1 Real-World Issues in DC/DC Converters M. T. Thompson, 2008 Power Electronics Notes 07B Some Real-World Issues in DC/DC Converters © Marc Thompson,

1Real-World Issues in DC/DC Converters

M. T. Thompson, 2008

Power Electronics Notes 07BSome Real-World Issues in DC/DC Converters

© Marc Thompson, 2006-2008

Marc T. Thompson, Ph.D.Thompson Consulting, Inc.

9 Jacob Gates RoadHarvard, MA 01451

Phone: (978) 456-7722 Fax: (888) 538-3824

Email: [email protected]: http://www.thompsonrd.com



Summary• Efficiency• Component selection (inductors, capacitors)• Off-the-shelf ICs• Electromagnetic interference (EMI) and EMI testing



Basic Buck Converter --- Efficiency

V o

DiL

LV cc

v c

+

-

C R

• This converter suffers from low efficiency at low output voltage, due to finite diode drop• For 1.2V output, a 0.4V diode drop results in 19% loss of efficiency for Vin = 4V• Power loss in diode is (1-D)VDIL assuming constant VD



Synchronous Buck Converter --- Efficiency• This is the basic buck with an additional controlled switch to lower losses when diode is on• Higher efficiency than standard buck; more complicated control; typically >10% more efficiency than standard buck• Need to be very mindful of timing in the two switches (shoot–through)

Synchronousswitch



Synchronous Buck Converter --- PSIM

“Dead time” circuit ensures that Q1 and Q2 are never on at the same time

Note: 0.5 microsecond dead time is probably excessive



Synchronous Buck Converter --- PSIM• Note that after turnon of the diode/switch the voltage drop decreases from 0.5V to 0V. The MOSFET drop is lower than the diode drop Diode on Sync. switch on



Other Losses in DC/DC Converters• Switch ON resistance (conduction loss)• Turn on/off switching loss• Diode ON voltage• Diode reverse recovery (if any)• Diode reverse current• Inductor ESR• Capacitor ESR• Losses in MOSFET gate driver• PWM and control system power dissipation• Etc., etc.



Boost Converter Efficiency• Assume lossy inductor



Boost Converter Efficiency• Assume lossy inductor



Boost Converter Efficiency vs. Duty Cycle

• Calculate efficiency; assume Rind/RL = 0.1

Efficiency vs. D, nonideal boost, Rind/RL = 0.1

00.10.20.30.40.50.60.70.80.9

1

0 0.2 0.4 0.6 0.8 1

D

Eff

.



Boost Converter Output Voltage vs. D

• Calculate effect on output voltage assume Rind/RL = 0.1

Vo/Vi vs. D, ideal and nonideal boost, Rind/RL = 0.1

0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6 0.8 1

D

Vo

/Vi

Ideal boost

Nonideal boost



Pulse-Width Modulation (PWM) in DC-DC Converters

st

control

V

vD

ˆ



Converter Input Filter• Added to reduce ripple component drawn from input source

Reference: Kassakian, et. al., Principles of Power Electronics, pp. 107-109



Converter Input Filter (cont.)


• If RC >> T, almost all the ripple current passes through C



Converter Input Filter (cont.)




Example 1: 170W Buck Converter Filter Caps

• Po = 170 Watts. Average input current = 10A• Let’s look at the switch current and the RMS current rating on the input filter capacitor (Cf) and output filter capacitor (Cload)

Main inputfilter capacitor(22 F)

Main outputfilter capacitor(330 F)



Example 1: Buck Converter Input Filter• Switch current is very discontinuous; average = 10.3A, RMS = 12.35A• Look at input capacitor current; RMS capacitor current = 4.8A• Need to spec cap with this RMS value

Switch current

Input capacitor current



Example 1: Buck Converter Output Filter• RMS current in Cload = 0.27A Output capacitor current



Example 2: Capacitor ESR• Real-world capacitors have equivalent series resistance (ESR)• This ESR may have dominant effect on output voltage ripple• Note: sometimes we need to worry about the equivalent series inductance as well



Example 2: Effects of Capacitor ESR• Without ESR, output ripple is 24 mV-pp• ESR has increased ripple to approximately 30 mV-pp



Impedance of CapacitorThe impedance of an ideal capacitor is:

CjZ idealcap

1,

For a real-world capacitor (ignoring dielectric loss) the impedance is:

Cj

RCjLCLjR

CjZ realcap

)1(1 2

,

The magnitude of the impedance is:

C

RCLCZ realcap

222

,

)1(



Impedance of CapacitorAn impedance plot of an electrolytic capacitor, comparing the ideal with the actual impedance is shown below for C = 100 F, L = 25 nH and R = 0.01.

