Introduction to DC-DC Converter

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Introduction to DC-DC Converter Cont.

• EE174 SJSU
• Tan Nguyen

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SWITCHING MODE POWER SUPPLY (SMPS)

• The switching-mode power supply is a power supply
  that provides the power supply function through
  low loss components such as capacitors,
  inductors, and transformers -- and the use of
  switches that are in one of two states, on or
  off.
• It offers high power conversion efficiency and
  design flexibility.
• It can step down or step up output voltage.
• The term switchmode was widely used for this type
  of power supply until Motorola, Inc., who used
  the trademark SWITCHMODE TM for products aimed at
  the switching-mode power supply market, started
  to enforce their trademark. Switching-mode power
  supply or switching power supply are used to
  avoid infringing on the trademark.
• Typical switching frequencies lie in the range 1
  kHz to 1 MHz, depending on the speed of the
  semiconductor devices.
• Types of SMPS
• Buck converter Voltage to voltage converter,
  step down.
• Boost Converter Voltage to voltage converter,
  step up.
• Buck-Boost or FlyBack Converter Voltage-Voltage,
  step up and down (negative voltages)
• Cuk Converter Current-Current converter, step up
  and down
• These converters typically have a full wave
  rectifier front-end to produce a high DC voltages

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SIMPLE SWITCHING MODE POWER SUPPLY
 
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PULSE WIDTH MODULATION (PWM)

• The switch control signal, which controls the on
  and off states of the switch, is generated by
  comparing a signal level control voltage vcontrol
  with a repetitive waveform.
• The switching frequency is the frequency of the
  sawtooth waveform with a constant peak.
• The duty ratio D can be expressed as

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THE BUCK CONVERTER

• The buck converter is known as voltage step-down
  converter, current step-up converter, chopper,
  direct converter.
• The buck converter simplest and most popular
  switching regulator.

The Buck Converter Circuit Diagram

• DC-DC Buck Converter Module 4.5-14V to 0.8-9.5V
  6A Adjustable Set-Down Regulator
• Size30mm(L)18mm(W)14(H) mm

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THE BUCK CONVERTER
If switch VSW is close and open as shown below.
Observe output Vo and current spike at ISW every
time the switch is close.
During switch is close Vi Vo, when switch is
open at T1, Vo starts to discharge, with higher
capacitor C value ? slower discharge rate
(improve ripple).
Use high capacitor C value to improve the output
voltage Vo ripple, but still have issue with
huge current spike ISW when switch is close can
easily burn out the switch.
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THE BUCK CONVERTER
The current spikes ISW can be controlled by
adding an inductor (L) between the switch (SW)
and the capacitor (C ). Since Inductor acts as a
storage energy. When the switch is close, the
inductor will absorb the energy and when the
switch is open, the inductor will supply that
energy to the capacitor result in smooth out ISW.
However, there is another issue with inductor
current path. The inductor current must have
continuous to flow through the inductor but
during the switch open, there is no current path
for the inductor current. To fix this issue,
adding a free-wheeling diode to the circuit below
it, this will keep the inductor current continues
to flow.
Free-wheeling Diode
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THE BUCK CONVERTER SUMMARY

• Two Mode of Operations
• Continuous Conduction Mode Inductor current IL
  does not reach zero, when output current IO is
  very large.
• Discontinuous Conduction Mode Inductor current
  IL will reach zero, when output current IO is
  very small.

• LC low-pass filter to pass the DC component
  while attenuating the switching components.
• diode is reversed biased during ON period, input
  provides energy to the load and to the inductor
• energy is transferred to the load from the
  inductor during switch OFF period
• Interchange of energy between inductor and
  capacitor is referred as flywheel effect.
• in the steady-state, average inductor voltage is
  zero
• in the steady-state, average capacitor current is
  zero

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THE BUCK CONVERTER CONTINUOUS MODE

1. Continuous Conduction Mode Inductor current IL
   does not reach zero, when output current IO is
   very large.

 
 
 
 
 
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THE BUCK CONVERTER DISCONTINUOUS MODE
2. Discontinuous Conduction Mode Inductor
current IL will reach zero, when output current
IO is very small.
When the switch is ON (short), Diode reversed
bias (open) VL Vi VO constant gt 0 When
the switch is OFF (open), Diode forwarded
bias VL VD VO VO constant lt
0 Calculate IL,max and relationship of Vo and Vi
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THE BUCK CONVERTER EXAMPLE

• Given a buck converter design with fsw 200 kHz
  (TS 5 µsec), L 33 µH, C 10 µF, I0 1 A
  and D 50 duty cycle. Find
• VO if Vi 10 V in continuous mode
• Output current and voltage ripples
• Current IL,max
• VO if Vi 10 V in discontinuous mode
• Solutions
• VO D Vi 0.5 x 10 5V
• 
  (33 µH)-1 (10 5) x
  0.5 x 5 µsec 0.38 A
• VO 3.32 V

