Realization of Power Supply for Cased Hole Logging Tools

0 Introduction
Casing well logging instruments generally use single-core cables for transmission. The amount of data to be transmitted is not large, and this transmission method is simple, highly operable, and more importantly, the logging cost is low. The cable also needs to supply DC power to the well while transmitting data, which requires the 220V AC power on the ground to be converted into DC power first, transmitted to the well through the cable, and then converted into the various voltages required by the system through the DC-DC power supply. A large number of test results show that this method works stably and can reliably transmit data at a transmission distance of 6000m.

1 Power system structure
The power system structure of the cased well logging instrument is shown in Figure 1. The system consists of four parts: a ground transformer, a DC voltage regulator circuit, a DC-DC transformer, and a series voltage conversion circuit. The function of the ground transformer is to convert 220V AC into 90V AC, and then convert the 90V AC into DC through the DC voltage regulator circuit, and then obtain the series voltage through the DC-DC transformer.

2 Design of main functional modules
1) DC voltage regulator circuit
The DC voltage regulator circuit in this power supply system is shown in Figure 2. In the figure, the 90V AC voltage is rectified by the KC403 unidirectional bridge and then filtered by the second-order II type network to output a 90V DC voltage. It is worth mentioning that since the downhole data is uploaded through a 90V DC cable, in order to prevent the subsequent circuit from interfering with the front-end circuit, a 0.1μF capacitor is connected in parallel to each diode of the 90V rectifier bridge. These capacitors can absorb the switching noise of the diode very well. In the power filter circuit, the withstand voltage of the capacitor is generally twice the input voltage, and the capacitor is relatively large, so that the AC component is easier to filter out and the output voltage is smoother.

2) DC-DC power supply
Figure 3 shows the implementation circuit of the DC-DC power supply. In the figure, VD1, L1, C1 and C2 act as isolation circuits to block the transmission signal. The main chip of the power supply, UCC3842, is an integrated chip with single power supply, current forward compensation and single-channel modulation output. Its pin ① is the output of the error amplifier, and the external resistor and capacitor are used to improve the gain and frequency characteristics of the error amplifier; the pin ② is the feedback voltage input terminal, and the voltage of this pin is compared with the 2.5V reference voltage of the error amplifier in-phase terminal to generate an error voltage, thereby controlling the pulse width; the pin ③ is the current detection input terminal, and when the detection voltage exceeds 1V, the pulse width is reduced to make the power supply in an intermittent working state; the pin ④ is the timing terminal, and the operating frequency of the internal oscillator is determined by the external resistor and capacitor time constant, f=1.8/(RT×CT); the pin ⑤ is the common ground terminal; the pin ⑥ is the push-pull output terminal, which is a totem pole type inside, with a rise and fall time of only 50ns and a driving capacity of ±lA; the pin ⑦ is the DC power supply terminal, which has under- and over-voltage lockout functions, and the chip power consumption is 15mW; the pin ⑧ is the 5V reference voltage output terminal, with a load capacity of 50mA. In the figure, R7 and C5 form an oscillation network, and the 12V output voltage returns to the feedback voltage input terminal of U1 through R2 and R3. Resistor R11 detects the current in the coil, and after voltage division by R6 and R9, it is connected to the current detection input of U1. Pin 6 of UCC3842 is PWM output, which drives the switch tube VT1 to be turned on and off through the tubes VT2 and VT3. The rectification of the back-end diode and the capacitor filtering generate the ±15V and 5V required by the system.

3) Generation of series voltages
The circuit in Figure 4 generates the 3V, 1.5V, 1.25V and 1.75V voltages required by the system. In Figure 4(a), the +15V voltage is stabilized to 6.2V by the voltage regulator VD1, and then divided by R5 and R6 and passed through the op amp to generate a 3V voltage, and finally divided by R4 and R7 and passed through the follower to generate a 1.5V voltage.

Figure 4(b) divides the 3V voltage by resistors R12, R13 and R14 to generate 1.75V and 1.25V voltages, which are then passed through a follower to obtain 1.75V and 1.25V respectively and can be used.

3 Conclusion
This power supply system is used in many cased well logging instruments in service. Its excellent performance ensures that downhole instruments can work stably in high-temperature well conditions, and its low logging cost makes it more and more widely used in the field of oil logging.