AN50002

Automotive LED side light SEPIC DC-to-DC converter design
exampleRev. 2.0 — 10 May 2021

application note

Document information
Information

Content

Keywords

SEPIC converter, DC-to-DC, MOSFET, LED dimming

Abstract

This application note provides a general guideline of Single-Ended Primary Inductance Converter(SEPIC) DC-to-DC converter design using Nexperia Power MOSFET devices.

AN50002

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Automotive LED side light SEPIC DC-to-DC converter design example

1. Introduction
In modern automotive applications, electrical systems are widely used in vehicles. They usually
powered by lead-acid or Li-ion batteries whose output voltage varies with the State of Charge(SOC), temperature, load dump and other loading operations. This requires DC-to-DC converters
to accommodate a wide input voltage and generate a stable regulated output voltage for electricalloads. A Single-Ended Primary Inductance Converter (SEPIC) design can be a good fit for abattery powered system. It can generate output voltage higher or lower than the input voltage withless components comparing to other topologies. Additionally, its AC coupled capacitor inherently
separates the output from the input. LED lighting is one example where SEPIC converters can beused in an automotive application.

Fig. 1.

Automotive application - LED lighting

In this application note, a general guideline of SEPIC converter design using Nexperia PowerMOSFET devices will be described.

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2. SEPIC converter design example
A Synchronous SEPIC DC-to-DC converter design example is shown in Fig. 2. One of the featuresof SEPIC converters is that the currents flowing through the two inductors are proportional to eachother [1]. Hence, a coupled inductor can be used in the design for reducing PCB footprint size andsystem cost.
Design specifications:••••

Input voltage: 9 ~ 16 V
Output voltage: 20 V
Maximum output power: 20 W
Switching frequency: 380 kHz

Key features:•••

Reverse battery protection (RBP)
Synchronous topology improves converter efficiency
Configurable light load operation modes:• Burst mode improves efficiency but causes higher output ripple and switching noise• Forced continuous mode for lower output ripple and switching noise but higher switchingloss• Pulse-skiping mode comprises of the above two operation modes

Dimming(current return)LED dimming
control

Taillight LED string

Current sense
Regulated 20 V
DC output

12 V input
from battery

Overvoltage andreverse polarity
protection
T9 LFPAK33 MOSFETs

Fig. 2.

SEPIC converter PCB for LED lighting

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3. System design
SEPIC topology is a combined topology consisting of two building blocks as shown in Fig. 3 below.
At the input side, there is a common Boost configuration formed by an inductor and a power switch.
This eases the RMS current seen by input capacitor [1].
At the output side, a Buck block is formed by a synchronous MOSFET or diode and an inductor.
As these two inductors are being charged and discharged during the whole operation period, a
coupled inductor can be used to reduce size and cost.
With this configuration, a SEPIC converter is able to accommodate a wide input voltage range andprovide a regulated output.
aaa-033105

Fig. 3.

SEPIC converter building block

To ensure proper energy transfer and operation of a SEPIC converter, a coupling capacitor with low
ESR is required. The RMS current rating of this capacitor is proportional to output power, since all
output current is passing through coupling capacitor. And maximum input voltage plus some ripple
will determine the voltage rating. This capacitor is also the inter-connection between two building
blocks. Please refer to AC coupling capacitor section for more details.

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Automotive LED side light SEPIC DC-to-DC converter design example
The basic operation of a SEPIC converter is shown in Fig. 4 (a) below.
During the period that switch Q1 = ON the coupled inductor (two windings) and coupling capacitor
are being charged and energy is stored inside them. Current flow is indicated in Fig. 4 (b). In thisperiod, all output power is provided by output capacitor to maintain regulated output voltage rating.
When Q1 = OFF, and synchronous MOSFET Q2 = ON, Fig. 4 (c), energy is released frominductors and delivered to the load.
aaa-033109

a) simplified schematic
aaa-033110b) Q1 = ON; Q2 = OFF
aaa-033111c) Q1 = OFF; Q2 = ON
Fig. 4.

