It doesn't have to be silicon all the time. I have a Gallium-nitride device here at the Cumps lab that I'm going to try out. In some high voltage, high power designs, GaN FETs have advantages over Si. On the other hand they are also more difficult to drive.
The chip that I have contains a built-in smart GaN FET driver. That takes away the complexities of driving the power stage correctly. We're covering fairly new technology here. The documents are still marked technology preview. I received them from TI after attending a GaN seminar and answering right on the quiz.
In this post I'm doing a first try-out. I'm powering a 12V 1.25A load.
What's in the package
I'm using a TI LMG5200 GaN Half-Bridge Power Stage. That's a fancy title for what is in essence a 2-FET half-bridge setup with driver logic in a single package.
The driver stage takes care that the not-so-easy to-drive FETs are kept within their safe operation boundaries.
Unlike many integrated smart half and full bridges on the market, you can control the output of the high and low side separately.
Taken from the technology preview document lmg5200.pdf.
These are my very first steps, so I'll focus on trying the driver in this post. I'll cover specifications and technology in a follow up.
TI's GaN overview page, Gallium Nitride (GaN) Solutions | Overview for Gallium Nitride (GaN) Solutions, has some good info on the technology.
It's vendor biased info, but I bet you're smart enough to filter that out.
Starting Up the Half-Bridge
I'm using the package on an evaluation board. In essence, that board is an implementation of the reference design (all rules of the 'Layout Considerations for LMG5200 GaN Power Stage' document have been applied).
The only added functionalities are the provisioning for a stable bias supply, an output low pass filter and some logic to generate the high and low driver signals from a single PWM signal.
That evaluation board by itself is good for a few blogs.
The Start Up sequence
You need to power up the device in a particular sequence.
- First you need to bring up the bias voltage.
- Then you connect the PWM signal
- Last step is to bring up the input power source.
You shutting down in the reverse order.
The Set Up
My source for power and PWM - and the frequency counter and output voltage meter - is the old trusted (by many untrusted) Metex Universal System MS-9150 (I could do yet another blog about that system, and the hack I've done to it).
The 6 V bias is delivered by it's variable 0-30 V power supply.
20 V input is generated by wiring the fixed 5 V and 15 V supplies in series.
The PWM signal (5 V, 100 kHz -> 2 MHz) is coming from its function generator TTL output. I'm checking the frequency with its universal counter. And I'm verifying output volmtage with its built-in DMM.
I've connected a Rigol DS1052E to the PWM input and the output.
The load is an incandescent 12V 15W bulb.
As expected, everything works fine. Any other outcome would be a surprise. The task that I'm giving to the NaG driver is very well within its operating range.
The whole setup stayed nicely under control and nothing got hot on the board.
Below are the capture of DC and AC analysis of the output signal. Don't get carried away by the ripple on the AC signal. That's just fine.
We're not testing a regulated DC power supply here. This is a PWM signal that's filtered by a coil and some caps. It will perfectly manage the power sent to the load.
The initial exercise was a success. I'm now confident that I can properly drive the GaN IC, and I can start probing a bit deeper under the surface.
|Part 1: Preview|
|Part 2: Dead Time Capture|
|Part 3: Control Deadband with Hercules LaunchPad|
|GaN Point of Load converter 48V to 1V 50A - part 1: Design Overview|
|on TI E2E community: driving GaN with a simple PWM signal|