Application of DCDC power module in medical sensor

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Application of DC/DC power module in medical sensor circuit I. Introduction at present, with the continuous development of modern medical devices, especially the electronic instruments directly connected to the human body, in addition to the higher and higher requirements for the performance of the instruments themselves, the consideration of human safety has also attracted more and more attention, such as life monitor, mother and baby monitor, baby insulation instrument and other instruments in close contact with the human body, In other words, when using the instrument, the patient cannot cause electric shock or other risks to the human body due to the instrument used

II. Application of ECG detection circuit

below, take the ordinary ECG detection circuit as an example, as shown in Figure 1:

Figure 1: ordinary ECG detection circuit

Figure 1 is the practical circuit of ina115 in ECG detection circuit. The positive and negative inputs of the amplifier are respectively connected to the left arm (LA) and right arm (RA) of the human body through the ECG sensor, and the driving network composed of operational amplifier N and resistors R1-R4 is connected to the right leg (RL) of the human body to form a "floating ground". Due to the weak biological signal and polarization potential, the amplification factor of this stage should not be too high. It is generally taken between and can only be used as pre amplification. In order to detect safety, the later stage should be isolated by isolation amplification circuit, and the power supply voltage of this stage should be provided by isolation power module at best, so as to achieve complete isolation of power supply and signal. Figure 2: isolated ECG detection preamplifier circuit

the above circuit is an isolated ECG detection preamplifier circuit composed of t6235d and ina115 of Hanwei company, which has high input impedance, low leakage current The detection accuracy is high and meets the indicators of human safety, which can fully meet the requirements and standards for the use of medical devices. The circuit is generally used as a preamplifier circuit. The device is in direct contact with the human body, and the ECG signal is amplified through ina115. Finally, it is isolated and transmitted to the post amplifier circuit by iso122 to realize the isolation and amplification of human body signal, output and power supply

the working power supply of the amplifier shall be selected with sufficient safe creepage distance, high isolation voltage characteristics and very low isolation capacitance value, so as to better ensure the safety performance of the system. Here, it is recommended to use 6000vdc isolated G Series (constant voltage input, 6000vdc positive and negative voltage output) or H series products (constant voltage input, 6000vdc single voltage output) of jinshengyang company, Due to its extremely high isolation voltage characteristics (6000vdc) and extremely low isolation capacitance (≤ 10PF), this series of products are widely used in the silver industry of medical electronic equipment due to its high thermal conductivity, electrical conductivity and corrosion resistance. Using g1515s-1w, the ground wire in contact with the human body of ina115 is truly disconnected from the input power line, so as to achieve the effect of complete isolation, that is, the complete isolation of power supply and signal

III. selection of isolated DC/DC power supply module:

Description: the above scheme is made under the condition that the working voltage of the isolation amplifier is ± 15V. If the working voltage is ± 5V, ± 12 or other voltages, just select the product with the corresponding output voltage

IV. the following problems should be paid attention to in the process of using DC/DC power module:

1. The external capacitors C2 and C3 of DC/DC power module adopt tantalum capacitors with low internal resistance, and the capacitance value is recommended to be 0.47uf. Excessive capacitance value should be avoided to prevent the module from being damaged by overcurrent at the moment of startup due to excessive capacitance

2. In order to ensure the reliability of the module for long-term operation, the module should work above 10% of the rated power to prevent the unstable output voltage of the module due to no-load or light load from affecting the circuit operation. If the load power consumption is really small, the dummy load (resistance) of 10% of the rated power can be added between + VO and 0V and - VO and 0V at the output end in the way of external dummy load to improve the reliability of the product

3. Since the DC/DC power module is realized by switching oscillation circuit, it will also produce the interference of common mode and differential mode noise. For occasions with high requirements for ripple noise, the output end of DC/DC power module can be connected with passive LC filter network or RC (with large loss) to further reduce the ripple noise of power supply. However, it should be noted that the resonant frequency of L itself is much higher than the switching frequency of the module, The allowable current value should be more than twice the input current of the module, and the internal resistance should be small to reduce DC loss. For details, please refer to the following figure:

v. conclusion

the above mainly introduces the characteristics, application and precautions of DC/DC module power supply in the medical electronics industry, especially in the sensor detection circuit. At present, there are many DC/DC module power supplies in the market, but because medical electronics belongs to a special industry, the requirements for its power supply are very high. In order to ensure the safety performance of human body and equipment, The selected DC/DC module power supply is particularly important. (end)

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