GB2584823A

GB2584823A - Power supply

Info

Publication number: GB2584823A
Application number: GB1907001.0A
Authority: GB
Inventors: Baynham Tom; Baynham Lawrence
Original assignee: INDO LIGHTING Ltd
Current assignee: INDO LIGHTING Ltd
Priority date: 2019-05-17
Filing date: 2019-05-17
Publication date: 2020-12-23
Anticipated expiration: 2039-05-17
Also published as: GB201907001D0; GB2584823B

Abstract

A power converter apparatus for converting an AC input into a DC output includes a power converter having an input circuit for receiving the AC input, one or more power switching devices to rectify the AC input and an output circuit to receive the rectified input and to provide a modulated DC output. The power converter apparatus also includes a feedback circuit for providing a feedback signal to the power converter. The feedback circuit includes a sensor for sensing the modulated DC output to generate a DC output signal and a filter to filter the DC output signal to generate a filtered DC output signal. The arrangement avoids the need for an electrolytic capacitor. The feedback signal is generated based on the filtered DC output signal. The filter may be an analogue or a digital filter. A power factor correction controller may be used to control the DC output of the converter. The power converter apparatus may be an LED driver for a plurality of LED lamps.

Description

POWER SUPPLY

TECHNICAL FIELD

The present invention relates generally to power supply apparatus and methods, including power supplies that are suitable for driving light emitting diodes (LEDs), especially (but not necessarily exclusively) arrays of LEDs used for high power LED lighting applications such as street lighting and other outdoor lighting.

BACKGROUND

AC-connected (e.g. mains connected) LED lighting is commonly used in outdoor lighting applications such as street lighting, horticulture lighting, facility lighting, flood lighting, security lighting and the like. LED lighting offers high efficacy, reliability and long life compared with other, more traditional types of light source previously used for outdoor lighting applications. This in turn translates into energy savings, maintenance savings, and environmental sustainability. There is also the potential for greater optical control (more controllable source), dimming, instant on/off, and reduced rate of lumen depreciation (potential for long application life).

A majority of conventional LED drivers for high power applications have Switch Mode Power Supply (SMPS) power converters (AC-DC) with a two-stage configuration, typically with a power factor correction (PFC) stage (e.g. a Boost or Buck PFC stage) followed by a DC-DC converter stage (e.g. Flyback or Buck converter stage). These conventional power supplies necessarily incorporate capacitors between the stages, which are typically large electrolytic capacitors. Single stage converters are also known but conventionally also include large electrolytic capacitors to minimize modulation (ripple) on the DC output, which is necessary to enable the feedback to the SMPS controller to operate correctly.

Electrolytic capacitors are known to have relatively short operating lives compared to the lifetime of LED lamps. Thus, they are recognized as a significant limiting factor in the overall lifetime and maintenance of an LED lighting installation, with the LED drivers incorporating -1 -electrolytic capacitors typically having to be replaced after 5 or 6 years of service (compared with the typical 20+ years lifetime of the LED lamps themselves). Clearly, LED drivers that avoid the need for electrolytic capacitors would be desirable.

US2017279367 describes a high-power single-stage LED driver that does not require an electrolytic capacitor. It uses an AC-DC power converter along with a second output circuit (also including power switching components) that cancels out the modulation ("ripple") in the DC output that would otherwise be present in the output from the power converter.

SUMMARY

In general, embodiments of the present invention are concerned with providing AC-DC power converter apparatus that avoids the need for electrolytic capacitors, or other relatively high capacitance capacitors whilst enabling control of the output power. The approach proposed for embodiments of the present invention is to use as single stage power converter having an AC input and a DC output that is highly modulated (at twice the mains supply frequency, typically 100Hz or 120Hz with a mains AC input), due to the absence of a large capacitor (which would typically be an electrolytic capacitor). The power output from the power converter is controlled using a feedback loop that senses the output current and/or voltage. Signals representing the sensed output current and/or voltage are processed (through the use of analogue circuitry and/or the microprocessor) before being fed back to a controller within the power converter to reduce or remove the modulation (ripple), which would otherwise interfere with the regulation of the output power.

Embodiments of the invention are particularly suited for use as LED drivers but may also have other applications.

