Design of a quasi-resonant flyback switching power supply

Quasi-resonant conversion is a mature technology widely used in power supply design for consumer products. The new Green Power series controllers achieve ultra-low standby power consumption as low as 150 mW typical. This article will explain how the quasi-resonant flyback converter improves power supply efficiency and how to design a quasi-resonant power supply with the UCC28600 .

1 Conventional hard-switching flyback circuit

Figure 1 shows a conventional hard-switching flyback converter circuit. This discontinuous mode flyback converter (DCM) has three operating intervals in one working cycle: (t0 ~ t1) is the stage where the transformer provides energy to the load, at which time the output diode is turned on, and the current in the primary of the transformer flows to the output load through the coupling of Np:Ns, gradually decreasing.

The MOSFET voltage is composed of three parts: input DC voltage VDC, output reflected voltage VFB, and leakage inductance voltage VLK. At t1, the output diode current is reduced to 0. At this time, the primary inductance of the transformer and the parasitic capacitance form a weakly damped resonant circuit with a period of 2π LC. In the stagnation interval (t1~t2), the voltage on the parasitic capacitance will change with the oscillation, but it always has a considerable value. When the MOSFET conduction time begins at the next cycle t2 node, the charge on the parasitic capacitance (COSS and CW) will discharge through the MOSFET, generating a large current spike. Since there is a large voltage on the MOSFET when this current appears, the current spike will cause switching loss. In addition, the current spike contains a large amount of harmonic content, thereby generating EMI.

2 Implementation of Quasi-Resonant Flyback Design

By using a detection circuit to effectively "sense" the first minimum or valley of the MOSFET drain-source voltage (VDS) and only then start the MOS-FET conduction time, the current spike at the turn-on will be minimized because the parasitic capacitance is charged to the minimum voltage. This situation is often called valley switching or quasi-resonant switching. This power supply is a variable frequency system determined by the input voltage/load conditions. In other words, regulation is performed by changing the operating frequency of the power supply, regardless of the load or input voltage at the time, the MOSFET always remains turned on at the bottom of the valley. This type of operation is between continuous (CCM) and discontinuous condition mode (DCM). Therefore, the converter operating in this mode is said to operate in critical current mode (CRM). The MOSFET drain-source voltage in critical mode is shown in Figure 2.

There are many advantages to using a quasi-resonant switching scheme in a flyback power supply design:

(1) Reduce conduction loss

Since the MOSFET has a minimum drain-source voltage when it is turned on, the on-current spike can be reduced, which reduces the stress on the MOSFET and lowers the temperature of the device.

(2) Reduce the reverse recovery loss of the output diode

Since the rectifier tube on the secondary side is turned off with zero current, the reverse recovery loss is reduced, thereby improving the overall efficiency of the power supply.

(3) Reduce EMI

The reduction of the on-current spike and the frequency jitter during the quasi-resonance process will reduce EMI noise, which will reduce the number of EMI filters used and thus reduce power supply costs.

3  Design of tungsten lamp power supply based on UCC28600 controller

3.1  Main features of the UCC28600 controller

The main features of the UCC28600 controller include advanced green-mode control; quasi-resonant control with low EMI and low loss (valley switching); no-load loss less than 150 mW (low standby current); low startup current (maximum 25 μA); programmable overvoltage protection (input voltage and output voltage); built-in overtemperature protection with automatic restart after temperature recovery; current limiting protection: cycle-by-cycle power limiting and overcurrent hiccup restart; programmable soft start; and integrated green status pin (PFC enable terminal).

3.2 UCC28600 Working Principle

UCC28600 integrates UVLO comparator, high frequency oscillator, quasi-resonant controller and soft start controller, standby mode pulse skipping comparator, input and output overvoltage protection. Its internal structure is shown in Figure 3.

(1) UVLO comparator

The VDD voltage of UCC28600 starts at 13 V and shuts down when it is lower than 8 V. It has a 5 V hysteresis voltage, which can improve the operating stability of UCC28600.

(2) Internal oscillator

UCC28600 integrates a 40~130 kHz oscillator.

(3) Quasi-resonant controller and soft start controller

UCC28600 uses a quasi-resonant switching converter to improve conversion efficiency. It uses the transformer's excitation flux to detect the output voltage of the transformer winding during the switch-off period. If the voltage is low and in the trough of the oscillation, it can be confirmed that the transformer excitation flux is exhausted at this moment and the next cycle can be started. The quasi-resonant mode can be divided into critical conduction mode (CRM), discontinuous conduction mode (DCM) and frequency modulation mode (FFM).

