Frequency capabilities - Project Peta-pico-Voltron

One important parameter for the switching (square waveform) and waveform (user-defined waveform) outputs modes is the range of frequency that is achievable.

There are two parameters that influence the frequency range: the capabilities of the firmware, and the load connected to the power supply.

Firmware limitations

In Switching mode, the minimum frequency is 0.001 Hz when using the set_f() command from the library of functions, but the GUI limits the settable value to 0.01 Hz. The maximum frequency is 1000 Hz when using the set_f() command from the library of functions, but limited to 100 Hz in the GUI.

In waveform mode, the minimum frequency is 0.016 Hz (rounded to 0.02 Hz in the GUI), and the maximum is 100 Hz.

Load limitation

In practice, the upper frequency limit is determined by the load connected to the output. When a capacitive load (for example an electrostatic actuator, such as a dielectric elastomer actuator), the speed at which it will charge depends on the current delivered by the power supply and the capacitance of the load:

$\frac{dV}{dt}=\frac{i}{C}$

In switching mode, we ideally want a square waveform, but because of the equation above, the voltage can not transition immediately from the low level to the high level. In other word there is a limited slew rate, that makes the square wave look like a trapezoid wave. We can define the maximum achievable frequency, as that that leads to a triangular waveform. For any frequency higher than this value, we wouldn’t reach the full output amplitude. This maximum frequency is defined as:

$f_{max}=\frac{i_{max}}{2\,C\,V_{pp}}$

where imax is the maximum current output of the power supply, and Vpp is the peak-to-peak amplitude of the square waveform. This equation is shown on the following figure for different values of load capacitance and signal amplitude, and for 2 different output current, corresponding roughly to the two output current settings of the hvps-x.

Maximum frequency for a square wave (at the point where it becomes a triangular wave) as a function of load capacitance and peak-to-peak voltage. The frequency scale is given for two output current settings: 100 uA and 200 uA

As seen on this graph, for most of the values shown here, the limiting factor is the charging the load and not the firmware frequency setting.

For other waveforms (waveform mode can produce arbitrary output voltage functions) we need to consider the highest slope dV/dt of the waveform. Below is an example for a sinewave, which is very similar to the triangular case above:

Maximum frequency for a sine wave as a function of capacitive load, peak-to-peak voltage, and power supply output current

We recommend using the hvps-x in normal current mode. However, if you cannot reach the actuation frequency that you need for a specific load and amplitude combination, you have the opportunity to increase the output current to ~200uA by setting the current selection jumper on the PCB to position 5V.