Patent Application
20130343098
This disclosure generally relates to power converters and, more specifically, power converters including active leakage energy recovery.
Solar panels, also referred to herein as photovoltaic (PV) modules, generally output direct current (DC) electrical power. To properly couple such solar panels to an electrical grid, or otherwise provide alternating current (AC) power, the electrical power received from the solar panels is converted from DC to AC power. At least some known solar power systems use a single stage or a two-stage power converter to convert DC power to AC power. Some systems are controlled by a control system to maximize the power received from the solar panels and to convert the received DC power into AC power that complies with utility grid requirements.
However, at least some known solar power converters are relatively inefficient and/or unreliable. It is desirable for a solar power converter to operate at relatively high efficiency to capture as much energy from a PV module as possible. At least some solar power converters utilize an isolated DC/DC converter including a transformer. One of the loss factors in such converters is the energy loss associated with the leakage inductance of the converter's transformer. In some converters, the losses are proportional to the leakage inductance of the transformer. A greater leakage inductance leads to greater losses and, accordingly, to a lower total conversion efficiency. Some known designs attempt to recover the energy stored in the leakage inductance. These recovery mechanisms, however, are generally not satisfactory.
This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
One aspect of the present disclosure is a power converter assembly. The assembly includes an input for receiving a direct current (DC) power input, a flyback converter coupled to the input, and a leakage energy recovery circuit coupled to the flyback converter. The flyback converter includes a transformer having a plurality of windings. The leakage energy recovery circuit is configured to couple leakage energy from the transformer to the input in response to a voltage across at least one of the plurality of windings
Another aspect of the present disclosure is a photovoltaic (PV) power system. The PV power system includes a first converter configured to receive a direct current (DC) power input and provide a substantially DC power output. The first converter includes an input for receiving the DC power input, a flyback converter coupled to the input, and a leakage energy recovery circuit. The flyback converter includes a transformer having a primary winding, a secondary winding, and an auxiliary winding. The leakage energy recovery circuit includes a clamp circuit coupled to the primary winding of the transformer, and an auxiliary converter coupled to the auxiliary winding, the clamp circuit, and the input. The clamp circuit is configured to store leakage energy from the transformer. The auxiliary converter is configured to couple leakage energy from the clamp circuit to the input in response to a voltage across the auxiliary winding.
Yet another aspect of the present disclosure is a method of recovering transformer leakage energy in a power converter. The method includes storing transformer leakage energy in a clamp circuit, and selectively coupling the stored transformer leakage energy to an input of the power converter based on a voltage across an auxiliary winding of the transformer.
Various refinements exist of the features noted in relation to the above-mentioned aspects. Further features may also be incorporated in the above-mentioned aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above-described aspects, alone or in any combination.
Like reference symbols in the various drawings indicate like elements.
The embodiments described herein generally relate to power converters. More specifically, embodiments described herein relate to power converters including active leakage energy recovery circuits. Moreover, some embodiments described herein relate to power converters and methods of operating power converters for use with a photovoltaic (PV) power source.
Although described herein with reference to power converters for use with a PV source, the teachings of this disclosure may be utilized in power converters for any suitable use.
In this embodiment, power conversion system 100 includes a power converter 104 (sometimes referred to herein as a power converter assembly) to convert DC power received from power source 102, via an input capacitor 105, to an alternating current (AC) output. In other embodiments, power converter 104 may output DC power. This power converter 104 is a two stage power converter including a first stage 106 and a second stage 108. First stage 106 is a DC to DC power converter that receives a DC power input from power source 102 and outputs DC power to second stage 108. Second stage 108 is a DC to AC power converter (sometimes referred to as an inverter) that converts DC power received from first stage 106 to an AC power output. In other embodiments, power converter 104 may include more or fewer stages. More particularly, in some embodiments power converter 104 includes only second stage 108.
Power conversion system 100 also includes a filter 110, and a control system 112 that controls the operation of first stage 106 and second stage 108. An output 114 of power converter 104 is coupled to filter 110. In this embodiment, filter 110 is coupled to an electrical distribution network 116, such as a power grid of a utility company. Accordingly, power converter 104 may be referred to as a grid tied inverter. In other embodiments, power converter 104 may be coupled to any other suitable load.
During operation, power source 102 generates a substantially direct current (DC), and a DC voltage is generated across input capacitor 105. The DC voltage and current are supplied to power converter 104. In this embodiment, control system 112 controls first stage 106 to convert the DC voltage and current to a substantially rectified DC voltage and current. The DC voltage and current output by first stage 106 may have different characteristics than the DC voltage and current received by first stage 106. For example, the magnitude of the voltage and/or current may be different. Moreover, in this embodiment, first stage 106 is an isolated converter, which operates, among other things, to isolate power source 102 from the remainder of power conversion system 100 and electrical distribution network 116. More specifically, in this embodiment, first stage 106 is a flyback converter. The DC voltage and current output by first stage 106 are input to second stage 108, and control system 112 controls second stage 108 to produce AC voltage and current, and to adjust frequency, phase, amplitude, and/or any other characteristic of the AC voltage and current to match the electrical distribution network 116 characteristics. The adjusted AC voltage and current are transmitted to filter 110 for removing one or more undesired characteristics from the AC voltage and current, such as undesired frequency components and/or undesired voltage and/or current ripples. The filtered AC voltage and current are then supplied to electrical distribution network 116.