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Buck Converter Design Example
For a buck converter, R1 ohm, Vd40 V, V05 V,
fs4 kHz. Find the duty ratio and on time of
the switch.
D V0 /Vd 5/40 0.125 12.5 Ts 1/fs
1/4000 0.25 ms 250 µs Ton DTs 31.25
µs Toff Ts ton 218.75 µs When the switch
is on VL Vd - V0 35 V When the switch is
off VL -V0 - 5 V I0 IL V0 / R 5
A Id D I0 0.625 A
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Power Losses in a Buck Converter

• There are two types of losses in an SMPS
• DC conduction losses.
• AC switching losses.

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DC conduction losses in Buck converter

• The conduction losses of a buck converter
  primarily result from voltage drops across
  transistor Q1, diode D1 and inductor L when they
  conduct current.
• A MOSFET is used as the power transistor.
  The conduction loss of the
  MOSFET IO2 x RDS(ON) x D,
• where RDS(ON) is the on-resistance of MOSFET
  Q1.
• The conduction power loss of the diode IO VD
  (1 D), where VD is the forward voltage drop
  of the diode D1.
• The conduction loss of the inductor IO2 x
  RDCR,
• where RDCR is the copper resistance of
  the inductor winding.

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Power Losses in a Buck Converter
Therefore, the conduction loss of the buck
converter is approximately PCON_LOSS (IO2 x
RDS(ON) x D) (IO VD 1 D) (IO2 x RDCR)
Considering only conduction loss, the converter
efficiency is

• Example
• For 12V input buck supply ? 3.3V/10AMAX output
  buck supply.
• Use 27.5 duty cycle provides a 3.3V output
  voltage.
• Vout Vin x D 12 x 0.275 3.3 V
• MOSFET RDS(ON) 10 mO
• Diode forward voltage VD 0.5V (freewheeling
  diode)
• Inductor RDCR 2 mO
• Conduction loss at full load
• PCON_LOSS (IO2 x RDS(ON) x D) (IO x VD x 1
  D) (IO2 x RDCR)
• (102 x 0.01 x 0.275) (10 x
  0.5 x 1 0.275) (102 x 0.002)
• 0.275W 3.62W 0.2W
  4.095W
• Buck converter efficiency

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AC Switching Losses in Buck Converter

1. MOSFET switching losses. A real transistor
   requires time to be turned on or off. So there
   are voltage and current overlaps during the
   turn-on and turn-off transients, which generate
   AC switching losses.
2. Inductor core loss. A real inductor also has AC
   loss that is a function of switching frequency.
   Inductor AC loss is primarily from the magnetic
   core loss.
3. Other AC related losses. Other AC related losses
   include the gate driver loss and the dead time
   (when both top FET Q1 and bottom FET Q2 are off)
   body diode conduction loss.

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Basic Nonisolated DC/DC SMPS Topologies
BUCK COVERTER
 
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Basic dc-dc converters and their dc conversion
ratios M(D) V/Vg.
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Mobile Device Using Linear versus Switch-Mode
Regulator
 
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DC-DC Converter Technology Comparison
Parameter Linear Regulator Switching Regulator
Efficiency Low High
EMI Noise Low High
Output Current Low to Medium Low to High
Boost (Step up) No Yes
Buck (Step down) Yes Yes
Size Small Large
Cost Inexpensive High cost
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Sample of Linear and Switch-Mode Regulator Output
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References http//en.wikipedia.org/wiki/DC-to-DC_
converter https//www.jaycar.com/images_uploaded/
dcdcconv.pdf Linear Technology - Application
Note 140 buck converter tutorial
abuhajara http//www.smpstech.com/tutorial/t03top.
htmSWITCHINGMODE Notes from Fang Z. Peng Dept.
of Electrical and Computer Engineering
MSU https//www.google.com/webhp?sourceidchrome-i
nstantrlz1C1OPRB_enUS587US587ion1espv2ieUT
F-8qpictureofnoiseonbuckoutput https//www
.google.com/url?satrctjqesrcssourcewebcd
2ved0CCQQFjABahUKEwj329-J4YvIAhVLy4AKHZiyADYur
lhttp3A2F2Fusers.ece.utexas.edu2Fkwasinski2
F_6_EE462L_DC_DC_Buck_PPT.pptusgAFQjCNH1PIzP73b3
t11mgGhnUBBg-sVNXgcadrja http//ecee.colorado.e
du/ecen4517/materials/Encyc.pdf https//www.value
tronics.com/Manuals/Lambda_20linear_versus_switch
ing.pdf