Ampisu

Output ripple is an important performance metric for any lab power supply, especially when powering sensitive analog circuits. I recently measured the ripple of Ampisu under full load conditions.

The measurement was done using a Rigol MSO5074 oscilloscope with the following setup:

• Output: 7.5 V, 500 mA

• Load: 15 Ω resistive load

• Scope settings:

  • AC coupling

  • 1× probe

  • 20 MHz bandwidth limit

Under these conditions, the measured ripple was:

• 8.44 mV RMS

• 39.5 mV peak-to-peak

This corresponds to less than 1% ripple, which is a good result for a power supply of this size.

The relatively low ripple is mainly due to the linear post-regulator, which filters the output of the switching converter.

In principle the ripple could be reduced further by using larger output capacitors or adding an LC filter, but that would require more space and is difficult to accommodate in a design this compact.

You can find out more about the project and sign up for updates on Ampisu's webpage: https://ampisu.nessie-circuits.de/

Today I assembled what I expect will be the final Ampisu prototype! Here’s what changed compared to the previous revision:

• Added mounting holes to secure the PCB to the aluminum enclosure

• Implemented a proper grounding scheme with a high-resistance path (10 MΩ ∥ 1 nF) from output ground to chassis ground

• Reworked the PCB around the output pin headers for better visibility and access

For low volumes, I prefer assembling prototypes myself rather than outsourcing — it’s usually faster and more cost-effective. That said, this board isn’t trivial: 201 SMD components (72 unique parts) adds up. Using my 3D-printed stencil printer, a Fritsch manual pick-and-place, and a Vapor Phase oven, it still took about three hours to complete a single board.

How do you handle prototype builds? Do you lean toward external assembly services, or do you prefer hands-on assembly?

Here’s the block diagram showing Ampisu’s architecture.

Let’s start with the input. Ampisu is designed to work with any standard USB port on a computer. The USB-C connector supports input currents of up to 3 A. The available current is advertised by the USB host via a resistor network, which Ampisu reads accordingly. USB is also used for communication: the device enumerates as a composite USB device, providing a serial interface for log output and a WebUSB-compatible interface that accepts SCPI commands.

An RP2040 microcontroller handles all host communication, controls the voltage and current limits of the two configurable output channels, and continuously measures output voltage and current. It also drives the 3-channel RGB LED controller for status indication (and a bit of light show).

A central design aspect of Ampisu is galvanic isolation. Isolation protects the host from accidental miswiring and enables flexible output configurations, such as series connection for higher or negative voltages. The system is therefore split into four galvanically isolated domains: the USB host domain, which contains the microcontroller, and three separate output channel domains. These domains are connected only through the flyback converter’s transformer and isolated I²C links.

Each of the two configurable channels includes a dual-channel DAC that generates the reference voltages for the constant-current source and the linear voltage regulator. This allows precise, fine-grained control of both output voltage and current limits. A dual-channel ADC measures the actual output voltage and current.

The auxiliary channel provides a fixed 3.3 V output with a maximum power of 500 mW.

I’ll release the full schematics after the crowdfunding campaign ends.
Until then, which aspects of Ampisu would you like to learn more about? Let me know in the comments.