PWM Boost System Start-Up with Current Limiting
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Solution Overview
Problem
Conventional PWM boost systems face challenges in successful start-up, particularly with heavy loads, due to inrush currents and oscillation of output voltage, which prevents the DC output voltage from reaching the desired level.
Innovation Solution
Incorporation of a current limit circuit and an enabled comparator during the pre-oscillation period to control inductor current and ensure stable voltage generation, along with a start-up method that adjusts the PWM signal based on error voltage and feedback, allowing for smooth transition from pre-oscillation to PWM periods.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the PWM boost system is started with a conventional design, then the system can operate normally, but inrush current occurs during start-up and the DC output voltage oscillates and cannot reach the desired level when connected to heavy loads
Solution Approach 1:
The patent applies preliminary action by enabling the comparator during the pre-oscillation period before the main PWM operation begins. This allows the system to prepare and control the inductor current in advance, preventing inrush current from occurring when the system transitions to full operation. The comparator is enabled via an enable signal that activates it only during the pre-oscillation period, ensuring proper current limiting before the main power stage engages.
Solution Approach 2:
The patent introduces an intermediary mechanism by using the comparator as a mediator between the control system and the power stage during start-up. The comparator compares the inductor current with a reference current and generates an error signal that regulates the current through feedback to the PWM circuit. This intermediary control prevents direct inrush current by mediating the power transfer during the critical start-up transition.
2Reliability
If the PWM boost system is started with a conventional design, then the system can operate normally, but the DC output voltage oscillates and cannot reach the desired level when connected to heavy loads
Solution Approach 1:
The patent implements feedback control by using the comparator to continuously monitor the inductor current during the pre-oscillation period and feed back the error signal to the PWM circuit. This feedback mechanism ensures that the inductor current is regulated within safe limits and prevents oscillation of the output voltage. The feedback loop is active only during the critical start-up period when stability is most challenging, and is disabled once the system reaches steady state.
Solution Approach 2:
The system performs preliminary voltage regulation during the pre-oscillation period by enabling the comparator before the main PWM operation. This preliminary action establishes stable current control and prevents voltage oscillation from the outset, ensuring that when heavy loads are connected, the voltage can steadily reach the desired level without oscillating.
3Object-generated harmful factors
If the comparator is enabled during the pre-oscillation period, then inrush current is eliminated, but the system complexity increases due to additional control circuits
Solution Approach 1:
The patent applies periodic action by enabling the comparator only during the pre-oscillation period rather than continuously. The enable signal activates the comparator temporarily during start-up when it is needed, and disables it during normal operation when it is not required. This periodic activation reduces the overall complexity burden by limiting the comparator's operational window to only when necessary, rather than requiring continuous complex control.
Data Source
AI summary
A PWM (pulse width modulation) boost system includes a boost circuit, a voltage dividing circuit, a comparator, a PWM circuit, a pre-oscillator, and a current limit circuit. A start-up method of the PWM boost system includes (1) providing an error voltage; (2) generating a PWM signal according to the error voltage; (3) controlling a switch in a boost circuit with the PWM signal, so as to control an inductor current flowing through a boost inductor in the boost circuit; (4) charging a capacitor in the boost circuit with the inductor current, wherein charges stored in the capacitor define a DC output voltage; and (5) providing a feedback voltage according to the DC output voltage to adjust the error voltage.