Power Electronics Europe Feb/March 2023

https://www.allegromicro.com/en POWER DENSITY 13 www.power-mag.com Issue 1 2023 Power Electronics Europe between the high-voltage and low-voltage domains, while providing system and human protection from hazardous direct current. Each gate driver must provide a specific control voltage for each of the power switches within the design. The voltage needed for one switch typically differs from the voltage needed for another. Because device switching speed and output quality greatly affect the power conversion, matching the requirements of the switches to the gate drivers is crucial to achieving the desired system performance. Before selecting a gate driver, the cost, size, weight, reliability, and efficiency of the power-conversion system must be weighed with consideration for the influencing factors, including the environmental conditions presented at the location where the gate driver is placed, component count, and design complexity. The wide variety of conditions that a gate driver will face heavily influence performance. These conditions vary depending on where the gate driver is located—on the high-voltage side of a bridge, on the low-voltage side of a bridge, in the hostile environment of an inverter, or in a charger. In power- conversion circuitry, component count and design complexity also heavily influence device performance. Larger designs result in longer signal paths, which adds parasitics, so smaller gate drives can improve performance. Also, because every component is a potential point of failure, gate drives with reduced component count can improve reliability. With the vast selection of gate drivers available on the market today, designers have a plethora of options. However, the choice of gate driver is only a starting point. In many applications, it can be challenging to identify a suitable source of power to enable the gate driver to drive the gates of the switches at the required speeds, as discussed next. To meet requirements, power-conversion designs often include multiple bias power supplies that are isolated from the control ground. For a solution that includes multiple tradeoffs in performance, the design challenges that arise from these architectures can consume a significant amount of valuable engineering time, which makes it critical to choose the right gate driver technology early in the design process: The right gate driver can have a big impact on the success of a system. Understanding the tradeoffs of power rails, auxiliary supplies, and bootstrap circuitry In conventional designs, external DC-DC bias supplies are used to drive the gates of the power transistors. Sometimes, application requirements can force the addition of eight or nine separate bias supplies, with each supply adding transformers and other bulky components that degrade reliability and increase solution size. Although the load and stress added by these components can be minimized by design, including these components hinders reliability because every component is a potential point of failure. The added bias supplies not only bring new points of potential for failure, but they must also be sized for the highest operational switching frequency, which results in less-efficient operation at lower frequencies. All things considered, the many complex components and circuitries used in conventional designs add to system cost, size, and weight and reduce reliability. The task of adding an extra power rail to a design is often more complicated than it first seems. Use of any power supply adds stray capacitance between the control ground and the source terminal of the semiconductor power switch that the gate driver is controlling. For a switch located in the high side of a full bridge, the additional stray capacitance is likely to create common- mode and electromagnetic interference, which brings new design challenges. Also, stray capacitance can allow for the flow of high-voltage current spikes, which can interfere with correct functional operation. These issues are likely to occur even if the power device is driven across an isolation barrier or if the extra power rail is derived from an additional winding on the main system transformer. Furthermore, using an additional winding complicates the design of the main system transformer and can have negative implications when it comes to safety compliance. A lower-cost alternative method of providing an additional power rail to serve as the external bias supply for the isolated gate driver is to use bootstrap circuitry. The essential components of the bootstrap circuit are a diode, a capacitor, and—in some cases—a resistor. A bootstrap circuit is arranged so that, when the low-side switch is turned on, the capacitor is charged via the diode almost to the level of the supply voltage (Vdd). The capacitor subsequently provides the drive power for the high-side switch. Although this process may seem straightforward, it brings a fresh set of challenges. The diode requires a reverse voltage rating that is higher than the high- voltage bus. In designs with high- frequency switching, this diode must also be rated for fast recovery. The capacitor must be small enough to recharge quickly, but large enough to supply the required gate charge for the switch without becoming discharged too quickly. The resistor, if required, must limit the spikes of charging current flowing into the bootstrap capacitor. This is because all bootstrap designs have the potential for false overcurrent tripping, which would occur if the charging current in the low- side current sense signal to the controller were to spike with sufficient amplitude at the capacitor. To prevent false overcurrent tripping, the amplitudes of such potential spikes must be limited. For that purpose, a resistor is often added in series with the diode. This resistor comes with difficult tradeoffs. Increasing resistor values requires a larger capacitor, which takes more time to charge at startup and thereby delays the availability of high-side drives; yet, if the capacitor is too small, supplying the required gate charge to the high-side switch will come with too much droop in the gate voltage. Together, these requirements typically result in a need that can only be filled by large and relatively expensive components. Considering the tradeoffs, there is no such thing as an ideal bootstrap circuit. At best, a bootstrap circuit provides a workable tradeoff between multiple design variables. Although this workaround has often been accepted as a required means to an end, it does not satisfy today’s top-notch power- conversion requirements. The ideal solution for the electrification needs of today might lie in finding a way to eliminate the need for bootstrap circuits and separate power supplies. However, most industry resources have not been devoted to research and development of gate drive systems but rather to wide-bandgap power switches. Driving these switches requires gate drive systems that can fully leverage the potential advantages of these new technologies, including the tremendous benefits that come with the elimination of bootstrap circuits and separate power supplies. New approach—Embedding the bias supply system within the driver A new approach to driving wide-bandgap power switches is to simplify the way the power switches are driven. Some devices now embed the bias supply system within the driver. These devices need no external auxiliary supply as the bias supply system replaces the need for the external auxiliary supply, eliminating the additional components and design resources associated with auxiliary supplies. In one such approach, the