SWITCHING MODE POWER SUPPLY (SMPS) TOPOLOGIES



OVERVIEW, COMPARISON AND SELECTION GUIDE
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Switch mode power supply (SMPS) circuits contain networks of energy storage inductors and capacitors as well as power handling electronic switches and rectifiers. Their particular arrangement is referred to as a topology. There are about a dozen main converter topologies used in practical power design. The table below summarizes and compares electrical features and characteristics of the main switching regulator circuits. This chart is followed by the converter topology selection guide.
Converter topology Diagram DC transfer
function
(Vout/Vin)		 Max
switch
voltage		 Peak switch current Max
rectifier voltage		 Average
rectifier current		 Switch utilization
ratio (SUR)

NON-ISOLATING CONVERTERS
Buck Buck converter diagram
 D

(0<D<1)		 Vin Iout Vin Iout×D Vout/Vin
Boost Boost converter diagram
 1/(1-D)

(0<D<1)		 Vout Iout×Vout /Vin Vout Iout Vin/Vout
Flyback (inverting) or buck-boost Buckboost diagram -D/(1-D)

(0<D<1)		 Vin+|Vout| Iout×
(1+|Vout|/Vin)		 Vin+|Vout| Iout |Vout|/Vin
Ćuk Cuk converter diagram -D/(1-D)

(0<D<1)		 Vin+|Vout| Iout×
(1+|Vout|/Vin)		 Vin+|Vout| Iout |Vout|/Vin
Sepic Sepic converter diagram D/(1-D)

(0<D<1)		 Vin+Vout Iout Vin+Vout Iout Vout/
(Vin+Vout)

ISOLATING CONVERTERS
Flyback Flyback converter diagram √(2Pout×
LpF/Vin)

(0<D<1)		 Vin+Vout
×(Np/Ns)		 D×Vin/Lp×F Vout+ (Vin×Ns/Np) Iout
2-switch
flyback		 2-switch flyback converter diagram √(2Pout×
LpF/Vin)

(0<D<0.5)
 Vin D×Vin/Lp×F Vout+ (Vin×Ns/Np) Iout D/4
Forward Forward converter diagram Ns/Np×D

(0<D<0.5)		 2×Vin Iout×Ns/Np Vin×Ns/Np D1: Iout×D

D2:
Iout(1-D)		 Vout/2Vin ×Ns/Np
2-switch
forward		 2-switch converter diagram Ns/Np×D

(0<D<0.5)		 Vin Iout×Ns/Np Vin×Ns/Np D1: Iout×D

D2:
Iout(1-D)		 Vout/2Vin ×Ns/Np
Active
clamp
forward		 Active clamp forward converter diagram Ns/Np×D

(0<D<1)		 Vin/(1-D) Iout×Ns/Np Vin×Ns/Np D1: Iout×D

D2:
Iout(1-D)		 Vout/Vin× (1-Vout×Np/ Vin×Ns)
Half-
bridge		 Half-bridge converter diagram Ns/Np×D

(0<D<0.5)		 Vin Iout×Ns/Np Vin×Ns/Np 0.5×Iout Vout/2Vin ×Ns/Np
Push-
pull		 Push pull converter diagram 2Ns/Np×D

(D<0.5)		 2×Vin Iout×Ns/Np 2Vin×Ns/Np 0.5×Iout Vout/4Vin ×Ns/Np
Full bridge Full bridge converter diagram 2Ns/Np×D

(0<D<0.5)		 Vin Iout×Ns/Np 2Vin×Ns/Np 0.5×Iout Vout/2Vin ×Ns/Np
Phase shifted
full bridge		 Phase shifted ZVT converter diagram 2Ns/Np×D

(0<D<0.5)		 Vin Iout×Ns/Np Vin×Ns/Np 0.5×Iout Vout/2Vin ×Ns/Np
Notes:
1. All formulas are given for ideal circuits. Ripple currents, voltage spikes, diodes voltage drop and power losses are excluded.
2. Flyback equations are given for discontinuous mode of operation.
3. SUR is total switch utilization ratio defined as SUR=Pout/n×Vmax×Imax, where n- the number of power switches in the circuit, Vmax and Imax- their peak voltage and current.

TOPOLOGY SELECTION CONSIDERATIONS

. There is no single topology, which is best for all applications. The right SMPS topology for a given application should be selected based on specific requirements for the power supply design including cost, size, time factors, and expected production volume. For example, for low-volume designs, the engineering expenses may be more important than BOM cost. In this case, you may want to choose a straightforward approach in which you are most experienced. For a high-volume production, you'll want to put extra engineering efforts in developing new solutions, minimizing component cost and assembly labor. When the other functional requirements are typical, the power level is usually the main factor that determines the topology. As an illustration, the table below shows the topologies I would prefer for a downstream DC-DC converter in an offline switching power circuit depending on its output power level. This selector guide is given for the power sources with output voltages below 60V running off 120 to 400V DC-link (which is typical for rectified AC input line voltage or the output of a PFC boost). Note, all the information here is just a basic guidance, which is based on the author's personal view. It does not constitute a professional or a legal advice (see our Disclaimer linked below).
0-100 W, Iout<5 A 0-100 W, Iout>5 A 200-400 W 400-1200 W 1200-3000 W
Single switch flyback Yes
 - - - -
2-switch flyback Yes - - - -
Single switch forward Yes Yes - - -
2-switch forward Yes Yes Yes - -
Half bridge - - Yes Yes -
Full bridge - - - Yes -
ZVT full bridge - - - Yes Yes
Note: At power levels above 3000W consider using multiple interleaved converters

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