Copyright Cirrus Logic, Inc. 2013(All Rights Reserved)

Cirrus Logic, Inc.http://www.cirrus.com

Application Note

Single Stage Output Ripple Current and theEffect on Load Current in an LED Driver

1. Introduction

LED lighting requires a switched-mode power supply (LED driver) to provide the low-voltage DC required for theLEDs from the AC mains supply. Figure 1 illustrates the sinusoidal AC line voltage pulse width modulated to a lowvoltage for the output of the LED driver. To determine the amount of output ripple in a given design, a thorough un-derstanding of the physics behind the PFC converter and the very nature of sinusoidal current in the AC line is re-quired. To approach the output ripple voltage analysis, first assume that the power factor of the converter is equalto 1. Second, assume that there is enough output capacitance to maintain the operation of the LEDs above the for-ward voltage of the LED string and that the output voltage of the converter is a DC voltage.

This application note details the interaction of the output capacitor with AC line voltage and the effect of this on out-put current ripple. The design example (see section “Design Example” on page 3) demonstrates the relationship be-tween a specified ripple current and the RC filter circuit. The CS1615 example (see section “CS1615 Example” onpage 5) uses the Cirrus Logic CRD1615-8W reference design to illustrate the attenuation of the output current ripple.See the CS1615/16 Single Stage Dimmable Offline AC/DC Controller for LED Lamps data sheet for more detailsabout the CS1615/16 IC. See the CRD1615-8W 8 Watt Reference Design and CRD1616-8W 8 Watt Reference De-sign data sheets for more details regarding the reference designs. For more information, see section “References”on page 6.

LED

LightingCOUT

AC/DCLED

DriverAC Line

Voltage

IOUT

Figure 1. Rectified AC on Output

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2. Design Process

Since the assumption is that the power factor is equal to 1 and the current obtained from the AC line is sinusoidal,then the troughs of a rectified sinusoid line voltage deliver a current to the load that is also in a trough.

2.1 Design ConsiderationsThe resulting current delivery to the LED load is equivalent to a [sin(t)]2 function. The squared sine is trans-formed into a double angle equivalent using the trigonometric identity in Equation 1:

The power-reduction function is partitioned into two components: a DC component, and an AC component attwo times the line frequency. The amplitude of the AC component is equal to the amplitude of the DC component(see Figure 2). Analysis indicates that a relationship exists between the RMS current and the DC current of theoutput current IOUT. The RMS of the output current IRMS is calculated using Equation 2:

Equation 2 can be used to identify the DC portion, current IDC, of the output current, which is a sine wave witha DC offset. Equation 3 defines the DC current to be approximately 82% of the RMS value of the outputcurrent IOUT.

For the purpose of determining the ripple on the load current ILED flowing through the LEDs, only the AC com-ponent is of interest. The AC component of the output current IOUT has an amplitude equal to the amplitude ofthe DC current IDC and must be reduced by placing additional capacitance COUT on the output of the converter.

t 2sin

1 2t cos–2

---------------------------------- 12---

12--- 2t cos–= = [Eq. 1]

IRMS12--- 1

2--- 2t cos–

RMS

12--- 2 1

2--- 1

2--- 2

+ 0.61= = = [Eq. 2]

IDC

IRMS------------ 0.5

0.61----------- 0.82= = [Eq. 3]

IDC

t

IDC

t

COUT RLED ILEDIOUT

+

-

IOUT

IPK

ILED

Figure 2. Model of LED Driver Output Circuit

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For a given capacitance COUT and resistance RLED, the amount of attenuation can be calculated by recognizingthat the output stage can be modeled as an ideal AC current source with an RC filter for the output stage.Equation 4 defines the transfer function of the RC network:

Since the AC line voltage is rectified, creating a ripple current with a frequency that is double the line frequency,the RC filter in Equation 4 is evaluated at two times the line frequency fline:

To determine the required output capacitance that provides a specified output ripple current, the output charac-teristics of the LED string must be understood at the full current operating point. To calculate the attenuation ofthe RC circuit with a chosen LED resistive load, the magnitude of Equation 5 needs to be determined:

Solving for capacitance COUT using Equation 7:

The important parameters for capacitor COUT are the RMS current rating and voltage rating. For a single stageLED driver, it is probable for high RMS currents to flow into the RC filter. As a result, the output capacitor COUTmay need to be selected for its RMS current rating, as over-current stress can degrade the output capacitor’sreliability. Although the voltage rating is directly related to the RMS current rating, verify that the output capacitorCOUT is rated for an output voltage plus some guard band.

