Introduction to a Quasi-Resonant Flyback Controller

Power adapter is a power supply conversion device for small portable electronic devices and electronic appliances. It can be divided into AC output type and DC output type according to its output type; it can be divided into wall-plug type and desktop type according to the connection method. It is widely used in telephone handsets, game consoles, language repeaters, walkmans, laptops, cellular phones and other devices.

Table 1 shows the latest EPA 2.0 Level V standards for external power adapters . The table highlights average efficiency and no-load and light-load power consumption.

Table 1 EPA 2.0 Level V Standards for External Power Adapters

To this end, Infineon has developed a new QR PWM IC ICE2QS03G with features such as digital frequency reduction, active burst mode and foldback correction for green power adapter solutions.

2 Comparison of CCM, DCM and QR working modes

Flyback converters are widely used in AC/DC power supplies, especially for power supplies with output power below 150W. Single-switch flyback converters have three basic operating modes: continuous conduction mode (CCM), discontinuous conduction mode (DCM) and quasi-resonant (QR) mode. Each of these three operating modes has its own advantages and disadvantages.

2.1 Continuous Conduction Mode

Figure 1a shows a typical CCM operating waveform. The input power of the converter is:

(1)

Since the energy stored in the inductor is not completely transferred to the secondary side, the inductance required for CCM operation mode is usually higher than that required for DCM operation mode under the same conditions. In addition, higher inductance means that the primary side switch current has a lower AC/DC conversion rate, thus obtaining lower conduction losses. However, as the primary inductance value increases, the magnetic loss of the transformer also increases, so a trade-off needs to be considered between the switch conduction loss and the transformer conduction loss.

In addition, when the duty cycle is greater than 0.5, a slope compensation function needs to be added to avoid subharmonic oscillation. Since the on-time is shorter under high voltage input, the compensation value under high voltage is lower than that under low voltage. This will make the maximum output power under high voltage much higher than the maximum output power under low voltage. In fact, SMPS ICs using CCM operation mode have only one compensation curve for a specific design. If the design changes, the maximum power limit performance will also change.

2.2 Discontinuous Conduction Mode

Figure 1b shows the typical operating waveform of a flyback converter in DCM mode. The input power of the converter is:

(2)

As mentioned above, the energy stored in the inductor during the MOSFET on-time is completely transferred to the secondary side during the MOSFET off-time. The maximum power is only related to the inductance, switching frequency and peak current. For designs with fixed frequency, it is easy to limit the maximum input power of the system by keeping the maximum peak current constant under different input voltage conditions.

2.3 Free-running quasi-resonant mode

Figure 1c shows the typical operating waveform in QR mode. The input power of the converter is:

(3)

When the current on the secondary side of the transformer is zero, the primary main inductance resonates with the parasitic capacitance of the drain-source and the line, and the power switch is turned on only at the lowest point of the drain-source voltage. Under this condition, the switching frequency is determined by the output load and the input voltage. If the peak current limit remains unchanged, the switching frequency will increase significantly under high input voltage conditions. This will result in a very high maximum input power at high voltage.

Figure 1 Typical operating waveforms of a flyback converter in different operating modes

3 ICE2QS03G Features

The QR mode without frequency limiting has lower turn-on losses due to the lower on-state voltage. However, under light load conditions, the switching frequency is very high and the efficiency drops rapidly. Therefore, under these conditions, the switching frequency needs to be limited. Infineon's patented digital frequency reduction concept was developed.

3.1 Concept of Digital Frequency Reduction

For the QR mode of operation, the switching cycle consists of three parts: the on-time (Ton), the off-time (Toff), and the semi-resonant period (Tres). Based on the volt-second balance of the transformer primary-side inductor, Ton and Toff can be calculated using equations (4) and (5), and the resonant period can be calculated using equation (6). In equation (6), Cds is the drain-source equivalent capacitance of the MOSFET.

(4)

(5)

(6)

This explains why the switching frequency increases when the load decreases or the input voltage increases. This is undesirable in a switched-mode power supply, as high switching frequencies lead to high switching losses. In order to limit the switching frequency, Infineon has developed a digital frequency reduction method that ensures operation not in the first resonance valley, but in the second, third or even seventh valley - depending on the load conditions.

