How to design a high-efficiency LED driver circuit?

Design Features

1. The precise primary side constant voltage/constant current controller (CV/CC) eliminates the optocoupler and all secondary side CV/CC control circuits, and the highest efficiency can be achieved without the need for current detection resistors; Low-cost solution.

2. The automatic restart protection function can reduce the output power to below 95% under the condition of output short circuit or open loop.

3. Hysteresis thermal shutdown function can prevent power supply damage

4. Meet the efficiency requirements of CEC and Energy Star 2.0: Green package: halogen-free and RoHS compliant

Working principle

Figure 1 is a circuit diagram of a universal input 7.6 V, 700 mA constant voltage/constant current flyback power supply designed using LinkSwitch-II device LNK606PG, which is suitable for LED driver applications.

LNK606PG (U1) integrates the power switching device, oscillator, CC/CV control engine, and start-up and protection functions into the IC.

Diodes D1 to D4 rectify the AC input. Capacitors C1 and C2 filter the rectified AC. These capacitors, together with inductors L1 and L2, can also attenuate differential mode conducted EMI noise. Resistors R1 and R2 can damp the resonance oscillation generated between these capacitors and the inductor. The combination of the above design and Power Integrations’ transformer E-sheild™ technology allows the power supply to easily meet EN55015 Class B conducted EMI requirements with a margin greater than 10 dB without the need for Y capacitors. The fusible resistor RF1 is used to limit the inrush current generated during startup and acts as a fuse when the component fails due to excessive input current.

This power supply design utilizes U1's integrated constant current characteristics to drive LED loads and can work with maximum output power in constant current mode. IC U1's constant voltage mode can provide output overvoltage protection in the event of any LED open circuit failure.

When working in the constant current phase, U1 adjusts the output current by changing the switching frequency of the MOSFET. As the output voltage increases, U1 will increase its switching frequency. The output voltage is determined by the number of LEDs in the load. The values of resistors R5 and R6 determine the maximum switching frequency and output voltage. The inductance of the transformer ensures that the driver always works at maximum power.

If an output failure occurs, the power supply will operate in constant voltage mode and use the on/off control method to adjust the output voltage. This can provide automatic output fault protection and reduce power consumption in this case. The automatic restart function of IC U1 provides output short circuit protection.

IC U1 is fully self-powered through the BP (bypass) pin and decouples capacitor C4, while also providing high frequency decoupling. When the internal MOSFET turns on, U1 uses the energy stored in C4; when the MOSFET turns off, the internal 6 V regulator draws current from the drain pin. This eliminates the need for external bias windings. Adding an external bias winding will further reduce no-load power consumption.

The rectified and filtered input voltage is applied to one side of the primary winding of T1. The integrated MOSFET in U1 drives the other side of the transformer primary winding. D5, R3, R4 and C3 form an RCD-R clamp circuit to limit the drain voltage spike caused by leakage inductance. IC U1 automatically compensates for tolerance differences in the primary magnetizing inductance. The output power is directly proportional to the set primary inductance, and the output power change can be detected at the FB pin. When the output power changes, the switching frequency will be adjusted to compensate for inductance fluctuations.

Diode D7 (Schottky barrier diode is used to improve efficiency) is used for the secondary output of the rectifier transformer, and C7 filters it. Resistor R8 and capacitor C6 eliminate high-frequency conducted and radiated EMI. When no load is connected, the dummy load resistor R9 acts as a bleeder resistor for C7.

Three, design points

1. The creepage distance between the high-voltage pin and the low-voltage pin on U1 is very large, which can avoid arcing and further improve reliability, which is particularly important under high humidity or high pollution conditions.

2. Capacitor C7 has inefficient series impedance (ESR), which can reduce the output voltage ripple and eliminate the LC post-stage filter.

3. The feedback resistors R5 and R6 should have a tolerance value of 1%, which helps to strictly control the rated output voltage and constant current adjustment threshold at the center position.

4. Using external bias winding can further reduce no-load power consumption. Place the bypass pin capacitor close to U1 on the PCB.

5. Reduce the loop area of the clamp and output diode to reduce EMI.

6. Keep the AC input and the switch node at a certain distance to reduce noise coupling that may bypass input filtering.

7. Make sure that the peak drain voltage of the U1D pin is lower than 650 V, otherwise, it needs to be achieved by reducing the value of R3.

 

If you want to know more, our website has product specifications for LED driver, you can go to ALLICDATA ELECTRONICS LIMITED to get more information

How to design of energy-saving electric vehicle AC drive system based on boost DC-DC bidirectional converter and inverter?

 Main unit circuit design

1. Switch tube Su control circuit

According to requirements, the control chip needs to have functions such as soft start, overcurrent, and undervoltage protection. The system uses UC3842A from Motorola, which is a function that can complete feedback voltage comparison, error amplification, overcurrent protection, undervoltage protection and other functions. Current tracking type PWM control integrated circuit.

