Principle of DC Boost Circuit

DC boost technology is an engineering technology that converts non-adjustable DC voltage into adjustable or fixed DC voltage. One method is to use the energy storage function of the inductor and the filtering function of the capacitor to boost the voltage; the other method is to use high frequency Oscillation generates low-voltage pulses, which are then boosted to a predetermined voltage by a pulse transformer, and then pulse rectification technology is applied to obtain high-voltage direct current. The DC step-up process is realized by a conversion circuit that controls electric energy by means of switching regulation, that is, a DC-DC converter. The core component of the DC-DC converter is a switching converter composed of a crystal diode, an energy storage device (or transformer), a capacitor and an inductor. Its output circuit rectifies the current through a low-pass filter composed of capacitors. Realize the output of high voltage direct current.

The continuous update and improvement of DC boost technology has greatly affected the evolution of DC-DC converter topology. High power density, high efficiency, high performance, high reliability, low cost, and small size are the development directions of DC-DC converters. At present, DC boost technology has been widely used in battery-powered portable devices, high-power DC transmission technology, photovoltaic power plants and other fields, and has a good application prospect.

1. Principle and analysis of DC boost circuit

As a DC-DC converter that converts DC into another fixed voltage or adjustable voltage, the DC boost circuit is used in DC drive systems, charging and storage circuits, switching power supplies, power electronic conversion devices and various electrical equipment. common application. Subsequently, various conversion circuits such as step-down circuits, buck-boost circuits, and composite circuits have appeared. According to the topology of the circuit, it is mainly divided into isolated and non-isolated circuits.

1. Non-isolated circuit

The non-isolated topology includes BUCK circuit, BOOST circuit, CUK circuit, SEPIC circuit, etc. Among them, the BUCK circuit is a DC step-down circuit, and the BOOST step-up circuit is the most widely used, and it is also the basis of the latter two circuits. Therefore, this section focuses on the principle and characteristics of the BOOST boost circuit.

The basic topology of the BOOST boost circuit is shown in the figure below. The switch in the actual circuit is generally an IGBT element (insulated gate bipolar transistor, which is composed of a BIT (bipolar transistor) and a MOS (insulated gate field effect transistor). Composite fully-controlled voltage-driven power semiconductor devices, which have both the advantages of high input impedance of MOSFET and low conduction voltage drop of GTK) or thyristor components, and the corresponding control signals are pulses based on PWM technology generator to provide.

According to the analysis of the closing and opening of the switch s according to the analog circuit, combined with the basic knowledge of circuit theory, the output voltage can be deduced.

To sum up, the boosting process of the BOOST circuit is the energy transfer process of the inductor. When charging, the inductor absorbs energy; when discharging, the inductor releases energy. If the capacitance is large enough, a continuous current can be maintained during the discharge of the output. If this on-off process is repeated continuously. A voltage higher than the input voltage can be obtained across the capacitor.

The diode in the circuit is mainly used for isolation, that is, when the switch is closed, the voltage of the anode of the diode is lower than the voltage of the cathode, and the diode is cut off at this time, so that the energy storage process of the inductor does not affect the normal power supply of the output capacitor to the load: because in When the switch is turned off, the two superimposed energies supply power to the load through the diode. At this time, the diode is forward-conducting. It is required that the forward voltage drop should be as small as possible, so that more energy can be supplied to the load terminal as much as possible.

The non-isolated DC-DC topology is relatively simple, the number of components required for the circuit is small, and it is easy to design and control, but it is also limited by the input-to-output voltage ratio, which cannot be applied to occasions with a large voltage conversion ratio, nor can it achieve Electrical isolation requirements, these topologies are suitable for low power equipment.

2. Isolated circuit

The isolated topology mainly includes "flyback circuit", "forward circuit", "push-pull circuit", "half bridge circuit", "full bridge circuit" and related circuits derived from these basic circuits .