(4) Standby mode and pulse skipping comparator

When the power continues to decrease, UCC28600 enters standby mode; the frequency modulation mode (FFM) frequency drops to 40 kHz and no longer decreases; when FB is less than 0.6 V, the switch pulse output is turned off, and when FB is greater than 0.7 V, the switch pulse is output normally, thus obtaining the standby working state of the pulse skipping mode.

(5) Input and output overvoltage protection

The OVP pin is the overvoltage (line voltage and load voltage) input pin and the response pin for resonant start. This pin detects input overvoltage, load overvoltage and resonant conditions through the primary bias coil of the transformer. The overvoltage point can be flexibly adjusted through the resistor connected to this pin.

3.3 Technical specifications of tungsten lamp power supply

Input voltage: 95~260 V AC 50/60 Hz; Output voltage: 5 V; Output current: 4.3 A; Power output can be turned off remotely.

3.4 Power Supply Design Process

The circuit diagram of the tungsten lamp power supply is shown in Figure 4. The AC power supply is input from the upper left corner and is converted into a DC high voltage of about 130~360 V through the input power filter, rectifier bridge, and high-voltage capacitor. N14 and V30 form the main circuit on the high-voltage side, which chops the DC high voltage into a pulse voltage, couples it through a transformer, rectifies it through V12, and filters it into a DC voltage through the output capacitor.

3.4.1 Startup Circuit

Since the startup current of UCC28600 is very small, with a typical value of 12 μA, the power consumption of the startup resistor can be greatly reduced, so the startup resistor is composed of three 300 kΩ chip resistors in series. However, since the VDD pin needs a sufficient energy storage capacitor to prevent hiccups during operation, a problem is that the voltage rises too slowly when the VDD starts, and the power startup time is too long. The solution is to use a small capacitor for the VDD pin and a large capacitor for the reverse supply winding, and isolate the two with V34 (1N4148).

3.4.2 Remote Control Circuit

The remote control circuit is safely isolated by optocoupler TLP181. When the remote control signal is input to the CTL terminal and a current signal is added, the output terminal of the optocoupler is turned on, and the SS pin of UCC28600  is pulled down through V33 to turn off the drive signal of the MOSFET. The VDD voltage is pulled down through R32 to be lower than the startup voltage of UCC28600 to prevent the chip from being in the restart process all the time.

3.4.3 Feedback Circuit

The output voltage of TL431 is sampled and fed back to the input of the chip after isolation by optocoupler TLP181. The reference voltage of TL431 is 2.495 V. Through the voltage divider of R84 and R85, the output voltage is set at 11.5 V. Since the load is a fixed tungsten lamp power supply, the transient response of the power supply does not need to be considered. Therefore, the compensation capacitor of TL431 adopts simple Class I compensation, which is simple, stable and reliable.

3.4.4 Transformer Design

Assuming that at maximum load, UCC28600 operates in quasi-resonant mode, its maximum duty cycle occurs at the lowest input voltage. Under the condition of fixed input voltage and input power:

The primary winding uses 2×0.35 enameled wire, the secondary uses 125 μm copper foil, and the sandwich winding method is adopted. The center column of the magnetic core is opened with an air gap, so that the ALG is 275 nH/T2.

3.5 Test Data

3.5.1 Power Conversion Efficiency

The efficiency of the power supply under different input and output conditions is shown in Figure 5.

3.5.2 Switching tube waveforms in different states

The switching tube waveforms of the power supply in different states are shown in Figure 6.

As can be seen from Figure 6, when the output load is very small, the power supply works in pulse skipping mode, which can reduce switching losses and improve light-load power efficiency; as the load increases, the power supply begins to enter the frequency modulation mode. When fully loaded and the input voltage is high, the power supply works in a quasi-resonant mode with a higher frequency; if the input voltage is low, the working mode remains unchanged, but the switching frequency is reduced to maintain the switch tube turned on at the bottom of the waveform.

4 Conclusion

This paper proposes a design scheme of quasi-resonant flyback switching power supply based on UCC28600 controller , which uses quasi-resonant technology to reduce the switching loss of MOSFET. Practice has proved that the design of quasi-resonant flyback switching power supply based on UCC28600 has the characteristics of wide input voltage range, high output voltage accuracy, high conversion efficiency, low standby power consumption, etc. This power supply is applied to tungsten lamp power supply, with the highest efficiency reaching 86%, and has achieved good results.