The switch tube Su control circuit is shown as in Fig. 2. Its working characteristics are: ① The maximum power supply voltage Vcc = 30V, there is a 36V voltage regulator tube inside to effectively prevent damage caused by high-voltage intrusion; ② Undervoltage lockout function, the threshold of the starting voltage is 16V, the shutdown voltage is 10V, 6V The difference between startup and shutdown can effectively prevent the circuit from oscillating when working near the threshold voltage; ③ comes with a stable 5V reference voltage, output by pin 8 for external use, the output current is 20mA; ④ the output high level is 13.5 V (Vcc = 15V, when output current is 200mA), low level is 1.5V (when input current is 200mA); ⑤ High and low level rise and fall time is 100ns, current sampling signal (input from pin 3) When it is greater than 1V, the pulse width modulation latch flips, and the output pin 6 immediately drops from high level to low level. Therefore, by changing the size of the current sampling resistor, the threshold of the overcurrent protection action can be changed. ⑥ Current tracking characteristics: When the current flowing through the switch tube Su in Figure 1 increases, the sampling voltage on the sampling resistor R21 increases, and the signal entering pin 3 of UC3842A becomes correspondingly larger. At this time, it is adjusted by the internal adjustment circuit of 3842A. The duty cycle of the output pulse of pin 6 is correspondingly reduced, so that the output voltage of the DC-DC converter is reduced, and the current flowing through Su is correspondingly reduced, which plays a role in current protection.

Figure 1 switch tube Su control circuit

During the energy feedback, the switch tube Sd is in the working state. In order to ensure that the system energy is fully fed back, and at the same time avoid the switch tube Sd to withstand a large feedback current for a long time, the "multivibrator + high-frequency pulse transformer with a frequency of 20 kHz composed of 555 is used "To drive Sd.

Figure 2 is the drive circuit composed of "multivibrator + high frequency pulse transformer", in which the working frequency of the multivibrator composed of 555 is f = 1.43 / (R18 + 2R22) / C19. In this circuit, the detection and interlock circuit controls pin 4 of the 555 manifold. When pin 4 is high, that is, the detection circuit detects that the system should enter the energy feedback state, the multivibrator starts to output switching pulses to Sd; when pin 4 is low, the system is in electric operation, multivibration The oscillator does not output switching pulses to Sd.

2. Detection and interlock circuit

In this system, the detection and interlock circuit has an extremely important role. First, it determines whether it is necessary to switch the circuit operating mode from the electric operating state to the energy feedback state or from the energy feedback state to the electric operation state by detecting the voltage level at the output end of the DC-DC converter; second, it needs to detect and As a result of the judgment, the drive circuits of the electric operation switch tube and the energy feedback switch tube are controlled accordingly. Figure 3 shows the detection and interlock circuit. Its working principle is as follows: First, set the reference voltage of the comparator pin 2 according to the output voltage of the DC-DC converter during normal operation, and use a resistor divider to detect the DC- The voltage at the output of the DC converter. When the system is decelerating downhill or braking, the motor is in the power generation state, then the back-EMF of the machine is greater than the output voltage of the DC-DC converter, that is, the comparison voltage detected by the resistor divider is greater than the reference voltage , Making the comparator flip, pin 1 output is high, it forces the transistor Q1 to turn on, pull down the gate signal of the switch tube Su to low level; in addition, also because the voltage divider is provided to pin 2 of UC3842A When the voltage exceeds the internal reference voltage of chip one, it immediately turns off the pulse output of UC3842A. The two safely blocked the switch tube Su. At the same time, the high level of the pin 1 of the comparator enters the pin 4 of the multivibrator, turning on the multi-resonance oscillation circuit, so that the entire system enters the energy feedback state.

3. Main circuit intelligent power module IPM

In this system, the three-phase inverter circuit has a very important role. It not only provides the power supply voltage for the squirrel-cage asynchronous motor, but also controls the frequency conversion of the motor. In the past, the inverter circuit was mainly constructed with 6 separate IGBT units. It is necessary to provide a driving circuit, an overheat protection circuit, and an overcurrent protection circuit for each IGBT unit. The matching of IGBT units makes the design of variable frequency inverter circuits quite difficult. However, with the emergence of intelligent power modules (IPM), this situation has been greatly changed, especially in recent years, IPM is gradually replacing ordinary IGBT modules.

The IPM module is a high-performance high-power device integrating the IGBT chip as the main body, integrating the chip and its gate drive, control and overcurrent, overvoltage, overheat, short circuit, undervoltage lockout and other protection and fault detection circuits. It has the advantages of compact structure, small volume, stable performance, reliable operation and moderate price. Therefore, combined with the basic requirements of the electric vehicle driving device, the IPM module PS21255-E produced by Mitsubishi was selected as the inverter circuit in this system.

