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Flyback Power Converter

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FLYBACK POWER CONVERTER
Shubham Agarwal(11BEC082) Electronics and Communication Department Institute of Technology Nirma University, Ahmedabad

Abstract
The flyback converter is used in both AC/DC and DC/DC conversion with galvanic isolation between the input and any outputs. More precisely, the flyback converter is a boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. When driving for example a plasma lamp or a voltage multiplier the rectifying diode of the boost converter is left out and the device is called a flyback transformer. lesson is limited to the study of fly-back circuit of single switch topology.

II.

Basic Topology of Fly-Back Converter

I.

Introduction

Fly-back converter is the most commonly used SMPS circuit for low output power applications where the output voltage needs to be isolated from the input main supply. The output power of fly-back type SMPS circuits may vary from few watts to less than 100 watts. The overall circuit topology of this converter is considerably simpler than other SMPS circuits. Input to the circuit is generally unregulated dc voltage obtained by rectifying the utility ac voltage followed by a simple capacitor filter. The circuit can offer single or multiple isolated output voltages and can operate over wide range of input voltage variation. In respect of energy-efficiency, flyback power supplies are inferior to many other SMPS circuits but its simple topology and low cost makes it popular in low output power range. The commonly used fly-back converter requires a single controllable switch like, MOSFET and the usual switching frequency is in the range of 100 kHz. A two-switch topology exists that offers better energy efficiency and less voltage stress across the switches but costs more and the circuit complexity also increases slightly. The present

Fig.1 shows the basic topology of a fly-back circuit. Input to the circuit may be unregulated dc voltage derived from the utility ac supply after rectification and some filtering. The ripple in dc voltage waveform is generally of low frequency and the overall ripple voltage waveform repeats at twice the ac mains frequency. Since the SMPS circuit is operated at much higher frequency (in the range of 100 kHz) the input voltage, in spite of being unregulated, may be considered to have a constant magnitude during any high frequency cycle. A fast switching device (‘S’), like a MOSFET, is used with fast dynamic control over switch duty ratio (ratio of ON time to switching time-period) to maintain the desired output voltage. The transformer is used for voltage isolation as well as for better matching between input and output voltage and current requirements. Primary and secondary windings of the transformer are wound to have good coupling so that they are linked by nearly same magnetic flux. As will be shown in the next section the primary and secondary windings of the fly-back transformer don’t carry current simultaneously and in this sense fly-back transformer works differently from a normal transformer. In a normal transformer,

Fig.1

under load, primary and secondary windings conduct simultaneously such that the ampere turns of primary winding is nearly balanced by the opposing ampere-turns of the secondary winding (the small difference in ampere-turns is required to establish flux in the non-ideal core). Since primary and secondary windings of the fly-back transformer don’t conduct simultaneously they are more like two magnetically coupled inductors and it may be more appropriate to call the fly-back transformer as inductor -transformer. Accordingly the magnetic circuit design of a fly-back transformer is done like that for an inductor. The output section of the fly-back transformer, which consists of voltage rectification and filtering, is considerably simpler than in most other switched mode power supply circuits. The secondary winding voltage is rectified and filtered using just a diode and a capacitor. Voltage across this filter capacitor is the SMPS output voltage. For ease of understanding, some simplifying assumptions are made. The magnetic circuit is assumed to be linear and coupling between primary and secondary windings is assumed to be ideal. Thus the circuit operation is explained without consideration of winding leakage inductances. ON state voltage drops of switches and diodes are neglected. The windings, the transformer core, capacitors etc. are assumed loss-less. The input dc supply is also assumed to be ripple-free.

being higher). Thus with the turning on of switch ‘S’, primary winding is able to carry current but current in the secondary winding is blocked due to the reverse biased diode. The flux established in the transformer core and linking the windings is entirely due to the primary winding current. The flyback converter is an isolated power converter, therefore the isolation of the control circuit is also needed. The two prevailing control schemes are voltage mode control and current mode control. Both require a signal related to the output voltage. There are two common ways to generate this voltage. The first is to use an optocoupler on the secondary circuitry to send a signal to the controller. The second is to wind a separate winding on the coil and rely on the cross regulation of the design. The first technique involving an optocoupler has been used to obtain tight voltage and current regulation, whereas the alternative approach was developed for cost-sensitive applications where the output did not need to be as tightly controlled but up to 11 components including the optocoupler could be eliminated from the overall design. Also, in applications where reliability is critical, optocouplers can be detrimental to the MTBF (Mean Time Between Failure) calculations.

IV.

III.

