Design and implementation of full voltage switching power supply circuit based on APFC-free

introduction

Compared with traditional linear power supplies, switching power supplies have inherent advantages such as small size, light weight, and high efficiency. Therefore, in recent years, more and more people have been studying switching power supplies, and corresponding technologies have emerged in an endless stream. Researching low-cost, reliable, and highly compatible switching power supplies has become the goal of many power supply design engineers. This article proposes a low-cost full-voltage design solution without APFC for high-power switching power supplies. This solution uses an automatic voltage doubling method to effectively reduce the range of the DC input voltage of the fire transformer, thereby greatly reducing the cost of the power supply.

Full voltage power supply

Statistics of AC voltage around the world can be divided into:

Japan is represented by 100V, the United States is represented by 120V, Mexico is represented by 127V, China is represented by 220V, Europe is mostly 230V, and Australia is 240V. Therefore, the voltage of countries around the world is distributed in two voltage segments: 100V-127V and 220V-240V. That is, if the switching power supply can meet the requirements of these two voltage segments, it can be considered as a full-voltage switching power supply. The switching power supply that achieves full voltage can currently be roughly divided into: ordinary stepless type, APFC stepless type, and automatic voltage doubler type.

1. Ordinary stepless type

Ordinary stepless switching power supplies are widely used in low-power switching power supplies. In the low-power segment of less than 300W, designers usually adopt a full-range voltage solution of 100-240V while taking into account both structure and cost. Although the structure is simple, it places higher requirements on power devices (such as fire cows, switch tubes, and rectifier tubes). Since the increase in device parameters within a certain range does not have much impact on the price, it has a considerable cost-effectiveness in the low-power segment. As the power increases, the power supply puts forward new requirements for the power devices of each part, which has great difficulties in terms of price and technology.

2. APFC stepless type

APFC is active PFC that uses a dedicated PFC controller.

The circuit power components are composed of standard boost circuits, with dual feedback of voltage and current, where the voltage is in the outer loop and the current is in the inner loop. Therefore, APFC ensures a constant voltage at the output while making the current waveform a sine wave.

The benefits of APFC are obvious: ① Greatly improve the power factor; ② Compatible with the full range of input voltages from 100 to 240V; ③ Great improvement in EMC. Disadvantages:

① The volume and weight have increased; ② The power cost has increased by about 50%.

3. Automatic pressure doubler

In view of the various disadvantages of manual operation and the voltage laws of countries around the world, the automatic voltage doubler is improved on the basis of the manual voltage doubler to achieve automatic switching of input voltage in low-voltage countries. The automatic voltage doubler switch can use relays, MOSFET, IGBT, and thyristor. Since the design is used under the condition of 50-60Hz power frequency, the zero-crossing requirement and production cost are considered.

Select thyristor as the switch device. Thyristor has great cost advantage, and the response speed can meet the requirements.

System structure and principle

Basic indicators of the power supply: rated output 1200W, peak power 2400W; input voltage can be AC100-127V and 220-240V; output voltage is DC160V. While the system meets global voltage compatibility, it also has an ultra-low standby power consumption capability of less than 0.3 watts.

1. System structure

The whole system can be divided into the main power supply part to provide power to the power amplifier part. The auxiliary power supply is used for the primary control circuit and the secondary control circuit. The controller is used to realize the automatic voltage recognition and voltage doubling functions, and at the same time, it is combined with the MCU to realize the remote control wake-up system function. In the AC to DC rectifier part, the auxiliary power supply and the main power supply are designed to be independent power supplies. In the standby mode, the auxiliary power supply is separated from the main power rectifier part, which provides a hardware foundation for low standby power consumption.

2. Main power supply

2.1 Main power supply design

The main power supply adopts a phase-shifted full-bridge topology. The full-bridge circuit is easy to achieve high-power output, and the phase-shifted full-bridge, as an improved version of the full-bridge circuit, has more advantages in terms of overall efficiency. A resonant inductor is connected in series in the bridge circuit, and the resonant inductor resonates with the parasitic output capacitance Coss of the MOS tube. As a result, the voltage at the DS end is zero before the MOS tube is turned on, and zero-voltage turn-on is achieved. Because the zero-voltage turn-on of the MOS tube is achieved, the requirements for the drive circuit and the Qg constant of the MOS tube are reduced, and the device cost is also reduced. A dual-phase thyristor is used as a voltage doubler switch. The unidirectional thyristor can disconnect the power supply of the entire main power supply. When the thyristor is completely disconnected, all devices in the entire main power circuit have no current loop. Except for the extremely small leakage current of the thyristor itself, there is no power loss in the main circuit.

