LED lighting has become the focus of everyone's attention because of its high energy saving, long life and environmental protection. There have been many successful cases in the field of functional lighting in many parts of the country. China's semiconductor lighting application technology is gradually in the forefront of the world. With the encouragement of national and local government policies, many places are in outdoor lighting such as: street lights, landscape lighting, etc.; indoor lighting such as: subway, underground garage, museum; special occasion lighting : Such as low temperature lighting, miner's lamp lighting, car lights and other aspects are widely used. Some traditional lighting companies have begun investing in transforming LED luminaires. LED indoor lighting and application technology are advancing by leaps and bounds. Indoor lighting is undoubtedly a huge market, and the market prospects are beyond doubt. I believe that in 2010, it will occupy a large market share in home lighting.
The most common lamps for indoor lighting are E220, GU10, PAR30, PAR38 and other AC220V high-voltage direct input LED spotlights. E27, Gu10 LED spotlights need to be directly converted into DC LED constant current source to drive high-brightness LED light source. Integrated circuit products for single SoCs are not currently available, and most are switching power supply solutions with primary or secondary feedback. However, the original feedback scheme has the problem that the output current accuracy is not high, and is generally around +-5%. The flyback constant current drive scheme with secondary feedback provides an output current accuracy of +-2%.
There are many drive ICs for the flyback constant current drive scheme that can be used for secondary feedback. This article will introduce the offline LED spotlight design technology based on the PT4201 control chip.
1W-30W offline high brightness LED driver controller PT4201
The PT4201 is a high-brightness LED driver controller that operates in current mode and can drive 1W to 30W illumination or spotlights. It is suitable for a wide range of LED lighting and spotlight applications from 1W to 30W, including E27, PAR30, PAR38 and more. The high-brightness LED drive system based on PT4201's isolated optocoupler feedback has the advantages of high constant current accuracy, simple peripheral circuits, no flicker and low EMI radiation. The oscillation frequency of the controller under normal operating conditions can be accurately set by an external resistor. At the same time, the front side blanking circuit of PT4201 helps to overcome the voltage glitch at the moment when the external power device is turned on, which can effectively avoid the LED light flicker caused by the malfunction of the controller. Internal integrated current slope compensation improves system stability.
The PT4201 provides comprehensive protection to improve the reliability of LED lighting systems, including cycle-by-cycle overcurrent protection (OCP), VDD overvoltage protection (OVP), and VDD undervoltage protection (UVLO). The OUT output pulse high voltage is embedded in an 18V protected external power MOS. The short circuit protection prevents damage to the system when the LED load is shorted.
The PT4201 is available in a SOT-23-6 package with pinouts as shown in Figure 1.
Figure 1: PT4201 pinout
PT4201 basic function description
The PT4201 integrates a variety of enhancements and provides a low-cost solution for low-power LED lighting drives with its extremely low startup and operating current, multiple protection features.
Startup and UVLO:
The PT4201 is activated by charging a capacitor Chold connected to the Vdd pin via a resistor Rstart connected to the high voltage line. At the beginning of power-on, the voltage on the Chold capacitor is 0, PT4201 is in the off state, and the current flowing from Rstart charges Chold to raise the Vdd voltage. When the Vdd pin voltage reaches the chip startup voltage VDD-ON, PT4201 Start working, after the work, the Vdd current increases, and the auxiliary winding starts to supply power to the chip.
The optimized design of the startup circuit allows the VDD to consume very low current before the PT4201 starts. This allows the larger starting resistor Rstart to be used to improve overall efficiency. For general general-purpose input range applications, a 2Mohm, 1/8W resistor and a 10uF/50V capacitor form a simple and reliable startup circuit (Figure 2).
Figure 2: PT4201 startup circuit
Current feedback and PWM control:
The PT4201 uses an optocoupler to detect the output current of the LED string and achieve output current control by changing the duty cycle of the output pulse. As shown in Figure 3, when the LED current reaches the set value, the voltage drop of the LED current on the sampling resistor R2 reaches the on-voltage of the optocoupler light-emitting tube, the light-emitting tube is turned on to cause the FB voltage to drop, and the PT4201 changes according to the magnitude of the FB voltage. The output pulse duty cycle achieves a constant current output.