In a first aspect, the invention provides a power converter apparatus that converts an AC input into DC output, the apparatus comprising: a power converter having an input circuit for receiving the AC input, one or more power switching devices to rectify the AC input, an output circuit to receive the rectified input and to provide a modulated DC output, and a controller to control the one or more power switching devices; and -2 -a feedback circuit for providing a feedback signal to the controller of the power converter, the feedback circuit comprising a sensor for sensing the modulated DC output to generate a modulated DC output signal and processing means to process the modulated DC output signal to generate a filtered DC output signal, the feedback signal being generated based on the filtered DC output signal.

Taking this approach, tolerating the modulated DC output (modulated current / voltage) for providing power to a load (e.g. driving an LED array), there is no need to use an electrolytic capacitor to smooth the DC output. The issue that then arises, however, is that the modulated DC output is not suitable in its raw form to provide the basis for a feedback signal to the controller within the power converter as the fluctuations in the signal would make regulation of the LED power impossible. To address this issue, the DC output signal is processed to significantly reduce or eliminate the modulation, thus providing a usable feedback signal.

The processing means that process the modulated DC output signal can take any of a number of suitable forms that operate to give the desired reduction (or elimination) of the modulation. Suitable examples include analogue filters and digital filters, e.g. digital filters within a microcontroller (that may also perform other functions).

If the power converter apparatus is to be used for an application in which the modulation of the DC output would be a problem, a capacitance (e.g. an electrolytic capacitor, or other type of capacitor) can be connected externally across the load to which the power converter is supplying power. Advantageously, such an external capacitor can be located far away from any heat generating parts such as the power supply components or the LEDs. This will help preserve the life of the capacitor. Additionally, such an external capacitor can be retrofitted to the device without any changes to the circuit of firmware.

In some embodiments, the power converter comprises: a power stage that includes the input circuit, the one or more power switching devices and the output circuit; and the controller (e.g. a power factor correction controller) to control the one or more switching devices to control the power of the DC output. The feedback signal is provided as an input to the controller.

In some embodiments, the feedback circuit comprises a microcontroller. -3 -

In some embodiments, the microcontroller takes the filtered DC output signal from analogue filters as an input, generates the feedback signal based on the received filtered DC output signal and outputs the feedback signal to the controller in the power converter.

In some embodiments, the microcontroller may itself process the raw (unfiltered) signals representing the voltage and/or current levels, the microcontroller carrying out digital filtering to determine the average current and/or voltage levels.

Whatever form of filtering is used, both average voltage and average current values are preferably obtained, and the microcontroller uses these values to calculate the actual power level.

In some embodiments, the unfiltered (modulated) signals representing the voltage and current levels are provided to the controller (along with the filtered signals in the case that analogue filters are used). The controller can then determine the actual power levels (based on average current and voltage), and also the controller can determine the level of modulation and therefore can know how much external capacitance is fitted. This is useful for: Adjusting any internal functions or calculations for improved accuracy based on the capacitance value fitted, and For monitoring the capacitance on the output and using this for predictive maintenance, as when a capacitor degrades it typically loses capacitance over time.

In some embodiments an analogue circuit is added to generate the minimum and/or average and/or maximum values from the signal representing the voltage and/or current. The microcontroller can use these signals to determine one or more of the average power level and the level of external capacitance fitted.

In some embodiments, the microcontroller is configured to generate the feedback signal based on a comparison of a value of the filtered DC output signal (preferably an actual power value calculated from the filtered voltage and current signals) with a reference value representing a desired output power level for the DC output. The reference value (which may be referred to as a target power level) can, in some examples, be a function of one or more internal settings for: -4 -constant light output (CLO) which starts the product dimmed and slowly increases the power over the product life to counter the degradation of the LED output, pre-programmed part night dimming schedule (e.g. dimming between midnight and 5am based on switching times) over temperature dimming protection, which dims the LEDs down if high temperatures are measured which could risk damaging the product In some examples the target power level can be based on an input from a DALI controller to tell the product to dim.

In some embodiments the microcontroller is configured to operate based on an iterative set and check algorithm in which the microcontroller sets a value for the feedback signal and subsequently checks the value of the filtered DC output signal. Typically, due to delays introduced by the filters, a period of time will elapse after a new value is set for the feedback signal before the filtered DC output signal settles to properly reflect the impact of the change. For this reason, the set and check algorithm cannot operate at a high frequency. Rather, the microcontroller is configured to check the value of the filtered DC output signal after a predetermined delay time has elapsed following the value of the feedback signal having been set. This gives time for the signal to settle following the change.

