Design components for flyback converters
The production method of flyback converter includes the following commonly used components:
- Flyback transformer
- switch
- rectifier
- wave filter
- Drive switch
- control device
A flyback converter is a switching converter with relatively few components, which is relatively easy to manufacture and design.
A flyback converter is an isolated switching converter that can be configured as a buck or boost converter. Most mobile phones, tablets, and laptops use flyback converters.
(Flyback Converter Schematic)
1. Flyback transformer
Transformers can transfer energy from the primary to the secondary. On the other hand, flyback transformers store energy in the primary magnetic field and transfer it to the secondary magnetic field after a certain period of time.
A transformer consists of at least two inductors, called the secondary coil and the primary coil, wound in a coil frame with a core in the middle. The magnetic core determines the magnetic flux density, which is an important parameter for transferring electrical energy from one winding to another. Voltage regulator phase determination, points displayed in the primary and secondary windings.
2. Switch
The function of a switch is to conduct and close the primary circuit, allowing the transformer to magnetize and demagnetize. This switch is controlled by the PMW signal from the selected controller.
3. Rectifiers and filters
The rectifier rectifies the voltage on the secondary winding into pulsating direct current. Another function of a rectifier or diode is to disconnect and connect the load from the secondary winding. The rectified voltage is then filtered out by a capacitor to increase the DC level and can be used for the intended application.
There is no buffer circuit in the above circuit diagram, but most of the time, a flyback converter requires a buffer to counteract voltage spikes on the switch or diode.
Principle of flyback converter
(1)The working principle of flyback converter when the switch is turned on
Schematic diagram of flyback converter - When the switch is turned on, the current will flow from Uin to the primary ground to charge the primary winding and store energy. At this point, the diode is reverse biased and there is no current flowing through the secondary winding. The load demand is provided by the output capacitor (Cout).
(Flyback conversion schematic (switch on))
(2) Current variation of flyback converter - when the switch is conducting
Current variation of flyback converter (switch conduction)
(3) Working principle of flyback converter - when the switch is conducting
(Schematic diagram of flyback converter (switch conducting))
When the switch is turned on, the primary will charge and there will be current flowing. According to KVL,
Vin – VL – Vs = 0
Assuming an ideal state where the switch voltage drop (Us) is 0,
- Vin – VL = 0
- VL=input voltage
- VL = Lp di / dt
- di = ( VL / Lp ) X dt
- VL = Vin， therefore
- di=(Vin/Lp) X dt:
Perform calculations on the formula:
(electric current)
Stored energy:
(Energy storage）
Working principle of flyback converter when the switch is turned off
(1) Schematic diagram of flyback converter - when the switch is closed
When the primary switch is disconnected, the primary winding will resist sudden changes in current and then reverse the polarity of the winding. This will cause forward bias of the output diode. The energy stored in the primary will be transferred to the secondary and load through diodes, and at this time, the output capacitor will replenish the charge.
（Circuit diagram of flyback converter (switch off)）
（2) Current variation of flyback converter - switch off
（Flyback converter current transformer (switch off)）
（3) Working principle of flyback converter
（Circuit diagram of flyback converter (switch off)）
If the switch is turned off, the permission of the flyback converter will be concentrated in the secondary, and when the switch is turned off, the secondary current flows.
According to KVL,
VL_Secondary VD Vout=0
In an ideal situation, the voltage drop of the secondary diode is zero:
- VL_Secondary - Vout=0
- VL_Second=Vout
- VL = Ls di / dt
- Di=(VL_Stage/Ls) X dt
- VL_Second=Vout, therefore
- Di=(Vout/Ls) X dt through integration,
(electric current)
(electric current)
Transferred energy:
(Transferred energy)
Vsec： The voltage on the secondary winding is exactly equal to the output voltage
Ls： Secondary inductance of transformer
T: Cycle of PWM signal (1/Fsw)
Ton： The time when the switch is turned on
Flyback conversion topology
The advantages of flyback conversion topology structure are: easy to apply, flexible, and can be used for SMPS (switch mode power supply) design.
The waveform and current characteristics of the topology structure of the flyback converter are shown below.
（Waveform and current characteristics of flyback converter topology structure）
Flyback topology SMPS
The topology of flyback SMPS requires relatively few components and can be used for AC or DC power supply. MOS transistors are used in the circuit. The operation of the flyback SMPS structure depends on the MOS transistor. The flyback SMPS topology operates in continuous or intermittent mode.
(Flyback topology SMPS）
Design of SMPS flyback transformer
The following is a simple circuit diagram of an SMPS flyback transformer. The advantage of an SMPS flyback transformer is that the current does not flow through both the primary and secondary windings simultaneously.
（Design of MPS flyback transformer）
Tips for using flyback converters
The application of flyback converter circuits is very extensive:
- DC-DC power supply
- telecom
- LED lighting
- Power over Ethernet (PoE)
- Capacitor charging
- Battery charging
- Solar micro inverter
- AC/DC power supply
Design Example of LM5160 Flyback Converter
(1) LM5160 Electrical Characteristics
- 4.5V to 65V wide input voltage range
- Integrated high side and low side switches
- No need for external Schottky diodes
- 2A maximum load current
- Adaptive constant conduction time control
- No external loop compensation
- Fast transient response
- Optional forced PWM or DCM operation
- FPWM supports multi output Fly Buck
- Almost constant switching frequency
- The resistance can be adjusted to 1 MHz
- Program soft start time
- Pre bias startup
- ± 1% feedback reference voltage
- LM5160A allows external VCC bias
- Inherent protection function of robust design
- Peak current limit protection
- Adjustable input UVLO and hysteresis
- VCC and gate driven UVLO protection
- Thermal shutdown protection with hysteresis
This is just a reference, please refer to the datasheet of LM5160 for more details
LM5160 Datasheet PDF(2) LM5160 pin diagram
（LM5160 pin diagram）
(3) Absolute maximum rated value
（Absolute maximum rated value）
(4) Circuit diagram of flyback converter
（Circuit diagram of flyback converter）
(5) Working principle of flyback converter
The schematic above uses a large number of components, but in reality it is not that complex. The C6, C7, and C8 input terminals are used to filter the input power supply, R6 and R10 are used for undervoltage locking, and R7 resistor is used for time related purposes. This pin can be programmed using a simple resistor.
The C13 capacitor connected to the SS pin is a soft start capacitor. AGND (analog ground), PGND (power ground), and PAD are connected to the power GND.
The C5 (0.01 uF) capacitor on the right is a bootstrap capacitor used for bias of the gate driver. R4, C4, and C9 are ripple filters, where R8 and R9 provide feedback voltage to the feedback pin of LM5160, and the ratio of these two resistors determines the output voltage. C10 and C11 are used for primary non isolated output filtering. One of the main components is T1, a coupling inductor, with a 60uH inductor on each side of the primary and secondary.
You can choose from the following specifications or other specifications of inductors:
- Turn ratio SEC: PRI = 1.5 : 1
- Inductance=60uH
- Saturation current=840mA
- Primary DC resistance=0.071 Ω
- Secondary DC resistance=0.211 Ω
- Frequency=150 kHz
C3 is used for EMI stability. D1 is the forward diode for converting the output, C1 and C2 are filtering capacitors, and R2 is the minimum load required for startup.
(6) Schematic and simulation diagram of flyback converter
（Schematic diagram of flyback converter）
The following is a simulation diagram of the flyback converter, which shows the load current and voltage:
(Simulation diagram of flyback converter)
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