Classification of SMPS
Non isolated converters
(Input and output share a common ground): Buck, boost and bock-boost converter
Isolated converters
(Use a transformer to electrically isolate
(use separate ground) input and output): Fly back, forward, half bridge and full bridge converter
Basic of Buck Converter
Step-down DC–DC switching regulator that reduces the
output voltage while proportionally increasing the output current to conserve
power.
• It operates using high-frequency switching of a semiconductor device
(typically a MOSFET) to achieve conversion efficiencies above 90%.
• The converter alternates the switch between fully ON and fully OFF states,
minimizing conduction and switching losses.
• Commonly utilized in computer and embedded systems to derive lower
supply rails (e.g., 1.8 V–4.2 V) from a standard 12 V DC bus.
• Typical applications include USB power regulation, DRAM voltage supply,
and CPU core voltage conversion.
• Serves as a core topology in modern switched-mode power supplies (SMPS)
where compactness, efficiency, and precise voltage regulation are required.
Role of the diode
A freewheeling diode, also known as a flyback diode, is connected with an inductive load
to provide a path for the inductor current when the driving switch is turned OFF
Modes of Buck Converter
Switch ON Condition
Current will flow through switch, inductor and load as well as partial current will flow through the capacitor. Inductor charges. The output voltage is approximately equal to the input voltage
Switch OFF Condition
Charged inductor will reverse its polarity and release its energy to maintain the output voltage across the load. (Inductor discharges)
• Current will flow through inductor, load and freewheeling diode as well as partial current will flow through the capacitor.
Volt-second balance
States that, in steady state, the average voltage across the inductor over one complete switching period is zero.
𝑉𝐿 (𝑂𝑁) × 𝑇(𝑂𝑁) + 𝑉𝐿 (𝑂𝐹𝐹) × 𝑇(𝑂𝐹𝐹) = 0
𝑉𝐿 = Inductor voltage
When Switch is on (using KVL),
𝑉𝐿 (𝑂𝑁) = 𝑉𝑠 − 𝑉𝑜
When Switch is off (using KVL)
𝑉𝐿 (𝑂𝐹𝐹) = −𝑉𝑜
(sign of 𝑉𝐿 (𝑂𝐹𝐹) is negative relative to its ON-state polarity)
Buck Converter:
Deravation of Vo from 𝑉𝐿 (𝑂𝑁) × 𝑇(𝑂𝑁) + 𝑉𝐿 (𝑂𝐹𝐹) × 𝑇(𝑂𝐹𝐹) = 0
D = Ton/T ∴ Ton = DT
D = (T-Toff)/T ∴ DT = T - Toff ∴ Toff = (1 - D)T
From Volt-second balance:
𝑉𝐿 (𝑂𝑁) = 𝑉𝑠 − 𝑉𝑜
𝑉𝐿 (𝑂𝐹𝐹) = −𝑉𝑜
∴ substituting in 𝑉𝐿 (𝑂𝑁) × 𝑇(𝑂𝑁) + 𝑉𝐿 (𝑂𝐹𝐹) × 𝑇(𝑂𝐹𝐹) = 0:
(𝑉𝑠 − 𝑉𝑜</sub) × DT + (−𝑉𝑜) × (1 - D)T = 0
Expand:
VsDT - VoDT - VoT + VoDT = 0
∴
VoT = VsDT
Dividing over T:
Vo = DVs
Buck converter:
Ampere-second balance and current derivation
Also called charge balance, states that, in steady state, the average current through a capacitor over one complete switching period is zero.
IC (𝑂𝑁) × 𝑇(𝑂𝑁) + IC (𝑂𝐹𝐹) × 𝑇(𝑂𝐹𝐹) = 0
When Switch is on (using KCL)
IC (𝑂𝑁) = IL - IO
When Switch is off
IC (𝑂FF) = IL - IO
Derivating IC (𝑂𝑁) × 𝑇(𝑂𝑁) + IC (𝑂𝐹𝐹) × 𝑇(𝑂𝐹𝐹) = 0 with the duty cycle:
IO = IL
Boost Converter
Step-up DC–DC switching regulator that increases the output voltage while proportionally decreasing the output current to conserve power.
• It operates using high-frequency switching of a semiconductor device (typically a MOSFET) and a diode–inductor–capacitor network to achieve conversion efficiencies above 90%.
• The converter alternates the switch between fully ON and fully OFF states, minimizing conduction and switching losses.
• _Commonly employed in battery-powered systems _where the available voltage must be raised to meet load requirements (e.g., boosting 3.7 V from a Li-ion cell to 5 V).
• Typical applications include portable electronics, solar power systems, LED drivers, and automotive power supplies.
• Serves as a core topology in modern switched-mode power supplies (SMPS) where compact design, high efficiency, and stable voltage boosting are required.
Modes of Boost Converter
Switch ON condition
• Current will flow through switch and
inductor.
• Inductor will get charged.
