ECEN5817, ECEE Department, University of Colorado at BoulderECEN 5817119.3 Soft switching in resonant convertersSoft switching can mitigate some of the mechanisms of switching loss and possibly reduce the generation of EMILosses due to high voltage and high current present in switch during transitions, e.g. due to diode reverse recoveryLosses due to shorting device capacitancesSemiconductor devices are switched on or off at the zero crossing of their voltage or current waveforms:Zero-current switching: transistor turn-off transition occurs at zero current. Zero-current switching eliminates the switching loss caused by IGBT current tailing and by stray inductances. It can also be used to commutate SCR’s.Zero-voltage switching: transistor turn-on transition occurs at zero voltage. Diodes may also operate with zero-voltage switching. Zero-voltage switching eliminates the switching loss induced by diode stored charge and device output capacitances.Zero-voltage switching is usually preferred in converters based on MOSFETsECEN 5817219.3.1 Operation of the full bridge below resonance: Zero-current switchingSeries resonant converter exampleL+–VgCQ1Q2Q3Q4D1D2D3D4+vs(t)–is(t)+vds1(t)–iQ1(t)Current bi-directional switchesZCS vs. ZVS depends on tank current zero crossings with respect to transistor switching times = tank voltage zero crossingsOperation below resonance: input tank current leads voltageZero-current switching (ZCS) occursECEN5817, ECEE Department, University of Colorado at BoulderECEN 58173Tank input impedance1ωCRe|| Zi ||f0ωLR0Qe = R0 /ReOperation below resonance: tank input impedance Ziis dominated by tank capacitor.∠Ziis negative, and tank input current leads tank input voltage.Zero crossing of the tank input current waveform is(t) occurs before the zero crossing of the voltage vs(t) – before switch transitionsECEN 58174Switch network waveforms, below resonanceZero-current switchingL CQ1Q2Q3Q4D1D2D3D4+vs(t)–is(t)+vds1(t)–iQ1(t)Conduction sequence: Q1–D1–Q2–D2Q1is turned off during D1conduction interval, without loss (same for Q4/D4)Q2is turned off during D2conduction interval, without loss (same for Q3/D3)ECEN5817, ECEE Department, University of Colorado at BoulderECEN 58175Turn-on transitions: significant switching lossesL CQ1Q2Q3Q4D1D2D3D4+vs(t)–is(t)+vds1(t)–iQ1(t)Q1turns on while D2is conducting. Stored charge of D2and of semiconductor output capacitances must be removed. Transistor turn-on transition is identical to hard-switched PWM, and switching loss occurs.ECEN 58176More on Diode Stored Charge and Reverse RecoveryTypical test circuit and parameter definitions in diode data sheetsECEN5817, ECEE Department, University of Colorado at BoulderECEN 58177Diode stored charge during recoveryECEN 58178“Snappy” and “soft-recovery” diodesAlso see textbook Section 4.3.2 and HW 1 problem 3 solutionECEN5817, ECEE Department, University of Colorado at BoulderECEN 58179ExampleReverse recovery time trr, maximum reverse recovery current IRRM, and reverse recovery charge Qrrdepend on diode forward current IFprior to turn off, rate of current decay dif/dt, and junction temperature TJDiode in IRGP50B60 (IGBT+diode): ultra-fast, “soft recovery”ECEN 581710Review of HW1 problem 3 examplegsgttTs+–DitidsMMsDsLsidiL = I+V_+vds_iLsitsg gsLCECEN5817, ECEE Department, University of Colorado at BoulderECEN 58171119.3.2 Operation of the full bridge above resonance: zero-voltage switchingSeries resonant converter exampleL+–VgCQ1Q2Q3Q4D1D2D3D4+vs(t)–is(t)+vds1(t)–iQ1(t)Operation above resonance: input tank current lags voltageZero-voltage switching (ZVS) occursECEN 581712Tank input impedance1ωCRe|| Zi ||f0ωLR0Qe = R0 /ReOperation above resonance: tank input impedance Ziis dominated by tank inductor.∠Ziis positive, and tank input current lags tank input voltage.Zero crossing of the tank input current waveform is(t) occurs after the zero crossing of the voltage vs(t) –after switch transitionsECEN5817, ECEE Department, University of Colorado at BoulderECEN 581713Switch network waveforms, above resonanceZero-voltage switchingL CQ1Q2Q3Q4D1D2D3D4+vs(t)–is(t)+vds1(t)–iQ1(t)Conduction sequence: D1–Q1–D2–Q2Q1is turned on during D1conduction interval, without lossZVS turn-on transitions for all transistorsTurn-off transitions are at non-zero currentECEN 581714Turn-off transition at non-zero current: hard switching?L CQ1Q2Q3Q4D1D2D3D4+vs(t)–is(t)+vds1(t)–iQ1(t)When Q1turns off, D2must begin conducting. Voltage across Q1must increase to Vg. Transistor turn-off transition is identical to hard-switched PWM. Switching loss may occur, but …ECEN5817, ECEE Department, University of Colorado at BoulderECEN 581715Soft switching at the turn-off transitionL+–VgQ1Q2Q3Q4D1D2D3D4+vs(t)–is(t)+vds1(t)–to remainderof converterClegClegClegClegConductingdevices:tVgvds1(t)Q1Q4D2D3Turn offQ1, Q4CommutationintervalX• Introduce small capacitors Clegacross each device (or use device output capacitances).• Introduce delay between turn-off of Q1and turn-on of Q2.Tank current is(t) charges and discharges Cleg. Turn-off transition becomes lossless. During commutation interval, no devices conduct.So zero-voltage switching exhibits low switching loss: losses due to diode stored charge and device output capacitances are eliminated.Note: with IGBTs, substantial Clegmay be requiredECEN 581716Resonant Inverter DesignApplications:Resonant inverters for lamp ballastsDC-AC side of resonant DC-DC convertersResonant inverter design objectives:1. Operate with a specified load characteristic and range of operating points• With a nonlinear load, must properly match inverter output characteristic to load characteristic2. Obtain zero-voltage switching or zero-current switching• Preferably, obtain these properties at all loads• Could allow ZVS property to be lost at light load, if necessary3. Minimize transistor currents and conduction losses• To obtain good efficiency at light load, the transistor current should scale proportionally to load current (in resonant converters, it often doesn’t!)ECEN5817, ECEE Department, University of Colorado at BoulderECEN 58171719.4 Load-dependent properties of resonant convertersECEN 581718Inverter output characteristicsECEN5817, ECEE Department, University of Colorado at BoulderECEN 581719Inverter output characteristicsECEN 581720Inverter output characteristicsThis result is valid provided that (i) the resonant network