In fact, there is a register inside the ICE2QS03G, called the ZC counter. This counter determines at which valley point the MOSFET is turned on. The value of the register can be adjusted by monitoring the feedback voltage. When the load current becomes smaller, the feedback voltage can be reduced through the control loop, thereby increasing the ZC counter value and reducing the switching frequency. When the load current increases, the ZC counter value will become smaller. Table 3 details the working principle of the change of the ZC counter, and Figure 2 uses three examples to illustrate how the ZC counter changes with the feedback voltage.

Due to the variable ZC counter and valley switching, the actual switching frequency of the converter decreases when the output load decreases, as shown in Figure 3.

Table 3 ZC adjustment method

Figure 2 Digital frequency reduction

Figure legend: Clock: clock; Up/Down counter: up/down counter; case 1: case 1; Case 2: case 2; Case 3: case 3

Figure 3 Switching frequency comparison between basic QR and Infineon QR

Figure legend: Switching frequency: switching frequency; Active burst mode: active burst mode; Free-running QR: free running QR; Output power: output power

3.2 Active Burst Mode (Patented)

Under light load conditions, the main losses are switching losses and transformer magnetic losses. Both are related to the switching frequency. Burst mode and skip mode are two widely used methods. By using burst mode and skip mode to reduce the light load switching frequency, energy efficiency can be greatly improved.

Figure 4 shows the operation of active burst mode. To enter burst mode, three conditions must be met. First, the feedback voltage must be below the preset threshold VFBEB, which sets the power to enter burst mode. Second, the value of the ZC up/down counter must be equal to 7, ensuring that the converter is in light load condition. Finally, the blanking time should be 24 milliseconds to avoid interference caused by some possible transients.

To exit the burst mode, the feedback voltage should be higher than the preset threshold VFBLB. During active burst mode operation, when the feedback voltage is higher than V FBBOn, the IC will start switching. When the feedback voltage is lower than VFBBOff, the IC will stop switching.

VFBBoff is 3.0V and VFBBOn is 3.6V. This voltage threshold is much higher than that of conventional burst mode, saving energy in the IC and feedback loop optocoupler. Due to its higher voltage level, it has excellent noise immunity. Compared to burst mode, this operation is more stable, resulting in higher energy efficiency.

Figure 4 Active burst mode operation: Enter burst mode; Burst On: Burst mode on; Burst off: Burst mode off; Leave Burst: Exit burst mode, Current limit level during burst mode: Current limit level during burst mode operation

3.3 Maximum power limit (with foldback correction function)

Pin is proportional to Ipk and fsw, and Ipk is limited by the current sampling limit Vcs. According to equation (4), we can see that fsw is proportional to Vin. When the line voltage increases, the converter input power becomes very large. When the line voltage increases, it is necessary to limit the current sampling level to limit the maximum input power. For ICE2QS03G, the input voltage information can be obtained through the current output from the ZC pin. This is because the auxiliary winding can sense a negative voltage proportional to the input voltage. Since the ZC pin is internally clamped to -0.3V, the output current of the ZC pin is proportional to the input voltage, as shown in equation (12). By adjusting the Vcs value, the maximum input power can be effectively limited. Figure 5 shows the detection circuit. This IC uses a digital comparison circuit. Figure 6 shows the maximum Vcs limit VS input voltage (proportional to Izc).

(12)

Figure 5 Foldback correction detection

Figure legend: Foldback point correction block: Foldback correction block; Current limitation: Current limitation

Figure 6 Vcsmax VS input line voltage

3.4 Loss calculation

Table 4 shows the distribution of various losses of a 65W adapter at 115V and 230V (AC)

Table 4 Loss distribution

Figure legend: Loss distribution: loss distribution; Power: power; Loss name: loss name

4 Conclusion

The conclusion of this article on the flyback converter is that the QR mode operation with digital frequency reduction can achieve extremely high energy efficiency under full load, medium load and light load conditions. Using the active burst mode feature, the standby power consumption can be limited to less than 100mW under 265V (AC) input voltage conditions. This makes it easy for the design to meet relevant standards such as EPA2.0 Level V.