“ Testing a 100kv / 20A Solid-State Fast Switch and Mod-Anode Regulator at the APS”. ASD – RF Group. “ Testing a 100kv / 20A Solid-State Fast Switch and Mod-Anode Regulator at the APS”.
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“Testing a 100kv / 20A Solid-State Fast Switch and Mod-Anode Regulator at the APS”
ASD – RF Group
Mercury - containing ignitron crowbars are commonly used to protect klystrons from arc damage. When an arc occurs, the crowbar closes and rapidly discharges the energy-storage capacitor.
An alternative way to protect a klystron is to use a switch that opens during an arc, using series arrays of insulated-gate bipolar transistors (IGBTs).
1) Zero resistance or forward voltage drop in the on-state.
2)Infinite resistance in the off-state.
3)Switch with infinite speed.
4)Would not require any input power to make it switch.
Choice involves voltage, current, switching speed, drive circuitry, load, and temperature effects.
Most desirable primary characteristics are fast switching speed, simple drive requirements and low conduction loss.
High Power MOSFET:
Has high input impedance and fast switching speed.
For low voltage applications, power MOSFETs offer extremely low on-resistance, RDS(on), and approach the desired ideal switch.
In high voltage applications, MOSFETs exhibit increased RDS(on) resulting in lower efficiency due to increased conduction losses.
Low conduction loss.
BJTs and IGBTs have similar ratings in terms of voltage and current.
The presence of an isolated gate in an IGBT makes it simpler to drive than a BJT. BJTs require that base current be continuously supplied in a quantity sufficient enough to maintain saturation.
•BJTs have a much slower performance than MOSFETs.
IGBT technology offers a combination of the low conduction loss of a BJT and the switching speed of a power MOSFET.
IGBTs, like MOSFETs, are transconductance devices and can remain fully on by keeping the gate voltage above a certain threshold.
The IGBT has high input impedance and fast turn-on speed like a MOSFET.
Although turn-on speeds are very fast, turn-off (tailing) of the IGBT is slower than a MOSFET. Until recently, the feature that limited the IGBT from serving a wide variety of applications was initial IGBTs had current fall times of around three microseconds.
Ch.1 = cathode voltageCh.2 = cathode currentCh.3 = diode currentCh.4 = over current faultInput = 80kV from spotknocker (fault caused by arc test – output to ANL dummy load)
Ch.1 = cathode voltageCh.2 = cathode current Ch.4 = over current faultInput = 100kV from spotknocker(fault caused by arc test – output connected to ANL dummy load)
Series opening switches have substantial advantages over crowbars in the protection of RF amplifiers (klystrons).
1) Nearly immediate resumption of operation after an arc, during klystron conditioning.
2) Solid – State opening switches use no ignitron crowbars, therefore no mercury.
3) IGBTs can be much less expensive than vacuum tubes due to expected longer lifetimes.
4) Removal of crowbars can significantly lower stresses on upstream power components.
5) Opening switches are made with excessive voltage capability, so the switch can operate even if several devices fail (IGBTs always fail shorted).
6) Because the energy – storage capacitor does not discharge during an arc, the RF could be turned again immediately after the arc clears. It is possible the beam may be retained during an arc.