Totem Pole PFC Control Circuit for Low-Side Current Sensing
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Solution Overview
Problem
Conventional totem pole PFC circuits face inefficiencies due to the need for high voltage isolation devices, which increase costs, and are unable to operate in Continuous Current Mode (CCM) due to incomplete detection of inductor current, resulting in low circuit efficiency.
Innovation Solution
A constant on-time scheme is implemented in the totem pole PFC circuit, where the main switch is kept on for specific periods based on current valley detection, allowing the detecting resistor to be placed at the low side, eliminating the need for high voltage devices and enabling operation in CCM and Discontinuous Current Mode (DCM).
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a detecting resistor is coupled to high voltage terminals for current detection, then current detection is achieved, but high voltage isolation devices are required increasing circuit cost
Solution Approach 1:
Instead of detecting current at the high voltage side, the patent inverts the detection approach by placing the detecting resistor at the low voltage side (ground reference). The current detection is achieved by measuring the voltage across the resistor connected to the synchronous switch, which operates at low voltage, thereby eliminating the need for high voltage isolation devices while maintaining detection functionality.
Solution Approach 2:
The patent introduces a synchronous switch as an intermediary element that allows the detecting resistor to be placed at the low voltage side. The synchronous switch acts as a mediator that connects the high voltage inductor current path to the low voltage detection circuit, enabling current measurement without direct high voltage exposure.
2Device complexity
If a detecting resistor is placed at ground reference, then high voltage devices are eliminated, but only decreasing part of inductor current can be detected preventing CCM operation
Solution Approach 1:
The patent applies dynamic control by adjusting the turn-off timing of the synchronous switch based on the detected current valley point. By dynamically controlling when to turn off the switch during the decreasing current phase, the system can detect the complete inductor current waveform characteristics and enable CCM operation, overcoming the limitation of static detection approaches.
Solution Approach 2:
The patent implements a feedback mechanism where the detected current signal from the detecting resistor is fed back to the control circuit. This feedback allows the control circuit to identify the current valley point and adjust the switching timing accordingly, enabling accurate current measurement and CCM operation while maintaining the simple low-side detection configuration.
Data Source
AI summary
A totem pole PFC (Power Factor Correction) circuit, having: a first switch, a second switch and a control circuit. When the totem pole PFC circuit works in CCM (Continuous Current Mode), the control circuit is configured to turn on a main switch when a current detecting signal indicative of an AC input current of the totem pole PFC circuit decreases to a current valley reference signal, and keep the main switch ON for a first on-time period. When the totem pole PFC circuit works in DCM (Discontinuous Current Mode), the control circuit is configured to turn on the main switch at a valley of a switching voltage after expiry of a time delay started from when the AC input current decreases to zero, and keep the main switch ON for a second on-time period.