Power Electronics Notes 07B Some Real-World Issues in DC/DC Converters Marc T. Thompson, Ph.D. Thompson Consulting, Inc. 9 Jacob Gates Road Harvard, MA 01451 Phone: (978) 456-7722 Fax: (888) 538-3824 Email: marctt@thompsonrd.com Web: http://www.thompsonrd.com © Marc Thompson, 2006-2008

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

Basic Buck Converter --- Efficiency 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) Synchronous switch

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

Boost Converter Output Voltage vs. D Calculate effect on output voltage assume Rind/RL = 0.1 Ideal boost Nonideal boost

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

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

Converter Input Filter (cont.) Reference: Kassakian, et. al., Principles of Power Electronics, pp. 107-109

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 output filter capacitor (330 F) Main input filter capacitor (22 F) 16

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 17

Example 1: Buck Converter Output Filter RMS current in Cload = 0.27A Output capacitor current 18

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 Capacitor

Impedance of Capacitor 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 15F, 35V = 9.2 mJoules; 4.7F, 16V = 0.6 mJoules

Ceramic Capacitors Used mainly for bypassing

Mica Capacitor Low capacitance, low loss

Film Capacitor

Inductors Toroids Planar spiral Surface-mount

Real-World Inductors Things to spec: inductance, RMS current, peak current, SRF, ESR, package type/dimensions Reference: www.coilcraft.com

Real-World Inductors Reference: www.coilcraft.com

Real-World Inductors Reference: www.coilcraft.com

Real-World Inductors Reference: www.coilcraft.com

Real-World Inductors Reference: www.coilcraft.com

Buck Regulator ICs Reference: www.national.com

Buck Regulator Internal (integrated) power switch Reference: www.national.com

Another Buck Regulator (Linear Technology) External MOSFET switches Reference: www.linear.com

Interleaved Buck Regulator (Fairchild) Fairchild FAN5090 Reference: www.linear.com

Multiphase Buck Regulator (Fairchild) Fairchild FAN5094 Reference: www.linear.com

Boost Regulator (Linear Technology) Internal switch Reference: www.linear.com

Another Boost Regulator (Linear Technology) Interleaved outputs Reference: www.linear.com

SEPIC (Linear Technology) Reference: www.linear.com

Cuk (Linear Technology) Reference: www.linear.com

Buck/Boost (Linear Technology) Reference: www.linear.com

Buck/Boost (Linear Technology) Reference: www.linear.com

EMI Let’s look at electromagnetic interference (EMI) We’ll focus on the switch current isw in the boost converter Reference: Alexander Kusko and Marc Thompson, Power Quality in Electrical Systems, McGraw-Hill, 2007

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

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

Testing for Conducted EMI Line impedance stabilizing network (LISN) Reference: Alexander Kusko and Marc Thompson, Power Quality in Electrical Systems, McGraw-Hill, 2007

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

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

EMI Filter Reference: On Semiconductor

Typical EMI Spectrum from a DC/DC Converter In this case, fsw ~ 300 kHz DC 1st 2nd 3rd Reference: Power Integrations, Inc., “Techniques for EMI and Safety,” Application Note AN-15, June 1996 , www.powerint.com

Conducted EMI Standards Covers 150 kHz – 30 MHz Reference: Power Integrations, Inc., “Techniques for EMI and Safety,” Application Note AN-15, June 1996 , www.powerint.com

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

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

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

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

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

Example 4: Buck Simulation Results Switch current In this case, switch current = supply current

Example 4: Buck Simulation Results Switch current spectrum; fsw = 200 kHz 1st 3rd 2nd

Example 5: Non-Ideal Buck Simulation with Input Filter

Example 5: Non-Ideal Buck Simulation with Input Filter Switch current Supply current Capacitor current

Example 5: Buck Simulation Results Switch current spectrum; fsw = 200 kHz Switch current Supply current Capacitor current