Converter waveforms over a cycle.
The Basics
A quasi-resonant converter (QRC) works on the basis of using a resonant switching element. When the FET is then turned on, the element generates a resonant pulse which is then filtered by the output LC like a traditional switching converter. In a QRC, the width and amplitude of the pulse is fixed and the converter is variable frequency (i.e. send more pulses per second to provide more power). By using a resonant pulse, the switching element will naturally go into a zero current state, at which point the FET is turned off ZCS avoiding switching loss. To better understand the operation of this circuit, we can simplify our model slightly (assume output filter is large and act like a constant current source) and break up its operation into four sub-intervals.
A quasi-resonant converter (QRC) works on the basis of using a resonant switching element. When the FET is then turned on, the element generates a resonant pulse which is then filtered by the output LC like a traditional switching converter. In a QRC, the width and amplitude of the pulse is fixed and the converter is variable frequency (i.e. send more pulses per second to provide more power). By using a resonant pulse, the switching element will naturally go into a zero current state, at which point the FET is turned off ZCS avoiding switching loss. To better understand the operation of this circuit, we can simplify our model slightly (assume output filter is large and act like a constant current source) and break up its operation into four sub-intervals.
Equivalent circuit for the first interval.
Interval 1
Immediately after the FET turns on, Lr's current begins to ramp up linearly. D1 is still conducting and the resonant node is tied to ground. This initial state lasts until the current through Lr increases to the load current, causing D1 to switch off.
Immediately after the FET turns on, Lr's current begins to ramp up linearly. D1 is still conducting and the resonant node is tied to ground. This initial state lasts until the current through Lr increases to the load current, causing D1 to switch off.
Equivalent circuit for the second interval.
Interval 2
After D1 switches off, the resonant capacitor begins to charge, and the resonant circuit begins in full. The current and voltage oscillate at their natural frequency, as the current is being used to both charge the capacitor and provided to the load. This state continues until the current in Lr hits zero at which point, D2 turns off and the FET can be turned off ZCS.
After D1 switches off, the resonant capacitor begins to charge, and the resonant circuit begins in full. The current and voltage oscillate at their natural frequency, as the current is being used to both charge the capacitor and provided to the load. This state continues until the current in Lr hits zero at which point, D2 turns off and the FET can be turned off ZCS.
Equivalent circuit for the third interval.
Interval 3
In this final state of the resonant switch, the voltage falls linearly as the resonant capacitor discharges and provides the current for the load. Once the resonant node hits zero, D1 turns on and pins it to ground.
In this final state of the resonant switch, the voltage falls linearly as the resonant capacitor discharges and provides the current for the load. Once the resonant node hits zero, D1 turns on and pins it to ground.
Interval 4
This final interval provides the free variable for the converter, adjusting the time between pulses. The value for the off time is dependent on the length of the resonant pulse, the voltage you want to output, and the current being drawn (meaning the control law will be load dependent).
This final interval provides the free variable for the converter, adjusting the time between pulses. The value for the off time is dependent on the length of the resonant pulse, the voltage you want to output, and the current being drawn (meaning the control law will be load dependent).