SIMO Power Supply Soft Start Using PFM-to-PWM Transition
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing single inductor multiple output (SIMO) power supplies face high inrush current and overvoltage issues during start-up, which can damage components, and current soft start methods require additional components increasing complexity and cost.
Innovation Solution
A soft start method for SIMO power supplies that operates in pulse frequency modulation (PFM) modes before transitioning to pulse width modulation (PWM) mode, limiting inrush current by gradually ramping up the inductor current without needing additional components or clock signals, thus reducing the likelihood of overcurrent and overvoltage conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional soft start components are added to limit inrush current, then component protection is improved, but device complexity and cost increase
Solution Approach 1:
The power supply system performs soft start automatically using its existing inductor and control circuitry. The controller detects startup conditions and naturally limits inrush current through controlled switching sequences without requiring external soft start components. The system serves its own protection needs using inherent components.
Solution Approach 2:
The existing inductor, which is already required for power conversion, is made to serve dual purposes: both power storage and soft start current limiting. The control circuit uses the same switching elements for both normal operation and startup protection, eliminating the need for dedicated soft start components and reducing overall device complexity.
2Loss of energy
If PWM mode is used immediately at startup, then power efficiency at heavy loads is improved, but inrush current damage risk increases
Solution Approach 1:
Before engaging PWM mode for high-efficiency operation, the system first performs a preliminary startup sequence in PFM mode. This preliminary action gradually charges the inductor and builds output voltage at a controlled rate, preventing inrush current. Only after this preparatory phase does the system transition to PWM mode, ensuring safe startup conditions are met first.
Solution Approach 2:
The system dynamically switches between operating modes based on startup conditions. During startup, it operates in PFM mode with naturally limited current. Once output voltage reaches a threshold, it transitions to PWM mode for efficient heavy load operation. This dynamic adaptation allows the system to optimize efficiency when needed while preventing harmful inrush current through mode-dependent operation.
Data Source
Figure 1
Figure 2
Figure 3
AI summary
A method is provided for soft starting a single inductor multiple output (SIMO) power supply. The method includes selecting operation in a pulse width modulation (PWM) mode. A first pulse frequency modulation (PFM) mode is enabled to supply a first load with a first voltage and the power supply begins to ramp up the output voltage. After the output voltage has reached a desired value in the PFM mode, the PFM mode is disabled. Then, operation is enabled in the PWM mode. The SIMO power supply then supplies a current to one or more loads in the PWM mode.