Low-cost, high-PF, high-brightness-adjustable LED driver design

This article will introduce an LED that has small size, low cost, high PF and high TRIAC brightness adjustment performance. This design uses a single-stage power factor correction (PFC) inverter converter with primary-side constant power control. It implements primary-side constant power control in a single-stage inverter topology without an optocoupler. It can work at high and low voltages and can provide a constant current of 350mA to drive 6 series LEDs.

This article introduces a reference design for a 7W AC/DC LED lighting driver with TRIAC dimming. The solution uses a single-stage power factor correction (PFC) inverting topology with primary-side constant power control. The article introduces the complete analysis and design of the power converter. Finally, we provide you with experimental results obtained based on a 7W application. This design can be easily adapted to other applications with simple modifications.

The PMP4304A reference design is a TRIAC dimming single-stage power factor correction LED driver using the TI TPS92210 LED lighting power controller. This LED application is mainly targeted at PAR bulb replacements, which have small size, low cost, high PF and high TRIAC dimming performance.

The solution uses a single-stage power factor correction (PFC) inverter converter with primary-side constant power control. It implements primary-side constant power control in a single-stage inverter topology without an optocoupler. This driver can operate with either high-line AC or low-line AC. The output can provide a constant current of 350mA to drive 6 LEDs in series.

LED lighting driver working principle

Power Factor Corrected Single Stage Inverting Converter

This single-stage power factor correction converter uses an isolated inverting AC/DC topology, which rectifies the AC input line voltage into a DC output of an input sinusoidal current. The single-stage inverting topology is widely used as an isolated LED solution because of its very low BOM cost and high efficiency.

Figure 1: Single-stage inverting converter

Traditional single-stage inverter solutions all use transition mode to adjust constants on time to achieve PFC function. However, the transition mode inverter topology is not a natural PFC because the duty cycle and frequency often change. Therefore, the accuracy of PF and THD under such conditions is not high.

However, the primary side constant power single-stage inverter is a natural PFC

When the RMS of Vin changes, the duty cycle changes in the opposite direction. When the RMS of Vin is limited, the duty cycle no longer changes. Therefore, when the system is stable, the duty time and duty cycle are constant.

At the same time, in order to maintain constant power, the system is kept at the same switching frequency.

Since Ton, L, f, and Vin are all constants, the input current is naturally sinusoidal according to Equation 2.

On the other hand, the input power is also a constant in Equation 3.

In summary, we can see that in this application, the primary side constant power single-stage solution has certain advantages over the traditional solution. First, the primary side constant power solution is a natural PFC, and its PF and THD are better than the traditional solution. Second, as the name implies, the primary side constant power solution is only controlled by the primary side. Therefore, the optocoupler can be excluded, thus achieving a low BOM cost.

TPS92210 controller and system operation

As far as the TPS92210 controller is concerned, there is an OTM pin, which can control the Ton time by connecting a resistor to it; details are as follows:

To achieve constant power control on the primary side, we use the following circuit, as shown in Figure 2.

Figure 2: Feedforward circuit for primary-side constant power control

Assuming Vin_rms = x, the relationship between Ton and Vin_rms can be calculated as follows:

This formula can be simplified as Equation 7:

In order to meet the requirement of constant power control on the primary side (Vrms *Ton= K), select B=0. At the same time, A and C can be selected according to the input power.

7W Off-line Constant Power LED Lighting Driver Design

Design Specifications

Table 1: Electrical design specifications

Schematic

Figure 3: PMP4304A schematic

PCB Layout

Figure 4: PCB assembly diagram – layer 1

Figure 5: PCB assembly diagram - layer 2

efficiency

Figure 6: Efficiency vs. input voltage

Line voltage regulation

Figure 7: Output current vs. input voltage

Power Factor

Figure 8: Power factor vs. input voltage

TRIAC brightness adjustment performance

Table 2: Output current of different brightness adjuster conduction angles

Figure 9: Output current versus dimmer conduction angle

Figure 11: Input current and input voltage at different conduction angles

in conclusion

This article analyzes a primary-side constant power control single-stage inverting LED driver and introduces the advantages of using primary-side control based on TPS92210. At the same time, we also implement a real 7W design. It demonstrates the many benefits of the TPS92210 solution, such as small form factor, low cost, high PF, and high TRIAC brightness adjustment performance.