**Project:** A power supply in a laboratory is broken. Upon inspection, it was found that the full bridge controlled by UC3875 needs repair.
**Phenomenon:** During initial testing, one of the power transistors was found to be damaged. Since no exact replacement was available, similar power-rated transistors were used as a substitute. After powering on, everything worked fine at low input voltage, but when the input voltage increased, the drive signal became unstable and the frequency jittered.
**Solution:** Increasing the gate resistance of the power transistor resolved the issue. The system operated normally after this adjustment, and the power supply was successfully repaired.
**Analysis:** The new transistors had different parasitic parameters compared to the original ones. This caused faster switching, which introduced more noise into the circuit. At higher voltages, this noise was strong enough to interfere with the control circuit’s operation.
**Notes from Experience:**
1. Always be careful when soldering components. Poor solder joints can be hard to spot and are dangerous. Pay attention to the direction of diodes—especially in bridge rectifiers. I once mistakenly soldered a diode in the wrong direction, which caused the filter capacitor to experience reverse voltage, a serious safety hazard.
2. When debugging, if you need to use flying wires for signals, twist the signal and return lines together. If left untwisted, they can act like an antenna, picking up unwanted interference.
3. In bus power supplies, use both large electrolytic capacitors and high-frequency ceramic capacitors for filtering. This ensures better noise suppression at both low and high frequencies.
**Project:** UC3845 dual-switch forward converter
**Phenomenon:** After turning off the two MOSFETs, the DS voltages were very different, not matching the expected half-voltage. Suspecting mismatched MOSFETs, we swapped their positions, but the problem remained. It seemed unrelated to the MOSFETs themselves.
**Solution:** We adjusted the driver timing to ensure both switches turned off simultaneously. This improved the situation slightly, but the voltages still didn’t balance.
**Analysis:** Two factors could be responsible: differences in PCB parasitic capacitance between the two positions, and slight timing mismatches in the driver output.
**Project:** UC3845 flyback with auxiliary winding feedback
**Phenomenon:** The main output voltage had a significant overshoot during startup, while the auxiliary winding used for feedback did not.
**Solution:** A resistor was added in series with the auxiliary winding. Reducing its value significantly reduced the main output overshoot.
**Analysis:** The feedback voltage came from the auxiliary winding, and the resistor created a voltage difference. This difference affected the output due to coupling through the transformer.
**Project:** NCP1014 with optocoupler feedback
**Phenomenon:** A previously working board started showing incorrect output voltage after re-soldering a component.
**Solution:** Replaced the 431 with the same model used in other parts of the board, and the issue was resolved.
**Analysis:** The original 431 had a very low minimum operating current (in the microamp range), so the design didn’t account for it. When replaced with a TI 431, which required 1 mA, the circuit failed.
**Project:** ICE1PCS01-controlled boost PFC
**Phenomenon:** At most input voltages, the current waveform was clean with low ripple. However, at around 220V, the high-frequency ripple suddenly increased.
**Solution:** Using an AC source, the ripple was observed under all voltages.
**Analysis:** The auto-transformer used for voltage regulation had leakage inductance. At 220V, the regulator bypassed the leakage, causing the ripple to increase.
**Project:** UC3845 dual-switch flyback
**Phenomenon:** The drive signal was unstable, with constant jitter and transformer noise. Adjustments had no effect. Oscilloscope analysis showed jitter in the sawtooth wave from the UC3845.
**Solution:** Separated the ground and power of the control circuit, and connected them at a single point. This stabilized the signal and eliminated the noise.
**Analysis:** Layout is critical in power supplies, especially grounding. Keeping the signal and power grounds separate and connecting them at a single point prevents high-frequency currents from interfering with the control circuit.
**Project:** UCC3895 current-mode phase-shifted full-bridge with double current rectification
**Phenomenon:** Transformer bias occurred.
**Solution:** Made the secondary PCB traces thicker and connected them to the inductor of the current doubler. The magnetic imbalance disappeared.
**Analysis:** Current doubler circuits can have imbalanced average inductor currents. Even small differences in DC resistance from PCB traces can cause transformer bias.
**Project:** 431 + optocoupler feedback
**Phenomenon:** Output voltage regulation was poor, dropping significantly with increasing load. The voltage difference between the sampling point and output was small.
**Solution:** Added a small capacitor between the reference pin and cathode of the 431. This improved the regulation.
**Analysis:** The reference pin of the 431 was being disturbed, leading to poor feedback performance.
**Project:** IR1150 boost PFC
**Phenomenon:** The switching frequency was 100 kHz, but the input current had a 1 kHz ripple. The X capacitor was also making noise.
**Solution:** Adjusted the EMI filter parameters.
**Analysis:** The EMI filter was resonating at the wrong frequency, causing the ripple.
**Project:** Flyback synchronous rectification
**Phenomenon:** The voltage peak on the synchronous rectifier was too high and couldn’t be suppressed.
**Solution:** Replaced the synchronous MOSFET with one that has a fast recovery body diode.
**Analysis:** The long reverse recovery time of the body diode caused large voltage spikes.
**Project:** IR1150 PFC
**Phenomenon:** During temperature testing, the MOSFET reached 80°C, but previously it went up to 110°C.
**Solution:** Found that the gate resistor was incorrectly soldered as 100R instead of 10R.
**Analysis:** A high gate resistance caused excessive power loss, leading to higher junction temperatures. Even though the case temperature was low, the actual junction temperature exceeded the MOSFET’s limit.
**Note:** A high gate resistance leads to insufficient drive power, which can damage the MOSFET. Ensure proper gate resistance is used. If the PCB trace inductance is too high, it can resonate with Cgs, causing voltage spikes. Adding a resistor can help dampen this oscillation.
**Project:** L4981 PFC
**Phenomenon:** No-load startup resulted in unstable oscillation, with frequency varying significantly. Higher input voltage made it worse.
**Solution:** Carefully examined the PCB and found a power line close to the control circuit. Isolated the copper trace near the control section, eliminating the interference.
**Analysis:** The D-pin of the MOSFET generated a large dv/dt, causing common-mode interference. Keep the control circuit away from high dv/dt nodes.
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