I have this plan to script an automated test setup. I want to check the behaviour of GaN power FETs. I'll need to control the input signals that drive the FETs, the load that the FETs see, and the measurement instruments. To make this possible, I bought a license for LabVIEW Home Edition.
I used the coupon code that's available if you listen to The Amp Hour. |
I'd like to automate measuring the behaviour of power FETs in a switched design. What's the effect of the switch frequency on the efficiency?
How short can I make the dead band before the circuit becomes unstable? What are the minimum and maximum duty cycle?
The Building Blocks
The lab setup has five main parts:
- the device under test. This is a switching power device (half-bridge, buck converter, ...) and the PSU to deliver the source power.
- a laptop with LabVIEW. That combination will set the input conditions for the device under test. It will also collect data from the test setup.
- the input controller. A microcontroller that generates the PWM input control signals based on LabVIEW commands.
- the load. Controlled by LabVIEW, it will stress the device under test.
- the measuring instruments. An oscilloscope and the same load that we can also query for data.
Device under Test (DoT)
This can be any switching power regulator that accepts PWM control signals (either one simple PWM, or separate signals for a high and low side).
In this setup, I 'm going to use GaN FET power drivers in half-bridge configuration. I can replace them with other devices like buck, boost, etc..
The power supply is also part of this block. It 'll deliver the ummph to the switching device. It's going to be set to fixed values in the test setup.
Laptop with LabVIEW
This is the heart of the system. It drives the inputs to the DoT. It sets up the parameters of the electronic load.
In the other direction, it 'll retrieve the measured values of the load.
It configures the oscilloscope and queries back measured values and wave forms.
LabVIEW can alter the input and load conditions. It can then measure the effects of those changes to the DoT.
Input Controller
A microcontroller that has three duties.
It has to generate simple PWM signals, where we can indicate frequency and duty cycle.
It also has to be able to generate more complex separate high and low side signals with dead band for finer controll of the switchin devices.
And it has to be able to communicate with LabVIEW. The program will tell what type of signal needs to be generated, and what the frequency, duty cycle and dead time should be.
The Load
An electronic programmable load. Has to be able to set its value based on LabView commands.
In combination with the input controller, we can check how the switching device behaves for different frequencies, duty cycles and loads.
The Measuring Instruments
The same load can play a role in this setup to. LabVIEW can query the voltage at the point of load and the current.
My oscilloscope will also be used. What it's going to measure is dependent on the facts I'm after.
It can be voltages, waveforms, frequencies, ripple, ...
LabVIEW will set the horizontal sensitivity, the time base, the trigger setup, etc... automatically.
It will query the results back and collect the info in a document.
For those setups where a waveform is useful, I'll also try to grab that with LabVIEW.
Anything Missing?
Yes. In the ideal world, the power supply would also be controlled by the test script.
And I'd like to have a meter linked into the equation. Ideal if I want to measure efficiency.
(I have a DMM with a RS232c interface. But I'd have to acquire a usb converter and the meter isn't worth any investments.)
I'm currently blind (or better: LabVIEW is blind) for what's happening on the power input of the DuT.
Not an ideal situation.
But it's better to get going than to muse about improvements.
I can always use the controllers ADC inputs to build makeshift voltage or current sensors that talk to LabVIEW.
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