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.
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.
- 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.
Renewable energy sources
Circuits for use in industrial, commercial, medical, defense, and aerospace settings
Name: Jeff Jackson
Phone: (831) 459-3976