Must read! Detailed explanation of commonly used drive circuits for power supplies

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Must read! Detailed explanation of commonly used drive circuits for power supplies [Copy link]

In the previous article, we talked about the sampling principle and sampling conditioning circuit of the power supply. After the digital power supply control core collects the input and output parameters, it uses the control algorithm to analyze and generate a PWM control signal. The PWM signal will be amplified and isolated by the drive circuit, and then connected to the power switch device to complete the output control of the power supply. This article will mainly explain the drive circuit of the power supply.

1. Overview of the drive circuit
1. Function of the drive circuit
The drive circuit is located between the main circuit of the power supply and the digital control core. Its essence is to amplify the PWM signal generated by the digital control core to drive the power switch device to open and close. A good drive circuit can improve the reliability of the digital power supply, reduce the switching loss of the device, improve the energy conversion efficiency and reduce EMI/EMC.

2. Classification of drive circuits

The driving circuit is divided into direct grounding driving and floating grounding driving according to the grounding type of the power device. In the direct grounding driving circuit, the ground terminal potential of the power device is constant, and commonly used ones include push-pull driving and totem pole driving. The ground terminal potential of the power device of the floating grounding driving will float with the change of the circuit state. The typical floating grounding driving circuit is the bootstrap driving circuit, which connects the driving circuit with the device ground reference control signal through a level shifting circuit. The bootstrap capacitor CBST, the totem pole bipolar driver and the conventional gate resistor can all be used as level shifting circuits. In addition, some driver chips have built-in bootstrap circuits, which can directly connect the bootstrap signal to the reference terminal of the power device.
The driving circuit is divided into isolated driving and non-isolated driving according to the circuit structure. An isolated driving circuit refers to a driving circuit that contains devices with electrical isolation functions such as optocouplers, transformers, and capacitors. Non-isolated driving circuits do not have an electrical isolation structure, and mostly use resistors, diodes, transistors or non-isolated driving chips.
3. Common drive circuit forms
1) Direct drive
The direct drive circuit is a drive circuit composed of single electronic components (such as diodes, triodes, resistors, capacitors, etc.) connected together. There is no electrical isolation in the circuit, and it is mostly used in low-power drive occasions with simple functions. In complex digital power systems, direct drive circuits have been gradually eliminated due to low integration and high failure rate.
2) Isolation drive
The circuit contains isolation devices, and commonly used ones are optocoupler drive, transformer drive, and isolation capacitor drive. Among them, the optocoupler drive circuit has the characteristics of simplicity, reliability, and good switching performance. The transformer drive circuit can not only play a driving role, but also be used for voltage isolation and impedance matching.
3) Dedicated drive integrated chip
Currently, dedicated drive chips are widely used in digital power supplies, and many drive chips have built-in protection and isolation functions. According to the number of power devices it controls, the drive chip can be divided into single-drive chip and dual-drive chip. Among them, dual-drive chips are usually used in power topologies such as half-bridge and full-bridge because a pair of complementary control signals are required. Single-drive chips are more suitable for power topologies such as buck, boost, and flyback.

2. Commonly used drivers for power switch tubes

1. MOSFET driver

MOSFET is commonly used in small and medium power digital power supplies, and its driving voltage range is generally between -10 and 20 V. MOSFET does not require high power from the driving circuit. In low-frequency applications, it can be driven directly by a triode, while in high-frequency applications, it is often driven by a transformer or a dedicated chip.

1) Transistor drive circuit

The transistor drive circuit is the most basic MOS transistor drive circuit. The following takes the N-MOS transistor drive circuit as an example. As shown in the figure, when the control core outputs a high level, the transistor Q1 is turned on, the control electrode (G) of the N-MOS tube Q2 is pulled low, and the MOS tube is turned off; when the control core outputs a low level, the transistor Q1 is turned off, the resistors R3 and R4 divide the power supply (V+), and the MOS tube is turned on and reaches saturation. The G electrode voltage is:

2) Push-pull drive circuit

When the driving capability of the power IC is insufficient, a push-pull drive circuit can be used. The push-pull drive circuit can improve the current supply capability and quickly charge the gate input capacitor. As shown in the figure, the push-pull drive circuit includes a PNP transistor and an NPN transistor, and adopts a complementary output. When the input is high, the upper tube NPN is turned on, the lower tube PNP is turned off, and the driving MOS tube is turned on; when the input is low, the upper tube NPN is turned off, the lower tube PNP is turned on, and the driving MOS tube is turned off.

