Design of offline LED spotlight based on PT4201

LED lighting has become the focus of widespread attention for its high energy saving, long life and environmental protection. There are successful cases in the field of functional lighting in many places in China. China's semiconductor lighting application technology has gradually taken the lead in the world. With the policy encouragement of national and local governments, many places have been widely used in outdoor lighting such as street lights, landscape lighting, indoor lighting such as subways, underground garages, museums, special occasions lighting such as low temperature lighting, mining lamps, and car lights. Some traditional lighting companies have begun to invest in transformation LED lamps . 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. It is believed that it will occupy a large market share in home lighting in 2010.

The most common lamps for indoor lighting are LED spotlights with direct AC220V high voltage input, such as E27, GU10, PAR30, and PAR38. E27 and Gu10 LED spotlights require an LED constant current source that directly converts AC into DC to drive the high-brightness LED light source to emit light. Currently, single SoC integrated circuit products are not available , and most of them use a switching power supply solution with primary or secondary feedback . However, the primary feedback solution has the problem of low output current accuracy, which is generally around +-5%. The flyback constant current drive solution with secondary feedback can achieve an output current accuracy of +-2%.

There are many driver ICs that can be used for secondary-side feedback flyback constant current drive solutions . This article will introduce the offline LED spotlight design technology based on the PT4201 control chip in detail.

1W-30W offline high brightness LED driver Controller PT4201

PT4201 is a high-brightness LED driver controller that works in current mode and can drive 1W to 30W lighting or spotlights. It is suitable for various LED lighting and spotlight applications from 1W to 30W, including E27, PAR30, PAR38, etc. The high-brightness LED driver system based on isolated optocoupler feedback of PT4201 has the significant advantages of high constant current accuracy, simple peripheral circuit, no flicker and low EMI radiation. Under normal working conditions, the oscillation frequency of the controller can be accurately set by external resistors. At the same time, the front-side blanking circuit of PT4201 helps overcome the voltage burrs at the moment of turning on the external power device, which can effectively avoid the LED light flicker caused by the misoperation of the controller . The internally integrated current slope compensation function improves the system stability.

PT4201 provides complete protection functions 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 the 18V to protect the external power MOS. The short-circuit protection function prevents damage to the system when the LED load is short-circuited.
PT4201 is available in SOT-23-6 package, and the pinout is shown in Figure 1.

Figure 1: PT4201 pinout

PT4201 basic function description

PT4201 integrates a variety of enhanced functions, and with its extremely low startup and operating current, multiple protection functions, it provides a low-cost solution with excellent performance and reliability for low-power LED lighting drivers.

Startup and UVLO:
PT4201 starts up by charging the capacitor Chold connected to the Vdd pin through a resistor Rstart connected to the high-voltage line. At the beginning of power-on, the voltage on the Chold capacitor is 0, and the PT4201 is in the off state. The current flowing from Rstart charges Chold to increase the Vdd voltage. When the Vdd pin voltage reaches the chip startup voltage VDD-ON, the PT4201 starts to work. After working, the current flowing into Vdd increases, and the chip starts to be powered by the auxiliary winding.

The optimized startup circuit makes VDD consume only very low current before PT4201 starts, so a relatively large startup resistor Rstart can be selected to improve the overall efficiency. For general universal input range applications, a 2Mohm, 1/8W resistor and a 10uF/50V capacitor can form a simple and reliable startup circuit (Figure 2).

Figure 2: PT4201 startup circuit

Current feedback and PWM control:

PT4201 uses an optocoupler to detect the current of the output LED string and achieves the purpose of output current control by changing the output pulse duty cycle. 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 optocoupler light-emitting diode conduction voltage, and the light-emitting diode conducts to reduce the FB voltage. PT4201 changes the output pulse duty cycle according to the size of the FB voltage to achieve constant current output.

Figure 3: Optocoupler circuit

LED open circuit:
When the LED load is open circuit, the current flowing through the voltage regulator produces a voltage drop on the resistors R1 and R2, turning on the optocoupler light-emitting diode, which reduces the FB of PT4201. When FB is reduced to a certain level, PT4201 enters the burst mode, and the entire system enters the low power consumption mode. Therefore, it is safe to open the LED lamp.

LED short circuit and sampling resistor short circuit protection:
When the LED load is short-circuited, the voltage across the optocoupler light-emitting tube is equal to the output voltage. Since the output power is very small, the entire system is safe. When the sampling resistor is short-circuited, since the voltage across the optocoupler light-emitting tube is zero, the light-emitting tube is not turned on, causing the FB voltage to quickly climb to the protection threshold. When Rosc is 100Kohm, PT4201 will automatically shut down after 32mS.

Working frequency setting:
The Rosc pin of PT4201 provides convenience for setting 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 setting resistor follows the following relationship: Fosc=6500/Rosc. FOSC is in KHz and Rosc is in Kohm.

When working normally, PT4201 will periodically change the PWM operating frequency to perform frequency jitter. The periodically changed frequency extends the EMI conduction interference to a wider spectrum range, thereby reducing the EMI interference in the conduction segment.

Figure 4: Operating frequency setting

Current sampling and leading edge blanking:

One of the functions of the CS pin of PT4201 is to sample the external MOSFET current for current slope compensation, and the other is to provide cycle-by-cycle MOSFET overcurrent protection. 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 CS and the voltage on FB jointly determine the PWM pulse duty cycle.
In each PWM conduction cycle, when the voltage on the CS pin exceeds the internal threshold voltage, the MOSFET will be turned off immediately to prevent overcurrent from damaging the device. 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 size of the sampling resistor. The internal overcurrent threshold value is related to the PWM duty cycle. When the PWM duty cycle is 0, the overcurrent threshold value is 0.80V.

