What Is an SMPS?

A switched-mode power supply (SMPS) uses high-frequency switching to transfer electrical energy efficiently from an input source to a regulated output.

Unlike a conventional linear power supply, an SMPS does not normally dissipate the excess input voltage continuously as heat. Instead, a switching transistor rapidly controls the energy transferred to inductive and capacitive components.

DC / Rectified AC
       │
       ▼
   Switching
   MOSFET / IGBT
       │
       ▼
 Transformer /
 Inductor
       │
       ▼
   Rectifier
       │
       ▼
 Output Filter
       │
       ▼
    DC Output

Common SMPS topologies include buck, boost, buck-boost, flyback, forward, push-pull, half-bridge and full-bridge converters.

Output Power

The output power of a DC switching supply is:

Pout = Vout × Iout

Example — 24 V at 5 A

Pout = Vout × Iout

Pout = 24 × 5

Pout = 120 W

The supply must therefore deliver approximately 120 W to the load.

Input Power and Efficiency

An SMPS consumes more power at its input than it delivers at its output because of switching, magnetic, conduction and other losses.

η = Pout / Pin

Therefore:

Pin = Pout / η

where efficiency η is expressed as a decimal.

Input Current

Once input power is known, approximate input current can be calculated from:

Iin = Pin / Vin

Buck Converter

A buck converter reduces a higher DC input voltage to a lower output voltage.

For an ideal buck converter operating in continuous conduction mode:

Vout = D × Vin

Therefore:

D = Vout / Vin

where D is the duty cycle.

Buck Converter Duty Cycle Calculator

Boost Converter

A boost converter produces a higher output voltage than its input.

For an ideal boost converter in continuous conduction mode:

Vout = Vin / (1 - D)

Therefore:

D = 1 - Vin / Vout

Boost Converter Duty Cycle Calculator

Buck-Boost Converter

The basic inverting buck-boost converter can produce an output whose magnitude can be either higher or lower than the input.

For an ideal converter:

|Vout| = Vin × D / (1 - D)

Therefore:

D = |Vout| / (Vin + |Vout|)

Buck-Boost Duty Cycle Calculator

Switching Frequency

Switching frequency is the rate at which the power switch turns on and off.

f = 1 / T

where:

• f = switching frequency in Hz
• T = switching period in seconds

Duty Cycle and Switching Time

Duty cycle is the fraction of each switching period during which the power switch is on.

D = Ton / T

or:

Ton = D × T

Inductor Ripple Current

In a buck converter operating in continuous conduction mode, the inductor current rises while the switch is on and falls while the switch is off.

The inductor ripple current can be estimated from:

ΔIL = (Vin - Vout) × D
      ───────────────────
          L × f

where:

• ΔIL = peak-to-peak inductor ripple current
• L = inductance
• f = switching frequency

Buck Inductor Calculator

Required Buck Inductance

The required inductance can also be calculated when the desired inductor ripple current is known:

L = (Vin - Vout) × D
    ─────────────────
       ΔIL × f

Output Capacitor Ripple

The output capacitor reduces voltage ripple produced by the switching current.

A simplified capacitive ripple estimate is:

ΔV ≈ ΔI / (8 × f × C)

This simplified relationship is useful for an initial estimate in a buck converter, but actual output ripple also depends on capacitor ESR and the inductor current waveform.

Output Capacitor Calculator

Required Output Capacitance

Capacitor ESR Ripple

The ESR of the output capacitor can contribute additional ripple:

VESR ≈ ΔIL × ESR

Therefore, a capacitor with very low capacitance but extremely low ESR and a capacitor with high capacitance but higher ESR can have different ripple characteristics.

MOSFET Switching Loss

An SMPS power switch dissipates energy during switching transitions. A simplified switching-loss estimate is:

Psw ≈ 0.5 × V × I × (tr + tf) × f

where:

• V = switched voltage
• I = switched current
• tr = rise time
• tf = fall time
• f = switching frequency

This is a simplified estimate. Real switching losses also depend on gate charge, Miller plateau, driver impedance, diode recovery, parasitic inductance and the actual switching waveform.

