Buck/boost LED constant current drive circuit diagram

Some electronic equipment and household appliances do not require the use of switching power supplies with completely isolated input and output. For example, the driving power supply of DC motors, the regulated power supply in air conditioners, frost-free refrigerators and microwave ovens, etc. The driving power supply of LED lighting itself is an isolated system, so it can be powered by non-isolated switching power supply, but this kind of switching power supply is required. The circuit is simple, low cost, high power conversion efficiency and reliability, and easy to use and maintain. Linkswitch TN series products provide 7 non-isolated circuit connection methods. Considering the safety of load devices and the characteristics of LED requiring constant current power supply mode, this design chose the sixth wiring method, that is, positive terminal voltage reduction. / Boost (Buck - Boost) constant current drive circuit.

The positive terminal buck/boost (Buck Boost ) constant current drive circuit is shown in the figure. Its main features are as follows: negative pressure output; when used as a step-down type | Uo | < UI; the circuit is simple and the cost is low. Debugging and production are simple; the accuracy of the output voltage Uo is about ±10%; even if the MOSFET in the power IC is damaged, the input voltage will not be added to the output and damage the load.

As shown in the figure, the basic working principle of the circuit is as follows: the input circuit is composed of fusible resistors R1, VD1, VD2, C1, CZ and filter inductor u; the power adjustment circuit is composed of LNK3OGP, energy storage inductor L2, VD3, R2, C4 and C5. When LN K30 6 is turned on, the input current I forms a loop through (LNK306) D → (LNK306) S → L2 → C2 (—), and the input current IIN is stored in the inductor L2 (the inductor current is according to UI/LI, linear increase), the polarity is upper positive and lower negative; when LNK306 is turned off, because the inductor current cannot mutate (the inductor current decreases linearly according to UI/L1), the polarity is lower positive and upper negative; the energy stored in L2 passes through the lower end of L2 (Ground)→LED (Load)→VD3→R2→The upper end of L2 forms a loop. VD3 is the rectifier; C5 is the output filter capacitor; R2 is the current limiting protection resistor; C4 and R2 are connected in parallel to eliminate noise interference; VD4 and VD5 form a simple voltage stabilizing circuit. Load current r. The voltage drop generated on R2 is Us = IOR2. Us is divided by R3 and R4. The divided voltage obtained on R4 is used as a feedback voltage to the FB terminal of LNK306. The normal operating voltage of the FB terminal is 1.65V. It can be adjusted by adjusting R3 Use the voltage dividing ratio with R4 to adjust the output current. The FB terminal of LNK306 controls the on-time of the internal MOSFET according to the feedback voltage, that is, the output current is adjusted by skipping cycles. When the output current increases, the voltage drop generated by Io on R2 also increases linearly, and the voltage division value on R4 also increases, so the current IFB flowing into pin FB will also increase. If the current IFB>49μA, then the subsequent cycle will be skipped until IFB (49μA; vice versa. Therefore, many cycles will be skipped when the load is lightened, and fewer cycles will be skipped when the load is increased. If an output overload or short circuit fault occurs, the Linkswitch-TN switch enters automatic reset. During the startup phase, the output power is reduced to POM×6%, thus limiting the average output power.

The composition of this circuit is very simple, with only 17 components including LN K30 6. The selection principle of the main components is: L2 is an energy storage inductor, using an inductor with a ferrite core to reduce costs and reduce audio frequency. Noise, in order to make the inductor current work in CCM mode, the inductance is selected as 1.5mH. Freewheeling diode VD3: This design uses continuous mode operation and requires trr≤35ns. UF4005 is an ultra-fast recovery diode with trr=30ns, which can meet the requirements of the working mode. Feedback resistor R3 and bias resistor R4: The resistor divider composed of R3 and R4 should keep the voltage of pin BB at 1.65V. The accuracy of this resistor directly affects the level of the feedback voltage, which in turn affects the constant current value. Therefore, choose a resistor with ±1% accuracy.