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1. INTRODUCTION
In general, a flyback converter is much more demanding on component ratings than would be a forward converter of the same power. In particular, the ripple current requirements for output rectifiers, output capacitors, transformers, and switching transistors are much larger.
However, the increased cost incurred for the larger components will be offset by a reduction in circuit complexity, since output inductors will not be required and there is only one rectifier diode for each output line.
In flyback applications, components will be selected to meet the particular voltage and current needs of each unit. The designer must remember, however, that even for the same output power rating, different modes of operation impose different stress conditions on the components. The recommendations for the selection of power components in the following section, although particularly suitable for the flyback converter shown in Part 2, Sects. 1 and 2, generally apply to all flyback converters.
The graphs and components shown are included for illustration only; it is not intended to suggest that they are necessarily the most suitable. Similar suitable components are available from many manufacturers.
2. PRIMARY COMPONENTS
2.1 Input Rectifiers and Capacitors
There are no special requirements imposed on the input rectifiers and storage capacitors in the flyback converter. Hence these will be similar to those used in other converter types and will be selected to meet the power rating and hold-up requirements. (See Part 1, Sect. 6.)
2.2 Primary Switching Transistors
The switching transistor in a flyback supply is very highly stressed. The current rating depends on the maximum load, efficiency, input voltage, operating mode, and converter design. It will be established by calculating the peak collector current at minimum input voltage and maximum load. In the examples shown in FIG. 2.4, the peak collector current ranges from three to six times the mean current, depending on the operating mode.
The maximum collector voltage is also quite high. It is defined by the maximum input voltage (off load), the flyback factor, the transformer design, the inductive overshoot, and the snubbing method.
For example, when working from a nominal 110-V ac supply line, the maximum input is typically specified as 137 V rms. For this input voltage the maximum off-load DC header voltage Vcc (using the voltage doubler input circuit) will be:
Vcc =2√2 Vin
where Vcc = Vin
Vin =maximum ac input voltage, rms
In this example,
Vcc =137 x 42 x 389V
Typically the flyback voltage is at least twice Vcc, or 778 in this example. Hence, allowing a 25% margin for inductive overshoot, the peak collector voltage will be 972 V, and a transistor with Vcex rating of 1000 V would be selected.
In addition to meeting these stressful conditions, the flyback transistor must provide good switching performance, low saturation voltage, and have a useful gain margin at the peak working current. Since the selection of the transistor will also qualify the gain, it defines the requirements of the drive circuit. Hence, the selection of a suitable power transistor is probably the most important parameter for efficiency and long-term reliability in flyback converters.
Note: To avoid secondary breakdown, current tailing, and overdissipation in high-voltage bipolar transistors, it is essential that the correct drive and load line shaping be used.
Suitable drive circuits, waveforms, secondary breakdown, and tailing problems are discussed in Part 1.
3. SECONDARY POWER COMPONENTS
3.1 Rectifiers
The output rectifier diodes in flyback converters are subject to a large peak and rms current stress. The actual values depend on the load, conduction angle, leakage inductance, operating mode, and output capacitor ESR. Typically the rms current will be 1.6 to 2IDC, while peak currents may be as high as 6IDC. Because the precise conditions are often unknown, the calculation of diode currents is difficult, and empirical methods are recommended.
For the initial prototype breadboarding, select diodes of adequate mean and peak rating. Fast diodes with a reverse recovery time not exceeding 75 ns should be used.
The final selection of the optimum rectifier diodes should be made after measurement of the prototype secondary rectifier currents. Attempts to calculate the rms and peak diode currents are generally not very accurate, as it is difficult to predict the various effects of leakage inductance, output loop inductance, pcb track and wiring resistance, and the ESR and ESL of the output capacitors. These parameters have a very significant effect on the rectifier rms and peak current requirements, particularly at low output voltages, high frequency, and high currents. These measurements can be made in the following way.
