Choosing Cores for different SMPS topologies

The selection of the magnetic components (transformer cores) to be used in a Switch Mode Power Supply (SMPS) design is the most important element of that process. During this, understanding the topologies of SMPS’ is crucial as the designer must consider the trade-offs between them. This article will give you an overview of these tradeoffs.

There are numerous circuits which convert AC or DC input to the required DC output, and here I will only address the most common and popular. Three broad basic topologies can be identified, they are: Flyback, Forward, and Push-pull.

CompareFlybackForwardPush-pull
Recommended power ratingLow to medium power up to 200W, with wide output voltage range.Medium to high power up to 500W in the low output voltage rangeHigh power >100W at the high voltage range
Advantages– Simple circuit (low cost)
– Lowest component count
– Low leakage losses
– Several easily regulated output voltages
– Higher power range than flyback converter
– Mid component count
– High efficiency
– Power capacity up to the kW range
– Small choke
– High efficiency
– Low EMI complexity
Disadvantages– High eddy current losses in air gap area
– Large transformer core with air gap required
– Average EMI performance
– Exact requirements on the components
– Two magnetic components required
– Large choke needed
– Demagnetization winding required
– Two magnetic components needed
– Complex winding required
– High component count
Core/material requirements– Low losses at high temperatures
– High saturation with low dependence on the temperature
– Gapped core with high accuracy on “AL”
– Low losses at high temperatures
– Low eddy current losses at high frequencies
– Generally, ungapped cores
– Low losses at high temperatures
– Low eddy current losses as the operation “F” is up to 500kHz and above
– Generally, ungapped cores
Table summarizing advantages and disadvantages of Flyback, Forward, and Push-pull topologies.

The following table shows which core types for your transformer could be considered suitable for the different converter topologies

Core TypeFlybackForwardPush-pull
E cores++0
ETD cores0++
EC cores0+
U cores+00
RM cores0+0
EP cores+0
P cores+0
Toroids++
+ Favorable
0 Average
– Unfavorable

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