Since the transformer used currently in the converter block may be problematic in terms of availability and construction, I have decided to design a compatible converter using a choke.
The tube has to be powered with a voltage of ca. 400 V. Because the MC34063 has not been originally designed to work with such a high voltage, we have to use an external transistor working as a key. Such a use of the MC34063 is nothing new: a similar schematic diagram can be found in a STMicroelectronics datasheet.
In the diagram, the key function is represented by Q1. Because of low supply voltage (5 V), it was required to use a specific type of a transistor, namely the ZVN0545G (or the ZVN0545A) transistor as well as the BAT81 diode as the D1 diode. The transistor (T1) allows for rapid discharge of the Q1 gate. Just as it is in the converter using a transformer, the output voltage depends here on the R2...R4 voltage dividers.
The choke used in the prototype has been chosen on an experimental basis: it is a vertical choke having an inductance of 1.5 mH and a current strength of 300 mA. There appear to be no contraindications to the use of a choke with a lower current strength – at least ca. 100 mA. Using a choke with another inductance is possible but it is not recommended as this may cause system instability.
The power consumption of such a converter block is ca. 10 mA in idle mode and ca. 28 mA during the simulation of a short circuit at the tube. The values have been selected in a way so as to meet the electrical specifications of most USB ports supplying power to the sensor.
The converter prototype has been connected to the sensor and the device has been tested for a couple of weeks under various conditions and on different computers. During the testing period, the device was operating properly and stably.
However, since we do not have access to specialist measuring equipment, the module described requires further measurements and tests before it can be officially implemented in the project. We will keep you informed on the progress of the works on the module.
If we use an external source of supply with a higher voltage for powering the converter block, we can employ elements that are more easily available. At voltages between 9 and 12 V, the IRF830 (or similar) may be used as Q1 and the 1n4148 ‒ as D1. The value of the choke is not critical either and it is possible to use a choke having an inductance between 1 and 2.2 mH or even an inductance outside this range. In such a case, an amateur electrical engineer can find most of the parts needed in an old ATX power supplier. For the sake of an experiment, we have used chokes from old energy-saving bulbs.
Elaborated by Marcin 'Kotfryc' Sedzikowski
Many thanks to Svistak, Tobas and Troll81 for help with testing.