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I am trying to charge a transmission line (Z0=4.4 ohms, propagation delay=1.4 ns) by a HV (1-1000 V, 600 mA) power supply through a 1700 V rated IGBT (S1). The charged transmission line gets discharged through a photo conductive switch (S2) to a 4.4 ohm resistive load. There is another identical transmission line between the load and S2. The IGBT is triggered by ISO5852s based gate driver evaluation board. The timing signal for S1 and S2 is provided by an external pulse delay generator. The simplified circuit diagram and the timing diagram for S1 and S2 are shown in the figure below.

When I probe the transmission line placed before S2 after S2 is triggered off (after 150.5 us), I am getting a huge oscillation which is bigger than the HV charging voltage (50 V) although S1 (IGBT) and S2 is off at that time. As a result of that, the transmission line is being charged again although it should be isolated from the power supply at that time as S1 and S2 are off at that time. I am also getting oscillation across the transmission line after S2 (ideally I should get voltage output only for the period S2 is on and a little bit time after that). I am wondering what is the source of these oscillations. The wire that is connecting the collector terminal to the HV supply is about 100 cm long connecting wire. Is it a reason of this oscillation? However, if I remove the IGBT (S1) and charge the transmission line directly, I do not see such high ringing after S2 is triggered off.

Any helpful suggestion will be appreciated. I am also attaching the oscilloscope output here (yellow color-voltage across transmission lien, blue color-voltage after S2). Voltage across transmission line and load Circuit schematic and timing diagram

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  • \$\begingroup\$ Do you understand what transmission lines will do when you interrupt a load current? Do you understand that transmission lines produce reflections? \$\endgroup\$ – Andy aka Jun 18 at 15:24
  • \$\begingroup\$ I am not really expert in transmission line. From my understanding, I am trying to charge the transmission line (TL) like a capacitor, cutting off the power supply and waiting for some time for triggering S2, so that the stored charge in the first section of the TL can be discharged to the matched load through the second portion of the matched TL. Are those reflections coming from the load end during the recovery time of the photo conductive switch? What would be your suggestion in this case? \$\endgroup\$ – Allison_81 Jun 18 at 15:30
  • \$\begingroup\$ what rings at 1,500KHz? 0.1uH and 0.1uF are close \$\endgroup\$ – analogsystemsrf Jun 18 at 15:32
  • \$\begingroup\$ @analogsystemsrf, the yellow waveform is the one that rings close to at 1,500 kHz. It is the voltage across the transmission line that is placed between the IGBT (S1) and the photo conductive switch (S2). The ringing happens right after S2 is triggered on for a short amount of time (close to 15-20 ns). Can you kindly tell what inductance and capacitance are you referring to by 0.1uH and 0.1uF? \$\endgroup\$ – Allison_81 Jun 18 at 15:56

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