Converter 200 includes transformer 206 having a primary winding P1, a secondary winding S1, and an auxiliary winding P2. Primary, secondary, and auxiliary windings P1, S1, P2 are magnetically coupled together, but electrically isolated from each other. Primary winding P1 is connected to input 202 and to main switch Q1. In this embodiment, switch Q1 is a MOSFET. In other embodiments, switch Q1 may be any other suitable switch.
Converter 200 is generally operated as a flyback converter as known in the art. In general, switch Q1 is switched on and off to store and release energy in transformer 206. More specifically, when switch Q1 is closed (also referred to as switched on), current flows through primary winding P1 and energy is stored in the core (not shown in
In general, the leakage inductance of transformer 206 potentially results in a power loss described by:
Ple=½Fs*Le*Ipk2 [1]
Where “Ple” is the leakage energy that may be lost due to leakage inductance, “Fs” is the switching frequency of converter 200, “Le” is the leakage inductance of transformer 206, and “Ipk” is the peak current through the switch Q1 when it turns off.
Converter 200 includes a leakage energy recovery circuit (LERC) 208 configured to recover leakage energy to reduce losses due to leakage inductance. LERC 208 includes a clamp circuit 210, also known as an RCD snubber, and a converter circuit 212 (also sometimes referred to as an auxiliary converter). Generally, the leakage energy from transformer 206 is transferred to clamp circuit 210 when Q1 turns off to clamp the leakage energy to a voltage below the breakdown voltage of switch Q1. More specifically, the leakage energy is transferred to capacitor C3 through diode D1. In some known clamp circuits, the leakage energy is dissipated across a resistor, such as resistor R1. In converter 200, resistor R1 is not used to dissipate all of the leakage energy and may be sized to handle less power than resistors in some known clamp circuits. Moreover, in some embodiments, LERC 208 does not include resistor R1.
During operation of converter 200, capacitor C3 experiences relatively large current pulses from the leakage energy. By so connecting capacitor C3 to capacitor C1, rather than connecting capacitor C3 to ground, the voltage stress of capacitor C3 is reduced and a capacitor with a lower voltage rating may be used. In other embodiments, capacitor C3 may be connected to ground.
Converter circuit 212 is a buck converter comprising switch Q2, inductor L1, and diode D3. In other embodiments, converter circuit 212 may be any other suitable converter topology. In this embodiment, switch Q2 is a MOSFET. In other embodiments, switch Q2 may be any other suitable switch. Converter circuit 212 transfers energy from clamp circuit 210, and more specifically from clamp capacitor C3, to input 202. Transfer of energy from clamp circuit 210 to input 202 is controlled by switch Q2. When switch Q2 is conducting (also referred to as being switched on), energy from capacitor C3 is coupled to input 202. More specifically, switch Q2 couples the leakage energy stored in capacitor C3 to input capacitor C1 through inductor L1.
Control of switch Q2 (i.e., turning switch Q2 on and off) is achieved through auxiliary winding P2 on transformer 206. Switch Q2 couples leakage energy from clamp circuit 210 to input 202 in response to a voltage across auxiliary winding P2. Auxiliary winding P2 is coupled to the gate and source of switch Q2. In operation, when switch Q1 turns off, auxiliary winding P2 applies a voltage to switch Q2. The number of turns on auxiliary winding P2 is sufficient to substantially match the turn on voltage limits on the gate of Q2. Thus, when switch Q1 turns off, switch Q2 turns on. Gate resistor R2 aids in controlling the gate waveform of switch Q2.
In this embodiment, converter 200 is operated in a quasi-resonant or boundary conduction mode. In other embodiments, converter 200 is operated in any other suitable mode including, for example continuous conduction mode, discontinuous conduction mode, etc. In quasi-resonant mode, the flux in the flyback transformer 206 resets to zero at the end of every switching cycle. Accordingly, energy in transformer 206 does not accumulate as it could in some other modes of operation. The volt seconds balance timing on inductor L1 is the same as on transformer 206, but inverted. Thus, induced current through and corresponding voltage across auxiliary winding P2 is suitably timed for the turn on and off of switch Q2. Unlike some known systems, no additional control chip is needed to provide on/off timing control of Q2. The clamp voltage automatically adjusts along with operation of the flyback converter from no load to full power. If the clamp voltage is too high, converter 212 simply transfers more energy to input capacitor 202 until the voltage drops to just above the flyback voltage.
Power converters in accordance with this disclosure have reduced losses due to leakage inductance and accordingly higher efficiencies. Leakage energy which is lost and dissipated as heat in some known converters is instead recycled and returned to the input of the converter. Power converters in accordance with this disclosure may have lower losses, reduced voltage stress on components, lower operating temperature, and higher circuit reliability due to the lower stresses as compared to some known converters. Moreover, in some embodiments, the leakage energy recovery circuit does not use any separate control signals to operate, but rather naturally tracks operation of the flyback converter when the converter is operated in quasi-resonant or boundary conduction mode.
When introducing elements of the present invention or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
