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I've got an industrial SMPS for a laboratory optical microscope that failed. When probing to measure for voltages I accidentally destroyed some components.

In the attached image I hand traced the circuit to help me understand the operation to better narrow down the fail point.

One of the components that was destroyed by my stupidity was the Zener diode in the gate driver circuit. Since the diodes were MELF package type I don't know the exact part number. Would it be safe to replace with a 10V zener or should it be higher like 15V?

Also destroyed were the PNP BJTs (BC32725.) The gate driver circuits are isolated through a small 1:1:1 transformer which is driven by the PWM circuit.

Gate driver circuit:

gate driver circuit

PWM,PFC and DC regulation:

enter image description here

isolated secondary side

enter image description here

enter image description here

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  1. The IXFH20N FET Vgs is rated for +/-30V but all you need is 10V to achieve the RdsOn specs, so you are just adding excess gate charge Q=CV. However being a half wave rectifier gate driver with unity gain as an emitter follower the low impedance is not be delayed by the extra gate voltage. The Zener is not conducting when the IXFH20N FET is switching.

The FU120 is rated for Vgs(max) =+/-20V

enter image description here

Reverse Engineering injected faults from a probe ground are hard to trace as it could have failed from over current during the short, or over-voltage from the flyback energy stored in the shorted inductance or over-energy (mJ) from the transfer of shorted energy, depending on where and how details.

If repair is difficult, it may be easier to simply use a replacement supply with the equivalent constant current or CV @ I.

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  • \$\begingroup\$ Usually 22V from the supplies I’ve repaired over the years. \$\endgroup\$
    – Kartman
    Mar 1, 2022 at 9:32
  • \$\begingroup\$ This FET shows very little difference from Vgs = 9V to 10V especially when hot due to NTC of Vt. littelfuse.com/~/media/electronics/datasheets/discrete_mosfets/… \$\endgroup\$ Mar 1, 2022 at 13:35
  • \$\begingroup\$ Thank you both very much. What about the two diodes (zener and rectifier?) connected in parallel with the coil on the primary of the 1:1:1 ? The FU120 is rated for 40V Vds I believe. When I measured the auxiliary VDC it was around 35 or 36V before I screwed up the circuit. The FET FU120 was also destroyed and I still have a 3 ohm short from auxiliary VDC to ground but I haven't identified what other component is causing this. \$\endgroup\$
    – Modz2300
    Mar 2, 2022 at 0:47
  • \$\begingroup\$ FU120 has a Vdss= 100V min infineon.com/dgdl/… \$\endgroup\$ Mar 2, 2022 at 0:55
  • \$\begingroup\$ I've added the other schematic images which show a portion of the secondary side. The connections labeled 'K01' through 'K25' are header sockets on the board where a separate microcontroller circuit plugs in. I haven't drawn out the schematic of that board yet but I think that probably controls the constant voltage/current output at connections 'M01' through 'M09'. The 'Mxx' connections are the DC outputs to other circuit boards of the microscope. \$\endgroup\$
    – Modz2300
    Mar 2, 2022 at 1:26

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