2.2 Design ExampleThe LED driver’s input AC line voltage is 120VAC with a line frequency fline equal to 60Hz. The RC filter transferfunction defined in Equation 5 on page 3 is evaluated at two times the line frequency fline:

For a design specification of 250mA RMS output current IRMS, there is a DC component and a peak sinusoidalcomponent. Since the amplitude of the DC current IDC is equal to the amplitude of the sinusoidal peakcurrent IPK, Equation 3 on page 2 is used to calculate the peak output current IPK:

H 11 jRC+------------------------= [Eq. 4]

H 4fl ine 11 j 4fline RC+-------------------------------------------= [Eq. 5]

H 4fl ine 11 j 4fl ine RC+-------------------------------------------

1

1 4fl ine RLED COUT 2+---------------------------------------------------------------------------------= =

[Eq. 6]

COUT

1

H 4fl ine 2-------------------------------- 1–

4fl ine RLED----------------------------------------------------=

[Eq. 7]

H 4fl ine 11 j 4fl ine RC+------------------------------------------- 1

1 j 754 RC+-----------------------------------= = [Eq. 8]

IPK 0.82= IRMS 0.82 250mA 205mA= = [Eq. 9]

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For the design example, the maximum ripple current Iripple specification is ±40% on the RMS output current of250mA, which is equal to 100mA peak. Equation 10 derives the attenuation factor required by the RC filter toreduce the output peak current IPK to the specified maximum ripple peak current of 100mA:

To determine the required output capacitance COUT for the RC filter, the forward current versus forward voltagecharacteristics of an LED light engine using ten Philips LUMILEDS DS61 LED assembled in series is required.For example, assume that the full-scale output current of the converter is 250mA. The chart plotted in Figure 3can be used to estimate the effective resistance of one LED by creating the tangent at the operating point.Therefore, at the intersection of the operating point of 250mA and the LED current-to-voltage curve, the esti-mated effective resistance is approximately 0.46.

It should be noted that the forward voltage of an LED drops with temperature, which will affect the final ripplecurrent. Since the total output resistance for the LED string is approximately 4.6, Equation 7 on page 3 is usedto calculate the required output capacitance COUT:

2.3 CS1615 ExampleThe CRD1615-8W is a 120VAC, 60Hz LED driver with an output voltage of 27.9V and an output current of250mA. In the design of the CRD1615-8W, the output capacitor was selected as a standard value of 680F.

Iripple

IPK--------------- 100mA

205mA------------------- 0.49= = [Eq. 10]

0

100

200

300

400

2.4 2.45 2.5 2.55 2.6 2.65 2.7 2.75 2.8 2.85

Forw

ard

Cur

rent

(mA)

Forward Voltage (V)

250 mA

DimmeredgeTrailing-

Leading edge-Dimmer

Effective Resistance0.46 ~

Figure 3. Effective Resistance of a DS61 LED

COUT

1

H 4fl ine 2-------------------------------- 1–

754 RLED----------------------------------------------------

1

0.492

-------------- 1–

754 4.6----------------------------- 513F= = =

[Eq. 11]

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Equation 6 on page 3 is used to calculate the attenuation factor:

Since the RC network attenuates the sinusoidal component by 40%, the resulting AC current in the LEDs is cal-culated using Equation 10 on page 4:

Therefore the LED string current ILED is a 164mA peak-to-peak sinusoidal on a DC offset. For this design, thetypical current ripple is approximately ±33% on the 250mA RMS output current.

2.3.1 Measured ResultsThe measured output voltage of the CRD1615-8W is 28V at an RMS output current of 250mA. The switchingmode power supply is driving 10 Philips LUMILEDS DS61 LEDs in series. Figure 4 illustrates the attenuation ofthe RC filter implemented in the CRD1615-8W. The final ripple current on the LED current, I_LED, is measuredat approximately 170mA peak-to-peak.

H 4fl ine 1

1 754 RLED COUT 2+------------------------------------------------------------------------- 1

1 754 4.5 680F 2+--------------------------------------------------------------------------- 1

2.5--------= = = [Eq. 12]

Iripple IPK 0.4 205mA 0.4 82mA= = = [Eq. 13]

Figure 4. CRD1615-8W Measured Results

Notes: The current to voltage relationship is 100mA:20mVCurrent I_LED is representative of the current flowing through the LED string

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3. Summary

As a starting point, calculate the RC filter attenuation factor to reduce the ripple current before selecting the finaloutput capacitor. The variations in system components, specifically the LED load, determine the final LED driverspecifications and tolerances. Designers need to be aware that they have to account for this and use analytical andempirical evaluation to ensure that their designs meet specification.

4. Appendix

4.1 References• Cirrus Logic, 2013. “CS1615/16 Single Stage Dimmable Offline AC/DC Controller for LED Lamps,”

DS961F1, JUL 2013.

• Cirrus Logic, 2013. “AN375 Design Guide for a CS1615 and CS1616 Single Stage Dimmable Offline AC/DC Controller for LED Lamps,” AN375REV4, JUL 2013.

• Cirrus Logic, 2013. “CRD1615-8W 8 Watt Reference Design,” DS1002RD3, JUL 2013.

• Philips Lumileds, 2012. Luxeon Rebel ES. [online] Philips. Available at <http://pdf.directindustry.com/pdf/philips-lumileds-lighting-company/technical-datasheet-ds61/Show/30998-177894-_16.html> [Accessed 13 February 2013]

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4.2 CRD1615-8W Schematic

Figure 5. CRD1615-8W Schematic

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Revision History

Revision Date Changes

REV1 APR 2013 Initial release.

REV2 JUL 2013 Content updated using PCBA Rev C.

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