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SEPIC converter phases

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3.1. Power MOSFET
Power MOSFET selection plays a critical part for the whole design as it determines the wholesystem’s thermal design requirements, cost and efficiency. Theoretical analysis can be performed
with mathematical equations.
In the SEPIC topology, the voltage stress on the main and synchronous MOSFET is the sum of
maximum input voltage and regulated output voltage:(1)

During the main switch on state, both inductor currents will pass through the main control MOSFET[1]. Therefore, the peak and RMS current can be calculated:

(2)

(3)

During the main switch off state, both inductor current will through the synchronous MOSFET,
consequently the peak and RMS current are expressed as:

(4)

(5)

With current and voltage rating known, we can calculate the switching and conduction losses forthe main MOSFET:
(6)

(7)

Where ∆Vg is gate voltage sweeping range, and Ciss is the MOSFET input parasitic capacitance, Ig
is the gate drive sinking and sourcing current.
And for synchronous MOSFET the switching and conduction losses can be obtained by:
(8)

(9)

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Automotive LED side light SEPIC DC-to-DC converter design example
During converter design, theoretical analysis utilizes complicated mathematical equations. Some ofthese can be only approximations. For instance, the entire switching process of a MOSFET is nonlinear. There is always some discrepancy between theoretical calculation results and the measured
ones. In addition, in thermal aspect only steady state junction temperature can be estimatedthrough mathematical equations. It is very difficult to estimate instantaneous junction temperature
through theoretical equations since the current profile could be very complex. Several iterations ofcalculation are usually needed to choose the right MOSFET that meets the system efficiency, costand size requirement. For initial performance estimation, calculation can be verified by computer
simulation. Please refer to Nexperia interactive application note IAN50002 for an example SEPIC
converter simulation. In the embedded simulation, any voltage or current waveform can be viewed
using the probes available from the toolbar. For thermal estimation using RC models, please refer
to Nexperia application note AN11261 RC Thermal Models [2].

3.2. Duty cycle calculation
The following consideration is based on SEPIC converter operating in Continuous Conduction
Mode (CCM) [1].
In CCM operation, the duty cycle D of a SEPIC converter can be obtained as:(10)Where VD is the voltage across the diode or the synchronous MOSFET.

3.3. Inductor Selection
There are several factors affecting the choice of inductance value like RMS current, peak-to-peakcurrent ripple, current to ripple ratio (r), maximum input current (at minimum Vin), and switchingfrequency. One of the important factors among them is the current ripple ratio (r) which is definedas the ratio between inductor ripple current and average current. As a rule of thumb, the r value of
0.4 would be a good starting point [1]. A higher value will bring high stress to the input capacitor(see input capacitor selection in later section.) A lower value will require higher inductor energy
handling capability, which means thicker wire and larger physical size. Therefore, an inductor value
can be calculated by the following equations:

(11)

(12)

In practice, inductor RMS and peak current should also be considered to ensure proper operationand design efficiency. They can be obtained by:

(13)

(14)

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Automotive LED side light SEPIC DC-to-DC converter design example

(15)

(16)

3.4. Bootstrap circuit design
In order to fully turn-on synchronous high side MOSFET, a floating power supply rail is required
to provide at least 5 V above the MOSFET source voltage level. A bootstrap circuit formed by acapacitor, a diode and a series resistor (limiting the capacitor charging inrush current) is often
used in practice. The capacitance value should be selected according to MOSFET gate charge
requirement. In other words, this capacitor should provide full charge to the MOSFET gate.

3.5. AC coupling capacitor
One of the distinguishing advantages of a SEPIC converter is that input and output terminal isseparated by the AC coupling capacitor. However, the RMS rating of this capacitor is proportional
to the output power. Hence, this topology is mainly used in low and medium power application.
During main control MOSFET turn-on period, IL2 pass through coupling capacitor [1]. In turned-off
period, IL1 goes through coupling capacitor [1]. Therefore, the RMS current and capacitance of AC
coupling capacitor can be obtained by following equations:

(17)

(18)

Where IL1 and IL2 are average current of the inductor.
In addition, when the primary control switch is on, this coupling capacitor will be charged to input
voltage and some added ESR ripple. Therefore, the voltage rating should be higher than the
maximum input voltage.