In the PS21255-E module, the drains of the three tubes of the lower arm are at the same point, and this point is connected to the system ground through a small current detection resistor, which can be directly used as the + 15V power supply of the system ground as the reference point The drive control is performed, but the drains of the three tubes of the upper arm are not in place. You need to pass an external circuit. When the tube of the lower arm is turned on, the + 15V drive power of the lower arm charges the external capacitor at the same time. , The voltage drop of + 15V is maintained at both ends of the capacitor, and its low potential end is just connected to the drain of the upper arm IGBT tube. Therefore, the bootstrap function of the upper arm IGBT tube gate voltage is higher than the drain, very Drive the upper arm tube well.

 

4. SA866AE / AM and EEPROM parameter setting

The serial three-wire interface of SA866AE / AM can be connected with 256-bit or 1024-bit serial EEPROM, such as 93C06 or 93C46. All parameters are stored in EEPROM, and are automatically downloaded through the serial interface after reset. This system intends to use 93LC46 for parameter storage and interface with SA866AE / AM.

If you want to know more, our website has product specifications for converter and inverter, you can go to ALLICDATA ELECTRONICS LIMITED to get more information

What are the RS-422 and RS-485 interfaces and what are their characteristics?

 The half-duplex network composed of RS485 interface is generally a two-wire system, and shielded twisted pair transmission is mostly used. This wiring method is a bus topology. Up to 32 nodes can be connected on the same bus. We know that initially the data is a simple process of analog signal output, and later the instrument interface is an RS232 interface. This interface can implement point-to-point communication, but this method cannot realize the networking function, and the subsequent RS485 solves this problem. To this end, this article introduces the RS485 interface in detail in the form of question and answer.

1. What is the RS-485 interface? What are its characteristics compared with RS-232-C interface?

Answer: Since the RS-232-C interface standard appeared earlier, it is inevitable that there are deficiencies, mainly including the following four points:

(1) The signal level value of the interface is high, which is easy to damage the chip of the interface circuit, and because it is incompatible with the TTL level, it is necessary to use a level conversion circuit to connect with the TTL circuit.

(2) The transmission rate is low. In asynchronous transmission, the baud rate is 20Kbps.

(3) The interface uses a signal line and a signal return line to form a common ground transmission form. This common ground transmission is prone to common mode interference, so the anti-noise interference is weak.

(4) The transmission distance is limited. The standard value of the maximum transmission distance is 50 feet. In fact, it can only be used around 50 meters. In response to the shortcomings of RS-232-C, some new interface standards have emerged continuously. RS-485 is one of them. It has the following characteristics:

1) Electrical characteristics of RS-485: logic "1" is represented by the voltage difference between the two wires as +(2-6)V; logic "0" is represented by the voltage difference between the two wires as -(2-6)V . The interface signal level is lower than that of RS-232-C, it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, which can be easily connected with the TTL circuit.

2) The maximum data transmission rate of RS-485 is 10Mbps

3) The RS-485 interface is a combination of a balanced driver and a differential receiver, which enhances the ability to resist common mode interference, that is, has good resistance to noise interference.

4) The standard value of the maximum transmission distance of the RS-485 interface is 4000 feet, which can actually reach 3000 meters. In addition, the RS-232-C interface allows only one transceiver to be connected on the bus, that is, single station capability. The RS-485 interface allows up to 128 transceivers to be connected on the bus. That is, it has multi-station capability, so that users can easily establish a device network using a single RS-485 interface.

5) Because the RS-485 interface has good anti-noise interference, long transmission distance and multi-station capability, the above advantages make it the preferred serial interface. Because the half-duplex network composed of the RS485 interface generally only needs two wires, the RS485 interfaces are all shielded twisted pair transmission. The RS485 interface connector uses a DB-9 9-pin plug socket, the RS485 interface to the smart terminal uses DB-9 (hole), and the keyboard interface RS485 to the keyboard uses DB-9 (pin).

3. Points for attention in the network installation of RS-422 and RS-485

RS-422 can support 10 nodes, and RS-485 supports 32 nodes, so multiple nodes form a network. The network topology generally adopts a bus-type structure with terminal matching, and does not support ring or star networks. When constructing the network, the following points should be noted:

1. Use a twisted-pair cable as the bus to connect the nodes in series. The length of the lead from the bus to each node should be as short as possible to minimize the impact of the reflected signal in the lead on the bus signal. Shown are some common wrong connection methods (a, c, e) and correct connection methods (b, d, f) in practical applications. Although the three network connections a, c, and e are incorrect, they may still work normally at short distances and low rates. However, as the communication distance increases or the communication rate increases, the adverse effects will become more and more serious, mainly because The signal is superimposed on the original signal after being reflected at the end of each branch, which will cause the signal quality to deteriorate.

2. Pay attention to the continuity of the characteristic impedance of the bus, and signal reflection will occur at the point where the impedance is not continuous. The following situations are prone to this kind of discontinuity: different sections of the bus use different cables, or there are too many transceivers installed on a certain section of the bus close to each other, and the long branch line leads to the bus.

In short, a single, continuous signal channel should be provided as a bus.

 

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