Dual Capacitor Resonant Circuit

This invention proposes a dual-capacitor resonant circuit. A resonant capacitor is connected in series with a trnsformer-primary winding, and is referred to as ‘series-resonant capacitor.’ A second resonant capacitor is connected in paralle…

This invention proposes a dual-capacitor resonant circuit. A resonant capacitor is connected in series with a trnsformer-primary winding, and is referred to as a ‘series-resonant capacitor.’ A second resonant capacitor is connected in parallel with the transformer's secondary winding, and is referred to as a ‘shunt-resonant capacitor.’ The series-resonant capacitor stores adaptive resonant energy dependent on the input current (converter loading) and the shunt-resonant capacitor stores fixed resonant energy under all operating conditions. The shunt-resonant capacitor is designed to hold only a fraction of its rated resonant energy and is also used as a design parameter to adjust the overall resonant impedance of the L-C resonant circuit.

Abstract

In existing converters, either a shunt-resonant capacitor or a series resonant capacitor is used.

Using only shunt-resonant capacitors results in the following challenges:

  • A dedicated charging interval is required in every switching half-cycle, which does not contribute towards energy transfer and results in duty-cycle loss;
  • A shunt-resonant capacitor is designed only to hold resonant energy sufficient for its rated current condition. Therefore, the resonant energy is fixed for all loading conditions;
  • At reduced loading, reduced resonant energy is sufficient but the shunt configuration has no way to control resonant energy. A converter with two additional MOSFETs can help, but this arrangement leads to increased losses and is also more expensive;
  • At reduced loading, the duty-cycle loss increases significantly because the reduced current results in longer capacitor charging time. This severely restricts the operation range of the converter;
  • Smooth current commutation and ZCS are lost at overload conditions since the capacitor is designed for its rated-current condition;
  • The shunt-resonant capacitor is expected to hold its voltage/energy during the operating mode while the input inductor charges. However, a leakage path exists through the transformer winding parasitics, which can result in capacitor discharge.
  • The capacitor energy must be overrated to compensate for this loss, which further aggravates all the aforementioned issues.

Using only series-resonant capacitors results in the following challenges:

  • Precise control of resonant energy can only be achieved only by using additional switches, such as two additional reverse-blocking (RB) switches. However, this arrangement leads to increased losses and is also more expensive;
  • To satisfy the resonant condition, the series-resonant capacitor must be charged to a voltage higher than the reflected voltage across the transformer-primary;
  • The peak voltage rating of primary-side components (e.g. the switches and input inductor) is increased;
  • Series-capacitors also transfer energy to the output during the time interval when the resonant current commutation occurs. Therefore, the capacitor rating must be higher
  • At reduced loading, the capacitor will not have enough voltage across it to satisfy the resonant condition.
  • One potential option uses switching frequency as an additional control parameter without using extra switches. However, a reduction in switching frequency results in increased charging time and hence, higher voltage. However, ripple content increases and requires a larger filter due to varying switching frequencies.

This invention achieves a balance between the benefits and drawbacks of series and shunt-resonant capacitor configurations.

Advantages

  • Adaptive resonant energy is realized without requiring additional switches or variations in switching frequency. This results in reduced cost, losses, and filtering
  • As the shunt-resonant capacitor stores only a fraction of rated-resonant energy, the duty-cycle loss is reduced which improves the operation range of the converter;
  • The parasitic winding capacitance can be utilized as a shunt-resonant capacitor without using additional physical capacitors since a small capacitance value will be sufficient;
  • At a reduced loading rate, the series capacitor stores less resonant energy, and the shunt capacitor mainly support the commutation process. At a higher loading rate, the series capacitor stores more resonant energy and mainly supports the commutation process;
  • Under overload conditions, smooth current commutation and zero current switching are still maintained due to the series-resonant capacitor adaptively storing higher resonant energy;
  • The series capacitor need not be charged to a voltage higher than the reflected voltage across the transformer-primary. This reduces the peak-voltage rating when compared to converters using only series capacitors.

Potential Applications

Renewable energy sources

Power Supply 

Electric vehicles

Circuits for use in industrial, commercial, medical, defense, and aerospace settings 

Contact Information

Name: Jeff Jackson

Email: jjackso6@ucsc.edu

Phone: (831) 459-3976