Synchronous Rectifier Gate Control for Forward Converter Loss Reduction
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Solution Overview
Problem
Forward converters with synchronous rectification face issues such as excessive losses due to negative gate voltage and reverse charging of parasitic capacitance, which increase power consumption and heat generation, especially in high-power DC-DC converters with multiple parallel synchronous rectifier elements.
Innovation Solution
The configuration includes a transformer with multiple coils, a primary switching element, a choke coil, a smoothing capacitor, and commutation switching elements controlled by a separate commutation switch control voltage generating circuit, which prevents reverse charging and maintains a stable gate voltage, reducing losses by eliminating unnecessary current paths and minimizing resistance-induced losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the primary switching element is turned on while the commutator-side synchronous rectifier element is still on, then the parasitic capacitance is discharged to turn off the rectifier element, but this causes reverse charging and negative gate voltage that increase power losses
Solution Approach 1:
The invention applies preliminary action by turning off the commutator-side synchronous rectifier element before the primary switching element is turned on. This is achieved by detecting the voltage at the third winding and generating a turn-off signal for the commutator-side element in advance, preventing the reverse charging and negative gate voltage problems that occur when the primary element is turned on while the commutator-side element is still on.
2Reliability
If the switching element is turned on for a longer time to sufficiently discharge the parasitic capacitance, then the commutator-side synchronous rectifier element turns off completely, but this generates reverse voltage at the third winding and increases losses
Solution Approach 1:
The invention uses feedback by continuously monitoring the voltage at the third winding and using this information to control the timing of the commutator-side synchronous rectifier element turn-off. The control circuit detects when the voltage at the third winding indicates sufficient discharge of the parasitic capacitance, and automatically generates the turn-off signal at the optimal moment, eliminating the need for extended conduction time and preventing reverse voltage generation.
3Power
If multiple synchronous rectifier elements are connected in parallel to handle high power, then the current capacity increases, but the number of current paths increases causing excessive losses
Solution Approach 1:
The invention extracts and eliminates the harmful current paths in parallel synchronous rectifier elements by implementing individual turn-off control for each commutator-side element. By detecting the voltage at the third winding and controlling each element's turn-off timing independently, the invention prevents circulating currents between parallel elements, maintaining high current capacity while minimizing losses from multiple current paths.
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 configuration effectively reduces power losses and heat generation by preventing negative gate voltage and reverse charging, enhancing the efficiency of the forward converter's synchronous rectification process.
Implementation Method 1
when the primary switching element 2 is turned off, a voltage is generated at the third winding 4c to cause a current to flow toward the gate of the commutator-side synchronous rectifier element 6
Implementation Method 2
the parasitic capacitance between the gate and source of the commutator-side synchronous rectifier element 6 is charged and the gate voltage of the commutator-side synchronous rectifier element 6 increases to turn on the commutator-side synchronous rectifier element 6
Data Source
AI summary
A commutation switch turning-on control switching element is controlled with a voltage generated at a fourth coil (auxiliary coil) of a main transformer and controls application of a control voltage to a control terminal of a commutation switching element. A commutation switch turning-off control switching element is provided which is connected to the control terminal of the commutation switching element and, when the commutation switch turning-off control switching element is turned on, controls a voltage at the control terminal of the commutation switching element to turn off the commutation switching element. A control switching element drive circuit turns on the commutation switch turning-off control switching element at a time when a primary switching element is turned on.