The PS in question is from an IBM 5153 CGA monitor which was made by Tatung in 1985. The PS is a separate internal assembly that produces 115 VDC @ 0.5 A for the CRT monitor chassis. It's housed in a metal enclosure with ventilation holes.
The monitor is labeled as "120 VAC" (only) but I live in a 230 VAC country and would prefer to run it directly off the line without a stepdown transformer.
Looking at the design (SAMs schematic,) since the primary DC rail runs fixed in full-wave mode and the smoothing caps are rated at 400 WVDC but only see 153 VDC at a 110VAC line, I think this design was originally intended to run at worldwide line voltages, and in fact ran just fine on 230 VAC for an hour until I became uncomfortable with the additional heat coming off the monitor, not present when on 110 VAC.
I want to either reduce that heat loss by a design change, or at least improve the cooling.
Under bench testing with a dummy load and the unit open in still air, using thermocouples I've narrowed the main heat source down to one power resistor, identified in the attached schematic. I don't understand enough about SMPS design so need assistance to understand if there is a minor design change that would reduce the heat losses off that part. The 136 C measured would not normally be of concern, but it will be much higher in the enclosure, inside the monitor. Other power resistors and the main switching transistor run at the same temp as they did under 110 VAC, and there are no additional noises that make me think the unit is under stress.
Note, in the schematic the primary DC section is at the bottom and provides 153 V (or 306 V) to the switching circuit above via label "1".
The monitor is labeled as "120 VAC" (only) but I live in a 230 VAC country and would prefer to run it directly off the line without a stepdown transformer.
Looking at the design (SAMs schematic,) since the primary DC rail runs fixed in full-wave mode and the smoothing caps are rated at 400 WVDC but only see 153 VDC at a 110VAC line, I think this design was originally intended to run at worldwide line voltages, and in fact ran just fine on 230 VAC for an hour until I became uncomfortable with the additional heat coming off the monitor, not present when on 110 VAC.
I want to either reduce that heat loss by a design change, or at least improve the cooling.
Under bench testing with a dummy load and the unit open in still air, using thermocouples I've narrowed the main heat source down to one power resistor, identified in the attached schematic. I don't understand enough about SMPS design so need assistance to understand if there is a minor design change that would reduce the heat losses off that part. The 136 C measured would not normally be of concern, but it will be much higher in the enclosure, inside the monitor. Other power resistors and the main switching transistor run at the same temp as they did under 110 VAC, and there are no additional noises that make me think the unit is under stress.
Note, in the schematic the primary DC section is at the bottom and provides 153 V (or 306 V) to the switching circuit above via label "1".
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