Transcript Introduction to DC-DC Conversion – Cont.

Introduction to DC-DC
Converter – Cont.
EE174 – SJSU
Tan Nguyen
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
SIMPLE SWITCHING MODE POWER SUPPLY
42%
58%
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
t on v control
D
 ^
Ts
V st
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
Size:30mm(L)*18mm(W)*14(H) mm
THE BUCK CONVERTER
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).
If switch VSW is close and open as shown below.
Observe output Vo and current spike at ISW every
time the switch is close.
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.
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.
Freewheeling
Diode
THE BUCK CONVERTER SUMMARY
Two Mode of Operations:
1. Continuous Conduction Mode: Inductor current IL does not
reach zero, when output current IO is very large.
2. 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
THE BUCK CONVERTER CONTINUOUS MODE
1. Continuous Conduction Mode: Inductor current IL does not reach zero, when output current IO is very large.
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 > 0
When the switch is OFF (open), Diode forwarded bias:
VL = – VD – VO ≈ – VO = constant < 0
Calculate IL,max and relationship of Vo and Vi:
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:
a) VO if Vi = 10 V in continuous mode
b) Output current and voltage ripples
c) Current IL,max
d) VO if Vi = 10 V in discontinuous mode
Solutions:
a) VO = D Vi = 0.5 x 10 = 5V
b)
c)
d) VO = 3.32 V
= (33 μH)-1 (10 – 5) x 0.5 x 5 μsec = 0.38 A
Buck Converter Design Example
For a buck converter, R=1 ohm, Vd=40 V, V0=5 V, fs=4 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
Power Losses in a Buck Converter
There are two types of losses in an SMPS:
• DC conduction losses.
• AC switching losses.
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.
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 mΩ
• Diode forward voltage VD = 0.5V (freewheeling diode)
• Inductor RDCR = 2 mΩ
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:
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.
Basic Nonisolated DC/DC SMPS Topologies
BUCK COVERTER
Basic dc-dc converters and their dc conversion ratios M(D) = V/Vg.
Mobile Device Using Linear versus Switch-Mode Regulator
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
Sample of Linear and Switch-Mode Regulator Output
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.htm#SWITCHINGMODE
Notes from Fang Z. Peng Dept. of Electrical and Computer Engineering MSU
https://www.google.com/webhp?sourceid=chromeinstant&rlz=1C1OPRB_enUS587US587&ion=1&espv=2&ie=UTF8#q=picture+of+noise+on+buck+output
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCQQFjABa
hUKEwj329J4YvIAhVLy4AKHZiyADY&url=http%3A%2F%2Fusers.ece.utexas.edu%2F~kwasinski%2F_6_EE
462L_DC_DC_Buck_PPT.ppt&usg=AFQjCNH1PIzP73b3t11mgGhnUBBg-sVNXg&cad=rja
http://ecee.colorado.edu/ecen4517/materials/Encyc.pdf
https://www.valuetronics.com/Manuals/Lambda_%20linear_versus_switching.pdf