The use of active medical devices is extremely common, such as various types of monitoring equipment, magnetic resonance, electrocardiograph, infusion pumps and electric operating beds, all of which are inseparable from the "power supply." With the rapid development of electronic technology, switching power supplies are increasingly used in medical equipment . Not only greatly reduces the size of the product, reduces weight, and greatly reduces energy consumption and improves work efficiency. Qualified medical power supply should not only meet the requirements of GB9706.1-2007 and YY0505-2006 standards, but also meet GB/T17626.2-1998 (electrostatic protection capability, requirements up to 3 kV), GB/T17626.3-1998 (RF Radiation protection, required to reach 3 V/m), GB/T17626.4-1998 (voltage transient withstand capability, required to reach 1 kV), GB/T17626.5-1998 (net power surge withstand capability, required to reach 1 kV and 2 kV), GB/T17625.1-2003 (grid line harmonic requirements), GB/T17625.2-1994 (power line flicker requirements), and EN55011 series of electromagnetic compatibility standards.
At present, most medical device manufacturers use finished switching power supplies, and most supplier companies cannot provide medical "switching power supplies" that meet the requirements of the above series of standards. Therefore, it is necessary to briefly introduce the EMC design of medical "switching power supplies".
1 Correct selection of analog and logic active devices
The key to electromagnetic interference emissions and electromagnetic sensitivity is the choice of analog and logic active devices.
Active devices can be divided into tuning devices and basic band devices. The tuning device functions as a bandpass component, and its frequency characteristics include center frequency, bandwidth, selectivity, and out-of-band spurious response; the basic tie device functions as a low-pass component, and its frequency characteristics include cutoff frequency, passband characteristics, out-of-band rejection characteristics, and They are responsive, and they also have input impedance characteristics and balance imbalance characteristics at the input. Attention must be paid to the inherent sensitivity and electromagnetic emission characteristics of active devices. Active devices have two sources of electromagnetic emissions: conducted interference is transmitted through power lines, ground lines, and interconnects, and increases with increasing frequency; radiated interference is radiated through the device itself or through interconnects, and squared with frequency And increase. Transient ground current is the initial source of conducted and radiated interference. Reducing transient ground currents must reduce ground impedance and use decoupling capacitors.
The sensitivity characteristics of analog devices depend on sensitivity and bandwidth, while sensitivity is based on the inherent noise of the device. The sensitivity characteristics of logic devices depend on DC noise margin and noise immunity. The shorter the flip time of the logic device, the wider the spectrum occupied. For this reason, the rise/fall time of the signal should be increased as much as possible while ensuring the function.
2 PCB board design
Practice has proved that even if the schematic design of the circuit is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of the electronic device. For example, the two thin parallel lines of the printed board are too close together to form a delay of the signal waveform, forming reflection noise at the end of the transmission line; in addition, the interference caused by the consideration of the power supply and the ground line may cause the performance of the product. decline. Therefore, when designing the PCB design of the switching power supply, you should pay attention to the correct method.
Each switching power supply has four current loops: a power switch AC loop, an output rectifier AC loop, an input source current loop, and an output load current loop.
The AC circuit of the power switch AC loop and rectifier contains high amplitude trapezoidal currents. The harmonic components of these currents are very high. The frequency is much higher than the fundamental frequency of the switch. The peak amplitude can be up to 5 times the amplitude of the continuous input/output DC current. The transition time is usually It is about 50 ns. These two loops are the most susceptible to electromagnetic interference, so these AC loops must be routed before other traces in the power supply are routed. The three main component filter capacitors, power switches or rectifiers, inductors or transformers of each loop should be placed adjacent to each other, adjusting the component position so that the current path between them is as short as possible.
The input loop charges the input capacitor through an approximately DC current, and the input filter capacitor acts as a broadband energy storage—logic “groundâ€. The output filter capacitor is used to store the high frequency energy from the output rectifier while eliminating the DC energy of the output load loop. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current loops should be connected to the power supply only from the terminals of the filter capacitor. If it is not directly connected to the terminals of the capacitor, the AC energy will be radiated into the environment by the input or output filter capacitors.
The best design flow for establishing the layout of the switching power supply PCB is: placing the transformer, designing the switching power supply current loop, designing the output rectifier current loop, and connecting the control circuit to the AC power circuit.
3 layout of the printed line
The switching power supply contains high frequency signals, and any printed circuit on the PCB can function as an antenna. The length and width of the trace affects its impedance and inductance, which affects the frequency response. Even a trace through a DC signal can couple from adjacent traces to the RF signal and cause circuit problems (even re-radiation) Interference signal). Therefore, all printed lines that pass through the AC should be designed to be as short and wide as possible, placing all components connected to the trace and connected to other power lines at close proximity. When wiring, the direction of the power line and the ground line should be consistent with the direction of the current, which helps to enhance the anti-noise ability.
4 ground wire design
4.1 Selecting a single point grounding
Considering that the currents flowing back to the ground of the circuit are different, the change of the ground potential introduces interference, and the circulating current formed by the grounding circuit has a great influence on the interference. Therefore, a single-point grounding method is needed to stabilize the power supply and reduce self-excitation. Single point grounding means that several components ground of the power switch current loop are connected to the grounding pin, and the ground of several devices of the output rectifier current loop is also connected to the grounding pin of the corresponding filter capacitor. In the design, the grounding point of the same level circuit should be as close as possible, and the power supply filter capacitor of the current stage circuit should also be connected to the grounding point of the stage.
4.2 Thickened ground wire
The grounding potential changes with the change of current. If the grounding wire is too thin, the timing signal level of the electronic device will be unstable, and the anti-noise performance will be deteriorated. Therefore, every large current ground should use short and wide traces as much as possible. The thickness of the ground, power, and signal lines is: Ground > Power Line > Signal Line. If possible, the width of the ground wire should be greater than 3 mm.
Sometimes, a large area of ​​copper may be used or the unused portion of the printed board may be connected to the ground as a ground. The input and output circuits of the switching power supply "DC-DC conversion" should have a common reference ground. The two ground wires can be respectively coppered and then connected together to form a common ground.
Instrument
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Lateral resolution: 3.8 um
Axial resolution: 4I um
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