Curve Tracer using a class-D amplifier as waveform source - Page 1
Forum section: electronics / projects

author: hatte
author_rank: Contributor
timestamp: August 08, 2023, 03:52:10 pm
content: Hi,I am working on a curve-tracer design where step and ramp voltages will be controlled by a micro-controller via DACs and current will be measured by ADCs.The controller – probably a Raspberry Pi 3 or similar with ethernet – where all UI be presented on a browser via an embedded http server, allowing settings to be controlled (polarity, step offset and voltage/current, ramp voltage and current limit) and waveforms to be viewed (as svg) and saved. DACs and ADCs may either be controlled directly by the RPI or by an MCU with onboard DACs/ADCs interfacing to the RPI digitally using (possibly galvanically isolated) SPI/I2C.The DUT (BJT or FET) would be connected using a (14 pin) ZIF socket. Possibly A-B device testing could be allowed (for device pair matching) with a relay switching between them.The reason for interfacing via Ethernet+http server rather than USB is to have a cross-platform GUI without having to install any client software. Also, it provides galvanic isolation between the high voltage power/waveform sources and any connected client PC. It would also allow LXI and other interfaces to be implemented for automated operation.Power (<30W) can be supplied as 12V DC, via USB-C PD or a barrel jack. Internal voltages (+/- 50V) can be generated using DC-DC Boost controllers.Offset calibration can be performed digitally prior to each test.This proposed curve tracer design would allow the vast majority of semiconductor categorization to be performed using a compact, low cost, all-in-one device, eliminating the need for DIY rats-nest setups using a waveform generator and an oscilloscope.Generating the required waveformsTo drive the DUT, two voltage/current sources need to be generated:Ramp voltage (triangle/sawtooth wave at approx. 200Hz) with current limit, for collector/drain, delivering roughly 20W (e.g. 10V/2A, 50V/0.4AStep voltage/current, for base/gate drive, delivering up to 15V or 15mA, but controlled at mV/uA level. Operating at approx 200 steps/sThe above voltage/current sources should be bipolar (quadrant 1 and 2 operation), allowing testing of NPN/N as well as PNP/P-channel devices. They should also have common ground (connected to BJT Emitter or FET Source).I would like to avoid having to dissipate 20W power in the power drive circuit to prevent the requirement for active cooling and would therefore like the ramp (and possibly step) sources to be driven by a switching DC-DC converter.I have experimented with switching DC controllers (e.g. the TI LT3942 buck-boost controller), which works quite well, except for the bipolarity aspect which suddenly makes interfacing DACs and ADCs quite complicated.Using a Class-D (audio) Amplifier as a bipolar power/waveform sourceSo I have thought about using Class-D audio amplifiers which are quite capable of delivering the required waveforms and voltage/current, although high collector/drain drive voltages would require an additional output voltage boost stage to be added. Unfortunately, most class-D amplifiers have built-in DC blocking as DC is undesirable for audio power applications.I have searched through a LOT of class-D amplifier data sheets, but have been unable to locate any that explicitly states that they support DC input t-to-output operation. Some of them may, if you remove the input capacitive coupling, but as DC operation is not required/desirable for audio purposes, datasheets often do not show operation below 20Hz.Are any of you guys aware of an integrated class-D amplifier that will operate to DC (0Hz) with output voltages to at least +/-12V?The VBA Curce TracerI will be sttempting to provide controls similar to The VBA Curce Tracer (https://www.paulvdiyblogs.net/2022/06/the-vba-curve-tracer.html), although with higher minimums for step voltage/current (1mV/1uA) and lower maximum ramp voltage.Any ideas, advise and comments are welcome!

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: August 08, 2023, 04:21:56 pm
content: Hi hatte,nice idea, web-based interface, LXI, i like it.If it's just for sand - i don't like it. ;-)What resolution do you want to achieve?Good luck

author: hatte
author_rank: Contributor
timestamp: August 08, 2023, 07:43:51 pm
content: Quote from: Roehrenonkel on August 08, 2023, 04:21:56 pmWhat resolution do you mean?Step resolution would be mV/uA with offset.The voltage/current resolution would likely be 12 bit (1/4096), with a variable full scale (Vref), as that is readily available at very low cost.

author: Someone
author_rank: Super Contributor
timestamp: August 08, 2023, 10:37:45 pm
content: Quote from: hatte on August 08, 2023, 07:43:51 pmNow calculate the settling time of the output filtering for the class D, which will impact your desired dynamic response.

author: hatte
author_rank: Contributor
timestamp: August 09, 2023, 12:02:27 am
content: Quote from: Someone on August 08, 2023, 10:37:45 pmWith a (triangle wave) sweep frequency of 200Hz, and an amplifier switching frequency around 1MHz, I doubt the settling time of the class D output will be of any consequence.