Discussion on the Working Principle and Repair of PAC Module Power Supply

PAC modular switch power supply (PAC modular power supply for short) is a new type of electronic component that has developed rapidly in recent years and is currently widely used in program-controlled switches and microwave communication equipment. It meets various technical requirements for secondary power supplies for various digital circuits and analog circuits in communication equipment. Since most PAC modular power supply manufacturers regard it as a one-time use component during design and production, once a problem occurs, it will be scrapped as a whole, and the possibility of repairing it is not considered at all. In circuit assembly, many manufacturers install components on the printed board and then debug them. After the debugging is qualified, they are placed in a copper box with dual functions of heat dissipation and shielding, and then the entire circuit is cast into a whole with thermal conductive silicone rubber. Therefore, if the PAC modular power supply is damaged, it is very difficult to repair it. This article intends to make some preliminary discussions on the principle and maintenance of single-ended drive PAC modular power supply.

1. Working principle of PAC module power supply

After analyzing the actual circuits of dozens of PAC module power supplies, the author found that PAC module power supplies have roughly two basic working modes: one is pulse width modulation (PWM) driven switching power supply, which is characterized by a fixed switching pulse frequency and adjusting the duty cycle by changing the pulse width; the other is pulse frequency modulation (PFM) driven switching power supply, which is characterized by a fixed switching pulse width and adjusting the duty cycle by changing the switching pulse frequency. Although the working principles of the two are slightly different, the functions and effects are the same, and both can achieve the purpose of voltage stabilization. Except for very few products, PAC module power supplies almost all use PWM control .

There are two uses for toroidal high-frequency switching transformers or EFD-shaped switching transformers:

(1) By using different turns ratios between the primary and secondary, different voltages can be obtained in the secondary circuit;

(2) The primary and secondary DC paths are completely isolated. The switch tube Q basically uses a high-frequency power field effect tube, and the driver chip IC can output the modulation wave of the power field effect tube gate according to the load size and input voltage level, and also has functions such as over-current detection, over-voltage detection, and soft start.

The auxiliary functions of IC vary depending on its model. The 48V input voltage is chopped by the field effect tube Q through the primary of T and stepped down by the secondary of the high-frequency switching transformer to obtain a high-frequency rectangular voltage. After being rectified by the Schottky diode D, it is filtered by C2 to output the required DC voltage. IC is also called a width modulation wave generator. It is the core of the module power supply. After comparing and analyzing the feedback signal sent by the optocoupler with the reference signal inside the chip, it outputs a PWM pulse with adjustable width but fixed frequency to the gate of the field effect tube in order to adjust and stabilize the output voltage. For example, when the output voltage increases for some reason, the IC reduces the duty cycle of the drive pulse, causing the average voltage after chopping to decrease, resulting in a decrease in the output voltage, and vice versa. C1 is a timing capacitor , which is used to control the sawtooth waveform of IC7 pin.

2. Discussion on Repair of PAC Module Power Supply

Component-level repair of PAC module power supplies is a meticulous and careful job that cannot be done with any carelessness or slackness.

First of all, we should have a general rational understanding of the module power supply to be repaired, and understand the IC model, composition form and general block diagram of the circuit to be repaired. For the suspected local circuit, its core circuit should be drawn according to the actual printed circuit board circuit, especially for the double-sided printed circuit board circuit. The circuit must be carefully measured and mapped, otherwise it will cause trouble for the next step of fault analysis. Users with conditions can also temporarily remove the larger components on the entire PAC module power supply printed circuit board, input the printed circuit board into the computer with a scanner, and use PHOTOSHOP graphic editing software to perform circuit analysis, which can achieve twice the result with half the effort.

In repair practice, the following issues should be noted:

(1) Most PAC module power supplies are sealed with thermally conductive silicone rubber. When repairing, it is inevitable to peel off the thermally conductive silicone rubber. In view of the unique use of the colloid in fixing components, conducting heat, and preventing component oxidation and leakage in the module, we do not need to peel off the entire limb covered by the entire circuit. We only need to peel off the colloid covering the suspected local circuit to try to keep the original technical indicators of the repaired module. Thermally conductive silicone rubber is divided into transparent and non-transparent types. When peeling off non-transparent colloid, it is very easy to damage the surface of the sheet components because the components covered by the colloid cannot be seen. Any slight surface scratch will cause damage to the sheet capacitors and resistors , so the operation must be very careful. When I was repairing several PAC module power supplies, I found that the previous repairer accidentally scratched and smashed many components in the printed board when peeling off the thermally conductive silicone rubber of the PAC module power supply, making those modules truly disposable parts.