104

105

106

107

108

10-4

10-3

10-2

10-1

100

101 Impedance of ideal and nonideal capacitor

Freq., Hz

Imp

eda

nce

, O

hms

Resonance



Key Things to Specify when Selecting a Capacitor

• Temperature coefficient of capacitance• Equivalent series resistance (ESR)• Voltage rating• Ripple current• Series inductance• Polarized or non-polarized• Leakage/dielectric loss• Physical size



Electrolytic Capacitors• 25V rating



Tantalum Capacitors• 15F, 35V = 9.2 mJoules; 4.7F, 16V = 0.6 mJoules



Ceramic Capacitors• Used mainly for bypassing



Mica Capacitor• Low capacitance, low loss



Film Capacitor



InductorsToroids

Surface-mount

Planar spiral



Real-World Inductors• Things to spec: inductance, RMS current, peak current, SRF, ESR, package type/dimensions

Reference: www.coilcraft.com



Real-World Inductors
















Buck Regulator ICs

Reference: www.national.com



Buck Regulator

Reference: www.national.com

• Internal (integrated) power switch



Another Buck Regulator (Linear Technology)

Reference: www.linear.com

• External MOSFET switches



Interleaved Buck Regulator (Fairchild)


• Fairchild FAN5090



Multiphase Buck Regulator (Fairchild)


• Fairchild FAN5094



Boost Regulator (Linear Technology)


• Internal switch



Another Boost Regulator (Linear Technology)


• Interleaved outputs



SEPIC (Linear Technology)




Cuk (Linear Technology)




Buck/Boost (Linear Technology)




Buck/Boost (Linear Technology)




EMI

Reference: Alexander Kusko and Marc Thompson, Power Quality in Electrical Systems, McGraw-Hill, 2007

• Let’s look at electromagnetic interference (EMI)• We’ll focus on the switch current isw in the boost converter



EMI


r

d

tF

TF

1

1

2

1



Example 3: EMI


• Find the spectrum of the drain current for a boost converter operating at a switching frequency fsw = 500 kHz, a duty cycle D = 0.5, and with a switch risetime and falltime tr = 25 nanoseconds.

MHzt

F

kHzT

F

r

d

7.12)1025)((

11

318)101000)((

11

92

91



Testing for Conducted EMI


• Line impedance stabilizing network (LISN)



LISN

Reference: Keith Billings, Switchmode Power Supply Handbook, McGraw-Hill, 1999



EMI Filter




EMI Filter

Reference: On Semiconductor



Typical EMI Spectrum from a DC/DC Converter

Reference: Power Integrations, Inc., “Techniques for EMI and Safety,” Application Note AN-15, June 1996 , www.powerint.com

• In this case, fsw ~ 300 kHzDC

1st

2nd

3rd



Conducted EMI Standards

Reference: Power Integrations, Inc., “Techniques for EMI and Safety,” Application Note AN-15, June 1996 , www.powerint.com

• Covers 150 kHz – 30 MHz



Conducted EMI Testing

Reference: T. Curatolo and S. Cogger “Enhancing a Power Supply to Ensure EMI Compliance,” EDN, Feb. 17, 2005, pp. 67-73

• Differential mode capacitor





• Add bypass capacitors





• Differential-mode choke





• Common-mode filter



Radiated EMI Standards

• There are also radiated EMI standards• Radiated EMI is tested up to 1000 MHz

Reference: Texas Instruments, Inc., http://www.eetchina.com/ARTICLES/2001SEP/PDF/2001SEP21_AMD_POW_AN1789.PDF



EMI Testing Lab• EMI testing is often done

• In-house (pre-certification)• At specialty EMI testing labs (Chomerics, Curtis-Straus, etc.)



Example 4: Non-Ideal Buck Simulation



Example 4: Buck Simulation Results




• Switch current• In this case, switch current = supply current




• Switch current spectrum; fsw = 200 kHz

1st

2nd

3rd



Example 5: Non-Ideal Buck Simulation with Input Filter



Example 5: Non-Ideal Buck Simulation with Input Filter

Switch current

Supply current

Capacitor current




• Switch current spectrum; fsw = 200 kHz

Switch current

Supply current

Capacitor current

Documents

1 Real-World Issues in DC/DC Converters M. T. Thompson, 2008 Power Electronics Notes 07B Some Real-World Issues in DC/DC Converters © Marc Thompson,