3) Double-ended transformer coupled gate drive

The double-ended transformer-coupled gate drive circuit can drive two MOS tubes at the same time. It is mostly used in high-power half-bridge and full-bridge converters. Its circuit structure is shown in the figure. In the first cycle, OUTA is turned on, and a positive voltage is applied to the primary winding of the transformer, and the upper tube is induced and turned on. In the next cycle, OUTB is turned on (the turn-on time is the same as OUTA), and a voltage with opposite polarity is provided on the magnetizing inductance, and the lower tube is turned on. The circuit will generate two bipolar symmetrical gate drive voltage outputs, which meet the control requirements of the half-bridge circuit.

2. IGBT driver

IGBT is often used in the development of medium and high power digital power supplies, and its driving voltage range is -15~15V. IGBT driving circuits are divided into positive voltage driving and negative voltage driving. The difference between the two lies in the gate potential when turned off. Using negative voltage to turn off can avoid the risk of mis-turning on due to the lifting effect of Miller capacitance on the gate voltage, and can also speed up the turn-off speed, reduce the turn-off loss, and improve the withstand voltage to a certain extent. The driving circuit of IGBT generally uses a dedicated driver chip, such as Toshiba's TLP series, Fuji's EXB series, Infineon's EiceDRIVER series, etc. Here we take Toshiba TLP250 and Infineon 1ED020I12-F2 as examples for introduction.

1) Toshiba TLP250 chip

In low-performance three-phase voltage sources or inverters, current control is achieved by monitoring the DC bus current, and the detection results can be used for overcurrent protection of IGBTs. The requirements for IGBT drive circuits in this type of circuit are relatively simple. TLP250 produced by Toshiba is widely used in this scenario, and its drive circuit is shown in the figure. TLP250 has a built-in optocoupler, with an isolation voltage of up to 2500V, a rise and fall time of less than 0.5us, an output current of 0.5A, and can directly drive IGBTs within 50A/1200V. The driver is small in size and cheap in price, and is an ideal choice for IGBT driver chips without overcurrent protection.

2) Infineon 1ED020I12-F2 chip

Infineon's 1ED020I12F2 is a current-isolated single-channel IGBT driver chip with a typical output current of 2A, which can be used for 600V/1200V IGBT driving. It integrates a coreless transformer to achieve electrical insulation isolation and can be directly connected to a power microcontroller. At the same time, the chip has DESAT detection function for overcurrent and short-circuit protection, active Miller clamping function, and two-level shutdown (TLTO) function, and is often used in inverters and DC/DC converters.

3. Other power device drivers

In addition to the commonly used MOS tubes and IGBTs, some new power devices are also widely used in digital power supplies, such as SiC MOSFET and gallium nitride transistors (GaN FET). SiC Mosfet tubes have the characteristics of high blocking voltage, high operating frequency, strong high temperature resistance, low on-state resistance and small switching loss, and are suitable for high-frequency and high-voltage applications. The driving voltage range of SiC MOSFET is -5~20V. The design of its driving circuit should consider the requirements of driving level and driving current, the requirements of dead time setting, the protection function of the chip and anti-interference. Gallium nitride transistors are similar to silicon tubes and are also voltage-driven. Their gate-source driving voltage range is -5~6V. In order to obtain a smaller driving resistance, the driving high level of gallium nitride transistors is generally set at around 5V. Considering that the parasitic inductance of the loop under high-frequency working conditions will cause large driving oscillations, the safety margin of the driving voltage is very small. However, an important advantage of GaN over Si MOSFET is its excellent high-frequency performance.

This is all about the drive circuit of the power supply. I believe everyone has a preliminary understanding of the implementation method and working principle of the drive circuit. When actually designing the drive circuit, you can choose the appropriate drive circuit form according to the requirements of the usage scenario (power, frequency, protection, drive voltage/current, etc.). Later, we will continue to introduce another important type of power peripheral circuit-communication circuit.

tsgnge

Knowledge sharing, benefiting a lot

ZhuPinchao

Thank you for your attention, welcome to consult and learn from each other

花鼓舞

Knowledge sharing, benefiting a lot

kobe0203

The explanation is detailed and comprehensive, which is very helpful for people who make power supplies. Thank you for sharing