4.2W GU10 LED lighting driver using primary feedback (Part 1)

Abstract
This article will introduce you to a low-power LED lighting driver solution using TI's offline primary-side sensing controller TPS92310. Due to the use of a constant on-time flyback topology and primary-side sensing control, this solution can achieve high efficiency and good line and load regulation. For GU10 replacement LED bulbs, the reference design PMP4325 has a suitable small form factor (30mm×18mm×10mm), which can support common AC line input and 3 or 4 LED series outputs with a constant output current of 350mA. Experiments show that for LED lighting, this solution has good line and load regulation, high efficiency, and overall LED lighting protection.
 

1 Theoretical operation

1.1 TPS92310 Controller

Single-stage flyback is an attractive topology for LED lighting with lower power ratings. Single-stage flyback is widely used in LED lighting for the following reasons:
 
l Galvanic isolation reduces the overall bill of materials (BOM)
l High power factor using special control architectures (e.g., constant on-time control, etc.)
l Smaller form factor compared to other two-stage topologies
 
Although single-stage flyback has many advantages for LED lighting, there are still some issues that need to be addressed. These issues include:
 
l High power factor
l Stable line and load regulation for primary side feedback (PSR)
l LED open or short protection
 
The TI TPS92310 controller is a single-stage primary side sensing AC/DC controller for driving constant current of high brightness LEDs. It operates in zero current sensing transition mode (TM). The "on time" (TON) is almost constant within the line voltage half cycle. Therefore, it has inherent power factor correction (PFC) because the peak current of the main winding varies with the input line voltage curve. TON is regulated to regulate the LED current to a preset level, which is set by an external sense resistor. TON is also used in the control design of flyback, boost, and buck-boost converters. Such converters operate in transition mode and use fixed on-time control to achieve high power factor. In addition, TON can also be used to control buck converters operating in transition mode, whose general-purpose LED drivers use peak current control.
 
Primary-side sensing does not require the use of optocouplers and secondary-side circuits, resulting in fewer components and a more compact PCB solution. In addition, this controller also has cycle-by-cycle current limiting, output short-circuit protection, output overvoltage protection (OVP) or open LED protection, short LED protection, and thermal shutdown protection, all of which provide protection measures for LED lighting.
 

 1.2 Constant on-time control

In traditional boost power factor correction converters, the transition mode with constant on-time control is usually used to keep the input current in phase with the input voltage to obtain high power factor and low total harmonic distortion (THD).
 
For the single-stage flyback topology operating in transition mode, it is not inherently power factor corrected because the duty cycle and frequency are always changing during the shape cycle. Therefore, the power factor and total harmonic distortion are not ideal under this condition. Fortunately, the single-stage flyback topology operating in filter mode can still achieve high power factor and low total harmonic distortion using a fixed (constant) TON. As shown in Figure 1, the average input current is a nearly sinusoidal wave with the same phase as the input voltage.
 

Figure 1 Current waveform during TON and TOFF
 
In this design, the TPS92310 controller is configured in constant on-time control mode, and the switch on time can be fixed if a large capacitor is connected to the COMP pin to filter the 100-Hz line ripple of the single-stage flyback application. However, in order to reduce the size of the circuit board, this reference design is not a single-stage structure without power factor correction, so we use a small compensation capacitor just to maintain the stability of the control loop. Since the DC input voltage of the flyback structure is relatively stable, the turn-on time is almost fixed.
1.3 Constant current control of primary side detection

Based on this, Figure 2 shows the primary current, secondary current and Vds voltage, and the average output current Io is calculated as follows:

Where:
2 × Tdly = half of the ringing time on the MOSFET drain
N = transformer turns ratio of primary winding to secondary winding
Ip_pk = primary current
Is_pk = secondary current
Io = average output current (LED current)
 

Figure 2 Current and Vds voltage waveforms
 
To regulate the output current, the converter uses a PWM control circuit as shown in Figure 3. This circuit includes charging and discharging operating modes. The charging operating mode is controlled by the internal reference current IREF × time (TON + TOFF + 2TDLY). The discharging operating mode is controlled by the TOFF switch and the Ipk current source, which is proportional to the primary side peak current. The COMP voltage level can represent the gate drive TON.
 
During normal operation, if the discharge Q (Ipk × TOFF) is greater than the charge Q (IREF × (TON + TOFF +2TDLY)), the COMP pin voltage decreases, resulting in the gate output TON increasing on the next cycle. In addition, if the charge Q (IREF × (TON + TOFF + 2TDLY)) is greater than the discharge Q (Ipk ×TOFF), VCOMP rises, and the gate driver output TON increases on the next cycle. If the charge Q is equal to the discharge Q, the VCOMP voltage is stable. Therefore, when a large capacitor is connected to the COMP pin to filter the 100-Hz line ripple, a fixed on-time is generated in half a sine cycle, thereby achieving power factor correction. In the case where power factor correction is not used to maintain loop stability and only for flyback topology, a small capacitor can be connected to the COMP pin.
 

Figure 3 Charging and Discharging Block Diagram
 
The controller implements primary current feedback and regulation to maintain a constant output LED current. Figure 4 shows the block diagram of the TPS92310 controller. The red dotted line indicates a main control loop.

Figure 4 TPS92310 block diagram