Product designers must be able to balance multiple constraints – package size, cost, reliability, and time to market. The key challenge is to select a power supply suitable for the small space required for modern applications.
Compact, high-performance power levels rely on fast, reliable gate drive solutions. These solutions range from simple low-pressure side drives to fully isolated versions for high-pressure environments. For many designs, a floating, non-insulated gate driver provides an effective path to success.
The gate driver serves as an intermediate device to transfer low power control signals, typically from a microcontroller or a pulse width modulation (PWM) controller, to a high power switch that regulates the energy flow. These devices ensure clean, fast, and accurate switching to optimize power output.
To select a suitable gate driver, the voltage and current requirements, topology and switching frequency need to be evaluated. Well-matched drives provide high efficiency, timing accuracy and thermal stability, which are essential for high-performance, compact systems.
Benefits of half-bridge topology
The half-bridge topology is a widely used method in modern power conversion, enabling efficient voltage stabilization in compact designs. This topology relies on two high-speed switching devices, typically MOSFETs or Insulated Gate Bipolar Transistors (IGBTs), to alternating input voltages, supplying transformers in isolated designs, or directly supplying loads in non-isolated systems. This topology is valued for its efficiency and thermal optimization potential.
The gate driver IC is integral to the control of these switches and serves as the interface between the controller and the power stage. This IC converts the PWM signal into a high current drive signal, ensuring fast and accurate switching between the high voltage side and low voltage side transistors. This fast and efficient operation minimizes energy loss and improves overall system performance.
In a half-bridge circuit, the source of the high voltage side MOSFET is connected to the switch node, which moves rapidly between the ground (0 V) and the input voltage (e.g. 12 V, 48 V, etc.) according to the switching period. When a floating non-isolated gate driver is used, the high-voltage side driver will "float" with the voltage of the switch node, so as to achieve clean and efficient conversion.
When isolation is not required, but compact structure, speed and efficiency are prioritized, the floating ground non-insulated half-bridge gate driver is an ideal solution. These drivers are designed to control the high and low voltage side MOSFET switches, avoiding the complexity of isolation and ensuring accurate switching performance. Since this type of drive provides no power isolation between the control logic and the power level, it works best in a system where all components are in common ground.
A bootstrap capacitor is usually required to generate the required gate drive voltage for the high voltage side MOSFET. The capacitor is charged when the switch at LV side is on; When the switch on the high voltage side is opened, the capacitor supplies power.
When the low voltage side MOSFET is switched on, the switching node is pulled to ground allowing a small diode capacitor circuit to charge the bootstrap capacitor from the power rail. When the high voltage side MOSFET needs to be turned on, the driver drives the gate to a voltage higher than the switch node, typically 10 V to 15 V, using stored charges.
The designer must ensure that the opening frequency of the switch on the low voltage side is sufficient to charge the bootstrap capacitor. In high duty cycle applications, additional precautions may be required, such as selecting the appropriate capacitance value and minimizing the voltage drop across the bootstrap diode.
By using a bootstrap architecture and tracking the switch node voltage, the floating ground non-isolated half-bridge driver not only avoids the complexity of achieving isolation, but also ensures robust high-voltage side control. It is simple and efficient and is ideal for high frequency switching applications such as step-down and step-up converters, synchronous voltage regulators, motor drivers and Class D audio amplifiers.
Select proper gate driver IC
The selection of suitable gate drivers is essential to ensure efficient, reliable and safe operation of the power level, particularly in high speed switching applications such as step-down converters, motor drivers and solar power generation systems. While the fundamental principles of gate drive are widely used, certain selection criteria become especially important depending on system requirements.
For example, in solar conversion and battery-powered systems, the gate driver must adapt to large input voltage variations and changing load conditions. High voltage side rated voltage with sufficient margin is required to withstand full power rail fluctuation and ensure long-term reliability.
Common mode transient immunity (CMTI) is another major consideration. Fast switching events create steep voltage differences between the high voltage side and low voltage side MOSFETs, causing noise and ringing. Gate drivers with high CMTI perform more stably in an electrically noisy environment.
Peak drive currents are also important, particularly in high power applications. The driver must provide sufficient current to quickly charge the MOSFET gate and overcome parasitic capacitance to reduce switching losses and improve thermal performance.
Finally, dead time control plays a key role in the half-bridge topology. If there is no short delay between closing one switch and opening another, a breakdown occurs, i.e. two MOSFETs are on at the same time. Many gate drivers have built-in or adjustable dead time settings to prevent this problem and provide safe and efficient operation under varying load conditions.
LTC706x Series of ADI
The ease of use and high speed switching capabilities of floating, non-isolated half bridge drives are the best solution for many designs. Analog Devices, Inc. (ADI) offers a wide range of high voltage features designed for demanding applications.
The LTC706x Floating Ground Non-Isolated Half Bridge Gate Driver (Figure 1) of ADI provides a multifunctional solution to meet the needs of high-speed, high voltage power conversion. The compact package provides strict timing control, breakdown protection and powerful driving force to meet the demands of various applications from automobile to industrial control.