Startup Circuit With Depletion-Mode J-FET Reducing No-Load Dissipation
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Solution Overview
Problem
Switching power supplies face challenges in providing initial energy to control circuitry during startup when the input voltage is higher than the operating voltage of the control devices, leading to high power dissipation and increased no-load power losses due to inefficient startup circuitry.
Innovation Solution
A startup circuit using a depletion-mode junction field-effect transistor (J-FET) as a normally on switching device, combined with a bipolar transistor and a charge pump circuit, automatically provides startup charging current and disables it once the control circuitry is operational, reducing power dissipation by exploiting the J-FET's conductive and cutoff states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a resistor or current source is used to charge the startup capacitor, then the control circuitry can be enabled when the capacitor voltage reaches the operating voltage, but high power dissipation occurs during normal operation regardless of output loading
Solution Approach 1:
The patent applies the dynamics principle by making the startup circuit dynamically controllable through a control signal that disables the current source after startup is complete. The circuit transitions from an always-on state to a dynamically controlled state where the current source can be enabled during startup and disabled during normal operation, thereby reducing no-load power loss while maintaining reliable startup functionality.
Solution Approach 2:
The patent implements feedback by using a monitoring circuit that detects when the startup capacitor has reached its operating voltage and generates a control signal to disable the current source. This feedback mechanism ensures that the startup circuit automatically terminates its operation after completing its function, preventing continuous power dissipation during normal operation while guaranteeing reliable startup.
2Reliability
If the startup circuit is designed to provide sufficient charging current at minimum input voltage, then startup reliability is ensured, but power dissipation increases at maximum input voltage
Solution Approach 1:
The patent applies preliminary action by designing the startup circuit to complete its charging function during the startup phase before normal operation begins. The startup capacitor is charged to the required operating voltage in advance, and once charging is complete, the current source is disabled. This preliminary action ensures that all necessary startup energy is delivered before the power converter begins normal operation, preventing continuous power dissipation.
Solution Approach 2:
The patent implements periodic action by making the current source operable in periodic cycles - enabled during startup to charge the capacitor, then disabled during normal operation. The monitoring circuit creates this periodic operation by generating control signals that enable the current source only when needed for startup, thereby reducing average power dissipation while maintaining startup reliability.
3Extent of automation
If a J-FET is used as a normally on switching device, then automatic startup current provision is achieved without separately powered control circuitry, but the device requires sufficiently high gate voltage to turn off
Solution Approach 1:
The patent uses an intermediary approach by introducing a charge pump circuit as a mediator between the control circuitry and the J-FET gate. The charge pump generates the sufficiently high positive voltage needed to turn off the J-FET, acting as an intermediary that bridges the voltage gap. This allows the J-FET to function as an automatic normally-on switch during startup while providing a mechanism to disable it when needed, resolving the contradiction between automation and device complexity.
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 configuration effectively reduces no-load power losses and efficiently establishes the operating voltage for control circuitry during startup, minimizing power dissipation and improving overall efficiency of the power supply.
Implementation Method 1
a normally on switching device, such as a depletion-mode junction field-effect transistor (J-FET). Such a device has a source-drain channel that conducts current in the absence of a control voltage on the gate of the device, and this feature is exploited for use during startup when control circuitry is not yet operating. Additionally, the source-drain channel can be cutoff by application of a sufficiently high control or bias voltage to the gate of the device
Implementation Method 2
A charge pump circuit is arranged between a controller that generates a clock signal and the normally on transistor, and it includes a capacitor-diode network that responds to the clock signal by providing a higher-voltage signal to the gate of the normally on transistor
Implementation Method 3
an emitter switched current source employing a bipolar transistor as the main current-control element
Data Source
AI summary
A power converter startup circuit establishes an operating voltage for control circuitry during startup and is then disabled to reduce no-load power dissipation. The startup circuit has a normally on characteristic to automatically provide startup charging current for a startup capacitor. The control circuitry begins operating as the startup capacitor voltage reaches an operating value, and it generates an inhibitory signal that disables the startup circuit to stop the startup charging current and reduce power dissipation. The normally on characteristic is achieved by an emitter switched current source employing a normally on device such as a depletion-mode J-FET. A resistor divider network provides both biasing for the startup current source and a point of monitoring the power supply input voltage during steady state operation. A charge pump provides a voltage of the inhibitory control signal sufficiently above the auxiliary voltage to turn off the normally on switching transistor.