1. Introduction to DC-DC Conversion – Cont.
EE174 – SJSU
Tan Nguyen

2. 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.

3. SWITCHING MODE POWER SUPPLY (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

4. Simple switching-mode power supply Heater:
The heater turns on and off every several minutes to keep the room temperature constant.
Examples:
Vin = 12 Vdc and the load resistor R2 = 0.25 ohms. The objective is to open and close the switch so that the average voltage across R2 is 5 Vdc. The waveform of the voltage across R2 shown below.
Vo = Vin x D where D = Ton / (Ton + Toff) : Duty cycle

5. Pulse-width Modulator (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

6. 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.
DC-DC Buck Converter Module V to V 6A Adjustable Set-Down Regulator
Size:30mm(L)*18mm(W)*14(H) mm
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7. The Buck Converter converts dc from one level to another
the average output voltage is controlled by the ON-OFF switch
PWM switching is employed
lower average output voltage than the dc input voltage Vd depending on the duty ratio, D
D=ton/Ts
Average output:
Applications:
regulated switch mode dc power supplies
dc motor drives
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
in the steady-state, average inductor voltage is zero
in the steady-state, average capacitor current is zero

8. Buck Converter Inductor current iL flows continuously
Average inductor voltage over a time period must be zero
Assuming a lossless circuit

9. Buck Converter Assume large C so that Vout has very low ripple
 Flywheel circuit
Assume large C so that Vout has very low ripple
Since Vout has very low ripple, then assume Iout has very low ripple
Interchange of energy between inductor and capacitor is referred as flywheel effect.

10. Buck Converter in Continuous Conduction
What do we learn from inductor voltage and capacitor current in the average sense?
the average current through a capacitor operating in periodic steady state is zero
the average voltage across an inductor operating in periodic steady state is zero

11. Buck Converter in Continuous Conduction Switch closed for DT seconds
+ (Vin – Vout) –
Switch closed for DT seconds
Where D = Duty Cycle
T = Switching period

12. Buck Converter in Continuous Conduction
– Vout +
Switch open for (1 − D)T seconds
When switch open VL = - Vout, diode is closed (forward biased) so iL continues to flow. This is the assumption of “continuous conduction” in the inductor which is the normal operating condition.

13. Buck Converter If D is duty cycle average output voltage is
Since the average voltage across L is zero
The input/output again becomes
From power balance, 

18. Power Losses in a Buck Converter
There are two types of losses in an SMPS:
DC conduction losses.
AC switching losses.

19. 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 = = 12.5%
Ts = 1/fs = 1/4000 = 0.25 ms = 250 μs
Ton = DTs = μs
Toff = Ts – ton = 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 = A

20. 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.

21. 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 = 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 W + 0.2W = 4.095W
Buck converter efficiency:

22. Power Losses in a Buck Converter
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 = 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 W + 0.2W = 4.095W
Converter efficiency:

23. AC Switching Losses in Buck Converter
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.
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.
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.

24. Basic Nonisolated DC/DC SMPS Topologies
BUCK COVERTER

25. Basic dc-dc converters and their dc conversion ratios M(D) = V/Vg.

26. An example of Linear Regulator versus Switch-Mode Regulator
VBAT = 3.7 V nom, BIN_BB = 1.2 V
Load Current = 600 mA
Power delivered to load = 600 mA * 1.2 V = 720 mW
Power converted to heat = 20 mW * ((3.7/1.2) 1) = 1,500 mW
Total power consumed = 720 mW + 1,500 mW = 2,200 mW
32% goes to work, 68% goes to heating user hand and ear.
Switch-mode regulator:
VBAT = 3.7 V nom; BIN_BB = 1.2 V
Load Current = 600 mA
Converter efficiency = 90%
Power delivered to load = 600 mA * 1.2 V = 720 mW
Power converted to heat =720 mW * ((1/0.9) 1)=80 mW
Total power consumed = 720 mW + 80 mW = 800 mW
90% goes to work, 10% goes to heating user hand and ear.

27. Sample of Linear and Switch-Mode Regulator Output

28. References: http://en.wikipedia.org/wiki/DC-to-DC_converter
Linear Technology - Application Note 140
Notes from Fang Z. Peng Dept. of Electrical and Computer Engineering MSU