How Push-Pull DC-DC Converters Use the B-H Hysteresis Loop

This article explains how center-tapped push-pull DC-DC converters maximize power efficiency in isolated electronics by utilizing both halves of the transformer core hysteresis loop. By alternating current flow through two primary winding sections, the push-pull topology drives the magnetic flux in both positive and negative directions. This bidirectional excitation doubles the effective flux swing compared to single-ended converters, allowing for smaller, more efficient transformers that avoid premature core saturation.

The Transformer Core Hysteresis Loop

To understand the efficiency of a push-pull converter, it is necessary to understand the magnetic hysteresis loop, often referred to as the B-H curve. This curve plots the magnetic flux density (B) against the magnetizing force (H) of a transformer core.

The B-H loop consists of four quadrants. Single-ended isolated topologies, such as flyback and forward converters, only push current through the transformer in one direction. Because of this, they only operate in the first quadrant of the B-H curve. The magnetic flux rises from zero to a positive peak and then resets back to zero. This leaves the negative half of the core's magnetic capability completely unused.

The Center-Tapped Push-Pull Architecture

A center-tapped push-pull DC-DC converter is specifically designed to utilize the first and third quadrants of the B-H loop. The primary side of the transformer features a winding with a connection right in the middle, known as a center tap.

The circuit relies on three main primary-side connections:

The Center Tap: Connected directly to the positive input voltage source.
Primary Winding A (Start): Connected to the ground via Switch 1 (typically a MOSFET).
Primary Winding B (End): Connected to the ground via Switch 2.

The two primary windings have an equal number of turns and are wound in opposite magnetic directions relative to the center tap.

Bidirectional Flux Excitation (Push and Pull)

The utilization of the full hysteresis loop happens through the alternating operation of the two switches. They are driven by out-of-phase pulse-width modulation (PWM) signals and are never turned on at the exact same time.

1. The "Push" Phase (First Quadrant) When Switch 1 turns on and Switch 2 remains off, current flows from the positive input, into the center tap, through Primary Winding A, and down to the ground. This current generates a magnetizing force (+H) that drives the magnetic flux density in the positive direction (+B). The transformer core is now operating in the first quadrant of the hysteresis loop, transferring energy to the secondary side.

2. The Dead Time Before the next switch activates, both switches are briefly turned off. This "dead time" prevents a short circuit condition known as shoot-through, which would destroy the switches.

3. The "Pull" Phase (Third Quadrant) Next, Switch 2 turns on while Switch 1 remains off. Current now flows from the center tap through Primary Winding B in the opposite direction. Because the winding is oriented opposite to Winding A, this creates a negative magnetizing force (-H). The magnetic flux is actively pulled down through zero and into the negative region (-B), utilizing the third quadrant of the hysteresis loop.

Benefits of Full Hysteresis Loop Utilization

By swinging the magnetic flux from a positive peak (+B) to a negative peak (-B), the push-pull converter achieves twice the total change in flux (\(\Delta\)B) compared to a single-ended converter.

This bidirectional utilization offers three distinct advantages for isolated electronics:

Reduced Core Size: Because the magnetic core is worked twice as hard per switching cycle, a physically smaller transformer can be used to transfer the same amount of power.
No Reset Winding Needed: Single-ended forward converters require extra circuitry just to reset the core's magnetic flux to zero. In a push-pull converter, the opposite switch naturally resets and reverses the flux while actively delivering power to the load.
Lower Copper Losses: Utilizing the full B-H loop requires fewer turns of wire on the transformer to achieve the necessary inductance, which reduces electrical resistance and heat generation.

The Risk of Flux Walking

While using both halves of the hysteresis loop is highly efficient, it requires precise symmetry. If Switch 1 stays on slightly longer than Switch 2, or if one MOSFET has a higher resistance than the other, the positive and negative flux swings will be unequal.

Over several cycles, the magnetic flux will creep toward one end of the B-H curve. This phenomenon is called "flux walking" or staircase saturation. If the flux walks too far, the core saturates, the transformer loses its inductance, and the resulting current spike will destroy the switches. To prevent this, modern push-pull DC-DC converters use current-mode control, which monitors the current in each switch cycle-by-cycle and automatically adjusts the PWM signals to keep the hysteresis loop perfectly centered.