A new power supply topology for LED display modules

Abstract: A new power supply topology for LED display module is designed . After filtering, rectification and PFC correction, the 220 V/50 Hz (or 110 V/60 HZ) AC power generates a 400 V voltage; the 400 V voltage is transmitted to the auxiliary power module and multiple high-efficiency power modules in the module through the bus; the auxiliary power module receives the control command to switch the PFC circuit and the high-efficiency power module, and provides power to the small signal circuit; the high-efficiency power module then converts the 400 V voltage into the voltage required by the LED dot matrix module. This topology power supply makes the electromagnetic compatibility of the LED display module meet the limits specified in the relevant standards (GB17625.1-2003 or compatible with IEC1000-3-2), and the module PF value can reach more than 98%; at the same time, the entire power conversion efficiency of the LED display can reach more than 82%, which reduces the power consumption of the entire display compared with the traditional topology.

LED full-color display screen is a new type of large-sized indoor and outdoor electronic media, with the characteristics of large size, good environmental adaptability, high brightness, dynamic playback, etc. With the further improvement of LED cost performance, it can be widely used in advertising, stage, signboard, transportation facilities, public places and other fields. As an emerging light-emitting device, LED has the recognized energy-saving characteristics. However, the application of LED full-color display screen composed of a large number of LED lamps consumes huge energy; at the same time, a large number of LED modules constitute the LED display screen, and the LED module is powered by a number of switching power supplies. The harmonic distortion generated by such a large number of traditional switching power supplies (without PFC correction) can easily pollute the public power grid; therefore, electromagnetic compatibility and low energy consumption design are an important development direction of LED display screen technology.

1 Power supply topology of LED display module

The harmonic distortion has a very serious impact on the power grid when the traditional full-color LED display module topology is used for power supply (without PFC correction). Through the observation of a number of actual projects by the engineering department, the main impacts of the LED display system on the power supply network are: 1) The main harmonic currents are 3/5/7/9/11 times; 2) After the superposition of harmonics, the system's comprehensive power factor is low, usually below 0.75 , far below the requirements of the national standard; 3) The voltage and current waveforms are seriously distorted and are not standard sine waveforms. Harmonic distortion may have adverse effects on other sensitive equipment on the power grid, and even cause it to work abnormally, bringing about related legal liability issues. For example, harmonics caused a sudden power outage in the network server of a company in the same building, causing losses. Therefore, the use of PFC technology in the power supply topology of the full-color LED display module is a general trend and an inevitable result of the technological upgrade of the entire industry.

As shown in Figure 1, the power supply topology of the traditional full-color LED display screen generally uses the parallel output of several (m) switching power supplies, and then uses a 5 V DC bus to power the LED dot matrix module. For example, a static P16 full-color LED module generally uses 4 standard power supplies S-350-5 , and uses a parallel power supply bus at the 5 V output end, and then each LED dot matrix module draws power through the bus branch. This bus power supply method causes the 5 V power supply to consume energy and waste energy during transmission to the LED device. Usually, the current of the 5 V DC bus is relatively large. During the transmission process, part of the energy is converted into heat (P=(ILED)2R line) through the transmission line resistance and dissipated into the environment. On the other hand, due to the load change (change in pixel signal), the LED conduction current also changes. According to the voltage division principle: VLED=5V-(ILEDR line), although the switching power supply module S-350-5 outputs 5V+1%, the voltage transmitted to the LED dot matrix module is a changing voltage value, and as the LED current ILED changes more, the voltage value transmitted to the LED dot matrix module also changes more. When the load current increases to a certain value, the voltage transmitted to the LED pixel may be lower than the required voltage value for the LED to be fully turned on, and even affect the normal display function of the LED large screen.

After analyzing numerous experimental data, in order to overcome the many shortcomings of traditional display screen modules, a new power supply topology of LED display screen modules is proposed, as shown in Figure 2. The topology includes:

1) The AC input uses a common AC filter and PFC correction circuit to generate a 400 V DC voltage, and then the 400 V DC voltage transmits energy through the power bus in the module; because a 400 V DC power transmission is used, the current on the entire transmission bus is small. For example, a P16 (pixel pitch is 16mm) LED display module needs to input 400W of power, then the current transmitted on the 400 V DC bus is only 1 A, and the line resistance on the bus is only 0.1 Ω, so the calculated transmission loss is only 0.01 W, which can be ignored.

