# Isolated Switch Mode Power Supplies

## Overview

For regular non-isolated buck, boost, regulators, SEPIC and Ćuk regulators see the Switch Mode Power Supplies (SMPS) page.

Isolated SMPS topologies:

FlybackRequires no inductor (uses the transformer). Simple. Low cost.
FlybuckHigher freq. than flyback (= smaller components/better efficiency).
Forward

## Flyback Converters

A flyback converter is a type of isolated power converter. It can be thought of as a buck-boost converter with the inductor split to form a transformer. The basic schematic is shown below:

Flyback converters are unique in the fact that they use the transformer as both a transformer (to provide voltage/current conversion and isolation) and inductor (for storage of energy in it’s magnetic field). All other isolating SMPSs only use the transformer for voltage/current conversion and isolation, and require a separate inductor on the secondary side for performing the buck/boost regulation. The flyback converter to the most used isolating SMPS topology used1.

### Principle Of Operation

1. Switch $SW1$ closes. The primary of the transformer is connected directly to the input. The current ramps up linearly in the transformer, storing energy in it’s magnetic field. Since the voltage on the secondary is negative (note the polarity dots on the windings), diode $D1$ is reverse biased and does not conduct, leaving $C_{OUT}$ to supply energy to the load (which it does from energy stored in the previous cycle).

2. Switch $SW1$ opens. The primary current drops (very rapidly). Suddenly, the secondary voltage becomes positive, and $D1$ conducts. This supplies energy to both $C_{OUT}$ and the load.

The output voltage is given by Eq $\ref{eq:vout-vin-flyback}$.

\begin{align} \label{eq:vout-vin-flyback} V_{OUT} = \frac{V_{IN}}{N_{PS}} \frac{D}{1 - D} \end{align}

where:
$N_{PS}$ is the windings ratio from primary to secondary, $\frac{N_P}{N_S}$
$D$ is the duty cycle, and varies from $0$ to $1$. It is defined as:

\begin{align} D = \frac{t_{on}}{T} \end{align}

### Feedback

Without feedback, regulation for a flyback converter can be as good a 5-10% (assuming the input voltage is known and constant). If you need tighter precision of the output voltage, or if the input voltage varies wildly, you will need to add feedback.

The TL431 precision shunt voltage reference in tandem with an optocoupler is a popular way of providing feedback from the secondary side back to a flyback converter. As shown below, it’s REF pin is connected via resistor divider to $V_{OUT}$, and it sinks as much current through the optocoupler’s LED to keep $V_{REF}$ at $2.5V$, hence providing just enough feedback drive to regulate the output voltage.

Flyback converters are used extensively in Power over Ethernet (PoE) applications.

• LM3481