Floating Ground Non-Isolated Half-Bridge Gate Drivers

Abstract

Half-bridge topologies are popular for converting power for a variety of commercial and industrial applications. At the heart of this switch-mode arrangement is the gate driver IC, whose primary function is to use a pulse-width modulated signal to provide clean, level-shifted signals to the high-side and low-side MOSFET power switches.

This article highlights the key criteria engineers should consider when selecting a gate driver IC for an application. In addition to the essential voltage and current ratings, the importance of a high common-mode transient immunity and the need for an adjustable dead time are explained. For some use cases, the ability to galvanically isolate the gate driver IC from the MOSFETs is required, and this floating ground approach is showcased with a simple reference design.

Introduction

Power conversion is a core function of virtually every electronics design today. Ideally, any conversion from one DC voltage, say 9 V, to a different level, for example, 24 V, should be as efficient as possible, with minimal losses. To accommodate different voltages, current requirements, and power density, several circuit architectures, termed topologies, cater to a wide variety of applications. For DC-to-DC power conversion, these include buck, boost, buck-boost, half-bridge, and full-bridge topologies. Another consideration is whether or not the output needs galvanic isolation from the input voltage, leading to nonisolated and isolated conversion methods.

For high voltage and high current use cases, such as motor control and solar inverters, half-bridge and full-bridge DC-to-DC conversion techniques are prevalent.

Half-Bridge Power Conversion

A half-bridge arrangement utilizes a switch-mode approach to stepping up or down a DC input voltage. It uses two switching devices, typically MOSFETs or insulated gate bipolar transistors (IGBTs), to switch voltage inputs to a transformer (isolated) or directly to a load (nonisolated). A gate driver IC receives a pulse-width modulation (PWM) signal from a controller IC. It amplifies and shifts the signal to the level required to turn on or off the MOSFET power switches (termed high-side and low-side) rapidly so that power losses are kept to a minimum, improving the converter’s efficiency. Selecting a suitable gate driver IC for an application depends on factors such as operating converter topology, voltage, current ratings, and switching frequency. Choosing a gate driver with accurate and efficient switching characteristics is essential to achieving optimal conversion efficiency.

Choosing a Gate Driver IC

When selecting a gate driver IC, engineers must consider several key criteria. Some of these may depend on the application, such as solar energy conversion, where the gate driver may encounter a wide range of input voltages and power demands.

• High-side voltage: Depending on the application, the highside MOSFET will experience the full supply voltage, so the gate driver must be able to handle this with a high degree of safety margin.
• Common-mode transient immunity (CMTI): With a fast switching operation capable of generating high noise levels and the potential for high voltage differences between highside and low-side MOSFETs, selecting a gate driver with a high transient immunity characteristic is vital.
• Peak drive currents: For higher power designs, the gate driver needs to supply a high peak current to the MOSFETs to charge and discharge the gate capacitances quickly.
• Dead time: To prevent the MOSFETs from experiencing a shoot-through event when they both conduct simultaneously, a small dead time period between high-side and low-side switching is a crucial aspect of any half-bridge circuit. Selecting a gate driver with a configurable dead time is highly recommended to achieve optimal efficiency. Some gate drivers include a default dead time to prevent shootthrough from occurring.

Half-Bridge Driver with Floating Grounds and Adjustable Dead Time

The LTC7063 is one example of a half-bridge gate driver suitable for a broad range of high voltage and high current applications. It is suited for use in a variety of industrial, automotive, and telecom power systems. This device is a high voltage gate driver designed for driving N-channel MOSFETs arranged in a half-bridge configuration up to an input supply voltage of 140 V. The IC features powerful drivers capable of quickly charging and discharging large gate capacitances typically associated with high voltage MOSFETs. Also, an adaptive shoot-through protection feature monitors the voltage of the switching node and controls the driver outputs to prevent simultaneous conduction of the MOSFETs. This crucial feature prevents the potential for shoot-through currents and helps improve power efficiency.

Both the high-side and low-side MOSFET drivers of the LTC7063 are floating and allow a ground offset of up to 10 V between the IC and the output grounds. This floating architecture makes the driver outputs more robust and less sensitive to ground offset, noise, and transients. The floating ground capabilities make it an excellent choice for remote MOSFET control applications, and for high voltage, high current switched capacitor converters.

The safety and protection features of the LTC7063 include thermal shutdown, input undervoltage and overvoltage lockout circuits, and undervoltage lockout circuits for both high-side and low-side MOSFET drivers. Such features ensure the long-term reliability and robustness of any half-bridge application.

For efficient heat transfer, these gate drivers are enclosed in a thermally enhanced exposed pad package.

An LTC7063-Based Step-Down Converter Application with Remote Load

Figure 1 illustrates a 2:1 step-down converter design with remote load using the LTC7063. It operates from an input supply of up to 80 V and delivers ½ V_IN for a max load of 5 A. The PWM pin receives a 3-state logic signal from an external controller, and the gate of the high-side MOSFET is driven high when the PWM signal rises above the rising threshold. The low-side MOSFET is driven complementary to the high-side MOSFET. The hysteresis between the rising and falling threshold of the input signal eliminates false triggering of the MOSFETs. Both the high-side and low-side MOSFETs are pulled low during the hysteresis interval of the input signal. When the enable (EN) pin is high, both the top gate (TG) and bottom gate (BG) outputs correspond to the input PWM signal, and by pulling the EN pin to low, both the TG and BG outputs are pulled low.

Bootstrapping the BST-SW and BGVCC-BGRTN supplies ensures an efficient high-side and low-side driver operation without any additional isolated supply voltage, reducing both cost and the number of components on the board. The control waveforms and average output voltage results are shown in Figure 2.

The dead time between the TG and BG is low, and the BG/TG rise can be increased by adding a resistor between the DG pin and the ground. Shorting the dead time (DT) pin to ground results in a default dead time of 32 ns for this transition, and by floating the DT pin, the dead time extends to a maximum of 250 ns. This programmable dead time feature ensures even more robust shoot-through protection for high voltage applications.

To attain high efficiencies, it’s important to minimize switching losses. The 1.5 Ω pull-up resistance and 0.8 Ω pull-down resistance of both high-side and low-side MOSFET drivers turn on or off the switches quickly and prevent any cross-conduction currents, thus improving the efficiency. Figures 3 and 4 show the switching transitions between turn-on and turn-off, along with the dead time.

The fault (FLT) pin is an open drain output, and it is pulled low internally when the junction temperature of the LTC7063 reaches 180°C. It is also pulled low when the supply voltage for V_CC is below 5.3 V or above 14.6 V. For the application shown in Figure 1, the BGVCC-BGRTN and BST-SW floating voltages below 3.3 V trigger the fault condition and pull the FLT pin low. Once all faults have cleared, the FLT pin is pulled high with an external resistor after a 100 μs delay.

Table 1 shows variants in the LTC706x family, which have features similar to those of the LTC7063.

Conclusion

The LTC7063 is a high voltage N-channel half-bridge gate driver and is part of the LTC706x product family. Its unique dual floating ground architecture provides efficient driver outputs for ground offset and remote load applications and offers high noise immunity. The adaptive shoot-through protection and programmable dead time features eliminate any potential shoot-through currents, and the powerful MOSFET drivers minimize the power loss by enabling fast switching and helping achieve high efficiencies in high voltage, high current DC-to-DC applications.

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