SMPS Startup Circuit Using Current Amplification
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Solution Overview
Problem
Conventional methods for reducing startup time in switching power supplies either increase power consumption or compromise operating power, as they require altering the startup resistor or capacitor values, which are not desirable.
Innovation Solution
A controller for a switch mode power supply that amplifies the startup current through a startup resistor during the initial phase, allowing for faster startup without altering other circuit parameters, and separates the power switch from the startup capacitor during normal operation to prevent interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If a smaller startup resistor is used to speed up startup, then startup time is reduced, but power consumption increases
Solution Approach 1:
The patent applies preliminary action by using a dedicated startup current path that operates only during the startup phase. The startup transistor Q1 is enabled before normal operation to provide an amplified startup current through the startup resistor Rst, charging the startup capacitor Cvcc quickly. Once startup is complete, the startup transistor is disabled and normal operation takes over, preventing continuous power consumption through the startup resistor.
Solution Approach 2:
The patent segments the current path by creating separate startup and normal operation paths. The startup transistor Q1 and diode D4 form a dedicated startup current amplification path that is isolated from the normal operation path. This segmentation allows the startup resistor to have low resistance during startup without affecting normal operation power consumption, as the startup transistor blocks the resistor from the main circuit after startup.
2Loss of time
If a smaller startup capacitor is used to reduce startup time, then startup time is reduced, but sufficient operating power cannot be provided
Solution Approach 1:
The patent uses preliminary action by providing an amplified startup current through the startup transistor Q1 during the initial charging phase. This temporary current amplification allows the startup capacitor to charge quickly to the required voltage level even with a smaller capacitance value. The startup transistor acts as a current booster only when needed, enabling fast charging without requiring a large capacitor that would slow down startup.
3Device complexity
If the power switch remains connected to the startup capacitor during normal operation, then circuit simplicity is maintained, but interference with regulating function occurs
Solution Approach 1:
The patent applies preliminary action by preparing the isolation mechanism in advance through the diode D4. During startup, the diode is forward-biased and connects the startup capacitor to the power switch. Once startup is complete and the controller begins normal operation, the diode becomes reverse-biased due to the voltage difference, automatically isolating the startup capacitor from the power switch. This pre-configured isolation mechanism ensures that the regulating function is not interfered with during normal operation.
Solution Approach 2:
The patent introduces an intermediary element (diode D4) between the startup capacitor Cvcc and the power switch Q1. This diode acts as a one-way valve that allows current flow during startup but blocks it during normal operation. The intermediary provides automatic isolation without requiring complex control circuitry, maintaining simplicity while ensuring reliable separation of startup and regulating functions.
Data Source
AI summary
A control circuit for a switch mode power supply (SMPS) includes a power switch for coupling to a primary winding of the power supply and a startup resistor coupled to an external input voltage and to a control terminal of the power switch. The control circuit also includes a controller. During startup, the controller is configured to cause the power switch to amplify a startup current from an external input voltage through the startup resistor and provide a startup power to the controller. During normal operation, the controller is configured to provide a power switch control signal to turn on and off the power switch for controlling a current flow in the primary winding and regulating an output of the power supply. The controller is configured to provide a current signal for driving an NPN power switch and to provide a voltage signal for driving an NMOS power switch.