Figure 3: Optocoupler circuit
LED open circuit:
When the LED load is open, the current flowing through the Zener diode creates a voltage drop across the resistors R1 and R2, causing the optocoupler to be turned on, reducing the FB of the PT4201. When the FB is reduced to a certain level, the PT4201 enters the burst mode and the entire system enters the low power mode. Therefore, the LED light is open and safe.
LED short circuit and sampling resistor short circuit protection:
When the LED load is short-circuited, the voltage across the photocoupler is equal to the output voltage. The overall system operation is safe because the output power is small. When the sampling resistor is short-circuited, the FB voltage quickly climbs to the protection threshold because the voltage across the photocoupler is zero and the LED is not conducting. In the case of Rosc 100Kohm, the PT4201 will automatically shut down after 32mS.
Working frequency setting:
The Rosc pin of the PT4201 provides a convenient way to set the PWM frequency. The PWM frequency can be set by connecting a resistor between the Rosc pin and GND (Figure 4). The relationship between the PWM frequency and the set resistance follows the relationship: Fosc=6500/Rosc. FOSC unit KHz, Rosc unit Kohm.
The PT4201 periodically changes the PWM operating frequency for frequency jitter during normal operation. The periodically changing frequency extends EMI conducted interference into a wider spectral range to reduce EMI interference in the conductive segment.
Figure 4: Operating frequency setting
Current sampling and leading edge blanking:
One of the functions of the CS pin of the PT4201 is to sample the external MOSFET current for current slope compensation, and the second is to provide cycle-by-cycle MOSFET overcurrent protection. The PT4201 samples the current flowing through the MOSFET by sampling the sampling resistor in series with the power MOSFET. The current flowing through the MOSFET is converted into a voltage signal on the sampling resistor Rcs. The voltage on the CS and the FB voltage together determine the duty cycle of the PWM pulse.
During each turn-on period of the PWM, the MOSFET will be turned off immediately when the voltage at the CS pin exceeds the internal limit voltage to prevent damage to the device due to overcurrent. The overcurrent threshold voltage and the current of the MOSFET can be determined by the following relationship:
IOC=Voc/Rcs
Where IOC is the MOSFET current, Voc is the overcurrent threshold voltage, and Rcs is the sampling resistor size. The internal overcurrent threshold is related to the PWM duty cycle. When the PWM duty cycle is 0, the overcurrent threshold is 0.80V.
Due to factors such as the reverse recovery time of the transformer secondary winding rectifier circuit and the parasitic capacitance of the primary winding, a very short duration spike is generated on the sampling resistor at the instant of each PWM cycle. For this reason, the PT4201 will shield the CS sampling input for a period of time TBLK after the MOSFET is turned on. During this time, the overcurrent protection is turned off and the external MOSFET is not turned off. This can avoid the voltage glitch generated on the sampling resistor at the moment the MOSFET is turned on, causing malfunction. This feature provided by the PT4201 eliminates the need for an RC filter for the current sampling circuit (Figure 5).
Figure 5: Omission of the RC filter
VDD overvoltage protection
When a serious fault occurs in the system, for example, in the case of an open photocoupler or an open feedback, the optocoupler output current approaches zero, causing the FB terminal voltage to rise. An increase in the FB voltage will cause the PT4201 to operate in an overcurrent protection state because there is excess current supplied to the load, and if the current required by the load is exceeded, the output voltage will rise rapidly. Since the voltage of the auxiliary winding is proportional to the output voltage, the increase of the output voltage causes the auxiliary winding voltage to rise and the VDD voltage to rise. When the PT4201 detects that the VDD pin voltage reaches the overvoltage protection point, the PWM is turned off. When the OVP is triggered, since there is no energy supply load and auxiliary winding, the VDD voltage and output voltage drop, and when it is lowered to the OVP release voltage, normal operation will be restarted. At this time, if the fault is removed, it will work normally. If the fault persists, it will re-enter the OVP protection state (Figure 6).
Figure 6: VDD overvoltage protection
OUT output driver:
The OUT pin of the PT4201 is used to drive the gate of the power MOSFET. Optimizing the drive capability of the design of the totem pole form output provides a good compromise between drive strength and EMI. At the same time, the output high potential of OUT is limited to 18V, which can protect the MOSFET from damage due to VDD rise. There is a resistor between the internal OUT and GND that can reliably set the gate of the external MOSFET to zero potential when the chip is not operating.
