Dear friends from Badcaps
I have been searching for a long time to repair sucessfully a battery charger taking in 230V AC and outputting normally 14V DC. Its power mosfet has been shorted ( Infineon 06N80C3), probably because the bulk storage rectifier capacitor had swollen and had no capacitance anymore and a very high resistance.. The mosfet short has naturaly blown its 0.15 ohms resistor then its driver, the UC3845 IC which was shorted between pins 5 (ground) and 7 (vcc) plus 8 (Vref). Another small resistor surrounding was also dead. So, I have replaced the bulk storage capacitor, the mosfet, the UC3845, the resistors but my smps was still dead. The vcc voltage powering the UC3845 was under its UVLO on threshold at around 7.8V instead of 8.5 which would have started the driver and the PSU. I have ordered some original (and expensive STM UC3845 IC) with no success and the same startup voltage of UC3845 under its on threshold was still present..... I have tried to change the UC3845 by a more modern UCC28C51 from Texas Instruments with a lower startup on threshold voltage but then my vcc was lower at 6.8V, preventing the mosfet driver IC and the PSU to start.... I have tried to change the mosfet with more sensitive mosfet with a lower Qg value but always with no luck.....
I have found on my internet search some people having a similar problem without finding a workaround. Some guys were lowering the value of the starup resistor ( originaly in my circuit 94Kohms) to 30 Kohms. But this has not worked for me. I have tested all the capacitors and change some but with no luck. I have looked at my PSU through my infrared camera: the startup resistor was hot and the UC3845 corner of the IC supporting pin 8 and the Vref output was slightly warmer: this pin was powering with 5V the NE555 IC which was timing pin 4 of the UC3845. So I decided to order some NE555 because the overvoltage that had destroyed the UC3845 could have been transmitted straightly to the NE555 IC and this IC seemed to drive the UC3845......
I have received yesterday my new NE555, I have changed it today and................ SUCCESS
The PSU was straight outputing 15V, the VCC of my UC3845 was at 17V. The PSU was able to power a 50W car bulb.....
I hope could help some guys in despair with their UC3845 PSU not turning on after repair
I have been searching for a long time to repair sucessfully a battery charger taking in 230V AC and outputting normally 14V DC. Its power mosfet has been shorted ( Infineon 06N80C3), probably because the bulk storage rectifier capacitor had swollen and had no capacitance anymore and a very high resistance.. The mosfet short has naturaly blown its 0.15 ohms resistor then its driver, the UC3845 IC which was shorted between pins 5 (ground) and 7 (vcc) plus 8 (Vref). Another small resistor surrounding was also dead. So, I have replaced the bulk storage capacitor, the mosfet, the UC3845, the resistors but my smps was still dead. The vcc voltage powering the UC3845 was under its UVLO on threshold at around 7.8V instead of 8.5 which would have started the driver and the PSU. I have ordered some original (and expensive STM UC3845 IC) with no success and the same startup voltage of UC3845 under its on threshold was still present..... I have tried to change the UC3845 by a more modern UCC28C51 from Texas Instruments with a lower startup on threshold voltage but then my vcc was lower at 6.8V, preventing the mosfet driver IC and the PSU to start.... I have tried to change the mosfet with more sensitive mosfet with a lower Qg value but always with no luck.....
I have found on my internet search some people having a similar problem without finding a workaround. Some guys were lowering the value of the starup resistor ( originaly in my circuit 94Kohms) to 30 Kohms. But this has not worked for me. I have tested all the capacitors and change some but with no luck. I have looked at my PSU through my infrared camera: the startup resistor was hot and the UC3845 corner of the IC supporting pin 8 and the Vref output was slightly warmer: this pin was powering with 5V the NE555 IC which was timing pin 4 of the UC3845. So I decided to order some NE555 because the overvoltage that had destroyed the UC3845 could have been transmitted straightly to the NE555 IC and this IC seemed to drive the UC3845......
I have received yesterday my new NE555, I have changed it today and................ SUCCESS
The PSU was straight outputing 15V, the VCC of my UC3845 was at 17V. The PSU was able to power a 50W car bulb.....
I hope could help some guys in despair with their UC3845 PSU not turning on after repair
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