3.6. Input and output capacitor
The SEPIC is a combined topology which has a BOOST part at input and BUCK configuration at
output. The input side has an inductor like a boost converter. Due to the existence of this inductor,
the input current is triangular and continuous. Therefore, the RMS current of the input capacitor canbe obtained by following equation. The voltage rating should be higher than the maximum input
voltage.

(19)

As the output capacitor is required to provide charge to the load during control MOSFET onstate. Large ripple current would be seen by the output capacitor. Hence, RMS current should be
calculated:

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Automotive LED side light SEPIC DC-to-DC converter design example(20)

In practice, any parasitic parameters like ESR or ESL of a bulk capacitor will contribute to output
voltage ripple. Thus, the output capacitor should be selected according to converter output ripple
requirements. The relationship between output voltage ripple and ESR as well as capacitance is
described by the following equations:

(21)

(22)

3.7. Output voltage
The output voltage can be set easily through the ratio of a potential divider. The internal feedbackreference voltage is 1.2 V.

(23)

3.8. Soft start-up
A soft start-up operation is realized by using a capacitor connected to SS pin of the controller IC.
In this case the controller used is the LTC3769 from Analog Devices. Other control ICs can beused. An internal current charger of 10 µA will continuously charge this capacitor until the voltage
reaches the preset feedback threshold value of 1.2 V.

(24)

3.9. LED dimming control
The LED dimming control (shown in Fig. 5) is realized by three sub-circuits formed by Nexperia
logic and transistor products. PWM generation is done by using a relaxation oscillator consists of
U2, RV1, and capacitor C29. The PWM frequency can be obtained by following formula:f = 1 ≈ 1
RC
Τ

(25)

The above equation decides the total duration of capacitor charge and discharge time. However,the charge time and discharge time can be altered by potential meter RV1. With slide more close
to pin 1 of RV1, it takes less time to charge capacitor, as charge current will go through diode D17(PMEG2005EJ) and small portion of RV1. While discharge time will be much longer, since larger

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Automotive LED side light SEPIC DC-to-DC converter design example
portion of RV1 resistance appears in the capacitor discharge path. This means lower duty cycle.
For increasing the duty cycle, move the slider toward pin 3 of RV1.
A logic AND gate (74AHCT1G08) is added in between output of the oscillator and MOSFET gate
driver for two purposes. The first one is increasing the current driving capability and the second one
is disabling the gate driver during over current fault event.
A pair of NPN and PNP transistor (in one package, BC846BPN) is used to drive the dimming
control MOSFET.
aaa-033113

Fig. 5.

PWM generation, duty cycle, enable and gate driver circuit
In order to sense current, a shunt resistor is connected in series with PWM controlled MOSFET.
The current through LEDs will convert to voltage signal and amplified by a factor of 33. A
comparator will compare amplified voltage signal with reference value 4 V. Once the amplified
voltage signal exceed threshold, signal ‘EN’ is pull down and disabling the gate driver. This state
will be held until a button is pressed.
aaa-033114

Fig. 6.

Dimming MOSFET, current sense, and latch circuit

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4. Schematics
aaa-033115

J1+12 VDC input
from battery

SEPIC converter

J2
20 V regulated
output

LED dimming control

J3
Dimming
current returnFig. 7.

Overview schematic
aaa-033116

Fig. 8.

PWM, duty cycle, current sense and latch circuits

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Automotive LED side light SEPIC DC-to-DC converter design example

4.1. SEPIC converter schematicVin

aaa-033117

Vout

At light load:
PLLIN_MODE = floating, Burst mode
PLLINMODE = GND, Burst mode
PLLINMODE = 1.2V_INTVcc-1.3V, Pulse-skipping mode
PLLINMODE = INTVcc, Force current continious mode
When OVMODE is tied to INTVCC
overvoltage protection is disabled and
top MOSFET gate (TG) is not forcedon during an overvoltage event

ILIM = INTVcc, Vsense= 100mV
ILIM = floating, Vsense = 75mV
ILIM = GND, Vsense = 50 mV

Fig. 9.

When OVMODE is tied to ground,
overvoltage protection is enabled and
TG is turned on continuously until the
overvoltage event is cleared.