For example, in an embodiment using analogue filters, a delay of at least 200ms will typically be used, more preferably at least 400ms. Where digital filtering is used, the delay could be less, for example less than 100ms or, in some cases no delay would be needed at all (if the digital signal processing was sufficiently quick).

Where there is full wave rectification of the AC input, the modulated DC output will have a modulation/ripple frequency that is double the line frequency of the AC input. For example, where the AC input is connected to mains power with a 50Hz frequency, the DC modulation will be 100Hz.

Whilst a 100Hz DC modulation may be undesirable for some applications, it can be tolerated for many lighting applications, including horticulture and streetlighting for example, where it has been present for many years in older lamp technologies. -5 -

The power converter apparatus of this aspect of the invention is particularly suited to be used as an LED driver, for example as the driver for outdoor lighting apparatus such as streetlights or horticulture lights.

In a second aspect, the invention provides a method for converting AC input power into 5 DC output power, the method comprising: output; using a power converter to rectify the AC input to generate a modulated DC signal; sensing the modulated DC output to generate a sensed modulated DC output filtering the sensed DC output signal to generate a filtered DC output signal; processing the sensed modulated DC output signal (optionally using a m crocontroller) to generate a feedback signal; and providing the feedback signal to the power converter.

Embodiments of the method of this aspect can include features corresponding to all of those discussed above in the context of the first aspect, including processing of the sensed voltage and current signals in the various ways discussed above.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 schematically illustrates an LED drive circuit that incorporates an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENT

An embodiment is described below by way of example with reference to the accompanying drawing.

In general terms, embodiments of the invention provide AC to DC power converters that do not require an electrolytic capacitor (or any relatively large capacitor). This is beneficial, as follows from the discussion further above, as it extends the lifetime of the power converter. -6 -

An embodiment of the invention is illustrated below with reference to an LED driver but the skilled person will understand that the power converter apparatus can be used to supply power to other types of load.

With reference to fig. 1, the LED driver includes a single stage power factor corrected power converter, including a power stage (e.g. a flyback converter) and a power factor correction (PFC) controller that controls the power stage. The power converter has main AC power as its input and generates DC output.

Unlike conventional power converters, no electrolytic or relatively high capacitance capacitor is used within the converter. Consequently, the DC output is modulated at twice the frequency of the AC input (100Hz in this example as the AC input frequency is 50Hz).

The modulated DC output can be used to directly power the LED load (e.g. an LED array in a streetlight). If for a particular application it is desirable to remove the 100Hz light modulation, then an electrolytic capacitor can be fitted externally across the LED load, either at the point of manufacture, or can be retrofitted at a later date as required.

In this example a DC reference voltage (feedback signal) is provided from the DC output to the power converter via a microcontroller. The microcontroller receives signals representing the voltage and current of the DC output. These inputs are heavily filtered to minimize the modulation on the DC signal, which would otherwise interfere with control of the power converter and consequently with the regulation of the LED current. In this example, the filtering is achieved with analogue filters. Alternatively, the microcontroller can digitally filter the signals.

The microcontroller can then use the filtered signals to determine whether the DC output is at a desired power level and, if it is not, adjust the power level accordingly by changing the magnitude of the feedback signal.

This approach works well in an application such as an LED driver, where the load is relatively constant, and is not changing wildly like the load presented by say a Class D guitar amplifier to its power supply. The lack of load transients means that a heavily filtered sense resistor voltage is fine.

It is necessary, however, for the microcontroller to take the filtering into account when "trimming" the LED current. That is, the micro cannot conduct an "iterative set and check" LED -7 -current regulation algorithm at a high frequency. Instead the microcontroller must set the dimming level, and then wait a predetermined time for the led current reading to settle out towards the new level before re-adjusting the LED current again (if needs be). It is to be noted that the "wait" is for the sense resistor filter to catch up, not the LED current itself If the delay in this "iterative set and check" algorithm was not long enough, then the LED current would oscillate wildly and never settle out.

Regulation is achieved, as discussed above, by the microcontroller reading the average LED current and the average LED voltage in order to ascertain the LED power. The microcontroller uses a calculation algorithm to set the dimming signal to approximately the value that is required. Subsequently, to take into account factors such as variation in mains input, operating temperature etc. the microprocessor "trims" the dimming signal to achieve the required LED power. As noted above, it does this with an iterative "set and check" algorithm i.e. read the led power, and if it is too low, increase demand, and vice versa, until the required power level is met.

