Often, the primary source of loss in an asynchronous switch-mode power supply is the non-idealities of the diodes. One way around this issue is to use an asynchronous switched-mode power supply (SMPS), where the diode is replaced with a controlled MOSFET switch. This method certainly increases efficiency; however, it comes at the expense of increased circuitry and the need for precise control.
Synchronous vs. asynchronous boost converters. Image used courtesy of Cadence
On the other hand, efficient asynchronous SMPS can be created by continually improving the characteristics of our non-ideal diodes. This approach is what Nexperia is taking.
This week, Nexperia released new trench Schottky rectifier diodes, which aim to significantly increase efficiency in asynchronous SMPS. Before diving into Nexperia’s new innovation, it is necessary to take a deeper look at diodes, more specifically Schottky diodes.
When engineers first learn about diodes, they almost always learn about them as perfectly ideal devices with no forward drop or leakage current. They certainly don’t hear about reverse recovery time (RRT). Unfortunately, the real world is not ideal, and these performance characteristics limit the efficiency of diodes, particularly in high-frequency switching applications like SMPS.
Graphical representation of reverse recovery time. Image used courtesy of ROHM Semiconductor
One source of loss for a diode is its forward drop, which varies between device types and represents an unwanted loss in the system. Furthermore, diodes tend to exhibit non-ideal leakage currents, which serve as a source of inefficiency.
Another source of loss for diodes in SMPS applications is switching losses. Real diodes cannot instantaneously go from being forward biased to being reverse biased; there are forward and reverse recovery times which delay this process. This non-ideal state of transitioning results in overlapping voltage and current during recovery times that can dominate losses if not carefully controlled.
With this in mind, how exactly do trench Schottky diodes factor into this conversation?
What is a Trench Schottky Diode?
Ideally, rectifier diodes should exhibit low forward voltage drop, high reverse blocking voltage, negligible leakage current, and low parasitic capacitance. Unfortunately, these often come with tradeoffs, making it hard to achieve a diode that exhibits all characteristics.
Planar (left) vs trench (right) Schottky diodes. Image used courtesy of Nexperia
A trench Schottky diode is a device that attempts to deplete the device’s drift region in the reverse direction and flatten the electric field profile along the drift region. This process is done by etching trenches into the silicon and filling them with polysilicon, thus creating a “trench” in the device.
Reverse recovery behavior of trench vs. planar Schottky diodes. Image used courtesy of Nexperia
This configuration’s result is a device with a good balance between reverse blocking voltage and leakage current. Another benefit is that its high current density results in a smaller effective junction capacitance. This benefit means that trench devices exhibit lower stored charges (Qrr), resulting in decreased reverse recovery times and ultimately reduced switching losses.
Nexperia’s New Devices
This week, Nexperia released their PMEGxxxTx devices, a new group to their family of trench diodes aimed at asynchronous SMPS.
According to their datasheets, these new diodes have impressive specs, with forward voltage drop of around 600 mV, leakage currents around 1 µA, and a reverse recovery step time of 17 ns. The company claims these specs result from the trench configuration, which allows for low reverse recovery charge and hence faster reverse recovery times.
The company sees these devices finding homes in applications that require high power and high-frequency switching like automotive, and specifically LED lighting. This family now offers devices ranging from 40 V to 100 V and up to 15 A in volume production, with 20 A devices available for sampling.
This advancement in more efficient diodes, especially for automotive applications, comes just in time with the global EV focus. With efficiency a major goal, more innovations are sure to come.