(2) There are many types of core component IC models of PAC module power supply. Common single-ended drive ICs are: UC3842, UC3845, TEA2018, pPC1094. The IC packaging forms are commonly DIP and LCC. In single-ended drive PAC module power supply, there are also circuit configurations that use double-ended drive chips to form single-ended drive forms. The functions and limit parameters of each pin of the driver chip IC can mostly be found in relevant technical information, so I will not go into details here. When repairing, if you encounter an IC that you are not familiar with or has its logo erased (often encountered when repairing imported equipment PAC module power supply), do not be impatient and be calm. You should analyze calmly and cooperate with advanced instrument detection methods to find the power pin, feedback pin, PWM output pin, timing pin (frequency setting pin), reference voltage pin, protection function input pin and state conversion pin of the driver chip, and carefully check and analyze the working conditions of the relevant IC components. The repair work is generally effective. Maintenance practice shows that when a PAC module power supply fails, the IC damage rate is very low, and most of the time, the problem is with peripheral components or power components. In view of this, if you are not sure, do not easily disassemble the IC chip to avoid artificially magnifying the fault. (3) When the same model of IC is used in PAC module power supplies produced by different manufacturers, the circuit configuration is mostly different. The same type of PAC module power supply may have the same fault phenomenon but the fault source may be very different. For example: in a module power supply assembled with UC3845 chip, the feedback signal input to IC pin ② can be taken from IC power supply pin ①, or the secondary output voltage of the high-frequency switching transformer can be sent to IC pin ② through the optical coupler VD. Therefore, when the output voltage is unstable, the idea of ​​judging the fault source is different. Similarly, the IC overcurrent detection pin ③ can be connected to the source current sampling resistor of the switching power supply field effect tube through a resistor to realize the current protection of the module power supply single function. A set of sampling amplifier circuits can be established using the "not" gate operational amplifier chip, and the IC pin ③ can be connected to the "or" gate output of the sampling amplifier to realize the overcurrent, overvoltage, and over-temperature alarm protection functions. Therefore, before implementing the entire circuit detection and fault analysis, you should pay attention to the distribution of each component, the direction of the IC's key pin signals, and truly understand the actual configuration of the circuit.

(4) The heat dissipation of power devices in the power module cannot be ignored. For example, the physical position of the power field effect tube and the Schottky diode in the TO-220 package in the module power circuit is to dissipate heat through close contact with thermal coupling silicone grease and the shell with dual functions of shielding and heat dissipation.

When replacing the above components after they are damaged, they must be installed strictly in the original position. During the process of installing the entire PAC module power board into the shell after repair, it is necessary to repeatedly check whether there is any obstruction in the heat conduction path when the device is working, and try to avoid a single failure. If this step is neglected, it will inevitably cause trouble, resulting in repeated damage to the replaced components during the use of the PAC module power supply after repair, causing the module to malfunction.

3. PAC module power supply fault troubleshooting example

[Example 1] Fault phenomenon: The South Korean SB-100PAC module power supply has no voltage output.