2) Auxiliary switching power supply module, which also extracts energy from the 400 V power bus. After conversion, it provides +5 V DC power to the signal control module of the module, and provides +12 V DC power to various auxiliary circuits of the module (electrical detection circuit , temperature and humidity detection circuit, ambient brightness detection circuit, cooling measure circuit, etc.). The auxiliary power supply module also supports communication interface and signal processing circuit interface, which is used to control the start and stop of PFC circuit and other switching power supply modules. This helps the intelligent management of the entire LED display system.

3) Several small switching power supplies distributed in the module, which obtain the necessary power from the 400 V DC bus, and then effectively transform the power supply voltage (Vr, Vg and Vb) required by the output LED dot matrix module; because the topology structure of red, green and blue three-way power supply is adopted, the specific voltage output value can be set according to the actual voltage requirements of the output LED device. Therefore, this power supply method can provide a more stable and reliable power supply for the LED dot matrix module, and provide a guarantee for high-quality LED display.

2 PFC regulation circuit and auxiliary switching power supply module

2.1 Power supply filtering and PFC regulation circuit

There are two major types of PFC at present. One is passive PFC (also called passive PFC), which mainly includes "inductor compensation" and "valley fill circuit " ; the other is active PFC (also called active PFC). The active PFC circuit is composed of inductors, capacitors and active electronic components ( diodes , MOS tubes and PFC controllers, etc.). It adjusts the waveform of the input current through a closed-loop control circuit and compensates for the phase difference between the current and voltage. The ripple of the active PFC output DC voltage is very small, and there is no need to use a large-capacity filter capacitor; and the active PFC can achieve a higher power factor (usually more than 98%).

The IEC1000-3-2 standard specifies the minimum value of harmonic distortion generated when the PFC circuit absorbs current from the grid after correction, and stipulates that the corrected current is approximately a sine wave, and the phase is consistent with the input mains. The circuit structure topology of the boost mode very cleverly realizes the correction of PFC. As shown in Figure 3, the amplitude and phase of the input voltage are input to an input end of the internal comparator of the PFC controller to control the current entering L to synchronize with the input voltage phase; at the same time, the voltage feedback input on the bulk capacitor is used to control the output voltage value of the PFC circuit; L, VD and SW form a basic boost circuit, generating a triangular wave current in L whose phase follows the input voltage phase. The current with a triangular wave waveform changes under the filtering effect of the filter capacitor of the input rectifier bridge stack, and becomes a sine wave current; the current amplitude of the triangular wave is controlled and limited by the sampling value of the controller's current limiting resistor . Therefore, after correction by the PFC circuit, the current absorbed from the mains is approximately a sinusoidal waveform in phase with the input voltage, which can be expressed as: K×1.414×Vac×sin(ωt), where K×1.414 is a constant, Vac is the voltage amplitude after the input AC voltage is rectified, and sin(ωt) is the current sinusoidal function that changes in phase with the input voltage. From this formula, it can be seen that the corrected current waveform is consistent with the input voltage, which well corrects the problem of current harmonic distortion.

Therefore, the latest LED display module power supply topology recommends that the PFC correction circuit adopts active correction technology, as shown in Figure 3. The active correction circuit (PFC part) is inserted between the input rectifier bridge and the power conversion power supply circuit. This inserted pre-processing device can provide a constant voltage output while absorbing current from the mains in a sine wave manner. It is actually a boost conversion topology. When the adjustment module works normally, the PFC correction circuit boosts the input mains voltage to about 400 V and stores the output energy in a large capacitor (Bulk).

NCP1653 is an integrated PFC regulation controller with the following features: compatible with IEC1000-3-2; continuous conduction mode (CCM); average current mode or peak current mode optional; fixed voltage output or follow-up boost operation; very few peripheral components; fixed switching frequency; soft start; VCC low voltage lock (hysteresis voltage range is 8.7 ~ 13.25 V); low voltage protection or shutdown; programmable overcurrent protection; programmable maximum power limit; thermal protection (hysteresis temperature range is 120 ~ 150 ℃); lead-free package.

