In recent years, the Silicon Photomultipliers (SiPMs) have
emerged as a promising photodetector in various applications such as
high energy physics, nuclear physics and medical instruments. One
such application is in the pixelated camera of Imaging Atmospheric
Cherenkov Telescopes (IACTs). IACTs are used to detect very
high-energy celestial gamma rays using Atmospheric Cherenkov
Technique. The SiPM gain is proportional to the overvoltage which is
the difference between applied bias voltage and breakdown
voltage. As the SiPM breakdown voltage increases with temperature,
the overvoltage and hence the gain decreases proportionally
(2-3%/°C) at a constant applied bias voltage. Also, the actual bias
voltage across the SiPM changes with load current due to voltage
drop across a series resistor in SiPM bias circuit, thus causing a
change in overvoltage and gain. To maintain a constant gain of the
SiPM, the applied bias voltage need to be adjusted to compensate for
the changes in temperature and load. We are developing a 256-pixel
imaging camera with SiPM as its photo-sensor. The camera will be
placed at the focal point of telescope and will be operated during
night in outdoor environment at Hanle, Ladakh, India. The night
temperature at Hanle typically varies by ∼ 10°C overnight
and ∼ 40°C (-20°C to +20°C) over the
year. Thus, gain of SiPMs, exposed to the environment, may vary
considerably during observations. A prototype 8-channel bias supply
board with real time temperature & load compensation is developed
to operate the camera SiPMs at fixed gain throughout the observation
night.
The voltage range of the bias supply for each channel is from 10 V
to 80 V with 5 mV resolution and current upto 4 mA. The output
voltage and current can be monitored with a resolution of
∼ 5 mV and ∼ 0.3 μA respectively. The single board
computer, Raspberry Pi, is connected to the bias supply board over a
7-wire Customized Serial Peripheral Interface (CSPI) for control and
monitoring. A multi-channel SiPM bias supply system is realized by
daisy-chaining 8-channel boards through CSPI to cater bias voltage
to all the pixels of the camera. The system is operated remotely
using Ethernet interface of Raspberry-Pi. This paper discusses the
design, development and performance of the prototype 16-channel
closed-loop, programmable bias supply system with built-in
compensation for changes in temperature and load.