COMPONENT DESIGN
How to Select a Transformer When Designing an Isolated Buck Converter
Yaxian Li, applications engineer, Analog Devices Abstract
This article explains how an isolated buck converter works and how to select a transformer, a pivotal step in designing an isolated buck converter. It discusses which parameters to consider, the mathematics that should be followed when choosing a transformer and how these parameters
How Does an Isolated Buck Converter Work?
An isolated buck topology, as shown in Figure 1, is similar to a generic buck converter. By replacing the inductor in a buck circuit using a transformer, we can get an isolated buck converter. The transformer’s secondary side has an independent ground.
Figure 2. On period equivalent circuit.
it can only change the current slope. The positive current decreases until 0 A, then the negative current increases.
transformer selection. Output current determines the thickness of the copper wire, while output power determines which transformer bobbin should be used. The permeability of the bobbin shows how much energy it can store and how much power it the inductor’s (transformer’s) ripple current. Duty cycle and switch frequency are used to
Figure 3. Off period equivalent circuit.
the Transformer?
Figure 1. An isolated buck topology.
During on-time, the high-side switch (QHS) is on and the low-side switch (QLS) is off. The transformer’s magnetising inductance
(LPRI) is charged up. The arrows in Figure 2 current increases linearly. The current
ramping slope depends on (VIN – VPRI) and LPRI. The secondary side diode, D1, is reverse biased during this time interval and loads
OUT to load. During off-time, QHS is off and QLS is on.
The primary inductor is discharged. The D1 is forward biased, and the secondary
OUTOUT is charged up in this time period. Turning off QHS and turning on QLS cannot change current direction;
When designing a converter, some cleared. It will determine which component will be used especially when choosing a transformer. • Input voltage range • Output voltage • Maximum duty cycle • Switch frequency • Output voltage ripple • Output current • Output power
Maximum duty cycle (D) is usually assigned in the range of 0.4 to 0.6. The minimum input
voltage (VIN_MIN) and maximum duty cycle will determine the primary output voltage
(VPRI). Then, the primary output voltage (VPRI) and secondary output voltage (VOUT) will determine the transformer turns ratio.
Output current (IOUT) and output power (POUT
10 FEBRUARY 2025 | ELECTRONICS FOR ENGINEERS Where D is the maximum duty cycle and
VIN_MIN is the minimum input voltage. From Equation 2, we can calculate the turns
ratio. In a non-isolated buck converter, the ripple current is the same on both sides of
calculate TON time, while VIN, VPRI, and ripple current determine the primary inductance. can lead to a large ripple current. A large ripple current may reach half of the H-bridge current limit and damage the MOSFET. This will lead to a large ripple voltage on the output capacitor due to its ESR and ESL. On the contrary, when an extremely small ripple current is needed, we need to use a high inductance value inductor (transformer). If the coil has many turns, this will require a bulky bobbin. The large inductance will limit loop bandwidth and reduce the dynamic response index.
Choosing a Transformer Energy is transmitted to secondary coil
only in TOFF time. The turns ratio can be determined by Equation 1:
Where VD is the secondary diode forward bias voltage. For VPRI, we usually assign a maximum duty cycle in the range of 0.4
to 0.6. The VPRI can be calculated using Equation 2:
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