Gate Drive Solution Options
Although optocouplers are commonly used for feedback isolation, their propagation delay performance is not fast enough to achieve the full benefit of the synchronous MOSFET gate-drive isolation circuit. Other drawbacks include unstable operating characteristics over temperature, device aging, and marginal common mode transient current (CMTI) resulting from a single-ended architecture with high internal coupling capacitance.
Gate drive transformers
Gate drive transformers have become a more popular method of providing isolated gate drive. Gate drive transformers are miniature toroidal transformers that are preferred over optocouplers because of their shorter delay times. They are faster than optocouplers, but cannot propagate a dc level or low-frequency ac signal. They can pass only a finite voltage-time product across the isolation boundary, thereby restricting ON time (tON) and duty cycle ranges. Transformer-based designs are inefficient, have high EMI, and occupy excessive board space.
An Optimum Isolated Gate Drive Solution: CMOS-based isolated gate drivers
Advancements in CMOS-based isolation technology have enabled isolated gate drive solutions that offer exceptional performance, power efficiency, integration, and reliability. Isolated gate drivers, such as Silicon Labs’ Si823x ISOdriver family, combine isolation technology with gate driver circuits, providing integrated, low-latency isolated driver solutions for MOSFET and insulated-gate bipolar transistor (IGBT) applications.
ISOdriver isolated gate driver solutions
The Si823x ISOdriver products are available in three basic configurations, including:
- high-side and low-side isolated drivers with separate control inputs for each output
- high-side and low-side isolated drivers with a single PWM input
- dual isolated driver
The Si823x ISOdriver family supports 0.5 A and 4.0 A peak output drive options and is available in 1 kV, 2.5 kV and 5 kV isolation ratings. The high-side/low-side versions have built-in overlap protection and an adjustable dead time generator (dual ISOdriver versions contain no overlap protection or dead time generator). As such, the dual ISOdriver can be used as a dual low-side, dual high-side or high-side/low-side isolated driver.
These devices have a three-die architecture that causes each drive channel to be isolated from the others as well as from the input side. This allows the polarity of the high-side and low-side channel to reverse without latch-up or other damage.
Maximizing System Efficiency
The switching mode in high-side/low-side drive applications must be “break-before-make” to avoid efficiency loss from both MOSFETs being on at the same time (i.e., “shoot-through current”). This time period between switch transitions where both switches are off is referred to as “dead time”. While an optimum amount of dead time can increase system efficiency, excessive amounts of dead time can reduce efficiency. High-side/low-side Si823x ISOdrivers have an integrated dead time generator that can be adjusted from 0.4 ns to 2μs using an external resistor, allowing the user to optimize dead time. The Si823x ISOdrivers also contain overlap protection.
While dead time optimization can increase efficiency by as much as +4%, additional efficiency gains can be achieved by arranging MOSFETs in parallel or by increasing gate driver strength to a single, larger MOSFET.
In both cases, a dual ISOdriver can be useful in providing additional drive capability. This is another advantage of the ISOdriver family of dual ISOdrivers as each have no restrictive, built-in overlap protection or dead time setting. The state of each driver output unconditionally follows that of its input as long as the device is powered. The two driver output circuits are isolated from each other and from the input, allowing the common-mode voltage of one driver to reverse polarity with respect to the other without damage (i.e., latch-up) or output errors.