RD008 - Full Report - Payton Group Report… · · 2013-12-12RD008 – 320W Push-Pull Converter August 16, 2007 RD008 - Full Report.doc Page 2 of 23 Contents 1 Introduction

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Title RD008

Description 320W Telecoms DC/DC PSU Input : 37Vdc to 60Vdc Output : 32V/10A

Date 16th August, 2007

Revision 1.1

RD008 – 320W Push-Pull Converter August 16, 2007

RD008 - Full Report.doc Page 2 of 23

Contents

1 Introduction ........................................................................................................................ 3 2 Specification ....................................................................................................................... 4 3 Schematic ........................................................................................................................... 5 4 Bill of Materials.................................................................................................................. 6 5 Custom Magnetics Design.................................................................................................. 7

5.1 Push-Pull Transformer................................................................................................ 7 5.1.1 Transformer Electrical Diagram......................................................................... 7

5.2 Output Inductor .......................................................................................................... 8 5.2.1 Inductor Electrical Diagram ............................................................................... 9

6 Design Analysis................................................................................................................ 10 7 PCB Layout ...................................................................................................................... 11 8 Measurement Results........................................................................................................ 12

8.1 Performance Measurements ..................................................................................... 12 8.1.1 Conversion Efficiency ...................................................................................... 12

8.2 Operating Waveforms .............................................................................................. 14 8.3 Primary FET Waveforms ......................................................................................... 14 8.4 Start-up Behavior...................................................................................................... 16 8.5 Load Transient Response ......................................................................................... 17 8.6 Output Voltage Ripple.............................................................................................. 18 8.7 Control Loop Characterisation ................................................................................. 20 8.8 Thermal Measurements ............................................................................................ 21

9 Appendix A – Modifications for tighter output tolerance ................................................ 22 9.1 Enhanced Regulation Performance .......................................................................... 22

10 Revision History ........................................................................................................... 23

RD008 - 320W Push-Pull Converter August 16, 2007


1 Introduction This report describes the design of a 320W Telecoms DC/DC converter. A push-pull forward converter running with current mode control using the LM5030 from National Semiconductor delivers high performance and small solution size. Custom planar magnetics from Payton have been used throughout to reduce component build height, increase efficiency and aid cooling. This report contains target specification, schematic, bill of materials, magnetics design information as well as a detailed design analysis. A full set of performance measurements is also included taken from the prototype unit shown in Figure 1. Measurements include conversion efficiency, power stage device temperature rise, line/load regulation, start-up behaviour, transient load response and loop gain/phase characteristic.

Figure 1 - 320W Push-Pull Forward Converter (Reverse side heatsink not shown). Unit measures 150mm x 70mm with maximum component height of 10mm.



2 Specification

Description Symbol Min Typ Max Units Comments

Input

Voltage Vin 37 48 60 VDC

Outputs

Output Voltage 1 VOUT1 30.4 32 33.6 V +/-5%

Output Current 1 IOUT1 0 10 A 10A continuous

Output Ripple Voltage 1 VRIPPLE1 1 V

Maximum Continuous Output Power Pout 320 W

Efficiency n 90 % Target Efficiency

Figure 2 - Converter Specifications



3 Schematic

Figure 3 - Push-Pull DC/DC Converter Schematic



4 Bill of Materials The bill of materials for this design is shown in Figure 4 below. This BOM excludes the PCB, fixing bolts, insulating material, aluminium interface plate and external heatsink.

Type Reference Quantity Description Manufacturer Manufacturer Part Number

C4, C27 2 100nF, 100V, X7R KEMET C1206F104K1RACC1 1 100nF, 50V, X7R AVX 08055C104KAZ2AC15 1 10nF, 1kV, X7R AVX 1210AC103KAT1A

C17, C18, C19, C20, C21, C22, C23, C24,

C25, C2610 1uF, 100V, X7R AVX 22201C105KAT1A

C3, C6 2 1uF, 16V, X5R Epcos B37641K9105K62C5, C10 2 2.2uF, 50V, X7R Kemet C1210C225K5RAC

C2 1 330pF, 50V, X7R Phycomp 2238 5801 5616C8 1 470nF, 16V, X7R Phycomp 2222 7801 5658

C13, C14 2 470pF, 200V, COG AVX 08052A471JAT2AC7 1 47nF, 50V, X7R AVX W2H15C4738AT1A

C11, C12 2 100uF, 50V BC Components 22215371101Connectors J9, J10 2 2-Way 22A Connector Phoenix Contact 17 33 57 0

