posted in Technology

RCC BUCK Constant Current LED driving

RCC stands for Ringing choke converter, which also means oscillating-blocking power converter. As a once very popular power converter structure, with the rapid development of the integrated circuit industry, it seems to have gradually faded out of mainstream applications.

Through the introduction of RCC-based BUCK constant current source circuit, this article explains the method and idea of using discrete components to achieve high precision and high efficiency constant current, and the application of RCC still has its huge potential to be tapped, and Strive to bring certain help to the design of conventional circuits and integrated circuits.

RCC is generally used in low-power flyback isolated power conversion applications. But if you carefully analyze its circuit structure and power loss factors (the analysis process is omitted), you can get the following main conclusions:

1.RCC can be well used in other applications except flyback isolation;

2.RCC can be well applied to high-power applications, especially BUCK and its derivative structures;

3.RCC BUCK can be insensitive to transformer parameters (such as primary inductance), and can make good use of the leakage inductance of the transformer and the distributed inductance of some circuits, which helps control the batch quality and reduce the design difficulty;

4.RCC BUCK can well solve the problem of low power and high efficiency;

5.The inherent advantages of RCC, such as simple structure, working in full energy transfer mode, and not afraid of short circuits, are well maintained in BUCK;

6.Corresponding to the constant power characteristics of flyback isolation, RCC BUCK has its own constant current characteristics, and does not require additional sampling, feedback and other devices; etc.

Accordingly, the RCC-based BUCK constant current source circuit described below is designed.

Circuit structure and function description

The following figure shows the circuit structure of common ground output:

The circuit structure is very simple, and it is easy for readers familiar with RCC to understand the working principle of the above figure. The following is a qualitative description of the main part:

  • Starting circuit: R1;
  • Peak current limit circuit: R2, R4, R5, Q1, Q2, among which Q1 and Q2 form a positive feedback fast turn-off circuit (that is, thyristor);
  • Zero current detection and gate protection circuit: R6, C2, D2;
  • Freewheeling circuit: D1;
  • Current sharing circuit: R3;
  • Output circuit: CD2, CD3, L1, R7, where R7 is a dummy load.

Some characteristics of the circuit can be seen:

1. The output current mainly depends on the value of R5;

2. When the output voltage is reasonable and the input voltage changes within a certain range, the output current does not change;

3. When the input voltage is reasonable and the output voltage changes within a certain range, the output current does not change;

4. When the withstand voltage of the output capacitor is greater than the input voltage, it can be no-loaded, and the vibration is almost stopped at no-load, and the loss is determined by R7 and D2;

5. The circuit will not be damaged when the output is short-circuited;

6. When the transformer inductance and other parameters are certain, the oscillation frequency is related to the input voltage, output voltage, output current, etc.;

7. The leakage inductance of the transformer and the energy stored in the distributed inductance of the transformer peripheral circuit will be released at the output end;

8. The output current is negatively correlated with temperature, which is just suitable for most LED application requirements;

9. The concept of "floating ground" that is "not practical" in traditional theory is used in the design.

Therefore, the circuit can achieve a good constant current output effect without adding additional current sampling and feedback components, but the disadvantage is that the input voltage range is not very wide.

(The circuit also has a common positive output structure, similar to this. )

Main design points

The main point and difficulty of the circuit design are the main inductance, the value of R2~R6, and the calculation of the turns ratio. It is possible to adopt experimental methods, but it will cause many unfavorable factors such as deviation of the center value; the calculation method is not difficult. , It should be noted that we should choose the correct reference potential, the aforementioned "floating ground".

Test and conclusion

According to the above-mentioned schematic diagram, the 3~100W LED driving experiment circuit is overlapped with conventional devices. In the case of AC150~250V input, the measured main parameters are:

1. Low power output efficiency can reach more than 92%, high power output efficiency can reach more than 99%;

2. Voltage adjustment rate ≤2%;

3. Load regulation rate ≤5%;

4. Long-term output short circuit or open circuit will not cause circuit damage, and the power consumption is extremely low;

5. LED light without stroboscopic;

6. Output π-type filtering to make the LED light more pure and heat less;

In summary

The design of RCC deformed circuit is feasible, BOOST and other circuits can also refer to the above ideas; RCC BUCK is a constant current drive circuit that can be practical and low-cost, which can effectively solve the problem of high-efficiency and high-quality LED driving at various power levels; If the main circuit is integrated, the LED drive circuit will be further simplified and optimized.


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