Flyback Converter

A flyback converter is an electrical device with the ability to generate a DC output voltage that can be either greater than or less than the DC input. If a buck converter, which steps down voltage is cascaded with a boost converter, which steps up voltage the result is a buck-boost converter. As its name implies, the buck-boost converter can either step down or step up its input voltage and is the basis of the flyback converter. The flyback converter differs from a buck-boost converter as it uses a coupled inductor, or a flyback transformer for electrical isolation between the output and input. This video will illustrate the construction of a flyback converter and investigate how changing the converter's operating condition effects its output.

To understand how a flyback converter works, begin with a buck converter in series with a boost converter. The switches in this circuit are turned on and off by a pulse-width modulated signal. The load at the output of the buck converter is a current sink, which in turn is the input of the boost converter. In this circuit, the boost converter must be inverted so the direction of the current through the current sink is consistent with the operation of each stage. As a result, the cascaded converter has an output polarity that is reversed compared to its input. The circuit can be simplified to this buck-boost converter configuration. When the switch is closed, the voltage source drives current through the inductor. This current increases linearly with time and creates a magnetic field that stores energy in the inductor. At this time, the diode is reversed biased and does not conduct, so only the capacitor supplies energy to the load. When the switch is open, current through the inductor must continue in the same direction causing the inductor to reverse polarity. Now the diode becomes forward biased and the inductor can deliver energy to the load while at the same time charge the capacitor. When the switch closes again, the cycle repeats. Replacing the inductor with a coupled inductor or flyback transformer, provides electrical isolation between the input and output which is necessary when grounds on the source and load sides must be separated. Moving the switch from the high side of the voltage source to the low side, simplifies electrical demands on the switch and the circuit that drives it. Finally, reversing the polarity of the coupled inductor or flyback transformer, and reversing the direction of the diode, enables the polarity of the output to match the input. The result is the basic flyback converter. Now that we've see how to derive the flyback converter from the cascade of a buck and boost converter let's investigate how its behavior changes with different operating conditions.

The output in this experiment is limited to 50 volts DC or less. Use only the specified duty cycles, frequencies input voltages, and loads. These experiments use the High Rel system's Power Pole board With switch S90 turned off, plug the signal supply into connector J90. Set the pulse with modulation selection jumpers J62 and J63 to the open loop position. Adjust the DC supply to 16 volts, but do not connect its output to the power pole board. Next, build the flyback converter circuit with the lower MOSFET and the flyback magnetic board. Adjust the load resistor to 10 ohms. Then connect it to the board potentiometer between terminals V2+ and com. Set switch selector bank S30 as follows PWM to bottom MOSFET use onboard PWM and switched load off. Connect the oscilloscope's differential probe between the gate and source of the lower MOSFET. Turn on switch S90 and observe the switching signal that turns the MOSFET on and off. Set RV60 to produce a switching frequency of 100 kilohertz. Set duty ratio potentiometer RV64 so the pulses have an on-time of five microseconds.

First, connect a regular probe between the gate and source of the lower MOSFET. Connect the differential probe across the load and connect the input DC supply to V1+ and com. The output voltage is a triangle wave resulting from the inductor and capacitor alternately supplying current to the load. The gate source voltage of the MOSFET is a digital pulse train. Measure the mean value of the output voltage and the on-time of the gate to source voltage then record the input current and voltage readings. Repeat this test with the pulse streams set to an on-time of one 2.5 and four microseconds which correspond to duty ratios of 0.1, 0.25, and 0.4, respectively. When the switch is closed, energy is stored in the inductor when the switch is open, energy is dissipated in the load. Ideally, the output increases with duty ratio however, for duty ratios above 0.5, stored energy is greater than dissipated energy, resulting in possible saturation of the core. To avoid residual energy storage, flyback converters are not operated above a duty ratio of 0.5.

Connect a regular scope probe to channel three of the oscilloscope. Clip this probe between CS1 and com to measure the input current. Observe the gate to source switching signal while adjusting potentiometer RV60 to produce a frequency of 70 kilohertz. Connect the DC power supply to input terminals V1+ and com. Observe the input current wave form and measure the mean input and output voltage. Record the frequency and duty ratio as well as the input current and voltage readings from the DC supply. Repeat this test after adjusting RV60 to a switching frequency of 50, 30, and 10 kilohertz with the duty cycle ratio fixed at 0.5. As the frequency decreases, the output ripple increases because the capacitor charge and discharge times also increase.

Flyback converters are typically used in isolated power supplies where the output must be galvanically isolated from the input. Preventing circuit damage in the event of failure and protecting users from hazardous voltages. Cell phone chargers convert the 120 volt AC main supply to an internal DC voltage that becomes the input to a flyback converter. The flyback converter in turn generates a five volt output to the standard USB connector that plugs into a cell phone and charges it. Galvanic isolation in the flyback converter protects both the cell phone and the user from contact with the 120 AC supply. In contrast, the cell phone likely uses a buck converter to reduce five volts from the charger to nominal 3.6 volts for its lithium-ion battery. For these safe low voltages, no isolation is necessary. The cathode ray tube in and older television or computer monitor uses an electron beam to illuminate phosphors on the screen. The horizontal deflection drive of CRT's often incorporates flyback converters operating in the step up mode. The flyback converter generates the high voltage that controls this beam and moves it to strike chosen points on the screen.

You've just watched JoVE's introduction to flyback converters. You should now understand how the flyback converter is related to boost and buck converters and how its behavior varies with operating condition. Thanks for watching.