The PFC correction circuit composed of NCP1653 according to the topology shown in Figure 3 is shown in Table 1 for the measurement comparison of harmonic distortion and power consumption, output voltage, output current, PF ratio, total harmonic distortion rate and conversion efficiency under AC 110 V input. The measurement comparison of harmonic distortion and power consumption, output voltage, output current, PF ratio, total harmonic distortion rate and conversion efficiency under 220 V input is shown in Table 2.

Table 1 Comparison of PFC correction circuit parameters at 110 V input

Table 2 Comparison of PFC positive circuit parameters at 220 V input

From the test data, the PFC part in the topology circuit of this article can achieve effective harmonic correction when the LED module load changes, and achieve a very high PF value. At the same time, the control of the NCP1653 power pin is used to enable the PFC circuit and bypass the power supply: When the power pin reaches 13.25 V, the PFC function starts; When the power supply pin is lower than 8.7 V, the PFC function stops, and the voltage after bridge rectification is directly bypassed and output to the bulk capacitor.

2.2 Design of auxiliary switching power supply

In addition to the LED dot matrix module, the LED display module also has a scanning signal control module, various detection circuit modules and a cooling processing module, and these modules are in normal working state. They may work when the display matrix is ​​on or off. Therefore, it is required that the power supply circuit is also powered 24 hours a day. Some system designs even use batteries as a backup power supply for the signal processing module when the power is off. Here, an auxiliary power supply is designed to power each functional module.

The NCP1207 controller has the following features: built-in 700 V withstand voltage MOSFET , with an on-resistance of 5.8 Ω at a junction temperature of 25 ℃; fixed frequencies of 65 kHz and 100 kHz for current mode; fixed peak current of 800 mA; Skip-Cycle operation mode at low peak current; built-in current source for clean, power-free startup timing; automatic recovery time base detection circuit with short-circuit protection; automatic recovery function of overvoltage of auxiliary winding; low voltage detection Brown-Out input function with programmable input voltage; programmable maximum power limit; internal frequency for improving EMI signal; duty cycle extended to 80%; input standby power consumption at no load is 85 mW@265 Vac; input standby power consumption at 500 mW load is 715 mW@230 Vac; the device is lead-free package.

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Since the scanning signal control module and various detection circuit modules are small signal processing circuits, the required power consumption is less than 15 W. Considering the energy conversion efficiency and input voltage adaptability, it is recommended to adopt the current mode, quasi-resonance, flyback feedback energy conversion topology architecture, and adopt synchronous rectification technology. Taking the NCP1207 controller as an example, the module auxiliary power module is designed as shown in Figure 4:

1) The power supply absorbs power from the Vbulk bus, and the DC voltage range is +120 to +400 V;

2) When the main switch tube VQ2 is turned on, the primary winding of the " transformer " stores energy, and when VQ2 is turned off, the energy is transferred to the secondary winding;

3) The primary auxiliary winding, on the one hand, rectifies and supplies power to the NCP1207 and PFC circuit chip, and on the other hand, provides a demagnetization signal to the pin of the NCP1207;

4) Resistor R4 limits the maximum current value of the switch tube;

5) The transformer secondary output voltage +5 V is the output main circuit, which is used to power the signal scanning module. It adopts synchronous rectification technology to reduce the reverse recovery loss of the rectifier diode .

6) The transformer secondary output voltage +12 V is the auxiliary output circuit, which is used to power the monitoring circuit and other functional modules. It also uses synchronous rectification technology to reduce losses.

7) The control signal from the receiving signal control board is coupled to the primary through the optocoupler to control the +15 V power supply of NCP1653 and realize the function of controlling the start and shutdown of the PFC circuit.

