Soft-Start Adapter for AC Electron Gun Inrush Current
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Solution Overview
Problem
Radiotherapy linac electron sources experience high inrush currents when first turned on, causing stress and potential premature failure of the cathode heater and associated wiring due to low resistance at room temperature, leading to electrical failures.
Innovation Solution
An alternating current (AC) current limiter is integrated into the circuit, either on the high-voltage primary or low-voltage secondary side of the filament transformer, utilizing a two-terminal network with a full-wave diode bridge and a current-limiting device, such as an N-channel depletion-mode MOSFET, to regulate the inrush current and maintain a safe operating level.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If voltage is applied to the electron source when the cathode is at room temperature, then the electron source can be turned on, but high inrush current flows due to low resistance causing stress and potential failure of the cathode heater and wiring
Solution Approach 1:
The patent applies a soft-start adapter before the filament transformer that gradually increases the voltage applied to the cathode heater from zero to the operating voltage over a predetermined time period. This preliminary action prevents the sudden application of full voltage that causes high inrush current, thereby reducing stress on the cathode heater and associated wiring while ensuring reliable operation.
2Speed
If full voltage is applied immediately to the electron source, then the system can start quickly, but the low resistance at room temperature causes excessive inrush current that stresses the cathode heater and wiring
Solution Approach 1:
The soft-start adapter implements a controlled voltage ramp-up sequence that gradually increases the voltage from zero to the full operating voltage over a predetermined time period. This preliminary action allows the cathode heater resistance to increase as it heats up, preventing excessive inrush current while still achieving relatively quick startup compared to traditional gradual heating methods.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The AC current limiter effectively limits the inrush current to a safe value, reducing the risk of electrical failure and ensuring stable operation by maintaining a low resistance after the cathode reaches normal operating temperature, thus extending the lifespan of the electron source and associated components.
Implementation Method 1
An N-channel depletion-mode MOSFET may be used as the current-limiting device in the AC current limiter
Implementation Method 2
A directly-heated cathode may be referred to as a 'filament' and an indirectly-heated cathode may be referred to as a 'cathode heater'
Implementation Method 3
an electron source that emits electrons by thermionic emission
Implementation Method 4
utilizing a two-terminal network with a full-wave diode bridge
Data Source
AI summary
A circuit to power an electron source includes a filament transformer comprising a primary side and a secondary side, a high-voltage transformer coupled to the filament transformer and the electron source, and an alternating current (AC) current limiter coupled in series with the primary side or the secondary side of the filament transformer. The AC current limiter includes a diode bridge and a current-limiting device in the diode bridge.