The basic working process of this circuit is as follows: high-frequency oscillation generates low-voltage pulses—pulse transformer step-up to a predetermined voltage value—pulse rectification to obtain high-voltage direct current.

Among them, the push-pull DC boost circuit is more widely used. At the same time, after the push-pull circuit is bridge rectified or full-wave rectified, the voltage ripple and current ripple of the output voltage are very small, so only a small energy storage is required. filter capacitor or energy storage filter inductor, an output voltage with very small voltage ripple and current ripple can be obtained. Therefore, the push-pull circuit is a switching power supply with very good output voltage characteristics.

The main disadvantage of the push-pull circuit is that when the system works for a long time or often runs at high power, the circuit is prone to unbalanced magnetic flux, that is, the phenomenon of magnetic bias, and it is easy to burn out the shutdown tube and related devices. Another disadvantage of the push-pull circuit is that the switch tube of the circuit has a large turn-off withstand voltage value. Compared with other circuits, its withstand voltage must be greater than twice the working voltage. Therefore, push-pull circuits are rarely used at high voltage input.

2. Application of DC boost circuit

DC boost technology meets the demand for higher DC voltage during operation of equipment using DC power sources such as batteries, and has been widely used in mobile phones, pagers and other wireless communication equipment, camera flashes, portable security detectors, electric mosquito swatters, etc. application. At the same time, the development of high-power DC transmission technology, photovoltaic power generation technology, and uninterruptible power supply (UPS) technology is also inseparable from the application of DC boost technology.

1. Application in LED driver

In the application circuits of electronic products such as mobile phones and digital cameras, it is usually necessary to drive the LED of the flash module or the LED of the display backlight through a boost circuit to adjust the brightness of the LED. The circuit for driving LED can generally be divided into two types: parallel drive and series drive. Parallel drive adopts the principle of capacitive charge pump voltage multiplication, and all LED loads are connected in series; series drive adopts the principle of inductive DC boost conversion (Boost circuit) to increase the total voltage of the system to meet the requirements of a single LED in series. The rated voltage demand of the load. The series drive circuit booster device is small in size, high in efficiency, and takes up less space, so it is widely used in mobile phones, digital cameras, PDA handheld devices, MP3 players, GPS receivers and other devices.

The inductive DC boost conversion of series LEDs applies the Boost circuit boost principle, and the boost device MP1518 chip of Monolithic Power Systems (MPS) is well applied to mobile phones and other equipment. In addition, under the condition of a certain input voltage, the voltage across the drive LED is related to the duty cycle of the signal. The greater the duty cycle of the signal, the greater the output voltage, and the greater the brightness of the LED. In practical applications, a PWM wave is often added to the enable pin EN, and the brightness of the LED is adjusted by changing the duty cycle of the PM W wave.

2. Application in solar photovoltaic power generation system

The power generation principle of a solar cell is to utilize the photoelectric effect caused when it is incident on a semiconductor. The basic characteristics of photovoltaic cells are similar to diodes, which are completely different from traditional power generation methods. There is no rotating part of the rotor and no electromagnetic interaction is used. Therefore, the current generated by it is direct current. The photovoltaic power generation system mainly includes solar panels, chargers, batteries, controllers, DC boost circuits, inverters, etc.

A typical photovoltaic power generation system consists of four parts: photovoltaic cell array, energy storage system, inverter, and DC control system. Among them, the electric energy emitted by single photovoltaic cells is very small, which is direct current. In order to meet actual needs and obtain a large enough power generation, it is necessary to connect single photovoltaic cells to form a battery pack, and then form a solar photovoltaic array by the battery pack. During the transmission and exchange of electric energy from the photovoltaic array to the energy storage unit, and then to your transformer unit, to maintain the efficient and safe operation of the system, a DC control system is required to adjust, protect and control the entire process.

