How to Choose a HV Power Supply Module for PMT？

1. Guess who I am?

Ask：I am the partner of the photomultiplier tube (PMT), and the PMT cannot work without me. Despite my small size, I can increase the input voltage by dozens, even hundreds of times.

Answer: High-voltage power supply module

High-voltage power supply module

（left：CC228 series and CC255 series；middle：CC238 series；right：CC356 series）

2. High-voltage power supply module

The main function of the high-voltage power supply module is to provide high voltage for PMT. At present, most of the high-voltage power supply modules used in PMT are DC-DC conversion. The working principle is to convert the input DC voltage into a high-frequency AC voltage, then boost the voltage through the transformer, and finally convert it into a DC high voltage output through rectification and filtering. In order to ensure the output high voltage stability, a feedback control circuit is necessary (Fig. 1).

Fig. 1 Schematic diagram of DC-DC high-voltage power supply module

3. Parameters of the high-voltage power supply module

Ⅰ Characteristic parameter：Such as the adjustment range, polarity, and output current of the high-voltage power supply module.

Ⅱ Quality parameter：These parameters reflect the quality of the high-voltage power supply module, such as power input voltage regulation、load regulation, ripple, temperature coefficient and stability.

4. How to choose the right high-voltage power supply module

Ⅰ Characteristic parameter selection

a) Polarity selection of high voltage：PMT can operate at either positive high voltage (Fig. 2) or negative high voltage (Fig. 3). When the negative high voltage is selected, the PMT can work in DC mode, AC mode or pulse mode, and when the positive high voltage is selected, the signal output needs to add capacitance, so the PMT can only work in AC mode or pulse mode.

Fig. 2 Working at positive high voltage

Fig. 3 Working at negative high voltage

b) Output high voltage selection：The maximum output high voltage of the high-voltage power supply module should not exceed the maximum working voltage of the PMT to prevent excessive voltage from causing damage to the PMT. In addition, the output high voltage needs to be continuously adjustable in order to set the operating point for the PMT.

c) Output current selection：The actual output current of the high-voltage power supply module is the voltage-divider current. Take Fig.2 as an example, the divider current(I_d) is approximately the high voltage(+HV) divided by the sum of the resistor values as given by the following equation (1):

In order to ensure stable and reliable operation, the current carrying capacity of the selected high-voltage power supply module is preferably 1.5 times of the current flowing through the voltage divider circuit（1.5×I_d）. For example, the maximum working voltage of CR131 is 1250V, and the total voltage divider resistance is 3.3MΩ, that is, the current flowing through the voltage divider circuit is 1250V/3.3MΩ=0.38mA, and then multiplied by 1.5 is 0.57mA, so the output current carrying capacity of the high-voltage power supply module is greater than 0.5mA.

Ⅱ Quality parameter selection

The current amplification of PMT is shown in equation (2):

Notes ：

μ is the current amplification;

A is determined by the number of dynode stages n and collection efficiency and the k;

V is the operating voltage;

K is determined by the structure and material of the dynode and has a value from 0.7 to 0.8;

n is the number of dynode stages.

It can be seen from equation (2) that the stability of the current amplification depends on the stability of the operating voltage. If the operating voltage is 1000V, the number of dynode stages is 10, k is 0.8, and A is 1. When the current amplification of PMT changes by 1%, the operating voltage V changes by 0.124% can be calculated by equation (2). Therefore, select a high-voltage power supply module whose electrical characteristics (input regulation, load regulation, temperature coefficient, ripple and noise, etc.) are at least 10 times better than the stability required for the PMT.

High-voltage power supply module quality parameter introduction:

a) Input voltage regulation: it is the percentage (%) of change in the output voltage during maximum voltage output relative to a given change in the voltage, usually defined by changing ±10% of the input voltage.

b) Load regulation: it is a measure of the ability of the output voltage to remain constant when the output current is changed by a change in the load. The load regulation is the difference between the output voltage at no load (zero output current) and the output voltage at full load (maximum output current), expressed as a percentage (%) of the output voltage at full load.

c) Stability: it is a measure of the change of the output high voltage over time. In the absence of external factors (such as input changes, load changes, environmental temperature changes, etc.), the high-voltage power supply module after a period of startup warm-up, in the specified time, the maximum change in the output high voltage.

d) Temperature coefficient: this indicates the output fluctuation rage (%/℃) under output conditions (mostly during maximum output) defined within the ambient operating temperature range.

e) Ripple: it is the variation (peak) in the output synchronized with the oscillator frequency of the high voltage pulse generator circuit. The characteristic value includes not only the variation (peak value) but also fluctuation components (noise) other than in the oscillator frequency. Some power supplies may show only the ripple component on the product tag or specs and not other fluctuation components, so care is needed when selecting a power supply for actual operation.

Recommended quality parameters of high-voltage power supply module are shown in table 1：

Table 1 Quality parameters of the high-voltage power supply module

High voltage power supplies of Bing Jing Hamamatsu photonics are shown in table 2：

Table 2 High voltage power supplies of Beijing Hamamatsu

Note：CC356 series is a high temperature and high-voltage power supply module, the working temperature is -40℃ ~ 175℃, mainly used in oil logging.