Switching Power Supply Startup Control Circuit
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Solution Overview
Problem
Conventional switching power supplies experience a step-wise increase in output voltage during the startup period, which can lead to power ON resetting and malfunctioning of the under-voltage locking out function, especially when there is a rapid rise in DC output voltage or transition to the overload mode.
Innovation Solution
A switching power supply with a control circuit that sets an overload mode during the startup period, using an initial state setting circuit to pull down the CS signal and pull up the FB signal, thereby inhibiting the generation of the driving signal and ensuring a monotonic increase in output voltage, while maintaining an oscillation frequency higher than the audible frequency band to prevent audible noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the switching power supply operates in normal mode during startup, then the output voltage rises quickly, but the voltage increases in a step-wise manner causing power ON resetting and malfunctioning of the under-voltage locking out function
Solution Approach 1:
The initial state setting circuit pre-sets the control circuit to overload detecting state before normal operation begins. By pulling up the FB signal and pulling down the CS signal during startup, the system forces an initial state that prevents step-wise voltage increases, allowing the output voltage to rise smoothly and monotonically from the beginning of operation.
Solution Approach 2:
The system dynamically switches between overload detecting state and normal operation mode during startup. The initial state setting circuit creates a temporary dynamic state where the control circuit operates differently than in steady-state, with modified signal levels that ensure stable voltage rise. After startup completion, the system transitions to normal operation mode.
2Object-affected harmful factors
If the oscillation frequency is reduced to prevent audible noise, then the switching frequency decreases, but the output voltage rise performance deteriorates
Solution Approach 1:
The system applies preliminary action by setting the initial state before normal operation, which modifies the control signals during startup. This preliminary state configuration allows the system to achieve stable voltage rise without requiring frequency reduction, thereby preventing audible noise while maintaining fast startup performance.
Solution Approach 2:
The initial state setting circuit changes the parameter states of control signals (FB signal pulled up, CS signal pulled down) during startup. This parameter change creates a different operational regime that decouples the relationship between oscillation frequency and voltage rise performance, allowing high frequency operation without audible noise.
3Device complexity
If the control circuit operates without initial state setting, then the device complexity is reduced, but the output voltage increases step-wise causing malfunctioning
Solution Approach 1:
The initial state setting circuit integrates multiple functions into a single component: it pulls up the FB signal, pulls down the CS signal, and sets the control circuit to overload detecting state simultaneously. This multi-functional approach achieves reliable startup control without adding significant complexity to the overall system.
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 solution ensures stable startup of the switching power supply without causing power ON resetting or malfunctioning of the under-voltage locking out function, while preventing audible noise issues by maintaining a stable output voltage rise and reducing oscillation frequency effectively.
Implementation Method 1
The switching power supply main body 1 switches the input voltage Vin through a primary winding Ta of a transformer T. The switching power supply main body 1 rectifies an AC voltage generated across a secondary winding Tb of the transformer T
Implementation Method 2
The switching power supply main body 1 rectifies an AC voltage generated across a secondary winding Tb of the transformer T with diodes DS2 and DS3
Implementation Method 3
The oscillator 21 generates a signal with a triangular waveform using charging and discharging process of a capacitor (not shown in the figure) contained in the oscillator 21
Implementation Method 4
The output voltage detecting circuit 3 delivers the FB signal to the power supply IC 2 through a photo-coupler PC
Data Source
AI summary
In some aspects of the invention, a switching power supply can include a control circuit that conducts feedback control of a pulse width of a driving signal for switching the switching element according to an output voltage and settle the output voltage to a specified voltage level; a frequency reducing circuit that is disposed in the control circuit and reduces a frequency of the driving signal for switching the switching element upon detection of an overload state and limits an output current; and an initial state setting circuit that sets the control circuit to an overload detecting state for a predetermined period of time in a startup period of the control circuit. The initial state setting circuit can pull down a control voltage for a soft start signal for soft starting to the ground potential to inhibit generation of the driving signal and pulls up a feedback voltage signal.