Monday, February 14, 2011

Weak current College】 【switching power supply design knowledge describes】.

<br> <br> 1 introduction of electronic products, especially military power supply design is a systematic project, not only to consider the power supply design parameters, but also the electromagnetic compatibility of electrical design, design, thermal design, security, design, .and three prevention design. Because any aspect of even the most minor negligence, this can cause the collapse of the entire power, so we should be fully aware of the power of the importance of product reliability design. <br> 2 switching power supply design .electrical reliability power supply mode select 2.1 centralized power supply system all output deviation as well as the transmission distance of different pressure difference resulting from reduced supply quality and application of a single power supply, power supply failure may cause system crashes. Distributed power supply system for .power supply unit closer to the load and improve the dynamic response characteristics, power quality, transmission loss is small, efficient, energy saving, high reliability, easy form N + 1 redundant power supply system, extended power is also relatively easy. So the use .of distributed power supply system can meet the requirements of high reliability of the device. 2.2 circuit topology selection switch power generally use the converter, Flyback, double transistor Flyback, dual converter, double forward-style, push-pull, half bridge, full bridge .eight topology. Converter, Flyback, double <br> Converter, push-pull switching pipe pressure at two times the input voltage above, if the 60% Derating, you make the switch easy selection of pipe. In push-pull and full-bridge .topology might appear one-way biasing saturated, so switching valve damage, and half-bridge circuit since have auto-resistance unbalance capacity, so this problem does not occur. Double transistor Flyback and half-bridge circuit switching valve's confined only to the power of .maximum input voltage, even by 60 per cent for use, select switch tube also easier. In high-reliability engineering general use both types of circuit topologies. 2.3 control strategy choice in small and medium-sized power supply, current-mode PWM control is .a large number of methods used, it is a voltage control type has the following advantages:-by-cycle current limit, voltage-mode control faster, from overcurrent, switching valve damage, significantly reduce overload and short circuit protection; excellent voltage regulation; .fast-transient-response; loop stability, easy compensation; ripple voltage control type. Production practice indicates that the current control-50W switching power supply's output ripple around the 25mV, over voltage control type. Hard switching technology due to the switching loss limit, .the switching frequency is generally 350kHz following, soft-switching technology is applied, the resonant principle at zero voltage switching components or zero current state on / off, switching loss is zero, thus increasing the switching frequency to megahertz level, the application of soft- .switch technology converter integrated PWM converter and resonant converter both advantages, close to the ideal characteristics such as low switching losses, constant frequency control, appropriate storage component size, wide control range and load range, but this technology mainly used for high-power power, .small and medium-sized power supply is still dominated by PWM technology. 2.4 components selection because components directly determine the reliability of the power supply, the components of the selection is very important. Components of the failure is mainly concentrated in the following four aspects: .(1) the quality of manufacturing quality problems caused by failure of work stress. Quality is not qualified to be through strict examination, when the application should use fixed-point production factory and mature product, does not allow the use of certified products. ( .2) components reliability issues components reliability issues that the basic problem of failure rate, this is a random nature of the failure, and the quality difference is that the components of the failure rate depends on work stress levels. To a certain level of stress, .the components of the failure rate is significantly reduced. Culling does not meet the requirements of the use of components, including electrical parameters not qualified, the sealing property is not qualified, the appearance is not eligible, poor stability, early failure, etc., .should be screening test, this is a non-destructive testing. By filtering allows components to reduce failure rate 1 ~ 2 orders of magnitude, of course, screening cost (time and cost) are large, but comprehensive maintenance, logistics support, the entire .frame with the discussion, or cost-effective development cycle, will not be extended. Power device main components of the screening test General requirements: ① resistance at room temperature according to the technical conditions for 100% test, excluding the non-conforming product. .② normal capacitor at room temperature according to the technical conditions for 100% test, excluding the non-conforming product. ③ connectors according to technical conditions of the various parameters of the sampling inspection. ④ semiconductor device filter in the following programs: eye checked → .epicenters → high temperature storage → high-temperature impact → power aging → high-temperature test → low temperature test → at room temperature after the end of the test screening should be calculated excluding the rate QQ = (n / N) × 100% type .: N - the total number of samples; n - the number of samples taken out; if Q over standard prescribed limit value, the number of components not hands-on, and all relevant provisions. In line with standard requirements, you will filter the .qualified components make paint dot callout, and then into the private storeroom for machine use. (3) design issues first is properly selected appropriate components: ① selection Silicon semiconductor devices as possible, less or no germanium semiconductor devices. ② the use of integrated circuit .and reduce the number of discrete devices. ③ switching valve selection MOSFET simplifies driver circuit, reduce losses. ④ output rectifiers as far as possible with soft recovery diode characteristics. ⑤ should choose metal packaging, ceramic packaging, glass encapsulation of devices. Prohibition of the .use of plastic packaging. ⑥ integrated circuit must be a class of goods or is compatible with standard B-1 MIL-S-19500 MIL-M-38510, more quality levels of the military. ⑦ design-time use relay, warranted should use .sealed relay contact good. ⑧ principle, selection of potentiometer, must be kept on the casting processing should be carried out. ⑨ absorption capacitors and transistors and output rectifiers distance should be very close, high frequency for flowedCurrent, it is easy to heat up, .so ask these capacitors with high frequency and low loss and high temperature characteristics. In humidity and salt spray, Aluminum Electrolytic Capacitor shell corrosion occurs, the capacity of drift, the leakage current increases, so the ship and humid environment, it is best not to .use Aluminum Electrolytic Capacitor. Due to the space particle bombardment, electrolyte will explode, Aluminum Electrolytic Capacitor also does not apply to aerospace electronics devices. Tantalum Electrolytic Capacitor temperature and frequency characteristics of a good, high and low temperature resistance, long storage time, performance .stable reliable, but tantalum Electrolytic Capacitor heavier, volume ratio low, not high pressure and high pressure varieties (> 125V) less, which is very expensive. About Derating design: electronic components of basic failure rate depends on work stress (including electrical, .temperature, vibration, shock, frequency, speed, crash, etc.). In addition to individual low stress failure of components, the other is expressed as the higher the stress of work, the higher failure rate. In order to reduce the failure rate .of components, so the circuit design is Derating design. Derating, in addition to reliability, regardless of size, weight, cost and other factors. Different devices have different Derating standard <br> The practice shows that most of the electronic components of basic failure .rate depends on electrical stress and temperature, thus reducing the amount also largely control both stress that the following is a common component of switch power Derating factor: ① resistance power Derating factor in 0.1 ~ 0.5. ② diode power Derating factor in 0.4 following, reverse .voltage in 0.5 ③ LEDs below..

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