Pac modular switching power (hereinafter referred to as pac module power) is a new electronic component that has developed rapidly in recent years and is now widely used in programme switches and microwave communications equipment. Various technical requirements for secondary power were met for digital and analog circuits in communications equipment. Since most pac modular power plants are designed to treat it as a single-use component, once a problem has arisen, the overall end of life does not take into account the possibility of repairing it. In circuit assembly, many manufacturers debug the components after they are mounted on the board, debug and then fit into a copper box with the dual effect of dispersing heat and shielding, and then water the entire circuit with a conductor-heat silicon rubber. Therefore, repairing the pac module power supply would be difficult if damaged. It is proposed to undertake some preliminary exploration of the rationale and maintenance of single-end pac modules。
I. Working principles of the pac module power supply
After an autopsy of the actual circuits of dozens of pac modules, it was found that pac modules had approximately two basic working methods: one was a pulse width (pwm) drive switch power, characterized by a fixed switch pulse frequency, which regulated the space ratio by changing the pulse width; the other was a pulse frequency (pfm) drive switch power, which was characterized by a fixed switch pulse width, which was adjusted by changing the switch pulse frequency. While both work in slightly different ways, their effects and effects are the same and can serve the purposes of stability. With the exception of a few products, almost all pac module power is controlled by pwm。
There are two uses for ring hf switch transformers or efd switches:
(1) use of primary and secondary different numbers to provide sub-group circuits with different voltage
(2) complete isolation of primary and secondary direct circulation. The switch q basically uses a high-frequency field effect tube, which drives the ic of the chip, which, in addition to the wide-widthing of the power field effect fence, can be controlled by weight size and input pressure high and low output, also functions such as overflow detection, overpressure testing and soft start。
The auxiliary functions of ic vary according to the type. 48v input voltage receives high-frequency rectangular voltage via primary field effect tube t q beam wave and high-frequency switch transformer sub-voltage, which is processed through the shortki diode d, and then through the c2 filter to output the direct current voltage required. Ic, also known as the wide-wave generator, is the core of the modular power source, which compares the feedback signals sent by the radial with the baseline signals within the chip, so that the pwm pulses, which can be moved in the width of the output but with a fixed frequency, reach the field effect fence to regulate and stabilize the output voltage. For example, when the output voltage is raised for one reason or another, the ic reduces the empty ratio of the driving pulse, which reduces the average voltage after cutting, leading to the output voltage, and vice versa. C1 is the time capacitor used to control the ic7 foot sawing waveform。
Ii. Pac module power repair exploration
The element-level repair of the pac module power supply was a delicate and careful exercise, which could not be half-wielding or negligent。
First, there should be an overall rational understanding of the power supply of the module, the ic model for the circuit application being overhauled, the form of its composition and the broad box chart. Local circuits that focus on suspicion should be mapped out on the basis of the actual plate circuits, especially for the two-sided printing panel circuits, which must be carefully mapped, otherwise the next analysis of the failure would be problematic. Conditional users may also temporarily remove larger parts of the entire pac module's powerboard and enter the printer scanner into the computer for circuit analysis using photoshop graphic editing software, with a multiplier effect。
In repair practice, the following issues should be noted:
(1) the bulk of the pac module power supply is sequestered with conductive silicon rubber, which is inevitably stripped when repaired. Given the unique use of glue in the module for stationary components, heat conductor, component oxidation and leaking, we do not have to strip the entire body covered by the entire circuit, as long as the suspected local circuit covered by the tape is removed, in order to keep the restored module as technical indicators as far as possible. Thermal silicon rubber is transparent and non-transparent. When detached from non-transparent gels, there is a high risk of damage to the surface of the fragments because of the lack of sight of the elements covered by the gel. Any minor scratches of the surface would result in damage to a sheet of electrical capacity and resistance, so the operation would be very careful. When repairing several pac modules, the writer found that the modules were truly one-off components because the former repairer accidentally cut multiple parts of the plates when stripping off the pac module's power-guided thermal silicon rubber。
(2) there are many types of core ic models for pac module power, and the most common ics for single-end drivers are: uc3842, uc3845, tea2018, ppc1094, and ic sealing forms are commonly dip and lcc types. In the single-end pac module power source, the two-end drive chip composition is also configured as a single-end drive. Most ic foot functions and limit parameters for drive chips are available in the relevant technical information and are not repeated here. In case of repairs, when encountering ics that are unfamiliar or without wordmarks (often encountered during the maintenance of pac module power for imported equipment), and in case of impatience, a calm analysis should be carried out and accompanied by advanced instrument tests to identify the power foot, feedback foot, pwm output foot, time foot (frequency-setting foot), base voltage foot, protective function foot and state-conversion foot functions, and to carefully verify and analyse the performance of the relevant ic components, the repair work is generally effective. Maintenance practice showed that pac module power had malfunctions and that ic had a very low rate of damage, mostly from peripheral or power components. In view of this, it is not quite certain that the ic chip should be easily removed in order not to be artificially extended. (3) when the same type of ic is used for pac module power produced by different manufacturers, the circuit configuration is mostly different. The same failure of the pac module power source may vary significantly. For example, in the modular power source assembled with the uc 3845 chip, the feedback signal entered for the ic2 foot can be taken from ic2 power supply of one foot, or the secondary output of the high frequency switch transformer can be delivered to ic2 feet via optical vd. Therefore, when the output voltage unstable failure occurs, the approach to determining the source of the failure is different. Similarly, an ic overflow test of three feet can achieve current protection for the single function of the modular power source by blocking access to a switch field effect tube sample resistance, and a sample magnification circuit can be set up using the "non" door amplifier chip to connect the ic3 foot to the "or" door output end of the sample amplifier to achieve overflow, overpressure, ultra-heating alarm protection. Therefore, prior to the implementation of the entire circuit detection and failure analysis, attention should be paid to the distribution of the components, the direction of the ic key leading signals and the actual configuration of the circuit。
(4) the problem of power plant dissipation in the power module cannot be ignored. For example, the power field effect tubes, the physical position of the to-220-sealed cylindrical diodes in the modular power circuits is dispersed by close contact with thermally integrated silicon resin and the outer casings with both shielding and heat dispersion functions。
When the above-mentioned components are replaced, they are to be fitted strictly at their original location, and during the entire pac module power plate repair process, it is necessary to check repeatedly whether or not there are obstacles to the heat transfer route during the operation of the device, so as to be as low as possible. A minor oversight of this step would have led to the burial of the after-effects, resulting in repeated damage to replacement components during the use of pac module power after repair。
