SWITCHING MODE POWER SUPPLY (SMPS) TOPOLOGIES



OVERVIEW, COMPARISON AND SELECTION GUIDE


Let's start with some definitions. What is a PSU 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 arrangement is referred to as a topology. There are about a dozen basic converter topologies used in practical power design. They can be classified into two major families: buck and flyback. 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 "off-state". Any practical circuit belongs to one of these two categories or their combination. Examples of the buck type are forward and bridge converters. Examples of the flyback type are inverting and boost regulators. Note that 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.

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.

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 DC-DC 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 DC-DC 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, power losses, MOSFETs and diodes voltage drops 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 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. 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. As an illustration, the table below shows the topologies 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). Note, all the information here is just a general guidance, which is based on the author's personal view.
0-100 W, Iout<5 A 0-100 W, Iout>5 A 100-400 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.

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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