After the digital power 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 power amplified and isolated through the drive circuit, and then connected to the power switching device to complete the output control of the power supply. This article will mainly explain the driving circuit of the power supply.

一、 Overview of drive circuit
1. The role 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 power of the PWM signal generated by the digital control core to drive the power switching device on and off. Excellent driver circuits can improve the reliability of digital power supplies, reduce device switching losses, improve energy conversion efficiency and reduce EMI/EMC.

2. Classification of drive circuits

The drive circuit is divided into direct ground drive and floating ground drive according to the component grounding type of the power device. In direct ground drive circuits, the ground potential of the power device is constant. Commonly used ones include push-pull drive and totem pole drive. The ground potential of power devices driven by floating ground will float as the circuit status changes. A typical floating ground drive circuit is a bootstrap drive circuit, which connects the drive circuit to the device ground reference control signal through a level shift circuit. Bootstrap capacitor CBST, totem pole bipolar driver, and regular 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 drive circuit is divided into isolated drive and non-isolated drive according to the circuit structure. Isolated drive circuits refer to drive circuits that include optocouplers, transformers, capacitors and other devices with electrical isolation functions. Non-isolated drive circuits do not have electrical isolation structures and mostly use resistors, diodes, transistors or non-isolated drive chips.

3. Common drive circuit forms

1) Direct drive
A direct drive circuit is a drive circuit composed of individual electronic components (such as diodes, transistors, resistors, capacitors, etc.) connected together. The circuit does not have electrical isolation and is mostly used in low-power driving situations 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, commonly used ones include optocoupler drive, transformer drive, isolation capacitor drive, etc. 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 can also be used for voltage isolation and impedance matching.
3) Dedicated driver integrated chip
At present, special driver chips are widely used in digital power supplies, and many driver chips have built-in protection and isolation functions. According to the number of power devices it controls, driver chips can be divided into single-drive chips and dual-drive chips. Among them, dual-drive chips are usually used in half-bridge, full-bridge and other power topologies because they require a pair of complementary control signals. The single drive chip is more suitable for power topologies such as buck, boost, and flyback.
二、 Commonly used drivers for power switch tubes
1. MOSFET driver
MOSFET is often used in small and medium power digital power supplies, and its driving voltage range is generally between -10~20V. MOSFET does not have high power requirements for the driving circuit. In low-frequency situations, it can be driven directly by transistors, while in high-frequency situations, transformers or special chips are often used for driving.
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, and the resistors R3 and R4 By dividing the voltage of the power supply (V+), the MOS tube is turned on and reaches saturation.
G pole voltage is:
       Vcc * R4
VG = —————
(R3 + R4)
2) Push-pull drive circuit
When the power IC driving capability is insufficient, a push-pull driving circuit can be used. The push-pull drive circuit can increase 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, using complementary outputs. When a high level is input, the upper tube NPN is turned on, the lower tube PNP is turned off, and the driver MOS tube is turned on; when a low level is input, the upper tube NPN is turned off, the lower tube PNP is turned on, and the driver 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 and is mostly used in high-power half-bridge and full-bridge converters. Its circuit structure is as shown in the figure.

In the first cycle, OUTA is turned on, applying a positive voltage to the primary winding of the transformer, and the upper tube is induced to conduct. In the next cycle, OUTB is turned on (the turn-on time is the same as OUTA), providing a voltage with opposite polarity on the magnetizing inductor, and the lower tube is turned on. The circuit will produce 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 drive circuits are divided into positive voltage drive and negative voltage drive. The difference between the two is the gate potential when turned off. The use of negative voltage turn-off can avoid the risk of misleading turn-on caused by the rise of the Miller capacitor on the gate voltage. It can also speed up the turn-off speed, reduce the turn-off loss, and improve the withstand voltage to a certain extent.
IGBT driver circuits generally use dedicated driver chips, such as Toshiba's TLP series, Fujifilm's EXB series, Infineon's EiceDRIVER series, etc. Here we take Toshiba TLP250 and Infineon 1ED020I12-F2 as examples.
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 IGBT overcurrent protection. In this type of circuit, the requirements for the IGBT drive circuit are relatively simple. The TLP250 produced by Toshiba is widely used in this scenario, and its drive circuit is as shown in the figure.

TLP250 has a built-in optocoupler, its isolation voltage can reach 2500V, the rise and fall times are less than 0.5us, the output current reaches 0.5A, and it can directly drive IGBTs within 50A/1200V. The driver is small in size and cheap, making it an ideal choice for IGBT driver chips without over-current protection.
2) Infineon 1ED020I12-F2 chip
Infineon's 1ED020I12F2 is a galvanically isolated single-channel IGBT driver chip. The chip output current is typically 2A and can be used to drive 600V/1200V IGBT. It integrates a coreless transformer to achieve electrical insulation isolation and can be directly connected to the power microcontroller.

At the same time, the chip has DESAT detection function for over-current and short-circuit protection, active Miller clamp function and two-level turn-off (TLTO) function, and is often used in inverters and DC/DC converters.
3. Other power device drivers
In addition to commonly used MOS tubes and IGBTs, some new power devices are also widely used in digital power supplies, such as SiC MOSFETs and gallium nitride transistors (GaN FETs).
SiC Mosfet tube has the characteristics of high blocking voltage, high operating frequency, strong high temperature resistance, low on-state resistance and small switching loss, and is suitable for high-frequency and high-voltage applications; the driving voltage range of SiC MOSFET is -5~20V, and its driving The circuit design should consider the requirements of drive level and drive current, the requirements of dead time setting, the protection function and anti-interference performance of the chip, etc.
Gallium nitride transistors are similar to silicon transistors and are also driven by voltage. Its gate-source driving voltage range is -5~6V. In order to obtain a smaller driving resistance, the driving high level of the gallium nitride transistor is generally set at about 5V. Considering that the parasitic inductance of the loop under high-frequency operating conditions will cause large driving oscillations, the safety margin of the driving voltage is very small. . But an important advantage of GaN over Si MOSFET is its excellent high-frequency performance.
That’s all about the drive circuit of the power supply. I believe everyone has a preliminary understanding of the implementation 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 introduce another important type of power supply peripheral circuit-communication circuit.