2.2 Voltage doubling structure and principle

The voltage doubling method is consistent with the principle of manual voltage doubling. When the AC voltage is in the 1st and 2nd quadrants, the current flows as follows (red track): AC+ -> D1 -> CAP1 -> K -> AC-. The power supply charges the capacitor CAP1, and its voltage will reach the AC peak. When the AC voltage is in the 3rd and 4th quadrants, the current flows as follows (green track): AC- -> K -> CAP2 -> D4 -> AC+. The power supply charges the capacitor CAP2, and its voltage will also reach the AC peak. Therefore, the rectified voltage will be twice the voltage when the switch is off.

The AC input voltage is AC100V-127V and AC220V-240V. From the formula, we can know that the voltage range after rectification is:

DC283-DC360V. Taking full account of device voltage division, such as capacitor ESR, switch tube voltage drop, and EMI device voltage drop, it can be considered that the rectifier conduction is about 60 degrees under heavy load conditions, and the voltage value can be considered to be: DC245V-DC360V. Compared with the normal full-voltage power supply voltage range (which will reach: DC122-DC360V), there is a significant attenuation.

3.3 Auxiliary power supply

The auxiliary power supply adopts the flyback RCD topology. The auxiliary power supply provides power for all control circuits. Since the overall power consumption is required to be less than 15W, an integrated solution of flyback topology is selected to achieve it.

It is an ideal choice in terms of both volume and cost control. The integrated solution often introduces a "hiccup" mode, which can easily control the power consumption within 0.3W. 3.4 Control Circuit

The control circuit is composed of a zero-crossing logic circuit, a voltage doubling logic circuit, a thyristor drive circuit, etc. Since the combination of unidirectional thyristors and bidirectional thyristors can cut off the rectifier post-stage circuit (including filter capacitors), theoretically the post-stage circuit has zero power consumption.

Combined with the auxiliary power adapter, the standby power consumption of the whole machine can be easily controlled within 0.5W, meeting the requirements of 'Energy Star'.

3.4.1 Zero-Crossing Circuit

Since there is no current blocking effect of NTC, the control circuit must also implement ZVS control. The voltage doubling control logic and ZVS control logic must be synchronized. The drive circuit uses an optical coupler for isolated drive, effectively avoiding the problem of inconsistent thyristor drive potential.

Comparator U1-B in Figure 2-4 can monitor the zero-crossing state in real time. To avoid multiple zero-crossing judgments, R101 is added to complete the zero-crossing logic self-locking. Figures 2-5 and 2-6 are the measured voltage and current waveforms.

Figure 2-5 shows the voltage and current waveforms of the NTC current limiting circuit when the power is turned on. Figure 2-6 shows the zero-voltage switch circuit, where the current is well controlled and has a current waveform from '0' to '1'.

The inrush current is also lower than that of the NTC current limiting circuit. The inrush current is significantly controlled and is not limited by the power-on interval. It can be arbitrarily limited by the number of switching times and frequency, and the effect is very obvious.

The automatic voltage doubling logic is generated before the zero-crossing logic. In Figure 2-4, the comparator U1-A monitors the input voltage in real time, and its output logic is in an "and" relationship with the zero-crossing logic. On the one hand, the voltage doubling logic circuit must be able to automatically implement the voltage doubling operation according to the input voltage, and at the same time, it must be able to effectively prevent interference waveforms from causing unnecessary system actions or even misoperation. For example: when the negative amplitude fluctuates greatly, the input voltage fluctuates, and this fluctuation is active within a certain range, so only the threshold needs to be set to allow voltage fluctuations within a certain range. What needs to be avoided during the startup process is that the circuit needs to avoid the voltage doubling misoperation caused by the voltage rising process and the voltage doubling misoperation during the normal drop of voltage during the shutdown process. Possible voltage doubling misoperation during fast switching operations.

3.4.2 Thyristor drive

The driving aspect of bidirectional thyristor is more sensitive to the working quadrant. Let the driving voltage direction be the horizontal axis and the current direction be the vertical axis. For bidirectional thyristor, the best working quadrant is quadrant 1, followed by quadrants 2 and 3. Quadrant 4 is usually not recommended.

When operating in the fourth quadrant, the loss of the thyristor reaches its maximum, and the stress it withstands for di/dt also drops sharply.

Therefore, using the second and third quadrant working ranges shown in the figure below can not only ensure the good performance of the thyristor, but also simplify the drive circuit.

in conclusion

This power supply has the characteristics of automatic voltage doubling, no NTC and ultra-low standby power consumption. In pursuit of environmentally friendly high-power switching power supplies , a new design idea is proposed and a new solution is given, which has strong practicality and commerciality.