Low-Voltage Startup Circuit for Threshold-Controlled Switching Converters
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Solution Overview
Problem
Conventional switching converters face challenges in starting up normally when the input voltage is low due to parasitic capacitance and threshold voltage limitations, leading to inadequate low-voltage startup capability.
Innovation Solution
A low-voltage startup circuit that includes a comparator and a substrate voltage control module to adjust the substrate voltage of the main switching transistor based on input voltage comparisons, reducing the turn-on threshold voltage and ensuring normal operation at low input voltages, and an operational amplifier to clamp the substrate voltage within a safe range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the conventional switching converter uses a main switching transistor with parasitic capacitance and threshold voltage, then the transistor can be manufactured with standard processes, but the startup voltage is limited and the transistor cannot turn on normally when input voltage is low
Solution Approach 1:
The substrate voltage control module performs preliminary action by adjusting the substrate voltage of the main switching transistor before the transistor is turned on. When the input voltage is low, the module pre-adjusts the substrate voltage to a first voltage level, which reduces the turn-on threshold voltage of the transistor in advance, enabling it to turn on normally despite the low input voltage condition.
Solution Approach 2:
The invention changes the substrate voltage parameter of the main switching transistor dynamically. The substrate voltage control module adjusts the substrate voltage between a first voltage (when input voltage is low) and a second voltage (when input voltage is normal), thereby changing the turn-on threshold voltage parameter of the transistor to adapt to different input voltage conditions.
2Reliability
If the substrate voltage is increased to reduce turn-on threshold voltage, then low-voltage startup capability is improved, but the transistor may enter latch-up state
Solution Approach 1:
The substrate voltage control module uses feedback from the input voltage detection to adjust the substrate voltage. When the input voltage is detected to be low, the module increases the substrate voltage to reduce the turn-on threshold. When the input voltage returns to normal, the module decreases the substrate voltage back to the second level, preventing the latch-up effect through continuous monitoring and adjustment.
Solution Approach 2:
The substrate voltage control module makes the substrate voltage dynamic rather than fixed. It continuously adjusts the substrate voltage between two levels based on the input voltage condition, making the transistor's threshold voltage adaptive to the operating conditions, thereby avoiding static voltage-induced latch-up problems.
Data Source
AI summary
A switching converter and a low-voltage startup circuit is provided. The low-voltage startup circuit includes a comparator and a substrate voltage control module. The comparator performs a comparison between an input voltage and a reference voltage and obtain a voltage detection signal according to a result of the comparison. The substrate voltage control module adjusts a substrate voltage of a main switching transistor according to the voltage detection signal, wherein when the voltage detection signal indicates that the input voltage is lower than/equal to the reference voltage, the substrate voltage control module increases the substrate voltage of the main switching transistor, and then a turn-on threshold voltage of the main switching transistor can be reduced, so that the main switching transistor may be normally turned on when the input voltage is low, and the low-voltage startup capability of the switching converter is improved.