Switching Loss Calculator

Conduction Loss

For a MOSFET with an approximate on-state resistance:

Pcond ≈ I² × RDS(on)

The actual loss depends on RMS current and the conduction duty cycle.

SMPS Thermal Dissipation

The total semiconductor loss contributes to temperature rise. A simplified junction temperature estimate is:

Tj = Ta + P × RθJA

where:

• Tj = junction temperature
• Ta = ambient temperature
• P = dissipated power
• RθJA = junction-to-ambient thermal resistance

SMPS Transformer

High-frequency SMPS transformers operate differently from conventional 50 Hz mains transformers.

The required number of turns depends on voltage, switching frequency, core area, flux density and topology.

For a simplified square-wave excitation:

N ≈ V × ton
    ─────────
    B × Ae

where:

• N = number of turns
• V = winding voltage
• ton = switch on-time
• B = allowed flux-density swing
• Ae = effective core area

The exact transformer equation depends on topology and waveform, so this simplified relationship should not be used as a complete high-frequency transformer design method.

SMPS Transformer Turns Calculator

Gate Drive

The switching transistor must be driven correctly to minimize switching loss and ensure reliable operation.

Important gate-drive parameters include:

• Gate voltage
• Gate charge
• Driver source current
• Driver sink current
• Rise and fall time
• Switching frequency
• Gate resistance
• Common-source inductance

A MOSFET gate is primarily capacitive. The driver therefore needs to source and sink current quickly to charge and discharge the gate.

Gate Charge Current

A simplified average gate-drive current estimate is:

Ig(avg) ≈ Qg × f

where:

• Qg = total gate charge
• f = switching frequency

SMPS Efficiency

An SMPS can achieve high efficiency, but total efficiency depends on all sources of loss:

• MOSFET conduction loss
• MOSFET switching loss
• Transformer copper loss
• Transformer core loss
• Rectifier loss
• Inductor loss
• Capacitor ESR loss
• Control circuit consumption

A simplified overall efficiency calculation is:

η = Pout / Pin × 100

Common SMPS Topologies

SMPS Design Considerations

A complete SMPS design requires considerably more than calculating duty cycle and component values.

• Input voltage range
• Output voltage range
• Output power
• Switching frequency
• Topology
• Transformer or inductor design
• MOSFET selection
• Gate-drive circuit
• Rectifier selection
• Output filtering
• Feedback compensation
• Current limiting
• Over-voltage protection
• Over-temperature protection
• EMI filtering
• Thermal management
• PCB creepage and clearance

High-Voltage SMPS Safety

Off-line SMPS circuits connected directly to mains can contain dangerous and potentially lethal voltages.

The rectified mains capacitor can remain charged even after the equipment has been disconnected from the mains.

• Use appropriate isolation.
• Use correctly rated components.
• Maintain adequate creepage and clearance.
• Use suitable fusing and protection.
• Discharge high-voltage capacitors safely.
• Do not connect an oscilloscope ground clip directly to a non-isolated mains circuit.

Common Mistakes

• Using ideal converter equations as a complete real-world design.
• Ignoring switching losses.
• Ignoring MOSFET conduction losses.
• Ignoring diode reverse-recovery losses.
• Ignoring transformer core saturation.
• Ignoring transformer copper losses.
• Choosing an inductor without checking saturation current.
• Choosing an output capacitor without checking ESR and ripple current.
• Ignoring gate-drive requirements.
• Ignoring PCB parasitics.
• Ignoring thermal management.
• Ignoring EMI and layout.

Key Points

• SMPS circuits transfer energy through high-frequency switching.
• Output power is Vout × Iout.
• Real efficiency is always affected by circuit losses.
• Buck duty cycle is approximately Vout/Vin in ideal continuous conduction.
• Boost duty cycle is approximately 1 − Vin/Vout.
• Inductor ripple depends on voltage, inductance, duty cycle and switching frequency.
• Output capacitor ESR contributes directly to voltage ripple.
• MOSFET switching and conduction losses must both be considered.
• High-frequency transformer design depends on topology and core characteristics.
• Thermal, EMI, protection and PCB layout are essential parts of SMPS design.