Principle of Operation

Continuous Versus Discontinuous Flux Mode of Operation

During its operation fly-back converter assumes different circuit-configurations. Each of these circuit configurations have been referred here as modes of circuit operation. The complete operation of the power supply circuit is explained with the help of functionally equivalent circuits in these different modes. As may be seen from the circuit diagram, when switch ‘S’ is on, the primary winding of the transformer gets connected to the input supply with its dotted end connected to the positive side. At this time the diode ‘D’ connected in series with the secondary winding gets reverse biased due to the induced voltage in the secondary (dotted end potential

A practical fly-back type SMPS circuit will have a closed loop control circuit for output voltage regulation. The controller modulates the duty ratio of the switch to maintain the output voltage within a small tolerable ripple voltage band around the desired output value. If the load is very light, very small amount of energy needs to be input to the circuit in each switching cycle. This is achieved by keeping the ON duration of the switch low, resulting in low duty ratio (δ). Within this small ON time only a small amount of current builds up in the primary winding. The off duration of the switch, which is (1-δ) fraction of the switching time period, is relatively large. The circuit operation changes from discontinuous to continuous flux mode if the output power from

the circuit increases beyond certain value. Similarly if the applied input voltage decreases, keeping the load power and switching frequency constant, the circuit tends to go in continuous flux mode of operation. For better control over output voltage, discontinuous flux mode of operation is preferred. However, for the given transformer and switch ratings etc., more output power can be transferred during continuous flux mode. A common design thumb rule is to design the circuit for operation at just-continuous flux mode at the minimum expected input voltage and at the maximum (rated) output power.

V.

A Practical Fly-Back Converter

The simplified and idealized circuit considered above essentially conveys the basic idea behind the converter. However a practical converter will have device voltage drops and losses, the transformer shown will also have some losses. The coupling between the primary and secondary windings will not be ideal. The loss part of the circuit is to be kept in mind while designing for rated power. The designed input power (Pin) should be equal to Po/η, where Po is the required output power and η is the efficiency of the circuit. A typical figure for η may be taken close to 0.6 for first design iteration. Similarly one needs to counter the effects of the non-ideal coupling between the windings. Due to the non-ideal coupling between the primary and secondary windings when the primary side switch is turned-off some energy is trapped in the leakage inductance of the winding. The flux associated with the primary winding leakage inductance will not link the secondary winding and hence the energy associated with the leakage flux needs to be dissipated in an external circuit (known as snubber). Unless this energy finds a path, there will be a large voltage spike across the windings which may destroy the circuit. The snubber circuit consists of a fast recovery diode in series with a parallel combination of a snubber capacitor and a resistor. The leakageinductance current of the primary winding finds a low impedance path through the snubber diode to the snubber capacitor. It can be seen that the diode end of the snubber

capacitor will be at higher potential. To check the excessive voltage build up across the snubber capacitor a resistor is put across it. Under steady state this resistor is meant to dissipate the leakage flux energy. The power lost in the snubber circuit reduces the overall efficiency of the fly-back type SMPS circuit. A typical figure for efficiency of a fly-back circuit is around 65% to 75%. In order that snubber capacitor does not take away any portion of energy stored in the mutual flux of the windings, the minimum steady state snubber capacitor voltage should be greater than the reflected secondary voltage on the primary side. This can be achieved by proper choice of the snubber-resistor and by keeping the RC time constant of the snubber circuit significantly higher than the switching time period. Since the snubber capacitor voltage is kept higher than the reflected secondary voltage, the worst-case switch voltage stress will be the sum of input voltage and the peak magnitude of the snubber capacitor voltage.

Practical Flyback Converter A Pulse Width Modulation (PWM) control circuit to control the duty ratio of the switch. In practical fly-back circuits, for closed loop output voltage regulation, one needs to feed output voltage magnitude to the PWM controller. In order to maintain ohmic isolation between the output voltage and the input switching circuit the output voltage signal needs to be isolated before feeding back. A popular way of feeding the isolated voltage information is to use a tertiary winding. The tertiary winding voltage is rectified in a way similar to the rectification done for the

secondary winding. The rectified tertiary voltage will be nearly proportional to the secondary voltage multiplied by the turns-ratio between the windings. The rectified tertiary winding voltage also doubles up as control power supply for the PWM controller. For initial powering up of the circuit the control power is drawn directly from the input supply through a resistor connected between the input supply and the capacitor of the tertiary circuit rectifier. The resistor ‘RS’ is of high magnitude and causes only small continuous power loss. In case, multiple isolated output voltages are required, the fly-back transformer will need to have multiple secondary windings. Each of these secondary winding voltages are rectified and filtered separately. Each rectifier and filter circuit uses the simple diode and capacitor as shown earlier for a single secondary winding. In the practical circuit shown above, where a tertiary winding is used for voltage feedback, it may not be possible to compensate exactly for the secondary winding resistance drop as the tertiary winding is unaware of the actual load supplied by the secondary winding. However for most applications the small voltage drop in the winding resistance may be tolerable. Else, one needs to improve the voltage regulation by adding a linear regulator stage in tandem or by giving a direct output voltage feedback to the control circuit.

Discontinuous mode has the following disadvantages, which limit the efficiency of the converter: 1) High RMS and peak currents in the design. 2) High flux excursions in the inductor

VII.

Applications

1) Low-power switch-mode power supplies (cell phone charger, standby power supply in PCs) 2) Low-cost multiple-output power supplies (e.g., main PC supplies…...

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