Flyback Converter Design

Flyback Converters

Before reading this section, please read the introduction.

The flyback, a popular structure The flyback converter is widely used in consumer products Ease of design, low-cost, well-known struture Poor EMI signature, bulky transformer, practical up to 150 W. My customer is designing a flyback converter using LM5155, and trying to simulate on PSPICE for TI. He is trying to simulate LM5155-Q1 FLYBACK STARTUP SIMULATION which is incorporated to reference design whose project file is LM5155-Q1TRANS.OPJ. However, he can't place a marker on the schematic. Lecture notes KFUEIT Document Linear circuit analysis GIKI Power Electronics Basic electric circuits notes EE-110 Electric Drives Prof P Gopakumar Scribd Electronics EE-355 EE-211 Power systems Video Tutorial Simulation EE-231 Flyback inverter Digital Logic Design Notes EE-434 Experimental work PSPICE Computational Electromagnetic Theory Grid.

All of the circuits in this tutorial can be simulated in LTspice^®. If you are new to LTspice, please have a look at my LTspice Tutorial

Sometimes an isolated voltage needs to be produced. For low power applications, a flyback converter is another form of dc/dc converter to fulfil this need.

Flyback converters are very similar to boost converters in their architecture and performance, except with a flyback converter the inductor is replaced with the primary of a transformer and the output is taken from the secondary. However, in the flyback architecture, the primary and secondary windings can be treated as 2 separate inductors, so the flyback converter’s operation is very similar to that of a boost converter.

The following tutorial describes how to design a boundary mode flyback circuit where the circuit operates on the boundary between continuous and discontinuous conduction.

A typical flyback converter is shown in FIG 1

FIG 1

This circuit converts 12V to a pseudo-isolated 5V and can support a load of 1A. This circuit is adequate to demonstrate the operation of a flyback converter, even though it does not offer full isolation – the feedback resistors R1 and R2 violate the isolation barrier.

Ignore components R4, D2, Q2 and C5. These just provide a linear regulator function to allow the LTC3873 to be used with high input voltages.

When the FET switches on, the current ramps up according to

as with a boost converter.

So in the case of FIG 1, the current ramps up at a rate of

or 400,000A per second. Thus if the MOSFET switches off after 1us, the current through the primary will have ramped up by 400mA.

When the MOSFET switches off, the voltage on both the primary and secondary windings increase in an effort to maintain the current flow. The winding that conducts first is the one in which the current flows and in the circuit in FIG 1, diode D1 conducts causing the energy to flow into the output capacitors C2, C6, C7.

In a boost converter, the inductor current ramps up linearly during charge and ramps down linearly during discharge. In a flyback converter, when the current ramps up in the primary, energy is built up in the transformer core. When the FET switches off this energy is dumped into the secondary allowing current to flow in the secondary, so the primary current collapses immediately to zero. This causes an immediate rise in current in the secondary, after which the current in the secondary ramps down linearly as it discharges. For the sake of analysis, it is often better to treat the primary and secondary transformer windings as 2 separate inductors where energy is built up in the primary and dumped into the secondary. Although a flyback converter can be considered as consisting of 2 separate inductors, classic transformer action still happens. When the FET switches off, the primary current collapses to zero causing an opposing rise in current in the secondary. The rise in secondary current is proportional to the collapse in primary current and is determined by the turns ratio of the transformer.

The discharge current in the secondary winding charges the output capacitor slightly after which the MOSFET switches on and the process starts again. The output voltage is monitored by the feedback resistors R1 and R2 and when the junction of these resistors reaches a certain point, the chip terminates the drive to the MOSFET.

Like with the boost converter, during charging the primary and secondary currents ramp up and down according to the equation

During the charging of the primary, L is the inductance of the primary. During discharge, L is the inductance of the secondary.

The LTspice circuit of FIG 1 can be downloaded here (right click over the link and save as a '.asc' file): LTC3873 Flyback Converter

The LTC3873 datasheet can be downloaded here: LTC3873 datasheet

We mentioned earlier that the primary current ramps up at 400,000A per second. Our LTspice simulation shows a current ramp of 408kA per second (see FIG 2).

The secondary inductance is 3.3uH, so the current ramps down at a rate of

or 1.66 million amps per second. In this case the output voltage is 5V and the voltage across the diode is 0.5V so the voltage across the secondary during discharge is 5.5V.

Our LTspice simulation shows a current ramp of 1600kA per second.

FIG 2 below shows the gate waveform (green), the primary current (blue) and secondary current (red).

FIG 2

The LTspice circuit in FIG 1 shows the transformer modelled as 2 inductors. The ratio of inductance is equal to the square of the turns ratio, so the turns ratio of this transformer is 3:1. Since a transformer is a purely passive device, when the voltage is stepped down, the current is stepped up by an equal amount and vice versa. Thus when the current stops in the primary (in FIG 2 this is approx. 1.4A), a current is produced in the secondary of 3x1.4A = 4.2A.

It is interesting to note that the value of di/dt is determined ONLY by the inductance value and the voltage across the inductor. The controller IC has nothing to do with setting the inductor ramp current.

Like with a boost converter, it is useful to know the duty cycle of the flyback converter.

We have already stated that the flyback converter is similar to a boost converter, so it makes sense to start with the duty cycle of a boost converter.

We determined that the duty cycle of a boost converter is

Now, with a boost converter when the FET switches off the voltage across the coil rises until the rectifier diode conducts. So the output voltage is equal to the sum of the coil voltage and the input voltage, so

From this

So, for a boost converter we can write

With a flyback converter, the output voltage (on the secondary) is referenced to ground, not the input voltage. Looking from the primary side, the voltage across the primary is equal to the output voltage divided by the turns ratio. So if the output voltage is 5V and we have a 10:1 turns ratio then the voltage across the primary is 50V.

So if we call Vout’ the output voltage as seen from the primary side we can say

In FIG 1, the turns ratio (primary to secondary) is 3:1. The output voltage is 5V, so the voltage across the secondary is 5.5V including the diode drop. When this is reflected to the primary this results in a voltage of 16.5V across the primary. With an input voltage of 12V, this results in a duty cycle of

In LTspice, we can measure the ON time as 3.5us and a period of 6.14us, giving a measured duty cycle of 57%.

With a boost converter, the current in the inductor must be continuous

Flyback Converter Design In Pspice Tutorial