SWITCHING MODE POWER SUPPLY (SMPS) TOPOLOGIES

OVERVIEW, COMPARISON AND SELECTION GUIDE

What is a power supply topology? Switch mode power supply (SMPS) circuits contain networks of energy storage inductors and capacitors as well as power handling transistors and rectifiers. Their particular configuration is referred to as a topology. Although there is a wide variety of converter topologies, only about a dozen basic ones are used in practical power design.
They can be classified into two major families: buck and flyback depending on the method of operation. In the buck family energy is transferred from the input to the load during the conduction cycle of a switching transistor. In the flyback family the energy is accumulated during "on-state" of a switch and delivered to the output during the "off-state". Any practical circuit belongs to one of these two configurations or their combination. Examples of the buck type are forward and bridge converters. Examples of the flyback type (besides transformer-isolated flyback) are buck-boost and boost regulators.

A complete off-line PSU normally contains several conversion stages. Usually it is comprised of a rectifier section followed by a PFC boost and one or more downstream DC-DC converters. Each of these stages is usually controlled by a separate controller IC, although there are also PFC-PWM combo ICs. The table below summarizes and compares electrical features and characteristics of the main single-stage switching regulator circuits. This chart is followed by the DC to DC regulator selection guide For a complete information about the main topologies and the design information get my handbook.

Converter topology Diagram DC transfer
function
(Vout/Vin)

NON-ISOLATING DC-DC CONVERTERS
Buck Buck converter diagram
 D

(0<D<1)
Boost Boost converter diagram
 1/(1-D)

(0<D<1)
Flyback (inverting) or buck-boost Buckboost diagram -D/(1-D)

(0<D<1)
Ćuk Cuk converter diagram -D/(1-D)

(0<D<1)
Sepic Sepic converter diagram D/(1-D)

(0<D<1)

ISOLATING DC-DC CONVERTERS
DCM Flyback Flyback converter diagram Vin2D2/
(2Lp×F ×Iout)

(0<D<1)
DCM 2-switch
flyback		 2-switch flyback converter diagram Vin2D2/
(2Lp×F ×Iout)

(0<D<1)
Forward Forward converter diagram Ns/Np×D

(0<D<0.5)
2-switch
forward		 2-switch converter circuit Ns/Np×D

(0<D<0.5)
Active
clamp
forward		 Active clamp forward converter schematic Ns/Np×D

(0<D<1)
Half-
bridge		 Half-bridge converter diagram Ns/Np×D

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

(D<0.5)
Full bridge Full bridge converter diagram 2Ns/Np×D

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

(0<D<0.5)
LLC
half bridge		 LLC converter diagram Ns/2Np

(at F=Fres)
Notes:
1. All formulas are given for ideal circuits. Ripple currents, voltage spikes, power losses, voltage drops in MOSFETs and diodes are excluded.
2. Transfer function for phase shifted bridge uses effective ("overlapping") duty cycle.
3. For LLC converter the formulas are given for operation at resonance.
4. 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 switching power supply 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 "textbook-based" approach in which you are most experienced. Well executed trivial design is better than poorly executed "fancy" one. Of course, while a basic design may be straightforward, meeting all of the safety and EMC regulatory specifications may still be a very time consuming task. 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 functional requirements are pretty much conventional, the power level is usually the main factor that determines the topology selection.
smps design handbook

As an illustration, the table below shows the configurations I would normally 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 are typical levels for rectified AC input line voltage or the output of a PFC boost). At power levels above 3000W I would consider using multiple interleaved converters to spread heat dissipation. Note, all the information here of course, is just a general guidance based on the author's personal view.

0-100 W,
Iout <10 A		 0-100 W,
Iout >10 A		 100-400 W 400- 1200 W 1200- 3000 W
Single-switch flyback Yes
 - - - -
2-switch flyback Yes - - - -
Active clamp flyback Yes - - - -
Single-switch forward Yes Yes - - -
2-switch forward Yes Yes Yes - -
Active clamp forward Yes Yes Yes - -
Half bridge - Yes Yes Yes -
LLC Half Bridge - Yes Yes Yes -
Full bridge - - - Yes -
Phase shifted ZVT full bridge - - - Yes Yes
Note: All info here is presented for reference only and does not constitute a professional advice- see a complete Disclaimer linked below.

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