Note: NP = not plated

SEPIC converter schematic diagram

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5. Printed Circuit Board
A 4-layer PCB has been designed to demonstrate and verify the Nexperia SEPIC LED converter
circuit.

Fig. 10. SEPIC DC-to-DC converter PCB
Fig. 11, Fig. 12, Fig. 13, Fig. 14 and Fig. 15 show the PCB layers.

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Fig. 11. PCB front silk screen layer

Fig. 12. PCB front copper layer

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Fig. 13. PCB ground copper layer

Fig. 14. PCB power copper layer

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Fig. 15. PCB bottom copper layer

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6. Bill of Materials
Table 1. BOM (Nexperia part)
Reference
Qty
Value

Part Number

Footprint

D1, D2

2

PTVS40VS1UR

PTVS40VS1UR

Diode_SMD: D_SOD123W

D3, D42

BZX84J-B10

BZX84J-B10

Diode_SMD: D_SOD-323F

D5

1

PMEG6010CEH

PMEG6010CEH

Diode_SMD: D_SOD-123F

D6

1

TDZ13J

TDZ13J

Diode_SMD: D_SOD-323F

D10

1

PMEG060V030EPD/NP

PMEG060V030EPD

Package_TO_SOT_SMD:
SOT1289

D15, D18

2

TDZ5V1J

TDZ5V1J

Diode_SMD: D_SOD-323F

D8, D16

2

PMEG1030EJ

PMEG1030EJ

Diode_SMD: D_SOD-323F

D13, D172

PMEG2005EJ

PMEG2005EJ

Diode_SMD: D_SOD-323F

Q1, Q22

BUK6Y14-40P

BUK6Y14-40P

LFPAK56

Q3, Q92BC817K-40H

BC817K-40H

Package_TO_SOT_SMD:
SOT-23

Q4, Q7, Q8

3

BUK9M20-40H

BUK9M20-40H

LFPAK33

Q10

1BC858B

BC858B

Package_TO_SOT_SMD:
SOT-23Q11

1BC846BPN

BC846BPN

Package_TO_SOT_SMD:
SOT-363_SC-70-6_
Handsoldering

U2

1

74LVC1G58GW-Q100

74LVC1G58GW-Q100

Package_TO_SOT_SMD:
SOT-363_SC-70-6_
Handsoldering

U4

1

74AHCT1G08

74AHCT1G08GW-Q100

Package_TO_SOT_SMD:
SOT-353_SC-70-5_
Handsoldering

Table 2. BOM (non-Nexperia)
Reference
Qty
Value

Part Number

Footprint

U1

1

LTC3769

LTC3769

TSSOP20U3

1AD8552ARZ

AD8552ARZ

Package_SO:
SOIC-8_3.9x4.9mm_P1.27mm

C6

1

47nF

06033C473KAT2A

Capacitor_SMD:
C_0603_1608Metric

C11

1

10nF

06033C103K4T2A

Capacitor_SMD:
C_0603_1608Metric

RV1

1

PRS11S-N20K-503B1

PRS11S-N20K-503B1

PRS11S

L1

1

MSD1278-103ML

MSD1278-103ML

Coupled_Inductor

C9, C38

2

100pF

C0603C101J3GAUTO

Capacitor_SMD:
C_0603_1608Metric

C22, C23, C24 3

10uF

GRT31CR61H106ME1L

Capacitor_SMD:
C_1206_3216Metric

C7

1

15nF

GCJ188R71E153KA1D

Capacitor_SMD:
C_0603_1608Metric

C29

1

2.7nF

GCD188R71H272KA1D

Capacitor_SMD:
C_0603_1608Metric

C20, C21

2

150uF

EEE-FK1H151GV

CP_Elec_10.5x12

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Reference

Qty

Value

Part Number

Footprint

R1, R2, R10,
11
R18, R28, R30,
R31, R33, R36,
R37, R40

1k

