1. Field of the Disclosure
The present invention relates generally to power supplies, and more specifically, the invention relates to power supplies that have a slow loop bandwidth.
2. Background
Power supplies are typically used to convert alternating current (“ac”) power provided by an electrical outlet into direct current (“dc”) to supply an electrical device or load. One important consideration for power supply design is the shape and phase of the input current drawn from the ac power source relative to the ac input voltage waveform. The voltage waveform of mains ac sources is nominally a sinusoid. However, due to the non-linear loading that many switching power supplies present to the ac source, the wave shape of the current drawn from the ac source by the power supply is non-sinusoidal and/or out of phase with the ac source voltage waveform. This leads to increased losses in the ac mains distribution system and, in many parts of the world, is now the subject of legislative or voluntary requirements that power supply manufacturers ensure the current drawn by the power supply is sinusoidal and in phase with the ac voltage waveform.
The correction of the input current waveform in this way is referred to as power factor correction (PFC). If the input ac current and voltage waveforms are sinusoidal and perfectly in phase, the power factor of the power supply is 1. In other words, a power factor corrected input will present a load to the ac source that is equivalent to coupling a variable resistance across the ac source. The effective resistance presented as a load to the ac source by the PFC corrected power supply is varied as a function of the rms voltage of the ac source in accordance with the power drawn by the PFC correct power supply output load. As harmonic distortion and/or phase displacement of the input current relative to the ac source voltage increase, the power factor decreases below 1. Power factor requirements typically require power factors greater than 0.9 and may have requirements for the harmonic content of the input current waveform.
Applications where switching power supplies must provide PFC include Light Emitting Diode (LED) lighting applications, which are becoming more popular due to the improved energy efficiency provided by LEDs compared to more traditional incandescent lamps. Since the brightness of light provided by LEDs is a function of the current flowing through them, the power supply also regulates the dc current provided to the LEDs, which form the output load to the power supply. The power supply control therefore combines the functions of dc output current regulation and also provides PFC by presenting a substantially resistive load to the mains ac source connected to the input of the power supply.
Output current regulation is typically achieved by sensing the current flowing in the LEDs and providing a feedback signal that is a function of the LED current to a power supply controller that regulates the flow of energy from an input to an output of the power supply. Switching power supplies will typically respond very quickly to fluctuations in current feedback information in order to regulate the LED current to be a smooth dc level.
As noted above, however, in order to achieve PFC, the power supply must present a load that is essentially resistive to the ac mains. Rapid changes in energy flow to regulate fast changes in LED current would corrupt the PFC performance and yield non-sinusoidal power supply input current waveforms and low power factor. Therefore, in order to achieve PFC, the power supply must be configured to respond slowly to fluctuations in current feedback information, which is often referred to as a slow power supply control loop or a low bandwidth loop. This slow loop functionality is normally achieved by introducing a large capacitance within the power supply control loop. The capacitance may for example be introduced at the output of the power supply to maintain a very stable dc output voltage at the output of the power supply that will tend to reduce any current fluctuations in the LED load.
In another example, the current in the LED is allowed to fluctuate but a large filter, typically comprising a large capacitance and resistance, is introduced in the feedback path between the LED current path and the power supply controller. This then filters the feedback signal such that the power supply controller is responding to a heavily filtered version of the power supply output current, which helps to prevent the controller making sudden demands for more or less energy flow from the ac mains input source.
Both of the above-described techniques to achieve PFC have the disadvantage of requiring physically large components in the power supply to slow the power supply control loop response. Typical applications for LED lights require that the power supply circuitry be as compact as possible as they often have to fit inside very small light bulb enclosures, sometimes referred to as in-bulb applications. Furthermore, large capacitors are a reliability and cost concern in such in-bulb LED lighting applications since temperatures inside the bulb are high requiring the use of expensive high temperature capacitors.