Speed Does Not Kill: Power Supplies with Fast Switching Frequency Get Safer

by Administrator llooper ‎05-16-2017 09:00 AM - edited ‎05-16-2017 09:02 AM

The more power generated by a smaller power supply, the more cost effective it is and the less room it takes in the design. Designers can improve W/mm3 with faster switching. Switching technology using Gallium Nitride (GaN) or Silicon Carbide (SiC) allows faster switching rates than are currently available—up to 20 times faster.

 

The technologies’ lateral structures, compared to vertical for silicon, make them low-charge devices capable of switching hundreds of volts in mere nano-seconds (ns). The challenge is that they require isolated gate drivers to work, and today’s technology typically cannot support the required noise immunity from their superfast switching.

 

Faster Switching - SMPS Want It

Fast power switching is most prevalent in switched mode power supplies (SMPS). SMPS convert their input power from ac to dc (ac-dc) or from dc to dc (dc-dc). In most cases, they also change voltage levels to suit the needs of the application.

 

SMPS Graphic.png

Typical ac-dc SMPS block diagram

 

The new GaN and SiC technology switches faster and is more efficient than current switches. But their faster switching causes higher switching transients as shown in the figure below with a typical 600 V high side rail.

Switching Transients.png

Switching transients in a power converter

 

GaN switching times are typically about 5ns, or about 10x - 20x faster than conventional systems. In this case, the 600 V high voltage rail results in a 120 kV/µs transient (600 V / 5 ns = 120 V/ns or 120 kV/µs).

 

CMTI – Common Mode Transient Immunity – the Key Isolated Gate Driver Spec

The isolated gate drivers controlling the power switches have to be designed to withstand these noise transients without creating glitches or latching-up. The ability of the driver to withstand these common mode noise transients is generally defined as CMTI (common mode transient immunity), and is expressed in units of kV/µs.

 

 

What are the Isolated Gate Driver Options?

Isolated gate drivers must preserve the integrity of the isolation from the primary to the secondary side. There are a number of isolated gate driver solutions available today:

 

  • Junction-isolated drivers – With 50 kV/µs for latch-up immunity (CMTI proxy), these are not suitable for new, fast-switching tech’s requirement.
  • Opto-coupled drivers – With 10-50 kV/µs for latch-up immunity (CMTI proxy), these are still not good enough.
  • Transformer-coupled drivers – Getting closer with 50-100 kV/µs for latch-up immunity (CMTI proxy), but these are still not adequate.
  • Capacitive-coupled drivers – She’s got it where it counts. These explicitly specify CMTI at 200 kV/µs (MIN) for signal integrity and 400 kV/µs (MAX) for latch-up immunity. These exceed the 120 kV/µs requirement.

 

There are other advantages for these drivers. Their latency (propagation delay) can be as much as 10x better than popular optocoupled gate drivers, and their part-to-part matching can be more than 10x better. This provides the designer with another key advantage—the system’s overall modulation scheme can be fine-tuned for maximum efficiency (W/mm3) and safety without having to accommodate specification slop. They also support lower voltage operation (2.5 V compared to 5 V) and a wider operating temperature range.

 

Plus, they offer advanced features such as input noise filters, asynchronous shutdown capability, and multiple configurations such as half bridge or dual independent drivers in a single package.

Finally, they are rated to 60 years of operating life at high voltage conditions, longer than any other comparable solution.

 

Power supply designers want to maximize their W/mm3 using the fastest power switching technology available. The latest GaN- and SiC-based switches are the fastest available technology today, but isolated require gate drivers with very high noise immunity (CMTI).

 

The Si827x isolated gate drivers from Silicon Labs meet GaN’s and SiC’s noise immunity requirements with margin to spare (120 kV/µs required, 200 kV/µs supplied).

 

Si827x.png