When the light is strong, the low-voltage direct current of the solar cell is directly provided to the DC boost circuit, and the battery is charged and stored through the charger; when the light is weak, the output power of the solar cell cannot meet the requirements of photovoltaic power generation. At this time, as an energy storage device The storage battery provides low-voltage DC power for the DC boost circuit, ensuring the continuity and stability of the photovoltaic power generation system. The DC boost circuit raises the low-voltage DC to 330V high-voltage DC, and then can get 50Hz/220V AC through the inverter. The output AC voltage and current are fed back to the controller through the detection circuit, and the controller can realize closed-loop control.

3. Application in photomultiplier tube

Photomultiplier tubes are widely used in various photoelectric detection instruments. Similarly, in the field of biomedical engineering, photomultiplier tubes play an irreplaceable role in photoelectric imaging detection and treatment instruments (such as PET). The power supply of the early photomultiplier tubes was boosted by a power frequency transformer and then rectified by voltage doubling. This kind of power supply has a large volume and poor effect. At present, the high-voltage power supply of the photomultiplier tube adopts the DC converter method, which is smaller in size and higher in efficiency than the rectification scheme after boosting the power frequency transformer, and has a good application prospect.

The input DC voltage is +12 V, the output voltage is adjustable, and the typical output voltage is +1200 V. Its main components are low-voltage DC power supply, PWM control circuit, power switch tube, high-frequency transformer, voltage doubler rectifier circuit, filter circuit and sampling feedback circuit. The +12 V DC input voltage provides working power for the control circuit and the transformer; the output signal of the PWM control circuit drives the power transistor to provide a low-voltage high-frequency square wave for the high-frequency transformer; the AC output signal of the high-frequency transformer is further obtained by a voltage doubler rectifier circuit Boost and rectify, output DC high-voltage signal: the high-voltage output signal is further filtered after passing through the filter circuit, effectively reducing the output ripple coefficient; the output voltage feedback circuit provides a voltage feedback signal for the PWM control circuit, through the pulse modulator The reference voltage of the error amplifier is compared to control the output pulse duty cycle of the pulse modulator, so as to achieve the purpose of regulating the output voltage.

Summary of boost circuit technology

This article describes the principles and applications of two main DC boost circuits. The BOOST circuit achieves the purpose of boosting the voltage through the switch tube and inductor energy storage. It is relatively simple, has high power density, low current, and uses a non-isolated topology; and because of this, it is itself a source of electromagnetic interference that may cause dysfunction of surrounding electronic equipment. The circuit using transformer boosting, represented by the push-pull DC boost circuit, adopts an isolated topology, and can further boost the voltage through the turns ratio and the subsequent voltage doubler rectifier circuit, which is more suitable for the input voltage. Low voltage, high voltage ratio, and isolation is required. The two have a wide range of applications based on their own characteristics. In the application circuits of electronic products such as mobile phones and digital cameras, it is usually necessary to drive the LED of the flash module or the LED of the display backlight through a boost circuit to adjust the brightness of the LED. In the photovoltaic cell, the low-voltage direct current converted from light by the direct current step-up circuit is charged and stored in the battery through the charger. Another example is that in the photomultiplier tubes widely used in various photoelectric detection instruments, the high-voltage power supply for the high-voltage electric field between the plates adopts the DC converter method, which is smaller than the rectification scheme after boosting the power frequency transformer. ,efficient.

The above is the principle and application of the DC boost circuit introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. If you have software and hardware function development needs for smart electronic products, you can rest assured to entrust them to us. We have rich experience in customized development of electronic products, and can evaluate the development cycle and IC price as soon as possible, and can also calculate PCBA quotations. We are a number of chip agents at home and abroad, including MCU, voice IC, Bluetooth IC and modules, wifi modules. We have hardware design and software development capabilities. Covering circuit design, PCB design, single-chip microcomputer development, software custom development, APP custom development, WeChat official account development, voice recognition technology, Bluetooth wifi development, etc. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS solutions, the development of Bluetooth audio, the development of children's toys, and the development of electronic education products.