D1, D4, D5, D6 4 120V, 100mA Diode Fairchild BAS19V

D2, D3 2 16A, 200V Diode International Rectifier MURB1620CT

L2 1 4.7uH, 18A Inductor Coilcraft SER2013-472MLCS1 1 100:1 CS Pulse PA1005.100T1 1 Custom (Planar) Payton 53035L1 1 Custom Inductor Payton 53044

R2, R10, R15 3 0R, 0.125W, 5%, 0805 Phycomp 2322 7309 1002R1, R4 2 10k, 0.125W, 1% Phycomp 2322 7346 1003

R6, R7, R8, R13 4 10R, 0.125W, 1% Phycomp 2322 7346 1009R19, R20 2 15R, 1W, 5% Tyco 352015RJT

R14 1 1k, 0.125W, 1% Phycomp 2322 7346 1002R5 1 200R, 0.125W, 1% Phycomp 2322 7346 2001R9 1 2k, 0.125W, 1% Phycomp 2322 7346 2002R11 1 4k7, 0.125W, 1% Phycomp 2322 7346 4702R17 1 4R7, 0.125W, 1% Phycomp 2322 7346 4709R12 1 56k, 0.125W, 1% Phycomp 2322 7346 5603R16 1 6R8, 0.125W, 1% Phycomp 2322 7346 6809

Q1, Q2 2 150V, 60A, 0.034R International Rectifier IRFSL52N15D

U2 1 2.5V Shunt Regulator National Semiconductor LM431

U1 1 Current Mode Push Pull Controller

National Semiconductor LM5030SD

U3 1 Opto 50% to 150% CTR Toshiba TLP181

Semiconductors

Capacitors

Diodes

Magnetics

Resistors

Figure 4 – BOM list for 320W DC/DC Converter



5 Custom Magnetics Design Custom planar magnetics were used in this design for the main power transformer and output inductor. Planar technology provides for a low build height, excellent electrical performance and also allows for effective thermal management. Payton were used to provide these parts and this section gives the details on the transformer and inductor.

5.1 Push-Pull Transformer The push-pull transformer has two primary windings and two secondary windings. Figure 5 below shows the Payton part.

Figure 5 - 320W Planar Push-Pull Transformer (Measures Approximately 40mm x 33mm x 12mm high)

5.1.1 Transformer Electrical Diagram

Figure 6 - Transformer Electrical Diagram



The transformer is designed to operate at 250kHz and has the electrical characteristics as shown in Figure 7 below.

Parameter Value Units

Winding Resistance (Pins 1-5) 3.7 mΩ Winding Resistance (Pins 7-9) 3.7 mΩ Winding Inductance (Pins 1-5) 190 µH Winding Inductance (Pins 7-9) 190 µH

Leakage Inductance (Pins 1-5 with Pins 7-9 Shorted)

0.13 µH

Interwinding Capacitance (Pins 3 to 8) 150 pF Breakdown Voltage (Pins 1-7) Minimum

1000 V

Breakdown Voltage to Core (Pins 1+7 to Core) Minimum 500

V

Figure 7 - Transformer Electrical Parameters

5.2 Output Inductor The high combined duty cycle of the push-pull topology allows for a small output inductance when compared to the standard single ended forward converter. A 5µH inductance was specified which results in a physically small inductor. Figure 8 shows the planar inductor which also includes an auxiliary winding to provide a bias supply for the LM5030 control IC.

Figure 8 - 5µH Planar Inductor (Measures Approximately 24mm x 20mm x 9mm high)



5.2.1 Inductor Electrical Diagram The electrical diagram of the output inductor with auxiliary winding is shown in Figure 9 below.

Figure 9 - Inductor Electrical Diagram

The main output inductance is provided between pins 2 and 6. The auxiliary winding steps the output voltage of 32V down by a factor of 3 to drive around 10V into the LM5030 to provide power for it during normal operation. The output inductor will be subject to a signal at twice the converter operating frequency due to the push-pull action of the power stage. This gives a 500kHz effective operting frequency for the inductor. The electrical parameters for the output inductor are given in Figure 10 below.

Parameter Value Units

L1 Resistance (Pins 2-6) 5 mΩ L2 Resistance (Pins 9-12) 5 mΩ L1 Inductance (Pins 2-6) 4.5-5.5 µH

L1 Inductance with DC bias (Pins 9-12, 16.7Adc) 4.25 min µH Breakdown Voltage (Pins 2-9) Minimum

1000 V

Breakdown Voltage to Core (Pins 2+9 to Core) Minimum 500

V

Figure 10 - Inductor Electrical Parameters



6 Design Analysis The design uses a push-pull forward converter running at 250kHz to allow for small magnetics and high conversion efficiency. The push-pull configuration results in an effective output frequency of 500kHz and high effective duty cycle to allow for minimal output inductance and capacitance. Figure 11 below gives the general operating parameters for the converter running at full power with 37Vdc, 48Vdc and 60Vdc input.