ERJ-3EKF1001V

Resistor_SMD:
R_0603_1608Metric

R9

1

10

ERJ-6GEYJ100V

Resistor_SMD:
R_0805_2012Metric

R11

1

57.6k

ERJ-3EKF5762V

Resistor_SMD:
R_0603_1608Metric

R12

1

12.1k

ERJ-3EKF1212V

Resistor_SMD:
R_0603_1608Metric

R13

1

200k

ERJ-3EKF2003V

Resistor_SMD:
R_0603_1608Metric

R14, R15, R32, 4
R34

33k

ERJ-3EKF3302V

Resistor_SMD:
R_0603_1608Metric

R17

1

523k

ERJ-3EKF5233V

Resistor_SMD:
R_0603_1608Metric

R24

1

0.1

ERJ-8CWFR020V

Resistor_SMD:
R_1206_3216Metric

R25, R26

2

0.02

ERJ-8CWFR020V

Resistor_SMD:
R_1206_3216Metric

R4, R6, R8,
6
R16, R23, R29

0

ERJ-3GEY0R00V

Resistor_SMD:
R_0603_1608Metric

R35, R41

2

100

ERJ-3EKF1000V

Resistor_SMD:
R_0603_1608Metric

R38

1

100k

ERJ-3EKF1003V

Resistor_SMD:
R_0603_1608Metric

R39

1

402k

ERJ-3EKF4023V

Resistor_SMD:
R_0603_1608Metric

R42

1

10k

ERJ-3EKF1002V

Resistor_SMD:
R_0603_1608Metric

SW1

1

EVQQ2K03W

EVQQ2K03W

EVQQ2

D7

1

LED

FR1112H-TR

LED_SMD:
LED_0805_2012Metric

D11

1

LED

VCDG1112H-4BY3C-TR

LED_SMD:
LED_0805_2012Metric

C2, C3, C5,
10
C13,C14,C15,
C16, C35, C36

4.7uF

CGA4J1X7R1V475K125AC

Capacitor_SMD:
C_0805_2012Metric

C10, C27, C28, 5
C33, C40

0.1uF

CGA3E2X7R1E104K

Capacitor_SMD:
C_0603_1608Metric

C8, C26, C34,
C37, C39

1uF

CGJ3E1X7R1E105K080AC

Capacitor_SMD:
C_0603_1608Metric5

7. Conclusions
A battery supplied electrical system e.g. for LED lighting, requires a DC-to-DC converter toaccommodate wide input voltage range. A Single-Ended Primary Inductance Converter (SEPIC)
is suitable for this kind of application. In this application note, a general design approach of SEPIC
DC-to-DC converter is presented. Mathematical equations should be used for initial components
sizing. During this process, several iterations are required for optimal system performance. With aidof computer simulation tools and Nexperia device SPICE and RC thermal model, a more accurate
initial estimation can be obtained.

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8. References
1. Switching Power Supplies A-Z, Second Edition – Sanjaya Maniktala
2. RC Thermal Models – Nexperia

9. Revision history
Table 3. Revision history
Revision
Date
numberDescription

2.0

2021-05-10

Correction to Equation 6 and Equation 8.

1.0

2021-02-01

Initial version

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10. Legal information
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modifications or additions. Nexperia does not give any representations or
warranties as to the accuracy or completeness of information included hereinand shall have no liability for the consequences of use of such information.