3.2 Rectifier Ripple Current Measurement Procedure
1. Connect a current probe of adequate rating in series with the output rectifier to be measured. (See Part 3, for suitable current probe design.)
2. Using the oscilloscope, observe the current waveform and note the peak current value.
3. Transfer the current probe to a true rms ammeter (e.g., a thermocouple instrument or true rms-reading instrument with a crest factor of at least 10/1) and measure the rms cur rent, making due allowance for the current probe and meter multiplying factors. These measurements should be carried out at maximum input voltage and maximum load.
Select diodes with appropriate peak and rms ratings.
3.3 Rectifier Losses
The actual power loss in the output rectifier diode of a flyback supply depends on a number of factors, including forward dissipation, reverse leakage, and recovery losses. The forward dissipation depends on the effective forward resistance of the diode throughout its forward conduction and the shape of the current pulse, both of which are nonlinear. (In practice the secondary current waveform is often very different from the ideal triangular shape normally assumed in calculations.) Consequently, it is often more expedient to measure the tempera ture rise of the diode in the prototype, and from this calculate the junction temperatures and heat sink requirements for worst-case conditions.
From temperature rise measurements carried out on rectifiers in several flyback sup plies (comparing the temperature rise caused by DC stress with ac stress conditions), it has been found that an approximate rectifier dissipation may be calculated, using the measured rectifier rms current (approximately 1.6 IDC) and an assumed forward voltage drop of 800 mV for silicon diodes or 600 mV for Schottky diodes. Adequate heat sinks based on these calculations should be provided for initial prototypes. (See Part 3, Sect. 16.)
4. OUTPUT CAPACITORS
Output capacitors are also highly stressed in flyback converters. Normally the output capacitors will be selected for three major parameters: absolute capacitance value, capacitor ESR and ESL, and capacitor ripple current ratings.
4.1 Absolute Capacitance Value
When ESR and ESL are low, the capacitance value will control the peak-to-peak ripple voltage at the switching frequency. Because the ripple voltage is normally small compared with the mean output voltage, a linear decay of voltage across the output capacitors may be assumed during the "off" period. During this period, the capacitor must deliver all the output current, and the voltage across the capacitor terminals will decay at approximately 1 V/µs/A (for a 1-µF capacitor). Hence, when the maximum "off" time, the load current, and the required peak-to-peak ripple voltage are known, the minimum output capacitance may be calculated as follows:
C = [toff IDC] / Vp-p
where C = output capacitance, µF
toff = off time, µs
IDC= load current, A
Vp-p = ripple voltage p-p
For example, for a 5-V, 10-A output line and an output ripple of 100 mV,
C = [18 x 10 -10 x 10] / 0.01 = 1800 uF
Note: Attempts to keep the peak-to-peak ripple voltage below 100 mV in a single-stage output filter will not be cost-effective. To obtain a lower output ripple, an extra LC stage should be fitted.
4.2 Capacitor ESR and ESL
Figure 4.1b shows the effect of the ESR and ESL (effective series resistance and inductance) of the output capacitor on the output ripple. In practice, the ripple voltage will be much greater than would be expected from the selection of the output capacitance alone, and an allowance for this effect should be made when selecting capacitor size. If a single stage output filter is used (no extra series choke), then the ESR and ESL of the output capacitors will have a significant effect on the high-frequency ripple voltage, and the best low-ESR capacitors should be used.
The beneficial effects of an additional output LC filter should not be neglected in fly back systems. Such filtering reduces output noise and allows the use of much lower cost ordinary-grade electrolytics of adequate ripple rating as the major energy storage elements.
(See Part 1, Sect. 20.)
4.3 Capacitor Ripple Current Ratings
In a flyback converter, the typical rms ripple current in the output capacitors will be 1.2 to 1.4 times the DC output current. (See Part 1.) The output capacitors must be capable of conducting the output ripple current without excessive temperature rise.
