Power Electronics Notes 07C Boost Converter Design Example 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, 2008

Summary Design a boost converter with the following specifications: Input voltage: 12V Output: 24V @ 1A, 24 Watts Continuous conduction mode Inductor and capacitors: selected from following datasheets Switching frequency 100 kHz Output voltage ripple < 50 mV-pp Evaluate output ripple and estimate efficiency of converter

Step-Up (Boost) DC-DC Converter Output voltage is higher than the input, without a phase inversion

Boost Converter Waveforms Continuous current conduction mode Switch closed: Switch open: Inductor Volt-second balance:

Boost: Limits of Cont./Discont. Conduction The output voltage is held constant For low load current, current conduction becomes discontinuous

Boost Converter: Discont. Conduction Occurs at light loads

Boost Converter: Effect of Parasitics The duty-ratio D is generally limited before the parasitic effects become significant

Boost Converter Output Ripple ESR is assumed to be zero Assume that all the ripple component of diode current flows through capacitor; DC component flows through resistor

Boost Converter 1st-Cut Design --- Inductor What is minimum inductor value to keep this converter in continuous conduction mode ? (I.e. this converter operates at the continuous/discontinuous conduction boundary) Average diode current: 0.5Ipk(1-D) = Io = 1A Ipk = 4A Lmin =(Vo – Vi)(1-D)T/Δi = (24-12)(0.5)(10-5)/4 = 15 µH For the diode, ID,rms = = 2.3A

Inductor Datasheet Use 22 µH (ESR = 0.085 Ohms) Note that series resonant frequency (SRF) is much higher than operating frequency Note that IRMS rating of this inductor is 2.7A

Boost Converter Current Waveforms

Boost Converter 1st-Cut Design --- Capacitor What is minimum capacitor value ?

Capacitor Datasheet Use 3 47 µF caps in parallel (35V, ESR = 0.9 Ohms)

MOSFET Datasheet This device is over-sized, but let’s use it anyway

1st Cut Design

Simulation Result --- Inductor Current Note that inductor ripple is about 3A peak to peak

Simulation Result --- Output Ripple Why is output voltage ripple so large ?

Simulation Result --- Analysis The culprit is capacitor ESR. Ripple current is 3A pp, divided into 3 capacitors. Ripple voltage = ripple current x ESR This is a problem with the boost converter --- large output ripple current makes sizing capacitor difficult Ripple  1 V pp

Mitigating Strategies Parallel up more capacitors, or find capacitors with even lower ESR Alternative strategy: use lower ESR caps with a post-filter

2nd Cut Design Lower ESR capacitors and an LC post filter added caps

2nd Cut Design --- Simulation Results Ripple  20 mV pp

2nd Cut Design --- Efficiency Estimate Losses due to: Inductor loss Switch conduction loss Switch switching loss Diode loss Capacitor ESR loss Gate drive loss

2nd Cut Design --- Efficiency Estimate

2nd Cut Design --- Evaluation FATAL DESIGN FLAW Note that ISAT rating of this inductor is 2.6A Peak current in inductor is 3.4A Therefore, this design will blow up

3rd Cut Design --- Replace Inductor Using next-size up Coilcraft inductor, Isat rating of 22 µH inductor is 7.0A, RMS rating is 3.5A, so this should be OK Inductor loss will be lower due to lower DC resistance This comes at the cost of a more expensive inductor, and more PC board space needed