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or completeness of such information and shall have no liability for theconsequences of use of such information. Nexperia takes no responsibility
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In no event shall Nexperia be liable for any indirect, incidental, punitive,
special or consequential damages (including - without limitation - lost
profits, lost savings, business interruption, costs related to the removalor replacement of any products or rework charges) whether or not suchdamages are based on tort (including negligence), warranty, breach ofcontract or any other legal theory.
Notwithstanding any damages that customer might incur for any reason
whatsoever, Nexperia’s aggregate and cumulative liability towards customer
for the products described herein shall be limited in accordance with theTerms and conditions of commercial sale of Nexperia.
Right to make changes — Nexperia reserves the right to make changes
to information published in this document, including without limitation
specifications and product descriptions, at any time and without notice. This
document supersedes and replaces all information supplied prior to the
publication hereof.
Suitability for use — Nexperia products are not designed, authorized or
warranted to be suitable for use in life support, life-critical or safety-criticalsystems or equipment, nor in applications where failure or malfunctionof an Nexperia product can reasonably be expected to result in personal
injury, death or severe property or environmental damage. Nexperia and its
suppliers accept no liability for inclusion and/or use of Nexperia products insuch equipment or applications and therefore such inclusion and/or use is at
the customer’s own risk.
Applications — Applications that are described herein for any of theseproducts are for illustrative purposes only. Nexperia makes no representation
or warranty that such applications will be suitable for the specified use
without further testing or modification.
Customers are responsible for the design and operation of their applications
and products using Nexperia products, and Nexperia accepts no liability forany assistance with applications or customer product design. It is customer’ssole responsibility to determine whether the Nexperia product is suitableand fit for the customer’s applications and products planned, as well as
for the planned application and use of customer’s third party customer(s).
Customers should provide appropriate design and operating safeguards tominimize the risks associated with their applications and products.
Nexperia does not accept any liability related to any default, damage, costs
or problem which is based on any weakness or default in the customer’s
applications or products, or the application or use by customer’s third party
customer(s). Customer is responsible for doing all necessary testing for thecustomer’s applications and products using Nexperia products in order toavoid a default of the applications and the products or of the application or
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AN50002

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Rev. 2.0 — 10 May 2021

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AN50002

Nexperia

Automotive LED side light SEPIC DC-to-DC converter design example

List of Tables
Table 1. BOM (Nexperia part)............................................17
Table 2. BOM (non-Nexperia)............................................ 17
Table 3. Revision history....................................................19

AN50002

application note

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Rev. 2.0 — 10 May 2021

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21 / 23

AN50002

Nexperia

Automotive LED side light SEPIC DC-to-DC converter design example

List of Figures
Fig. 1. Automotive application - LED lighting....................... 2
Fig. 2. SEPIC converter PCB for LED lighting..................... 3
Fig. 3. SEPIC converter building block................................ 4
Fig. 4. SEPIC converter phases.......................................... 5
Fig. 5. PWM generation, duty cycle, enable and gate
driver circuit........................................................................10
Fig. 6. Dimming MOSFET, current sense, and latch circuit 10
Fig. 7. Overview schematic................................................11
Fig. 8. PWM, duty cycle, current sense and latch circuits.. 11
Fig. 9. SEPIC converter schematic diagram...................... 12
Fig. 10. SEPIC DC-to-DC converter PCB.......................... 13
Fig. 11. PCB front silk screen layer................................... 14
Fig. 12. PCB front copper layer......................................... 14
Fig. 13. PCB ground copper layer..................................... 15
Fig. 14. PCB power copper layer.......................................15
Fig. 15. PCB bottom copper layer......................................16

AN50002

application note

All information provided in this document is subject to legal disclaimers.

Rev. 2.0 — 10 May 2021

© Nexperia B.V. 2021. All rights reserved

22 / 23

AN50002

Nexperia

Automotive LED side light SEPIC DC-to-DC converter design example

Contents
1. Introduction................................................................... 2
2. SEPIC converter design example................................3
3. System design.............................................................. 4
3.1. Power MOSFET.......................................................... 6
3.2. Duty cycle calculation.................................................. 7
3.3. Inductor Selection........................................................ 7
3.4. Bootstrap circuit design............................................... 8
3.5. AC coupling capacitor................................................. 8
3.6. Input and output capacitor...........................................83.7. Output voltage............................................................. 9
3.8. Soft start-up................................................................. 9
3.9. LED dimming control................................................... 9
4. Schematics.................................................................. 11
4.1. SEPIC converter schematic.......................................12
5. Printed Circuit Board................................................. 13
6. Bill of Materials...........................................................17
7. Conclusions................................................................ 18
8. References.................................................................. 19
9. Revision history..........................................................19
10. Legal information......................................................20© Nexperia B.V. 2021. All rights reserved

For more information, please visit: http://www.nexperia.com
For sales office addresses, please send an email to: salesaddresses@nexperia.com
Date of release: 10 May 2021

AN50002

application note

All information provided in this document is subject to legal disclaimers.

Rev. 2.0 — 10 May 2021

© Nexperia B.V. 2021. All rights reserved

23 / 23