The skilled person will understand that various modifications and additions can be made to the example described above without departing from the spirit and scope of the present invention. -8 -

Claims

CLAIMS: 1. A power converter apparatus that converts an AC input into DC output, the apparatus comprising: a power converter having an input circuit for receiving the AC input, one or more power switching devices to rectify the AC input and an output circuit to receive the rectified input and to provide a modulated DC output; and a feedback circuit for providing a feedback signal to the power converter, the feedback circuit comprising a sensor for sensing the modulated DC output to generate a DC output signal and processing means to process the DC output signal to generate a filtered DC output signal, the feedback signal being generated based on the filtered DC output signal.
2. The power converter apparatus of claim 1, wherein the processing means comprises one or more analogue filters.
The power converter apparatus of claim 1 or claim 2, wherein the processing means comprises one or more digital filters.
4. The power converter apparatus of any one of the preceding claims, wherein the power converter comprises: a power stage that includes the input circuit, the one or more power switching devices and the output circuit; and a controller to control the one or more switching devices to control the power of the DC output; wherein the feedback signal is provided as an input to the controller.
The power converter apparatus of claim, wherein the controller is a power factor correction controller. -9 -
The power converter apparatus of any one of the preceding claims, wherein the feedback circuit comprises a microcontroller, the microcontroller including an input, and an output for outputting the feedback signal, the microcontroller being configured to generate the feedback signal based on the filtered DC output signal.
The power converter apparatus of claim 6, wherein the microcontroller input is configured to receive a filtered DC output signal from analogue filters.
8. The power converter apparatus of claim 6, wherein the microcontroller input is configured to receive the modulated DC output signal and the microprocessor is configured to digitally filter the modulated DC output signal to generate the filtered DC output signal.
9. The power converter apparatus of any one of clams 6 to 8, wherein the microcontroller is configured to generate the feedback signal based on a comparison of a value of the filtered DC output signal with a reference value representing a desired output power level for the DC output.
10. The power converter apparatus of claim 9, wherein the filtered DC output signal includes average voltage and average current values that are used by the microprocessor to calculate an actual power value for comparison with the desired output power level.
11. The power converter apparatus of any one of claims 6 to 10, wherein the microcontroller is configured to operate based on an iterative set and check algorithm in which the microcontroller sets a value for the feedback signal and subsequently checks the value of the filtered DC output signal.
12. The power converter apparatus of claim 11, wherein the microcontroller is configured to check the value of the filtered DC output signal after a predetermined delay time has elapsed following the value of the feedback signal having been set.-10 -13. 14. 15. 16. 17. 18. 19. 20.
The power converter apparatus of any one of the preceding claims, wherein the modulated DC output has a ripple frequency that is double a line frequency of the AC input.
The power converter apparatus of any one of the preceding claims, wherein the modulated DC output has a ripple frequency that is double the line frequency of the AC input.
The power converter apparatus of any one of the preceding claims, wherein the power converter apparatus is an LED driver apparatus.
A lighting apparatus comprising a plurality of LED lamps and an LED driver apparatus according to claim 15.
The lighting apparatus of claim 16, wherein the lighting apparatus is a streetlight.
A method for converting AC input power into DC output power, the method comprising: using a power converter to rectify the AC input to generate a modulated DC output; sensing the modulated DC output to generate a sensed DC output signal; processing the sensed DC output signal to generate a filtered DC output signal; generating a feedback signal based on the filtered DC output signal; and providing the feedback signal to the power converter.
The method of claim 18, wherein the power converter comprises a controller to control the power of the DC output and the feedback signal is provided as an input to the controller.
The method of claim 19, wherein the controller is a power factor correction controller.
-11 - 21. The method of any one of claims 18 to 20, wherein the feedback signal is generated by a microcontroller, the microcontroller including an input for receiving the filtered DC output signal and an output for outputting the feedback signal.
22. The method of claim 21, wherein the microcontroller generates the feedback signal based on a comparison of a value of the filtered DC output signal with a reference value representing a desired output power level for the DC output.
23. The method of claim 21 or claim 22, wherein the microcontroller operates an iterative set and check algorithm in which the microcontroller sets a value for the feedback signal and subsequently checks the value of the filtered DC output signal.
24. The method of claim 23, wherein the microcontroller checks the value of the filtered DC output signal after a predetermined delay time has elapsed following the value of the feedback signal having been set.
25. The method of any one of claims 18 to 24, wherein the modulated DC output has a ripple frequency that is double a line frequency of the AC input.-12 -