Current-Mode Switching Power Supply Feedback Using Lower MOSFET Sensing
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current mode control type switching power supply devices face challenges in current feedback due to delay times and noise reflection, especially when the ratio of output voltage to input voltage is small, leading to instability and inability to perform current feedback with reduced pulse width of the switched voltage.
Innovation Solution
A current mode control type switching power supply device with a controller that senses current flowing in a second switch and controls both switches based on the sensed current, using a current sensing portion and a slope circuit to accumulate and reflect current information, reducing delay and noise impact, and generating a slope voltage that allows current feedback even with small output voltage ratios.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If current sensing is performed on the upper MOS transistor Q1 to achieve current mode control, then current feedback can be implemented, but a delay time occurs after the upper MOS transistor is turned on until current information is transmitted to the slope voltage
Solution Approach 1:
The patent introduces a current sensing circuit that senses current flowing in the lower MOS transistor Q2 instead of the upper MOS transistor Q1. This lower transistor serves as an intermediary element whose current sensing provides immediate feedback information to the slope circuit without the delay associated with upper transistor sensing, as the lower transistor current directly reflects the switch current during the on-state.
2Reliability
If current sensing is performed on the upper MOS transistor Q1, then current mode control can be achieved, but noise on the leading edge of the switched voltage is transmitted and reflected on the slope voltage
Solution Approach 1:
The patent uses the lower MOS transistor Q2 as an intermediary sensing point that is electrically isolated from the noise-prone switched voltage node. By sensing current in the lower transistor rather than directly at the switched voltage node, the system obtains current information without picking up the high-frequency noise that appears on the leading edge of the switched voltage.
3Speed
If the pulse width of the switched voltage is decreased to improve response speed, then faster response is achieved, but delay time and noise become dominant and current feedback cannot be performed
Solution Approach 1:
The patent implements preliminary current sensing by detecting the current in the lower MOS transistor Q2 during its on-state before the upper transistor turns off. This preliminary action captures the current information at the optimal moment, ensuring that feedback is established before the pulse width becomes too short to accommodate the sensing and processing delay.
Data Source
AI summary
A current mode control type switching power supply device includes a first switch having a first terminal connected to a first application terminal to which an input voltage is applied, and a second switch having a first terminal connected to a second terminal of the first switch and a second terminal connected to a second application terminal to which a predetermined voltage lower than the input voltage is applied. A current sensor is configured to sense current flowing in the second switch. A controller configured to control the first switch and the second switch, wherein the controller is configured to control the first switch and the second switch independently of a difference between the input voltage and an output voltage and in addition in accordance with the current sensed by the current sensor.