Q8Advanced Power Electronics

(a) Construction and Working of a ZVS Resonant Converter

A Zero-Voltage-Switching (ZVS) resonant converter is constructed using a switching bridge (commonly a half-bridge or full-bridge arrangement of MOSFET or IGBT switches, each typically paired with an anti-parallel diode and often an additional small parallel 'snubber' capacitor across the switch), connected to a resonant tank circuit (an inductor and capacitor, arranged in series, parallel, or a combination such as the widely-used LLC configuration) that in turn couples to the load, commonly through an isolation transformer in DC-DC power supply applications.

Working principle: the converter is operated at a switching frequency set (typically) above the resonant tank's natural resonant frequency, in a region where the resonant tank current lags the applied switch-bridge voltage. In this operating region, when a given switch is commanded to turn off, the resonant tank current (which was flowing through that switch) does not instantaneously stop but instead transfers to charge/discharge the small parallel capacitor across each switch in that leg — since this capacitor charging/discharging process is relatively gradual (limited by the finite resonant tank current available to charge it) compared to an idealized instantaneous switch transition, the voltage across the switch that is about to turn on rises/falls gradually, and by design reaches zero (or very close to zero) exactly by the time that switch is actually commanded to turn on. The switch's anti-parallel diode then naturally conducts briefly (carrying the resonant tank's current in the reverse direction, while holding the switch's own voltage at essentially zero), during which interval the switch itself can be turned on with essentially zero voltage across it and hence essentially zero switching loss at turn-on — this is the origin of the 'zero-voltage switching' name.

Output power/voltage regulation in a ZVS resonant converter is typically achieved by varying the switching frequency (moving further from or closer to the resonant frequency changes the tank's effective impedance and hence the power delivered to the load), rather than by conventional PWM duty-cycle variation, since maintaining the specific phase relationship needed for ZVS operation across the full required load range constrains how much freedom the duty cycle itself can be varied independently of frequency.

Dead-time requirement: to ensure the resonant tank current is genuinely given sufficient time to complete the capacitor charge/discharge transition and bring the incoming switch's voltage to zero before that switch is actually gated on, a small dead-time interval is deliberately inserted between the turn-off command to one switch in a leg and the turn-on command to its complementary switch — this dead time must be carefully tuned: too short, and the incoming switch may be turned on before its voltage has genuinely reached zero (resulting in a partial hard-switching loss, defeating the purpose of the ZVS design); too long, and the anti-parallel diode conducts for an unnecessarily extended period, increasing conduction losses in the diode and potentially allowing the resonant current to begin decaying or reversing again before the intended switch is actually turned on.

Load-range limitation of ZVS operation: a key practical limitation of basic half-bridge/full-bridge ZVS resonant converters is that true zero-voltage switching can typically only be maintained down to some minimum load current level — at very light load or no-load conditions, the resonant tank current available to charge/discharge the switch-parallel capacitors within the available dead-time interval becomes insufficient, causing the converter to lose ZVS operation and revert to conventional hard switching at light loads, precisely the operating condition where switching losses (as a fraction of the now-small delivered output power) would otherwise be most detrimental to overall light-load efficiency. This has motivated the development of more advanced resonant topologies (such as the LLC resonant converter, which adds an additional resonant inductor specifically to help maintain adequate magnetizing current for ZVS operation across a much wider load range, including light load and no-load conditions) that are now very widely used in commercial high-efficiency switch-mode power supplies precisely because they substantially extend the load range over which soft-switching benefits can be maintained.

(b) Parallel Resonant Inverter

In a parallel resonant inverter, the resonant capacitor is connected in parallel with the load (rather than in series with it, as in the series resonant configuration), with this parallel L-C-load combination driven by a current-source-like input (commonly achieved via a series inductor or a current-fed switching bridge arrangement feeding the parallel resonant tank). Because the capacitor is directly in parallel with the load, the parallel resonant configuration behaves, from the load's perspective, more like a controlled current source feeding into the resonant tank, and the voltage appearing across the load/tank rises to a resonantly-boosted value at frequencies near the tank's parallel resonant frequency — giving the parallel resonant inverter an inherent voltage step-up (gain) characteristic near resonance, which can be advantageous for applications requiring a higher output voltage than the switch/DC-link voltage would otherwise directly provide, or for loads (such as certain induction heating or high-voltage discharge lamp/ballast applications) that are naturally better matched to a current-driven, parallel-resonant excitation rather than the voltage-driven, series-resonant configuration. Similar to the series resonant case, output power control in a parallel resonant inverter is achieved primarily through switching frequency variation relative to the parallel tank's resonant frequency, with the specific voltage-gain and power-transfer characteristics differing from the series case due to the fundamentally different (parallel vs series) circuit topology and its correspondingly different frequency-response characteristic.

Behavior at light load, compared to series resonant: an important practical distinction between series and parallel resonant inverters concerns their behavior under light or no-load conditions — a series resonant circuit's current naturally tends toward zero as load resistance increases toward open-circuit (since the series tank current path is broken if no load current can flow), giving series resonant inverters a naturally self-limiting output under light load; a parallel resonant tank, by contrast, continues to circulate a substantial resonant current within the tank itself (between the parallel inductor and capacitor) even when the actual load draws very little or no current, since this circulating tank current is not directly interrupted by the load's own open-circuit condition — this means parallel resonant inverters can be subjected to significant voltage stress and continued internal power dissipation even at light or no load, a design consideration that must be carefully accounted for when selecting component voltage/current ratings and designing the light-load control strategy for a parallel-resonant-based power converter or induction heating system.