**Title: Haier BCD-553 Refrigerator Sometimes Fails to Operate and Display Shows No Indication**
**Fault Phenomenon:** According to the user, this refrigerator had been functioning well. However, recently, it started experiencing intermittent automatic shutdowns. During these shutdowns, the display would also go dark with no indication at all. The fault appeared irregularly; sometimes it wouldn't happen for days, while other times it occurred multiple times in a day, making normal use impossible.
**Fault Analysis:** Based on the user's description, this is an unstable soft fault. The refrigerator only occasionally stops working automatically, and when it does, the display goes dark with no indication. This kind of fault is usually related to the refrigerator’s power supply circuitry. It is likely due to poor contact or broken connections in certain components or wiring within the circuit, or due to degraded characteristics of some components leading to inconsistent performance. During inspection, focus should be placed on checking relevant circuit elements within the switch-mode power supply circuit.
The switch-mode power supply circuit in the Haier BCD-553 microcomputer-controlled refrigerator mainly consists of IC201, IC203, IC202, T200, and VT201. Among them:
- IC202 is a photocoupler.
- IC203 is a three-terminal fixed voltage regulator with a stabilization value of SV.
- IC201 model NCP1200P100 is a current-mode single-end PWM control switch-mode power integrated circuit with external switch tubes. Its internal circuit includes a 7.0mA high-voltage current source, reference voltage source, cycle comparator, variable frequency clock circuit, current detection comparator, RS latch, driver stage, overvoltage protection, overcurrent protection, and overload protection. It adopts an 8-pin PDIP plastic package form.
The working principle of the switch-mode power supply circuit in the Haier BCD-553 microcomputer-controlled refrigerator can be explained from the following two aspects:
(1) **Oscillation Circuit Startup:** After power-on, the AC220V voltage undergoes rectification and filtering to obtain a pulsating DC voltage of about 280V. This voltage is applied to the drain of the MOS200/FPQF5N60C switch tube through the primary winding of the switch transformer T200. The high-start voltage passes through the series-parallel resistors R215, R216, R217, and R218 to the HV end (pin ⑧) of the NCP1200P100 (IC201). The 7.0mA current generated by the high-voltage current source charges the external capacitor CE203 via pin ⑥. When the charging voltage rises to about 13V, the reference power supply starts providing bias for the control circuit. The clock circuit triggers the RS latch output, which is amplified by the driver and outputs a PWM pulse from pin ⑤ of IC201, driving the VT201 switch tube to operate.
(2) **Oscillation Process of the Oscillation Circuit:** After VT201 conducts, current flows through the MOS200 drain-source across R208-R212 (3.9Ω5), generating a voltage drop. This voltage is applied to pin ③ of IC201 via R214 (15kΩ), then after a 380ns delay blanking inside pin ③, it is applied to the inverting input terminal of the current comparator for comparison with the non-inverting input terminal voltage. When the voltage at pin ③ of IC201 reaches the potential at the non-inverting terminal, the comparator flips and triggers the RS latch to turn off the driver stage. Pin ⑤ of IC201 outputs a low level, turning off the power transistor VT1 until the rising edge of the clock pulse arrives again, triggering the RS latch to output a high level, driving the power transistor VT1 to conduct again. As a result, the switching power supply enters a periodic oscillation working state. Thus, the secondary side of the switch transformer obtains an induced voltage, which is rectified and filtered by VD202, VD203, and VD207, respectively outputting +16V, +5V, and +12V DC voltages required by the control circuit.
(3) **During Fault Occurrence:** Measuring the voltage across the CE201 filter capacitor shows that it is approximately 300V, indicating that the main rectifier filter circuit is operating normally.
(4) **Measuring Voltage Across CE203 Capacitor:** The measured voltage of about +13V DC across the CE203 capacitor is 0V, clearly indicating that the switch-mode power supply circuit has not started oscillating. However, during measurement, the display sometimes lights up, and the switch transformer emits a slight sound. This phenomenon suggests that the switch-mode power supply sometimes recovers oscillation. Suspecting that there might be poor contact or soft damage in the feedback power supply voltage branch provided by the switch-mode power supply control integrated circuit IC201 pin ⑥.
**Locating the Fault Point:**
(1) Disconnect the refrigerator's AC power supply and check CE203 capacitor, R202 resistor, VD201 rectifier diode, and VD205 voltage stabilizing diode. No obvious damage was found. Replacing these components one by one with new ones still resulted in occasional faults, leaving the repair process stuck.
(2) After careful consideration, since replacing components didn’t solve the problem, could there be any breaks in the circuit? Following this line of thought, a magnifying glass was used to carefully inspect the soldering conditions of each pin of IC201, the solder joints of pins ③ and ④ of the switch transformer T200, and the connection lines between pin ⑥ of IC201 and pin ④ of the switch transformer T200. No obvious cracks or fissures were found.
(3) Without further options, considering that the switch transformer's leads are prone to virtual soldering or de-soldering, the solder joints of pins ④ and ③ of the switch transformer T200 were re-soldered. After reconnecting the power and testing, the aforementioned fault did not reappear. After prolonged testing, everything remained normal, confirming that the fault had been resolved.
**Hint:** In this case, the fault was obviously caused by a hidden virtual soldering or de-soldering issue between the lead pins ④ or ③ of the switch transformer T200 and their solder points. For such hidden virtual soldering or de-soldering faults, if they cannot be detected visually, a common method is to re-solder the suspected component's solder joints to see if the problem can be solved. Sometimes, the fault is quickly eliminated, as seen in this example. Additionally, if the three groups of DC operating voltages output by the switch-mode power supply are all normal but the refrigerator cannot start and the display shows no indication, further inspection should focus on the main control circuit board.
**Source:** Beijing Haier Refrigerator Repair Phone http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3455410