Low-Side Transistor Isolation Biasing for Reverse Recovery Control
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Solution Overview
Problem
The reverse recovery charge in switching power conversion circuits, particularly in synchronous buck converters, significantly impacts efficiency as it cannot be effectively reduced through circuit means, becoming more critical with increasing operating frequencies.
Innovation Solution
A power conversion circuit with a low-side transistor placed in an isolation layer and a control method that involves reverse biasing the parasitic diode in the isolation layer before switching, using a third signal to manage the biasing state before the transistor turns off and on, thereby reducing reverse recovery charge.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the operating frequency of the power conversion circuit is increased, then the productivity is improved, but the reverse recovery charge impact on efficiency worsens
Solution Approach 1:
The patent applies preliminary action by reverse-biasing the parasitic diode in the isolation layer before the low-side transistor turns off. This pre-action removes minority carriers from the drift region in advance, so that when the transistor switches, there are fewer carriers to recover, thereby reducing reverse recovery charge and enabling higher operating frequencies with maintained efficiency
2Productivity
If the low-side transistor is turned off quickly, then the productivity is improved, but the minority carrier accumulation worsens
Solution Approach 1:
The patent applies preliminary anti-action by applying a reverse bias voltage to the parasitic diode through the isolation layer just before the low-side transistor turns off. This creates an opposing electric field that actively removes minority carriers from the drift region, counteracting the natural tendency of carriers to accumulate during rapid switching, thus reducing reverse recovery charge
3Loss of energy
If the isolation layer biasing is maintained during transistor on-state, then the reverse recovery charge is reduced, but the on-resistance increases
Solution Approach 1:
The patent applies periodic action by dynamically switching the biasing state of the isolation layer in synchronization with the transistor switching cycle. The isolation layer is reverse-biased only during the critical turn-off period to reduce reverse recovery charge, then the biasing is removed or reversed during the on-state to maintain low on-resistance, creating a periodic control pattern that optimizes both parameters at different times
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively minimizes reverse recovery charge, reducing on-resistance and improving conversion efficiency by managing minority carrier accumulation and recovery time.
Implementation Method 1
reverse biasing a parasitic diode formed by the isolation layer before the switching element is turned off
Implementation Method 2
the removal of the accumulated minority carriers is called reverse recovery charge
Implementation Method 3
a discharge current ID flows through the low-side transistor QL, the inductor L, and the load LD from the ground
Implementation Method 4
When the low-side transistor QL is turned off, the discharge current ID flows through the parasitic diode DP of the low-side transistor QL instead
Data Source
AI summary
A power conversion circuit includes a high-side transistor, a low-side transistor, and a driving circuit. The high-side transistor provides an input voltage to a switch node based on a first signal. The low-side transistor couples the switch node to a ground based on a second signal, and is deposited in an isolation layer. The driving circuit generates the first signal, the second signal, and the third signal, provides a third signal to the isolation layer, and generates the third signal based on the first signal and the second signal.