Design of switching power supply based on LDO

Power supply is an indispensable component of various electronic devices. The quality of its performance is directly related to the technical indicators of electronic devices and whether they can work safely and reliably. Currently, the commonly used DC regulated power supplies are divided into two categories: linear power supplies and switching power supplies. Since the key components inside the switching power supply work in a high-frequency switching state, the energy consumed by the switching power supply itself is very low. The efficiency of the switching power supply can reach 80%~90%, which is nearly doubled that of the ordinary linear regulated power supply. It has become the mainstream product of regulated power supplies.

The structure of the switching power supply

Figure 1 shows the schematic diagram and equivalent block diagram of the switching power supply, which consists of a full-wave rectifier, a switch tube Vi, an excitation signal, a freewheeling diode VD, an energy storage inductor and a filter capacitor C. In fact, the core part of the switching power supply is a DC transformer. Here we explain the DC converter and inverter as follows. Inverter, it is a device that converts DC to AC. Inverters are usually widely used in backup power supplies composed of level or battery. DC converter, it is a device that converts DC to AC and then converts AC to DC. This device is widely used in switching power supplies. A DC converter can be used to convert a DC supply voltage into multiple DC supply voltages with different polarities and values.

Advantages and disadvantages of switching regulated power supplies

Advantages of switching regulated power supply:

Low power consumption and high efficiency. In the switching voltage-stabilized power supply circuit in Figure 1, the transistor V, under the stimulation of the excitation signal, alternately works in the switch state of on-off and off-on, with a very fast switching speed, and the frequency is generally around 50kHz. In some technologically advanced countries, it can reach several hundred or nearly 1000kHz. This makes the power consumption of the switching transistor V very small, and the efficiency of the power supply can be greatly improved, and its efficiency can reach 80%.

Small size and light weight. From the principle block diagram of the switching voltage-stabilized power supply, it can be clearly seen that no bulky power frequency transformer is used here. Since the power dissipation on the adjustment tube V is greatly reduced, a large heat sink is omitted. Due to these two reasons, the switching voltage-stabilized power supply is small in size and light in weight.

Wide voltage regulation range. The output voltage of the switching voltage regulator is adjusted by the duty cycle of the excitation signal, and the change of the input signal voltage can be compensated by frequency modulation or width modulation. In this way, when the power frequency grid voltage changes greatly, it can still ensure a relatively stable output voltage. Therefore, the voltage regulation range of the switching power supply is very wide and the voltage regulation effect is very good. In addition, there are two methods to change the duty cycle: pulse width modulation and frequency modulation. The switching voltage regulator not only has the advantage of a wide voltage regulation range, but also has many ways to achieve voltage regulation. Designers can flexibly select various types of switching voltage regulators according to the requirements of actual applications.

The efficiency of filtering is greatly improved, which greatly reduces the capacity and volume of the filter capacitor. The operating frequency of the switching power supply is currently basically 50kHz, which is 1000 times that of the linear power supply, which makes the filtering efficiency after rectification almost 1000 times higher; even if half-wave rectification is used and capacitor filtering is added, the efficiency is also increased by 500 times. Under the same ripple output voltage, when using a switching power supply, the capacity of the filter capacitor is only 1/500 to 1/1000 of the filter capacitor in the linear power supply. The circuit form is flexible and diverse, including self-excitation and external excitation, width modulation and frequency modulation, single-ended and double-ended, etc. Designers can give full play to the strengths of various types of circuits and design switching power supplies that can meet different applications.

Disadvantages of switching regulated power supply:

The disadvantage of the switching power supply is the existence of serious switching interference. In the switching power supply, the power adjustment switch transistor V works in the switching state, and the AC voltage and current it generates pass through other components in the circuit to generate spike interference and resonance interference. If these interferences are not suppressed, eliminated and shielded by certain measures, they will seriously affect the normal operation of the whole machine. In addition, since the switching power supply oscillator is not isolated by the power frequency transformer, these interferences will be connected to the power frequency power grid, causing serious interference to other nearby electronic instruments, equipment and household appliances.