Minimum Nominal MaximumVin 37 48 60 V DC Input VoltageVout V DC Output VoltageIout A DC Output CurrentFs kHz Power Stage Switching FrequencyVd V Output Diode Conduction DropVds V Primary Switch Conduction DropLout uH Output InductanceLr ohms Inductor DC ResistanceRdson ohms Primary MOSFET On State Resistance

Ns Number of Secondary TurnsNp Number of Primary TurnsLp uH Magnetising Inductance

Po W Output PowerD 44.2% 33.9% 27.0% Operating Duty Cycle (Per Switch)Ir 1.66 4.26 5.99 Apk-pk Inductor Ripple CurrentImag 0.77 0.77 0.76 Apk-pk Primary Magnetisation CurrentTf 0.23 0.64 0.92 us Diode Freewheel TimeTon 1.77 1.36 1.08 us Primary FET on-time

Output Inductor Operating Parameters

Ir 1.66 4.26 5.99 Apk-pk Inductor Ripple CurrentIp 10.83 12.13 12.99 Apk Inductor Peak CurrentIrms 10.05 10.30 10.58 Arms Inductor RMS CurrentPcond 0.50 0.53 0.56 W Inductor Copper Loss

Primary FET Operating Parameters (Parameters per FET)

Irip 1.66 4.26 5.99 Apk-pk Reflected Ripple Current Ip 11.22 12.51 13.38 Apk Peak MOSFET CurrentIrms 6.91 6.09 5.47 Arms MOSFET RMS CurrentPcond 1.43 1.11 0.90 W MOSFET Conduction LossVds 74.00 96.00 120.00 V Peak drain-source voltage

Output Diode Operating Parameters (Parameters per diode)

Idave 5.00 5.00 5.00 A Output Diode Average CurrentIdpk 10.83 12.13 12.99 Apk Output Diode Peak CurrentPcond 2.50 2.50 2.50 W Output Diode Conduction LossesVrr 74.00 96.00 120.00 V Peak Diode Reverse Voltage

85

Transformer Design Parameters

320

General Operating Parameters

32102500.50.35

33

0.030.005

Figure 11 - Design Operating Analysis at Full Power



7 PCB Layout The PCB was realised with 2 layers of 2oz copper with all components surface mount on the top side of the PCB. Use of 2oz copper reduces conduction losses in the PCB tracks as well as providing better heat transfer to keep components cool.

Figure 12 – Top Side Silk Screen

Figure 13 - Top Copper

Figure 14 – Bottom Copper



8 Measurement Results Measurements were taken from a prototype unit mounted on a 1.1°C/W heatsink with natural convection cooling.

8.1 Performance Measurements

8.1.1 Conversion Efficiency Efficiency was measured as a function of output power for 48Vdc input. Figure 15 below shows the resulting efficiency profile.

80

82

84

86

88

90

92

94

96

98

100

0 50 100 150 200 250 300 350

Output Power (W)

Con

vers

ion

Effic

ienc

y (%

)

Figure 15 - Conversion Efficiency as a Function of Output Power at 48Vdc Input

Conversion efficiency remains very high over the full range of input voltages. Peak efficiency is in excess of 92%. Conversion efficiency was also measured as a function of input voltage with full load on the output (32V/10A). Figure 16 shows the resulting efficiency profile.



80

82

84

86

88

90

92

94

96

98

100

35 40 45 50 55 60

Input Voltage (Vdc)|

Effic

ienc

y (%

)

Figure 16 – Conversion Efficiency as a Function of Input Voltage at Full Load 32V/10A

Efficiency drops a little under high line conditions and this will be due to the lower operating duty cycle of the power stage leading to higher RMS currents. Efficiency is still well in excess of 90% under all conditions at full output power.



8.2 Operating Waveforms

8.3 Primary FET Waveforms The plots in this section show the drain-source voltage and sensed primary winding current for 36, 48 and 60Vdc input. Figure 17, Figure 18 and Figure 19 show the waveforms with 10A load current.

Figure 17 – 36Vdc Input and 10A Load. Q1 Drain-Source Voltage (CH1 at 50V/div) and Sensed Primary

Current (CH2 at an effective 7.2A/div). Timebase is 1us/div.







The drain-source voltage and sensed primary current were also measured at 48Vdc input with the peak load of 13A. Figure 20 shows the waveforms for this case.

