Flyback Converter

Overview

Source: Ali Bazzi, Department of Electrical Engineering, University of Connecticut, Storrs, CT.

A flyback converter is a buck-boost converter, which can both buck and boost. It has electrical isolation between the input and the output using a coupled inductor or a "flyback transformer." This coupled inductor enables a turns ratio that provides both voltage step-up and step-down capability, like in a regular transformer but with energy storage using the air-gap of the coupled inductor.

The objective of this experiment is to study different characteristics of a flyback converter. This converter operates like a buck-boost converter but has electrical isolation through a coupled inductor. Open-loop operation with a manually-set duty ratio will be used. An approximation of the input-output relationship will be observed.

Procedure

ATTENTION: This experiment is designed to limit the output voltage to be less than 50V DC. Only use duty ratios, frequencies, input voltage, or loads that are given here.

This experiment will utilize the DC-DC converter board provided by HiRel Systems. http://www.hirelsystems.com/shop/Power-Pole-Board.html

Information about the board operation can be found in this collections video “Introduction to the HiRel Board.”

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Results

Flyback converters are isolated buck-boost converters that can step up or step down the input voltage. The turns-ratio of the flyback coupled inductor or transformer aids in the stepping up or down process. Given that the switching frequency is high, the flyback transformer size is small and uses ferrite cores. If the input voltage is V_in and the output voltage is V_out_,V_out/V_in=(N₂/N₁)D/(

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Application and Summary

Flyback converters are typically used in isolated power supplies where the output side should have galvanic isolation from the input side. This is common in driving high-side power semiconductors such as MOSFETs and IGBTs whose gate drive circuits may require isolated DC supplies. Flyback converters are typically operated at high switching frequencies higher than 100 kHz, and have power ratings typically not exceeding 200 W.

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Tags

Flyback Converter DC Output Voltage Buck Converter Boost Converter Buck boost Converter Flyback Transformer Electrical Isolation Pulse width Modulated Signal Current Sink Cascaded Converter Configuration

ATTENTION: This experiment is designed to limit the output voltage to be less than 50V DC. Only use duty ratios, frequencies, input voltage, or loads that are given here.

This experiment will utilize the DC-DC converter board provided by HiRel Systems. http://www.hirelsystems.com/shop/Power-Pole-Board.html

Information about the board operation can be found in this collections video “Introduction to the HiRel Board.”

The procedure shown here applies to any simple flyback converter circuit that can be built on proto boards, bread boards, or printed circuit boards.

1. Board setup:

Connect the ±12 signal supply at the "DIN" connector but keep "S90" OFF.

Make sure that the PWM control selector is in the open-loop position.

Set the DC power supply at 16 V. Keep its output disconnected from the board for now.

Before connecting the load resistor, adjust it to 10 Ω.

Build the circuit shown in Fig. 3 by using the lower MOSFET and flyback magnetic board.

Note that the turns ratio N₁/N₂=2.

Connect "R_L" across "V2+" and "COM."

NEVER Disconnect the load during the experiment as the boost converter can become unstable and cause damage to the board.

Make sure the switch array for MOSFET selection (lower MOSGET), PWM selection, and other settings are correct to achieve a functional Fig. 3.

Figure 3. Flyback converter circuit

2. Adjusting the Duty Ratio and Switching Frequency

Connect the differential probe across the gate-to-source of the lower MOSFET.

Turn ON "S90." A switching signal should appear on the scope screen.

Adjust the signal time axis to see two or three periods.

Adjust the frequency potentiometer to achieve a frequency of 100 kHz (period of 10 µs).

Adjust the duty ratio potentiometer to achieve a 50% duty ratio (on-time of 5 µs).

3. Flyback Converter Testing for Variable Input

Connect the input DC power supply, which is already set at 16 V, to "V1+" and "COM."

Connect a regular probe to measure the input current at "CS1." Make sure the ground connector is connected to "COM."

Connect the differential probe across the load.

Capture the waveforms and measure the output voltage mean, input current peak, and input current mean.

Record the input current and voltage readings on the DC power supply.

Adjust the input voltage to 11 V, 13 V, and 15 V.

Repeat the above steps for each of these voltages.

Disconnect the input DC supply and adjust its output to 16 V.

4. Flyback Converter Testing for Variable Duty Ratio

Connect a regular probe across the gate to source of the lower MOSFET.

Connect the differential probe across the load.

Connect the input DC supply to "V1+" and "COM."

Capture the waveforms and measure the output voltage mean and on-time of the gate-to-source voltage (also the duty ratio).

Record the input current and voltage readings on the DC power supply.

Adjust the duty ratio to 10%, 25%, and 40%. Repeat the above steps for each of these three duty ratios.

Reset the duty ratio to 50%.

Disconnect the input DC supply.

5. Flyback Converter Testing for Variable Switching Frequency

Connect a regular probe at "CS1" to measure the input current.

Connect the differential probe across the load.

On the second oscilloscope, observe the gate-to-source voltage using a regular probe to adjust the switching frequency as needed.

Connect the input DC supply to "V1+" and "COM."

Adjust the switching frequency to 70 kHz.

Capture the waveforms from the first scope and measure the input current peak and output voltage mean.

Record the frequency and duty ratio from the second scope but do not capture its waveform.

Record the input current and voltage reading on the DC power supply.

Adjust the switching frequency to 50 kHz, 30 kHz, and 10 kHz (or minimum possible if 10 kHz cannot be reached).

Repeat the above steps for each of these three switching frequencies.

Turn OFF the input DC supply and "S90," and then disassemble the circuit.

Skip to...

0:06

Overview

1:04

Principles of the Flyback Converter

3:28

Setting up the HiRel Power Pole Board

5:12

Flyback Converter Testing for Variable Duty Ratio

6:38

Flyback Converter Testing for Variable Switching Frequency

7:42

Applications

9:12

Summary

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