February 25, 2025 - Lessons from the Lab: PVX Bias
This script is authored by BNC electrical engineer Cameron Simmons. It is an excerpt from a video that is linked [here]
In today's video we will be modifying a PVX-4151 high voltage pulse generator and so that its output can be biased up 1,000 volts.
The PVX-4151 is a bipolar pulse generator, so it has two high voltage inputs for positive and negative polarity. The power supply voltages cannot exceed 1,500 volts differential, but they can be asymmetrical about zero. For example, -500 V to 1000 V, -750 V to 750 V, 0 V to 1,500 V, etc.
What if we wanted to stay within the 1,500 V differential limit, but bias it up so it pulses from 1,000 V to 2,500 V? This is actually possible with a small modification.
There are many possible applications for a DC-biased, high-voltage output signal. One example might be parallel plates in a time of flight mass spectrometer. The experiment might require that both the plates would need to be held at a high voltage above ground and one will pulse slightly higher to achieve a certain velocity. If you know of other applications for an output like this, we would love to hear about it in the comments.
In order to do this, we will need two positive polarity high voltage power supplies. The lower 5 kV supply is going to act as the bias voltage. The issue here is that most high voltage power supplies cannot regulate voltages of the same polarity from separate sources. The higher voltage supply will try to regulate the lower voltage and pull it up to the same potential. In order for the bias supply to maintain its voltage, it must have a separate path to ground through a shunt resistor.
The most crucial part of this process is selecting the correct resistance for the shunt and ensuring it can handle the power dissipated in it. The resistance will be determined by the following equation:
R = Vbias / Ips
Power supply current draw can be extrapolated from the equation for power draw in a capacitive load:
P = C*V2*F
For our setup we will be pulsing between 1,000 V and 2,450 V and our load is a 50 pF capacitor connected with 2 feet of RG58 cable. The PVX-4151 has an internal capacitance of 200 pF that also must be factored in.
For my specific case, I will be using a 62 Mohm resistor from the negative supply to ground. I soldered together several 10 Meg, 1 watt resistors and added spade connectors to either side. I also have it wrapped in kapton tape just in case to prevent arcing. This should not be an issue but it's better to play it safe since we are technically exceeding the absolute voltage limit. The placement of the shunt is such that it can be easily removed and reinstalled as needed.
With our shunt resistor in place, we can now start slowly turning up the voltage on our Heinzinger EVO. The bias supply will start showing an output voltage regardless of the fact that we have not turned it up, this means our shunt is working.
The final output is a 1,500 V pulse from 1 kV to 2.5 kV.
If you would like to do this with your PVX unit, please reach out to us or visit the technical resources section on the PVX-4151 product page.