Figure 20 – 48Vdc Input at 13A Load. Q1 Drain-Source Voltage (CH1 at 50V/div) and Sensed Primary




8.4 Start-up Behavior Start-up behavior was measured with 48V input and a 3Ω resistive load on the output. Figure 21 below shows the output voltage rise profile and the drain-source voltage measured on Q2 just after input voltage is applied.

Figure 21 – Start-Up behavior with 48Vdc input. Output Voltage (CH1 at 10V/div) shows a monotonic

rise with zero overshoot whilst Q1 Drain-Source Voltage (CH2 at 50V/div) shows safe operating voltage levels during start-up due to soft-start behavior.



8.5 Load Transient Response Load transient response was measured with 48Vdc by stepping the output load current from 6.5A to 13A in 10µs. Figure 22 below shows the response of the output voltage to this step load change.

Figure 22 – Load transient response at 48Vdc input. Output voltage (CH1 at 1V/div) and output current

(CH2 at 5A/div) with a timebase of 1ms/div.

The step load test from 50% to 100% load shows that the control loop is well damped as expected. Peak overshoot/undershoot is less than 1V which allows the +32V output rail to remain within its 5% regulation window during aggressive load step changes.



8.6 Output Voltage Ripple The output voltage noise and ripple was measured at 10A load with 37V, 48V and 60Vdc inputs. Output ripple is measured as 200mVpk-pk, 400mVpk-pk and 600mVpk-pk for input voltages of 37Vdc, 48Vdc and 60Vdc respectively.

Figure 23 - Output Voltage Ripple (CH2, AC-Coupled at 200mV/div) with 37Vdc input and 10A Load







8.7 Control Loop Characterisation The loop gain and phase were measured at 48V input with full load on the output. Figure 26 below shows the resulting loop performance.

-30

-20

-10

0

10

20

30

100 1000 10000 100000

Frequency (Hz)

Gai

n (d

B)

-200

-150

-100

-50

0

50

100

150

200

Phas

e M

argi

n (D

eg)

Gain Phase

Figure 26 - Measured Loop Gain and Phase with 48Vdc input and 32V/10A Resistive Load

The cross-over frequency of 20kHz will give excellent load and line transient response. Current mode control results in a phase margin in excess of 100° which will result in a well damped recovery from transient conditions.



8.8 Thermal Measurements The operating temperature of key power stage components was measured as a function of output power with 48Vdc input. The PCB was mounted vertically on a 1.1°C/W heatsink with a local lab ambient of 24°C. Cooling was provided by natural convection. Figure 27 below shows the resulting temperature profiles.

0102030405060708090

100

0 50 100 150 200 250 300 350

Output Power (W)

Tem

pera

ture

(Deg

C)

Q1 Q2 T1 U1 D2C11 L1 Heatsink D3 Ambient

Figure 27 - Component Temperature Rise as a Function of Output Power at 48Vdc Input

Component temperature was also measured as a function of input voltage at full power 32V/10A. Figure 28 shows the temperature profile.

020406080

100120

35 40 45 50 55 60 65

Input Voltage (Vdc)

Tem

pera

ture

s (D

eg C

)

Q1 Q2 T1 U1 D2C11 L1 Heatsink D3 Ambient

Figure 28 - Component Temperature Rise as a Function of Input Voltage at Full Load

.



9 Appendix A – Modifications for tighter output tolerance In order to increase the accuracy of the output 32V rail, 0.1% feedback resistors and a 0.5% accurate LM431CIM reference were used in the circuit as shown in Figure 29 below.

Figure 29 – Revised Feedback Section to give Tight Output Tolerance

The component changes were made as per Figure 29 and the regulation measured again.

9.1 Enhanced Regulation Performance The input voltage was set to 48Vdc and the output current varied from 0 to 10A. Figure 30 below shows the load regulation is within 0.2% of nominal voltage which corresponds to a variation of +/-60mV.

95

96

97

98

99

100

101

102

103

104

105

0 2 4 6 8 10

Output Current (A)

Reg

ulat

ion

(% o

f Nom

inal

)

Figure 30 – Enhanced Regulation Accuracy



10 Revision History Date Revision Author Details 8th June, 2007 1.0 Iain Mosely First Draft

13th July, 2007 1.1 Iain Mosely Higher tolerance feedback resistors and reference used. Regulation measurements retaken

Documents

RD008 - Full Report - Payton Group Report… · · 2013-12-12RD008 – 320W Push-Pull Converter August 16, 2007 RD008 - Full Report.doc Page 2 of 23 Contents 1 Introduction