Due to the reverse recovery time of the transformer secondary winding rectifier circuit and the parasitic capacitance of the primary winding, a short-duration spike voltage will be generated on the sampling resistor at the moment 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 period, the overcurrent protection is turned off and the external MOSFET will not be turned off. This can avoid the voltage burrs generated on the sampling resistor at the moment of the MOSFET turning on, which may cause false operation. This function provided by the PT4201 can save the RC filter required for the current sampling circuit (Figure 5).

Figure 5: RC filter omitted

VDD overvoltage protection

When a serious fault occurs in the system, such as an open circuit of the optocoupler or an open feedback circuit, the output current of the optocoupler approaches zero, causing the voltage at the FB terminal to rise. The rise in FB voltage will cause the PT4201 to work in an overcurrent protection state, because there is excess current to supply the load. If the current exceeds the current required by the load, the output voltage will rise rapidly. Since the voltage of the auxiliary winding is proportional to the output voltage, the increase in output voltage causes the auxiliary winding voltage to increase, thereby increasing the VDD voltage. When the PT4201 detects that the voltage on the VDD pin reaches the overvoltage protection point, it will turn off PWM. When OVP is triggered, since there is no energy to supply the load and the auxiliary winding, the VDD voltage and the output voltage drop. When it drops to the OVP release voltage, it will restart normal operation. At this time, if the fault is removed, it will work normally. If the fault still exists, it will re-enter the OVP protection state (Figure 6).

Figure 6: VDD overvoltage protection

OUT output driver:

The OUT pin of PT4201 is used to drive the gate of the power MOSFET. The optimized design of the totem pole output drive capability makes 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 caused by the increase of VDD. There is a resistor between the internal OUT and GND, which can reliably set the gate of the external MOSFET to 0 potential when the chip is not working.

E27 3W offline spotlight solution based on PT4201

Figure 7: E27 3W offline spotlight solution based on PT4201

The E27 3W offline spotlight solution based on PT4201 is a typical flyback topology, using secondary feedback (i.e. optocoupler feedback) to improve the output current accuracy. Compared with the primary feedback circuit current accuracy of +-5%, the secondary feedback circuit accuracy is within +-2%, and the cost is only increased by 0.3RMB, but it provides convenience for mass production.

The application of 3W E27 generally connects 3 1W LEDs to the load, and the VF of each LED is about 3.4V+-0.2V. The general 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, surge protection). The voltage from the rectifier bridge is about 1.4XVin and the current is about 1A. C1 is a filter capacitor. The capacitance value can be selected to be about 1-3 times the load power. The 3W application here uses a 4.7uF capacitor. If too small is selected, it will cause large ripples . If too large is selected, the space is not allowed. The VDD end of PT4201 is initially powered by R4 after stepping down, and 18V is started. After starting, 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 circuit used to absorb the spikes generated when Q1 switches . Reducing R1 can improve the absorption effect, but it will lead to a decrease in system efficiency. It is recommended to adopt a compromise. The resistor R7 connected to the RI terminal of PT4201 is used to set the switching frequency. Here, the frequency is set to 65kHz. The CS terminal of 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, transformers 5 and 6 are turned off and on. The withstand voltage of D1 is the transformer input voltage/turn ratio + transformer output voltage. When Q1 is turned on, there is current at the 1st and 2nd terminals of the transformer, and the 3rd, 4th, 5th and 6th terminals are cut off. The withstand voltage of D1 is the transformer output voltage X turns ratio + transformer input voltage. D1, T1, and Q1 are the key factors affecting efficiency. The reverse withstand voltage of D1 and the turns ratio of T1 are mutually restrained. SR1100 on the right side of the circuit is a Schottky diode or a fast recovery diode can be used for rectification. When unloaded, R3 is a current limiting resistor, limiting the current on this branch to 10mA. D4 uses a 12V voltage regulator here, which plays a role of rectification and voltage limiting. It only works when unloaded. R2 is a shunt resistor. The current flowing through R2 is 10mA, and the voltage on the left end of R2 is 1V. When loaded, the voltage across R6 is about 1V. The output current is adjusted by selecting different resistance values. This is a 1X3W application, and the working current is set at about 300mA. U2 is an optocoupler. When the current on R6 increases, the current on the light-emitting diode increases. After the photoresistor senses it, the current is fed back to the FB end of PT4201. The voltage at the FB end decreases. PT4201 reduces the energy by adjusting the duty cycle, thereby reducing the current on R6. Since the current is sampled from the output end and fed back to the chip, such secondary feedback can fine-tune the current in real time, thereby improving the accuracy of the output current.

Figure 8: Photo of the 3W E27 solution. It is very small and can easily fit into an E27 lamp holder.

By changing some of the design data of this solution, various solutions of 5W, 7W, and 12W can be designed, and the working principles are similar. Since the application space of 5W-12W is large, it is allowed to add some auxiliary circuits on the basis of 3W. For example, adding anti-lightning devices, conjugate inductors that can improve EMC , PFC, ∏ filters, etc., can improve the ability of the entire circuit to pass EMC, work efficiency, and PFC.