Optimizing power stage design needs adaptable gate drive solutions for evolving technologies like Si, SiC, and GaN. This paper highlights the benefits of isolated DC/DC converters with programmable outputs for efficient, flexible, and future-proofgate drive power.

Abstract

As industries transition to higher switching frequencies and voltages to improve performance and minimize overall system size, designers are finding it increasingly challenging to balance cost, efficiency, and reliability. A critical aspect of power stage design optimization is selecting appropriate gate drive solutions that can adapt to the always-changing current and emerging transistor technology requirements — such as silicon (Si), silicon carbide (SiC), and gallium nitride (GaN). This paper explores the challenges associated with standard gate drive designs and highlights the benefits of using isolated DC/DC converters with programmable outputs to achieve efficient, flexible, and future-proof gate drive power for IGBTs, Si, and SiC MOSFETs.

Current Challenges in Power Stage Design

Power conversion system designers face a dilemma when selecting gate drive solutions for their power stages — fixed gate drive voltage solutions, while simple to implement, lack the flexibility to accommodate different transistor technology requirements. For instance, optimal gate voltages for IGBTs, Si and SiC MOSFETs vary, consequently requiring the use of gate drive circuits or a total power stage redesign for newer transistor generations.

IGBTs typically require a positive gate voltage between +15V and +20V to fully turn on. For rapid turn-off and false triggering prevention, IGBTs require a negative gate voltage between -5V and -15V. Si MOSFETs have lower gate voltage requirements in contrast, generally needing between +10V and +15V to turn on and between 0V and -5V to turn off. SiC MOSFETs, chosen for their high switching speeds and low on-state resistance, have gate voltages close to those of IGBTs — with some devices requiring up to +25V for optimal performance. Using fixed gate drive voltage solutions for multiple transistors can lead to suboptimal performance, increased loss, and failure due to insufficient or excessive gate voltages. Designers may consequently resort to using separate gate drive circuits tailored to each transistor type, which increases not only the overall system complexity, but also cost and board space.

Increasing the switching frequency and voltage to enhance both efficiency and power density introduces certain challenges. Higher frequencies require faster switching transitions, which can result in increased EMI and noise issues. Faster switching edges (high dv/dt and di/dt) can couple noise through the circuit’s parasitic capacitances, including the transistor package, PCB traces, and isolation barriers. This noise interferes with the gate drive circuit’s proper operation, leading to unintended switching and even increased power loss and device failure.

Using high-performance components to achieve faster switching comes at a premium, as designers must strike a balance between cost and performance based on the application and market requirements. For example, designers may opt for lower-cost Si MOSFETs or IGBTs in cost-sensitive consumer applications. This decision sacrifices efficiency and performance in favor of a cheaper solution. On the other hand, in high-performance industrial or automotive applications, using expensive SiC MOSFETs may be justified to achieve efficiency, reliability, and power density benchmarks.

The need to reduce overall system size is another significant challenge in power stage design. As power density becomes increasingly important, designers must find ways to achieve greater miniaturization and seamlessly integrate gate drive circuitry without compromising performance or reliability. Unfortunately, standard gate drive solutions rely on discrete components and separate power supplies, which can occupy valuable board space and complicate the design. Discrete gate drive circuits consist of the following: a gate drive IC, isolated power supply, passive components such as resistors, capacitors, diodes, and more. Carefully selecting and placing each component on the PCB can only be done after considering factors such as power dissipation, thermal management, and signal integrity. As the number of transistors in a power stage increases, so does gate drive circuitry complexity and size.

Isolated DC/DC Converters for IGBTs, Si, and SiC MOSFETs

Most isolated DC/DC converters available on the market come with fixed output voltages. This lack of flexibility makes it difficult to accommodate the gate voltage requirements of different transistor technologies, including IGBTs, Si, and SiC MOSFETs. As a result, designers may need to use multiple isolated DC/DC converters or resort to additional circuitry to achieve the required gate drive voltages, which increases system complexity, size, and cost.

To address this problem, designers are turning to programmable isolated DC/DC converters as a solution. These converters combine the functionality of an isolated power supply and a gate drive circuit into a single package, while offering the ability to adjust the output voltages to match the needs of the chosen transistor technology. By providing gate drivers with programmable output voltages for each transistor, designers can optimize turn-on and turn-off characteristics of their power stage while simplifying the overall design and reducing system size.

