1. Power Electronics Notes 07C Boost Converter Design Example
Marc T. Thompson, Ph.D.
Thompson Consulting, Inc.
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Harvard, MA
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© Marc Thompson, 2008

2. Summary Design a boost converter with the following specifications:
Input voltage: 12V
Output: 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

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

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

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

6. Boost Converter: Discont. Conduction
Occurs at light loads

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

8. 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

9. 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

10. 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

11. Boost Converter Current Waveforms

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

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

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

15. 1st Cut Design

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

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

18. 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

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

20. 2nd Cut Design Lower ESR capacitors and an LC post filter added
caps

21. 2nd Cut Design --- Simulation Results
Ripple  20 mV pp

22. 2nd Cut Design --- Efficiency Estimate
Losses due to:
Inductor loss
Switch conduction loss
Switch switching loss
Diode loss
Capacitor ESR loss
Gate drive loss

23. 2nd Cut Design --- Efficiency Estimate

24. 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

25. 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