ZVS Assist Circuit for Series Resonant Converters
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Solution Overview
Problem
Series resonant converters (SRCs) with fixed switching frequency struggle to achieve zero voltage switching (ZVS) across the entire range of load and input voltage, leading to inefficiencies and increased electromagnetic interference (EMI).
Innovation Solution
A ZVS assist circuit is introduced, comprising a ZVS inductance, switches, and diodes, connected between the switching node and the endpoint of the converter, which allows current through the inductance to change the voltage conditions for zero voltage switching of the switches in the switching leg, using a transformer and leakage inductance to reduce voltage stress and EMI.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If fixed switching frequency series resonant converters are used, then component count is reduced, but zero voltage switching cannot be achieved across entire load and input voltage range
Solution Approach 1:
The ZVS assist circuit performs preliminary action by pre-charging or pre-discharging the parasitic capacitance of the main switches before the main switching event occurs. This is achieved through auxiliary switches and capacitors that prepare the voltage conditions in advance, enabling ZVS across the entire operating range without requiring complex resonant frequency adjustment.
Solution Approach 2:
The patent introduces ZVS assist circuits as intermediary components between the main power switches and the resonant tank. These assist circuits, comprising auxiliary switches, capacitors, and inductors, mediate the voltage transition process to ensure zero voltage conditions are met before main switch turn-on, thereby extending ZVS capability across full load range while maintaining simple fixed-frequency operation.
2Device complexity
If conventional switching is used without ZVS assist circuit, then device complexity is low, but switching losses and EMI increase
Solution Approach 1:
The patent converts the harmful effect of parasitic capacitance in main switches into a beneficial element for achieving ZVS. The assist circuits utilize the inherent parasitic capacitance and resonate it with auxiliary inductors to create soft-switching conditions, thereby converting what would normally be a source of switching losses and EMI into the mechanism that enables lossless switching.
Solution Approach 2:
The ZVS assist circuits employ resonant oscillation at the switching frequency to create voltage conditions favorable for zero voltage switching. By introducing auxiliary inductors and capacitors that resonate with the parasitic elements, the circuit creates controlled voltage oscillations that ensure the main switches turn on at zero voltage, minimizing switching losses and EMI.
3Object-affected harmful factors
If ZVS assist circuit is added to enable soft switching, then switching losses and EMI are reduced, but device complexity increases
Solution Approach 1:
The patent applies local quality by implementing ZVS assist circuits only for the specific switching nodes that require soft switching, rather than redesigning the entire converter topology. The assist circuits are localized to individual switch nodes, using targeted auxiliary components to achieve ZVS only where needed, thereby minimizing overall circuit complexity while effectively reducing EMI and switching losses.
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
The ZVS assist circuit enables efficient zero voltage switching across the entire load range, reducing switching losses and EMI, while allowing the use of lower-cost silicon-based MOSFETs and Schottky diodes to minimize reverse recovery effects.
Implementation Method 1
a ZVS inductance, a first ZVS switch that allows current through the ZVS inductance of the ZVS assist circuit to change a voltage of the switching node to a condition for zero voltage switching of the first switch
Implementation Method 2
a ZVS capacitor connected in series with the ZVS inductance
Implementation Method 3
using a transformer and leakage inductance to reduce voltage stress and EMI
Data Source
AI summary
An apparatus for zero voltage switching includes a ZVS assist circuit connected between a switching node and a negative connection of a converter. The switching node is located between first and second switches of a switching leg of the converter. The converter is fed by a constant current source and feeds a constant current load. The ZVS assist circuit includes a ZVS inductance, a first ZVS switch that allows current through the ZVS inductance to change a voltage of the switching node to a condition for zero voltage switching of the first switch of the switching leg, and a second ZVS switch that allows current through the ZVS inductance to change the voltage of the switching node to a condition for zero voltage switching of the second switch of the switching leg. Current through the first ZVS switch is opposite current through the second ZVS switch.