| Electronics > Repair |
| SMPS Troubleshooting. About to give up... |
| (1/4) > >> |
| Milmat1:
Background, this is a SMPS board from a DRO system on a milling machine. It's an YCL PCB153 board, No schematic available. It is an KA3842 PWM chip. / triple output +5V, +36V, +6V (all outputs dead) At the bridge rectifier, I have 120VAC in and 165.3 VDC across the bulk filter cap. (100uf/400V) So I assume everything ion the primary circuit is OK (??). I have reverse engineered the part of the board dealing with the chip and found it different from any of the sample drawings on this chip. But still a basic SPMS design. I recapped the board, found an "Open" startup resistor (180K) and replaced it. Found the startup cap(86uf) had a very high ESR . I have checked all the silicon, i.e. Diodes, Zeners etc. An R/C circuit between pin 4 and pin 8 is supposed to set the output freq. They are fine. The chip will not start. pin 6 (output) is just dead and nothing going to the gate of the MOSFET. (And voltage across D/S is exactly the same as the DC across the BFC). EDIT: Also pin 8 should be a 5VDC refference voltage, it has no voltage present. Should this be present all the time when Vcc is present ? The Vcc to the chip (pin7) looks like the attached pic. Is this normal for a Vcc startup until the aux winding can take over ? How likely is it this chip itself has failed, is there a way to test the chip to confirm ? Any help or direction appreciated... Thanks, Matt |
| kripton2035:
there is usually a small cap (around 10uF) between pins 7 and 5. often this cap has high esr or too low capacity. it's often worth to try to replace it. |
| Milmat1:
There is an 82uf between pin 7 and pin 5 (gnd). It did in fact have very high ESR, but I replaced it. |
| Bud:
Check the chip for shutdown condition --- Quote ---Shutdown of the KA3842B can be accomplished by two methods; either raise pin 3 above 1V or pull pin 1 below a voltage two diode drops above ground --- End quote --- |
| Milmat1:
--- Quote from: Bud on August 16, 2022, 04:24:57 pm ---Check the chip for shutdown condition --- Quote ---Shutdown of the KA3842B can be accomplished by two methods; either raise pin 3 above 1V or pull pin 1 below a voltage two diode drops above ground --- End quote --- --- End quote --- Pin 3 = 105mv Pin1 = 6mv |
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