At present, due to the gap between domestic microelectronics technology, resistor and capacitor device production technology and magnetic material technology and some technologically advanced countries, the cost cannot be further reduced, which also affects the further improvement of reliability. Therefore, in my country's electronic instruments and mechatronics instruments, switching power supplies cannot be widely popularized and used. In particular, for high-voltage electrolytic capacitors, high-reverse-voltage high-power switching tubes, and magnetic core materials of switching transformers in switching power supplies without power frequency transformers, they are still in the research and development stage in my country. In some technologically advanced countries, although switching power supplies have made certain developments, there are still some problems in practical applications, which are not very satisfactory. This exposes another disadvantage of switching power supplies, that is, the circuit structure is complex, the failure rate is high, and the maintenance is troublesome. In this regard, if designers and manufacturers do not pay enough attention to it, it will directly affect the promotion and application of switching power supplies. Today, the main reason why switching power supplies are difficult to promote and apply is that its manufacturing technology is difficult, maintenance is troublesome, and the cost is high.

The switching power supplies currently sold on the market use bipolar transistors with a switching frequency of 100kHz and MOS-FETs with a switching frequency of 500kHz. The prominent disadvantage of switching power supplies is that they generate strong EMI. EMI signals have a wide frequency range and a certain amplitude. They will pollute the electromagnetic environment through conduction and radiation, and cause interference to communication equipment and electronic products. If not handled properly, the switching power supply itself will become a source of interference. When using a switching power supply as the input VIN of an LDO, pay attention to the LDO power supply rejection ratio and power consumption.

The power supply rejection ratio (PSRR) is an AC parameter that reflects the ability of the LDO output to suppress input ripples. Generally, the output and input frequencies are the same. The larger the PSRR value, the stronger the ripple capability of the LDO, which means that the input has little effect on the output. Although the power supply rejection ratio of the LDO is very strong, it is only very strong within a certain frequency range. Generally, the power supply rejection ratio between 50KHz and 200kHz is still very poor. Figure 4 shows the PSRR and frequency curve of SGM2007, and this frequency range is exactly the operating frequency of most switching power supplies. If the LDO load and input and output capacitors are not well matched, it is easy to cause LDO oscillation. This will cause instability in the entire LDO power supply system.

Most of the switching power supplies sold on the market have fixed voltage outputs, usually 5V outputs, while the most commonly used LDO outputs are 3.3V outputs. When the output of the switching power supply is used as the input of the LDO, there is a large voltage difference of 1.7V. If the LDO current is very large, such as 200mA, the temperature of the chip will be very high, the power consumption will be very large, and working at high temperature for a long time will affect the working life of the chip.

The power consumption of a low dropout linear regulator is mainly a function of the input voltage, output voltage and output current. The following equation can be used to calculate the power consumption under the worst case condition:

PD = (VINMAX-VOUTMIN)ILMAX. Where: PD = actual power dissipation under the worst case, VINMAX = maximum voltage on the VIN pin, VOUTMIN = minimum voltage output by the regulator, ILMAX = maximum (load) output current.

The maximum allowable power dissipation (PDMAX) is a function of the maximum ambient temperature (*AX), the maximum allowable junction temperature (TJMAX) (+125°C), and the junction-to-air thermal resistance (qJA). For a 5-pin SOT-23 package mounted on a typical double-layer FR4 electrolytic copper PCB, (qJA) is approximately 250°C/Watt.

PDMAX=(*AX- TJMAX)/ qJA

VINMAX=3.0V+10%, VOUTMIN=2.7V-2.5%, ILOADMAX=40mA, TJMAX= +125℃, *AX = +55℃

Actual power consumption PD=26.7mW, maximum allowable power consumption: PDMAX="280mW".

Conclusion

When the switching power supply is used as the input of a low-dropout linear regulator, attention must be paid to the ripple of the switching power supply, the effect of the switching frequency on the LDO, and the matching of the LDO load capacitance. The maximum power consumption should not be exceeded to affect the stability of the system.