• The diode is reverse-biased, preventing current flow to the output, so the output capacitor supplies the load. Voltage is maintained by the capacitor
Switch OFF condition
• The inductor’s current cannot stop suddenly, so the
charged inductor reverses its voltage polarity to keep current flowing and release its energy through the load.
• Current will be supplied by the source and the
inductor. Hence, current will flow through
inductor, load and freewheeling diode as well as
partial current will flow through the capacitor.
• The inductor voltage adds to the input voltage
How does the diode in Boost Converter flip from RB to FB?
Switch ON
The right end of the iductor (connected to diode anode) is pulled near ground through the switch. The diode’s cathode is sitting at the high output voltage (Vo, which is higher than Vin), so the diode sees this diffrence:
VD = Vanode - Vcathode = 0 - Vo
Switch OFF
Inductor reverses polatity.
VD = Vanode - Vcathode = (Vin + VL) - Vo
∴ Vanode > Vin (so diode is FB now)
Boost Converter:
Deravation of Vo as per Volt Sec Balance
Switch ON
Vs - VL(ON) = 0
VL(ON) = VS
Swtich OFF
Vs + ( - VL (OFF)) - Vo = 0
VL(OFF) = Vs - Vo
TON = DT
TOFF = (1-D)T
Substite:
VS ×DT + ( VO - VS) × (1-D)T = 0
VsDT - VoT + VsT + VoDT - VsDT = 0
VoT - VoDT = VsT
T(Vo - VoD) = VsT
Vo - VoD = Vs
Vo(1 - D) = Vs
Vo = - Vs / (1 - D)
Buck converter:
Ampere-second balance and current derivation
Also called charge balance, states that, in steady state, the average current through a capacitor over one complete switching period is zero.
IC (𝑂𝑁) × 𝑇(𝑂𝑁) + IC (𝑂𝐹𝐹) × 𝑇(𝑂𝐹𝐹) = 0
When Switch is on (using KCL)
IC (𝑂𝑁) = - IO (capacitor discharges through the load)
When Switch is off
IC (𝑂FF) = IL - IO
Derivating IC (𝑂𝑁) × 𝑇(𝑂𝑁) + IC (𝑂𝐹𝐹) × 𝑇(𝑂𝐹𝐹) = 0 with the duty cycle:
IO = (1 - D)IL
Buck-Boost Converter
Also called an non-isolated type converter or inverting regulator
Modes of operation in buck-boost converter
➢ Mode 1:
• Transistor Q1 is turned on, allowing the inductor to charge as current flows in a clockwise direction, consistent with the polarity indicated in the figure.
• During this period, the diode remains OFF
since it is reverse-biased.
• As a result, the voltage V appears across the
inductor L.
➢ Mode 2:
• Transistor Q1 is turned OFF, and the inductor releases its stored energy, causing its polarity to reverse.
• The released energy charges capacitor C2.
• During this interval, the diode becomes
forward-biased and therefore turns ON.
• This process continues until the inductor’s
stored energy is fully depleted.
Mode 3:
• Once the inductor has released all its energy, no current flows, and all active components are OFF.
• The diode D turns OFF as well.
• The capacitor now supplies current to the
load loop, as highlighted in the blue path.
• The voltage across the capacitor and output are negative with respect to the input; therefore, the circuit operates as an inverting regulator/converter.
Mode III: happens only when D < 0.5. Inductor stores less energy, releases all that energy
quickly during the OFF period. Inductor current reaches
zero before the next ON, so Mode III begins.
Buck-Boost Converter:
Deravation of Vo as per Volt Sec Balance
VL(on)Ton + VL(off)Toff = 0
VsDT + Vo(1 - D)T = 0
VsD = - Vo(1 - D)
Vo = - Vs (D / (1 - D) )
if D > 0.5 then is a buck converter, because output voltage V0 is less than input voltage Vs.
if D < 0.5 then is a boost converter, because output voltage V0 is greater than input voltage Vs.
Advantage/Disadvantages of SMPS
High efficiency (80–95%) — minimal power loss since the switch operates fully ON or OFF.
BUT Generates EMI/noise — high-frequency switching causes interference and requires filtering.
Compact and lightweight — small inductors can use due to high-frequency operation.
BUT Complex design — needs control circuitry, feedback, and protection circuits.
Wide input voltage range — can regulate output even with fluctuating mains.
BUT Higher initial cost — more components (switching ICs, inductors, filters).
Better voltage regulation — maintains stable output under load or input variations.
BUT Switching ripple and noise — unsuitable for very sensitive analog circuits without extra filtering.
Low heat generation — less dissipation compared to linear regulators.
BUT Design/layout sensitive — improper PCB layout may cause instability or oscillations.
High power density — more power delivered per unit volume.
BUT Difficult to troubleshoot — high-frequency operation
complicates repair and testing.
Multiple outputs possible — one converter can feed several DC rails.
BUT Slower transient response — may show voltage dips or spikes with fast load changes.
