Emitter-Switched BJT Base Drive for Fast Saturation Without Capacitors
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Solution Overview
Problem
Existing emitter switched bipolar junction transistor (BJT) circuits require capacitor storage elements, which limits their integration into integrated circuits and results in inefficiencies such as slower turn-off times and energy loss due to overdrive, as well as higher turn-on losses and energy loss during saturation.
Innovation Solution
A method and circuit for overdriving the base current of an emitter switched BJT, involving a comparator to monitor the collector voltage and inject a current pulse until saturation is achieved, followed by a fixed period of overdrive current to rapidly saturate the BJT, and then reducing the current to prevent overdrive, allowing for efficient saturation and energy recovery.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a large base current overdrive is used to rapidly saturate the BJT, then turn-on performance is enhanced, but excess base drive energy is stored in the base region causing slower turn-off and energy loss
Solution Approach 1:
The patent applies periodic pulsed base current to the BJT rather than continuous overdrive. The controller delivers controlled pulses of base current that are sufficient to saturate the transistor quickly, then stops. This periodic action allows the BJT to reach saturation rapidly without maintaining excessive base current that would store energy in the base region and cause slow turn-off. The pulsed nature of the base drive eliminates continuous energy loss while achieving fast turn-on.
2Loss of energy
If capacitor storage elements are used to recover storage charge, then energy recovery is enabled, but the circuit cannot be implemented in an integrated circuit
Solution Approach 1:
The patent extracts and eliminates the capacitor storage element from the circuit while retaining the energy recovery function. Instead of using a physical capacitor to store and recover energy, the invention uses a controller that actively manages base current delivery. The controller monitors BJT saturation and dynamically adjusts base current accordingly, enabling energy recovery through intelligent control rather than passive energy storage. This extraction of the capacitor simplifies the circuit for integrated circuit implementation.
Solution Approach 2:
The patent replaces the passive mechanical/electrical energy storage mechanism (capacitor) with an active control system. The controller uses feedback and timing to manage energy recovery through controlled base current pulses. This substitution transforms the energy recovery approach from a passive component-based system to an active control-based system, which is more suitable for integrated circuit implementation while maintaining or improving energy efficiency.
3Device complexity
If fixed base drive current is used for on-time operation, then circuit simplicity is maintained, but turn-on losses are higher due to slower saturation
Solution Approach 1:
The patent transitions from a static fixed base drive current to a dynamic base current that changes over time. The controller adjusts the base current magnitude and duration based on the BJT's saturation state and operating conditions. During turn-on, the base current is increased to rapidly saturate the transistor, reducing turn-on losses. During on-time operation, the base current is optimized for the required collector current. This dynamic adaptation maintains circuit simplicity while significantly reducing turn-on energy losses compared to fixed base drive.
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
This approach enables rapid saturation of the BJT with controlled overdrive current, reducing turn-on losses, energy loss, and allowing for energy recovery, while eliminating the need for capacitor storage elements, thus enabling integration into integrated circuits.
Implementation Method 1
A comparator is used to sense when the BJT is saturated by comparing the collector voltage to a reference voltage
Implementation Method 2
The output of the comparator is coupled to an input of a buffer. The output of the buffer is coupled to the base of the BJT
Implementation Method 3
The low voltage MOSFET Q2 is controlled by the high frequency PWM control signal and turns on or off current flowing through the emitter of the BJT Q1
Implementation Method 4
They may have lower saturation voltages for high voltage applications
Data Source
AI summary
An emitter switched bipolar transistor circuit includes a bipolar junction transistor (BJT) having a collector coupled to an output terminal, a metal oxide semiconductor field effect transistor (MOSFET) coupled to an emitter of the BJT, a bias voltage supply coupled to the base of the BJT, a buffer coupled to the base of the BJT, and a comparator. The comparator includes a first input coupled to the collector of the BJT, a second input coupled to a voltage reference, and an output coupled to an input of the buffer. The comparator is configured to receive a collector voltage of the BJT at the first input of the comparator, compare the received collector voltage with the voltage reference, and cause the buffer to inject a current pulse to the base of the BJT until the collector voltage is less than the voltage reference, indicating the BJT is substantially saturated.