Analysis and repair Check whether the module fuse is intact. After disassembling the module copper and gold, it was found that the same circuit space of the module was filled with gray silicone rubber, and the entire circuit configuration could not be observed. Use a hacksaw blade to make several suitable picks, and carefully remove the colloid along the component surface. After all the colloids were removed, the component configuration of the module power supply was fully revealed. The main core component is an LCC packaged unmarked IC, and the top of the IC is a gold-plated steel sheet, which is very delicate. After verification, it was confirmed that this chip should be a PWM switch pulse generator. After adding 48V voltage, it was found that all 16 pins of the unmarked IC had no voltage, which was obviously abnormal. For the convenience of analysis, the partial circuit was drawn according to the printed board. IC pin ② is the power pin, and pins ③ and ② are the PWM drive pulse output pins. At this point, the fault has become traceable. Check that the voltage of IC pin ③ is zero, and the voltage across R2 is 48V. After checking again, it is found that R2 is open. After replacing R2 with a new part, the voltage across R2 is still 48V. Check C1 and D1 and there is no problem. In-circuit test, the forward and reverse resistance of IC@pin to ground is 12Ω (using 500 type RΩ×1Ω range), confirming that the IC is damaged. Under normal circumstances, it is very difficult to repair the PAC module power supply after the unlabeled main control chip is damaged. However, when the author compared the actual mapping of the printed circuit board circuit with the local drawing , he found that the arrangement of the functional pins of this unlabeled IC seemed to be similar to the arrangement order of the functional pins of the TL494 pulse width control chip in the common DIP-16 package form. He immediately looked up the TL494 data and found that it was exactly the same as the guess. Therefore, a suitable space was selected in the PAC module power supply shielding box, and the TL494 chip label surface and the shielding box frame were fixed with glue. After connecting the corresponding pads of the TL494 functional pins and the original chip functional pins with thin soft wires, a 48V voltage was applied, and the PAC module power supply worked normally. After the repair, the PAC module power supply may have a lower overall thermal resistance index because the thermal conductive silicone rubber has been removed from the internal space of the component. However, practice has shown that the original index design has a margin. Even after the repair, the PAC module power supply can still work normally for a long time even if the thermal conductive colloid is no longer filled.

[Example 2] Fault phenomenon: The BM-2078PAC module power supply sometimes outputs normally, sometimes has no output, and sometimes has output but is not stable.

Analysis and maintenance: The PAC module power supply is a transparent colloid package. After removing the bottom box cover, you can directly observe that there are two chips in the circuit. The top label of the chip is all polished off. One is a DIP one-out package, and the other is an 8-pin SMD package. From the circuit configuration, the DIP-16 package chip must be a PWM pulse driver IC, and the other IC may be an amplifier, which plays a role in amplifying various feedback and protection signals. Taking advantage of the favorable opportunity when the module can sometimes work normally, test the waveforms of each pin of the main control chip when it works normally. It is found that the chip pin ⑿ is the PWM width modulation wave output pin, and the pin ⑦ is the sawtooth wave formation pin, also known as the capacitor timing pin. When working normally, the measured frequency is about 120kHZ. Wait until the PAC module power supply output is abnormal, and immediately use a dual-trace oscilloscope to simultaneously observe the PWM output waveform of the ⑿ pin and the sawtooth waveform of the ⑦ pin. It is found that the duty cycle of the PWM waveform of the ⑿ pin changes irregularly and greatly, and the period and amplitude of the sawtooth wave of the ⑦ pin also change accordingly. According to the cause and effect relationship, it is confirmed that the abnormal output of the chip's PWM drive pin ⑿ is due to the abnormal sawtooth wave generated by pin ⑦. Remove the chip capacitor C at pin ⑦ and measure that there is no leakage at both ends. Considering that the fault may be covered up due to the heating effect of the soldering iron head during the desoldering process, the capacitance value is about 1089pF measured with a capacitance meter. Simply solder a small 1000pF capacitor to the original position of C. After power-on, the PAC module power supply resumes normal operation. By the way, if this fault is not detected with a dual-trace oscilloscope, it is quite time-consuming and laborious to eliminate it. [Example 3] Fault phenomenon Beijing Jihui PAC module power supply has an abnormal increase in the primary power supply 48V, causing the 2A fuse at the input end of the module power supply in normal operation to burn out. After that, a 5A fuse is connected and power is turned on. Only a "pop" sound is heard in the module box, and the fuse is burned out again.

Analysis and maintenance After disassembling the shielded copper box of the module power supply, the internal circuit was found to be free of thermal conductive silicone rubber filling, which greatly facilitated maintenance. The power field effect tube itself was found to have exploded. After replacing the new part, the module power supply still could not work. After carefully checking the power pin voltage value of the main control chip UC3845 in the module, it was only 2.5V, and the voltage values ​​of other pins were all zero. After carefully checking the peripheral components of the UC3845 chip, there was no problem. After replacing the UC3845 with a new one, the module power supply worked normally.