For a more accurate assessment of the ripple current, the following measurement procedure is recommended. Using a current probe of adequate rating, measure the rms current in the output capacitor leads under full load at maximum line input. (A true rms meter should be used with the current probe.) Select a capacitor to meet the ripple requirements, making due allowance for frequency and temperature multiplying factors.
(See Part 3, Sect. 12.)
FIG. 4.1 (a) Secondary circuit of a flyback converter, showing parasitic
series components ESL and ESR. (b) Output waveforms, showing effect of parasitic
components.
5. CAPACITOR LIFE
Although the preceding measurements and calculations will give a good starting point for the selection of the optimum components, the most important parameter for long-term reliability is the temperature rise of the components in the working environment, and this should be measured in the finished product.
The temperature rise is a function of the stress in the component, heat sink design, air flow, and the proximity effects of surrounding components. Radiated and convected heat from nearby components will often cause a greater temperature rise in a component than internal dissipation. This is particularly true for electrolytic capacitors.
The maximum temperature rise permitted in the capacitor, as a result of ripple current and peak working temperature, varies with different capacitor types and manufacturers. In the component examples used here, the maximum rise permitted for ripple current is 8°C in free air, and it is this limitation that the manufacturer uses to establish the ripple current rating. The rating applies up to an ambient air temperature of 85°C, giving a maximum case temperature of 93°C.
Irrespective of the cause of the temperature rise, the absolute limit of case temperature (in this example, 93°C) must be used to establish the limits of operation of the unit. This should be measured at maximum rated temperature and load, in its normal environment.
The life of the capacitor at its maximum temperature is not good, and lower operating temperatures are recommended. If in doubt as to the temperature rise caused by internal ripple current (this can be very difficult to calculate with complex flyback waveforms), proceed as follows:
1. Measure the temperature rise of the capacitor under normal operating conditions away from the influence of other heating effects. (If necessary, mount the capacitor on a short length of twisted cable, inserting a thermal barrier between the unit and the capacitor.) Measure the temperature rise of the capacitor resulting from the ripple component alone, and compare this with a manufacturer's limiting values. The permitted temperature rise is not always given on the data sheets, but it can be obtained from the manufacturer's test and QA departments. The temperature rise allowed is typically between 5 and 10°C.
2. If the temperature rise resulting from ripple current is acceptable, mount the capacitor in its normal position and subject the power supply to its highest-temperature stress and load conditions. Measure the surface temperature of the capacitor, and ensure that it is within the manufacturer's rating. This way you can be sure to avoid thermal runaway, a possibility with electrolytic capacitors.
6. GENERAL CONCLUSIONS CONCERNING FLYBACK CONVERTER COMPONENTS
The power elements of a flyback system have been discussed in considerable detail. Careful attention to the ratings and operating conditions for every component is essential for good performance and reliable operation. To the power supply engineer, this will become second nature. The selection is a laborious process that cannot be avoided if the most cost-effective and reliable selections are to be made. Calculations can take the designer only part of the way, as much of the information critical to these selections is just not available without making the appropriate measurements.
The leakage inductance of the transformer, the track layout and size, the values of ESR and ESL of the output components, component layout, and cooling arrangements have a considerable influence on the stress and ratings of the components. These effects cannot be reliably predicted. When actual measurements are not made, a wide safety margin must be applied to the calculated values in the selection of component ratings.
Much of the optimization and proving measurements can be more easily carried out at the design approval stage and will be limited to those prototypes that are destined for final production.
If long-term reliable operation and cost-effective design are to be achieved, it is incumbent upon the cognizant engineer to ensure that the design is optimized before the product is finalized and that all necessary approval testing is carried out.
7. QUIZ
1. Explain the major parameters that control the selection of the switching transistor in a flyback converter.
2. What controls the selection of secondary rectifier diodes in a flyback converter?
3. Which parameters of the flyback converter control the selection of the output capacitors?