Using programmable isolated DC/DC converters offers the ability to independently control the positive (Vpos) and negative (Vneg) gate voltages for individual transistors. This flexibility allows designers to fine-tune gate drive voltages, ensuring full enhancement and rapid discharge of the transistor while minimizing switching losses and improving efficiency. By selecting specific Vpos and Vneg values, gate voltages can be kept within a transistor’s safe limits while maximizing the performance. For example, in an IGBT-based power stage, a programmable isolated DC/DC converter can be set to provide a Vpos of +15V and a Vneg of -8V, ensuring full enhancement during turn on and rapid gate capacitance discharge during turn-off. Similarly, in a SiC MOSFET design, the converter can be configured to offer a Vpos of +20V and a Vneg of -5V, optimizing the gate drive voltages for the specific requirements of the SiC device.

Providing a stable, well-regulated gate voltage supply, independent of the main power supply, is another benefit of isolated DC/DC converters. In typical gate drive circuits, the primary supply derives the gate voltage using a linear regulator or a bootstrap circuit. Linear regulators, while simple to implement, tend to have poor efficiency and greater power dissipation when there is a large difference between input and output voltages. Excessive power dissipation can lead to thermal management issues and may require additional heat sinks or cooling solutions. Bootstrap circuits, on the other hand, rely on a charge pump mechanism to provide the high-side transistor’s gate voltage in a half-bridge configuration. As such, carefully size the bootstrap capacitor to ensure an available sufficient charge to drive the transistor’s gate over the entire on-time. The duty cycle and switching frequency can affect the circuit’s performance, leading to voltage droop and instability.

The R24C2T25, a key product within this series, offers a range of features and benefits that make it suitable for powering IGBTs, Si MOSFETs, and SiC MOSFETs. Measuring just 7.5 x 12.83mm and packaged in a surface-mount SSOP-36 package with an integrated isolation transformer, RECOM’s RxxC2Txx series is suitable for designers looking to minimize their gate drive circuitry’s footprint. The RxxC2Txx series’ compact size and SMT compatibility is suitable for space-constrained applications and facilitates easy integration into existing designs. The SSOP-36 package also provides excellent thermal performance, with an exposed pad on the bottom of the package that allows for heat dissipation.

The R24C2T25 offers continuous 2W output power, ensuring ample power delivery to the gate drive circuitry. This high output-power capability enables the converter to drive larger transistors or multiple transistors in parallel, suitable for high-current applications including industrial motor drives, solar inverters, and electric vehicle traction inverters. With a 3kVAC/1 min isolation rating, the product offers robust isolation between the input and output stages. The R24C2T25 achieves high isolation voltage through proprietary transformer design and advanced insulation materials for reliable operation even in harsh environments. Moreover, the R24C2T25 features a high common-mode transient immunity (CMTI) rating of 150kV/µs. CMTI is a critical parameter in isolated gate drive applications that represents the converter’s ability to withstand rapid changes in common-mode voltage without causing signal corruption or damage to the device.In high-power systems, fast-switching transistors generate considerable common-mode transients, which can couple through the isolation barrier and disrupt gate drive signals. A high rating CMTI ensures that a converter maintains signal integrity and reliable operation even in the presence of these transients.

The R24C2T25 also features an ultra-low isolation capacitance of less than 3.5pF, crucial for minimizing common-mode noise and reducing the impact of high dv/dt transients on the gate drive signals. In high-power applications, the switching transistors can generate large.

voltage and current transients during turn-on and turn-off events. These transients tend to couple through the gate drive circuit’s isolation capacitance, causing false triggering, excessive ringing, and greater EMI. By minimizing the isolation capacitance, the R24C2T25 effectively reduces transient coupling, improving the gate drive circuit’s overall signal integrity and reliability. This low isolation capacitance enables faster switching speeds and higher frequency operation, as it reduces common-mode noise impact.

RECOM’s R24C2T25 programmable outputs provide flexibility in setting the gate drive voltages with a positive output range of +2.5 to +22.5V and a negative output range of -2.5V to -22.5V. RECOM accomplishes this feat by changing the feedback divider circuit’s resistor values — meaning that one power supply solution can provide +15/-9, +20/-5, +18/-4, +15/-3 or any other combination, as long as the combined output lies in the range of 18 to 25V. This programmability simplifies the design process with a single, optimized solution for current and future transistor generations, avoiding the need for multiple gate drive circuits or costly redesigns.

In terms of safety, the R24C2T25 incorporates several protective features to ensure robust and reliable operation in demanding environments, including: