SWITCHING MODE POWER SUPPLY (SMPS) TOPOLOGIES
OVERVIEW, COMPARISON AND SELECTION GUIDE
SMPS Home
| SMPS
Design | Power
Supply Tutorial | PC
PSU |
DC-AC
Inverters |
UPS | Thermal
Design | Transformers
| Electronic
Circuit
Let's start with some definitions. 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. Power train of a complete off-line PSU normally contains a rectifier section followed by one or more conversion stages. 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 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 DC-DC CONVERTERS |
|||||||
| Buck |  |
D (0<D<1) |
Vin | Iout | Vin | Iout×D | Vout/Vin |
| Boost |  |
1/(1-D) (0<D<1) |
Vout | Iout×Vout /Vin | Vout | Iout | Vin/Vout |
| Flyback (inverting) or buck-boost |  |
-D/(1-D) (0<D<1) |
Vin+|Vout| | Iout× (1+|Vout|/Vin) |
Vin+|Vout| | Iout | |Vout|/Vin |
| Ćuk |  |
-D/(1-D) (0<D<1) |
Vin+|Vout| | Iout× (1+|Vout|/Vin) |
Vin+|Vout| | Iout | |Vout|/Vin |
| Sepic |  |
D/(1-D) (0<D<1) |
Vin+Vout | Iout | Vin+Vout | Iout | Vout/ (Vin+Vout) |
ISOLATING DC-DC CONVERTERS |
|||||||
| Flyback |  |
√(2Pout× LpF/Vin) (0<D<1) |
Vin+Vout ×(Np/Ns) |
D×Vin/Lp×F | Vout+ (Vin×Ns/Np) | Iout |  |
| 2-switch
flyback |
 |
√(2Pout× LpF/Vin) (0<D<0.5) |
Vin | D×Vin/Lp×F | Vout+ (Vin×Ns/Np) | Iout | D/4 |
| Forward |  |
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 |
 |
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 |
 |
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 |
 |
Ns/Np×D (0<D<0.5) |
Vin | Iout×Ns/Np | Vin×Ns/Np | 0.5×Iout | Vout/2Vin ×Ns/Np |
| Push- pull |
 |
2Ns/Np×D (D<0.5) |
2×Vin | Iout×Ns/Np | 2Vin×Ns/Np | 0.5×Iout | Vout/4Vin ×Ns/Np |
| Full bridge |  |
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 |
 |
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 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 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 functional requirements are conventional, 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 |  |
- | - | - | - |
| 2-switch flyback | |
- | - | - | - |
| Single switch forward | |
|
- | - | - |
| 2-switch forward | |
|
|
- | - |
| Half bridge | - | - | |
|
- |
| Full bridge | - | - | - | |
- |
| ZVT full bridge | - | - | - | |
|
All info here is presented for reference only and does not constitute a professional advice- see a complete Disclaimer linked below.