In actual engineering applications, turning off and on the PFC function (achieved by changing the power supply to NCP1653) is of great significance. When the LED display is only undergoing system maintenance and the LED dot matrix module does not need to be lit, the PFC correction function and the distributed switch power supply module can be turned off to achieve energy saving; when the LED display is turned on and off, the control system opening and closing command signal can be received to realize the simultaneous opening or closing of each module of the entire LED display, greatly reducing the surge peak value of the entire LED display project when turning on and off, and avoiding dangerous impacts on equipment in the power grid. As shown in Figure 4, the opening and closing of VQ8 controls the opening and stopping of the PFC circuit.

3. Power design required for module dot matrix module

The module of this topology design is composed of several LED dot matrix modules, and correspondingly there are several LED power modules to supply power.

3.1 Variable voltage design of LED dot matrix module

Full-color LED dot matrix modules are generally composed of red, green, and blue LEDs, and the voltage conduction characteristics of these three types of LEDs are different. Generally, the conduction voltages of blue and green LEDs are close, so it is recommended that the blue and green LEDs be powered by the same voltage. Therefore, the power module uses two adjustable voltage outputs, Vred and Vblue-green, for power supply, as shown in Figure 5.

3.2 LED dot matrix module power supply schematic design

The switching power supply of the LED dot matrix module designed with NCP1207[2] as the main control device is shown in Figure 6: 1) The power supply absorbs electric energy from the Vbulk bus, and the DC voltage range is 370~400 V. The voltage divider circuit composed of R23, R25 and R28 samples the voltage value on the input voltage bus and then feeds it back to NCP1207 to achieve control; 2) When the main switch tube VQ14 is turned on, the primary winding of the "transformer" stores energy. When VQ14 is turned off, the energy is transferred to the secondary winding; 3) The primary auxiliary winding, on the one hand, rectifies to supply power to NCP1207, and on the other hand, provides a demagnetization signal to the pin of NCP1207; 4) Resistor R31 limits the maximum current value of the switch tube; 5) The feedback signal is obtained from the (VDC blue-green) output circuit, thereby maintaining a stable voltage output of (VDC blue-green LED). The transformer secondary (VDC blue-green) output voltage adopts synchronous rectification technology. The reverse recovery loss of the rectifier diode is reduced; 6) The transformer secondary output circuit also uses synchronous rectification technology to reduce losses, and then passes through an output adjustable Buck conversion circuit (with U5 as the adjustment core) to output a stable actual required power supply voltage (VDC red LED).

As shown in Figure 6, when the input voltage Vbulk is lower than +370 V (the appropriate value can be set according to the specific situation), the voltage obtained by voltage division makes VQ12A conduct to the ground, thereby turning off the output pulse of NCP1207, NCP1207 stops working, and the power supply stops supplying power to the LED dot matrix module; on the contrary, when the bus voltage rises to greater than +370 V, the PFC circuit works normally, VQ12A is turned off, NCP1207 works normally, and the power supply supplies power to the LED dot matrix module again.

Improving the power conversion efficiency of the LED dot matrix module power supply is an important part of this topology design. Therefore, the quasi-resonant soft switching technology of zero voltage conduction and zero current shutdown is adopted to reduce the switching loss of the main switch tube of the switching power supply; the synchronous rectification technology is adopted to reduce the reverse recovery loss of the output rectifier tube. At the same time, the LED dot matrix module power supply requires the miniaturization of the power module transformer design, which requires the power density of the magnetic component to be increased. The planar transformer has great advantages in reducing leakage inductance, AC impedance, etc., and because of its small size, it becomes an excellent magnetic component, which greatly improves the working state of the switching power supply. Therefore, the switching power supply module here uses a planar transformer.

4 Conclusion

Through the AC power filtering and PFC correction module, several subsequent DCDC conversion circuits are separated from the power grid, and PFC active correction is performed, thereby cleaning the power supply and eliminating harmonic pollution to the power grid; at the same time, 400 V high-voltage bus transmission is adopted to reduce transmission loss and improve conversion efficiency; an auxiliary power supply module is used to power the signal control module, PFC adjustment circuit and other functional circuits, and a control interface is provided, which can facilitate the system to manage the module power supply more effectively; at the same time, when the power supply system supplies power to the LED dot matrix module, the output voltage of each channel can be adjusted arbitrarily to meet the actual needs of the LED load, thereby reducing the useless power consumption of the LED dot matrix module caused by the same voltage.