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. 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.

Numerous circuits convert AC or DC input to the required DC output, and here we 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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