opamp

Introduction The world of digitally controlled switchmode power supplies has always held a certain allure for me: the idea that you could sample things like output voltage and current, apply some DSP techniques, and then based on that output a gate drive signal in real time, giving you whatever response you want out of your buck/boost converter just seems so… insanely cool. That being said, a lot of times these require a good understanding of the control loop architecture and parameters of switchmode power supplies to be able to properly stabilize, which has proven to be quite the barrier to entry....

Introduction I started off this project when I realised that the official Discovery BNC adapter board has too high an input capacitance to be used with generic scope probes, which usually have a compensation range of 5-35pF ish. Using scope probes is really necessary in some cases where low loading is desireable, since the quoted 1MΩ || 24pF input impedance of the Analog Discovery 3 loads down sensitive analog circuits quite significantly....

Before reading on, please refer to the previous part/MK1 of this project, otherwise some of the stuff here might seem a little odd. Schematic <?xml version="1.0" encoding="UTF-8" standalone="no"?> SVG Image created as AD3 Front End Amplifier-CH1.svg date 2023/12/18 09:48:42 Image generated by Eeschema-SVG C207 C207 1u 1u -2V5 -2V5 1 1 2 2 SW201 SW201 AC/DC AC/DC C203 C203 1u 1u V- V- 2 2 V+ V+ 5 5 1 1 + + 3 3 - - 4 4 U202 U202 C201 C201 10n 10n -2V5 -2V5 +2V5 +2V5 C208 C208 1u 1u R207 R207 402 402 R201 R201 1k 1k R208 R208 22 22 V- V- 2 2 V+ V+ 5 5 1 1 + + 3 3 - - 4 4 U201 U201 D201 D201 R202 R202 1M 1M C205 C205 1u 1u C204 C204 1u 1u R203 R203 100 100 R204 R204 402 402 R206 R206 402 402 +2V5 +2V5 C206 C206 1u 1u R205 R205 22 22 C202 C202 10n 10n 1 1 2 2 J201 J201 CH1 CH1 COM COM 1 1 NO NO 2 2 V- V- 3 3 IN IN 4 4 V+ V+ 5 5 IC201 IC201 TS12A4516DBVR TS12A4516DBVR OUT+ OUT+ OUT- OUT- V+ V+ V+ V+ V- V- U202 U202 V- V- 2 2 V+ V+ 5 5 1 1 + + 3 3 - - 4 4 -2V5 -2V5 SW201 SW201 AC/DC AC/DC 1 1 2 2 -2V5 -2V5 U201 U201 V- V- 2 2 V+ V+ 5 5 1 1 + + 3 3 - - 4 4 C206 C206 1u 1u IC201 IC201 TS12A4516DBVR TS12A4516DBVR COM COM 1 1 NO NO 2 2 V- V- 3 3 IN IN 4 4 V+ V+ 5 5 D201 D201 Check out the design files here Compared to the MK1 design, the gain stage is essentially identical....

When I got my Analog Discovery 3, I made the decision to also purchase the BNC adapter board. Trying it out with some OWON scope probes I got from Amazon, it unfortunately didn’t cut the mustard. This was the most square I could get by twiddling with the compensation adjustment. To prove that I haven’t goofed it somehow, this is at the opposite end of the compensation range: This waveform ain’t even square at this point....

The Analog Discovery 3 is a great debugging tool, but as I discovered here in a note on using it with 10x scope probes, the input capacitance was too high for the probes to be compensated correctly. Custom LNA design To combat this, I decided to design an amplifier that would have a low input capacitance so that we fall within the 5-35pF compensation which is typical of these 10x scope probes....

Background As I have only recently come to realize, inductor selection for boost converters is not a trivial matter at all. Every inductor I tried saturated way too early, causing progress for the SMPS Chronicles Boost Converter project to grind to a halt. After many frustrating afternoons, and several blown parts and melted breadboard holes, I had enough. Inspired by this video on the RF Man YouTube channel (really nice channel by the way), I decided to build a dedicated jig that would allow me to easily measure the saturation current of the any inductor that I was about to use....

The inspiration for this series of articles came from Microchip’s TB3103 and TB3104 , which outlined the implementation of current mode controlled buck and boost converters using the PIC16F753’s internal analog peripherals, which include an operational amplifier, a comparator, PWM, etc. As a hobbyist, I’ve always found stuff like transfer functions, bode plots (the more complex ones), and SPICE to be really intimidating. I am aware of what these are, and why they are used in circuit design, but I’ve never really sat down and worked with them before, much less explore their nitty gritty details....

With the addition of these 3 components, I had a fully functional boost converter! However, it is not without its (huge) caveats, and let’s just say that along the way, Murphy has paid me many, many visits. In Chapter 1 , we talked about the sawtooth generator, which connects directly to the an opamp responsible for generating a PWM gate drive signal. So, I thought it made sense to take a closer look at that next....

Being on a little analog synth nerd-out lately, I chose to kick things off by designing the sawtooth generator that will ultimately determine the switching frequency of the boost converter. The sawtooth wave generated is used to derive the PWM signal that drives the gate of the FET. By connecting it and a control voltage to a comparator, a PWM signal with a duty cycle directly proportional to the control voltage will be generated....