E27 3W offline spotlight solution based on PT4201
Figure 7: E27 3W offline spotlight solution based on PT4201
The PT4201-based E27 3W off-line spotlight solution application line is a typical flyback topology with secondary side feedback (ie, optocoupler feedback) to improve output current accuracy. Compared with the primary side feedback circuit current accuracy +-5%, the secondary side feedback circuit accuracy is within +-2%, the cost is only increased by 0.3RMB, but it provides convenience for mass production.
The application of 3W E27 is generally connected to three 1W LEDs, and the VF of each LED is around 3.4V+-0.2V. Typical current is 300mA-350mA. The working principle is shown in Figure 7. The AC85V-265V AC input is connected to the rectifier bridge through L1 (equivalent to a fuse, anti-surge). The voltage from the rectifier bridge is about 1.4XVin and the current is about 1A. C1 is a filter capacitor. The choice of capacitor value is about 1-3 times of the load power. Here, the application of 3W uses 4.7uF capacitor. If the choice is too small, the ripple will be large, and the space will be too large. Not allowed. The VDD terminal of the PT4201 is initially powered down by R4, and is started at 18V. After startup, it is powered by the auxiliary winding of the transformer, and the voltage is between 9-27V. R1, C3 and D2 are an RCD absorption loop that is used to absorb the spikes generated by the Q1 switch. Decreasing R1 can improve the absorption, but it will lead to a decrease in system efficiency. A compromise is recommended. The resistor R7 connected to the RI end of the PT4201 is used to set the switching frequency. Here, the frequency is set at 65 kHz. The CS terminal of the PT4201 is connected to the sampling resistors R8 and R9 to set the current. The transformer is an important component. It adopts a flyback topology. When Q1 is turned off, the transformers 5 and 6 are turned on. The withstand voltage of D1 is the transformer input voltage / turns ratio + transformer output voltage. When Q1 is on, there are currents at terminals 1, 2, and 3, 4, 5, and 6 are cut off. The withstand voltage of D1 is the output voltage of the transformer X turns ratio + transformer input voltage. D1, T1, and Q1 are the key factors affecting efficiency, and D1 reverse withstand voltage and T1 turns ratio are mutually restrained. The SR1100 on the right side of the circuit is a Schottky diode or can be rectified with a fast recovery diode. When no load is available, R3 is a current limiting resistor, limiting the current on this branch to 10mA. D4 uses a 12V voltage regulator here to function as a rectified voltage limit, which works only when no load is present. R2 is a The shunt resistor has a current of 10 mA flowing through R2 and a voltage of 1 V at the left end of R2. When the load is applied, the voltage across R6 is about 1V. The output current is adjusted by selecting the resistance value. This is the application of 1X3W, and the operating current is set at about 300mA. U2 is an optocoupler. When the current on R6 becomes larger, the current on the LED becomes larger. After the photoresistor is sensed, the feedback current reaches the PT4201FB terminal, and the voltage at the FB terminal becomes smaller. The PT4201 reduces the energy by adjusting the duty cycle. , which in turn reduces the current on R6. Since the sampling current is fed back to the chip from the output, such secondary side feedback, the current is fine-tuned in real time, and the accuracy of the output current is improved.
Figure 8: Photo of the 3W E27 solution, small in size, more than enough to put in the E27 lamp
By changing some of the design data of this scheme, various schemes of 5W, 7W, and 12W can be designed, and the working principle is similar. Due to the large application space of 5W-12W, it is allowed to add some auxiliary circuits to the 3W. Such as the increase of anti-lightning devices, can improve EMC conjugate inductance, PFC, ∠filter, etc., in order to improve the ability of the entire circuit over EMC, work efficiency, PFC. (Text / China Resources Converse Technology (Shanghai) Co., Ltd. Huang Yiwei)
references:
Powtech Offline LED Spotlight Solution
"PT4201Datasheet"
Night Light,Sensor Night Light,Acrylic Night Light,Mini Night Light
Table Lamp & Reading Light Co., Ltd. , http://www.nbledlamp.com