These days, LEDs are turning up in backlighting, general illumination, and other innovative applications. They provide many performance advantages over their incandescent counterparts. However, they’re also more expensive.
LED drivers based on the flyback power-supply topology are inexpensive. They also can provide an isolated output that complies with UL safety requirements in fixtures that aren’t double-insulated.
Further, it’s possible to include power factor correction (PFC) without adding greatly to the cost while enabling dimming with a standard triac-based dimmer. The flyback driver can meet Energy Star requirements from the U.S. Department of Energy as well, making it a very attractive option for offline LED-based light fixtures under 50 W.
The flyback converter is based on the boost converter principle. An offline flyback converter utilizes a single high-voltage switching MOSFET and coupled inductor to provide energy storage and transfer to an isolated secondary and single-diode rectifying output circuit.
In an LED application, the power-supply output current is regulated instead of the voltage unlike most power supplies. LEDs should be driven with a constant current for best stability and operating life. It is also essential to include open-load over-voltage limiting at the output since this type of switching power-supply circuit can produce very high output voltages (see the figure).
When the MOSFET switches on, the current in the primary of the coupled inductor shown in the figure rises linearly. During this phase a magnetic field builds up in the air gap in the center of the ferrite cores.
When the MOSFET switches off, the magnetic field collapses as its stored energy transfers to the load through the rectifier diode. The voltage at the inductor secondary rises to whatever level is required for current to flow and permit energy transfer.
A capacitor is included at the output to remove ripple. The pulse width of the MOSFET gate drive signal determines the amount of energy stored per switching cycle, which is controlled by means of an error amplifier that compares the LED current with a reference and either increases or decreases the pulse width to regulate energy transfer.
Flyback converters may be designed to operate in continuous or discontinuous modes. For simplicity and to aid PFC, Flyback LED drivers usually operate at the border between the two modes in critical conduction or transition mode. This means the switching cycle begins immediately after all of the energy stored in the inductor has been transferred to the output.
Current Regulation Methods
The output is isolated, so it’s necessary to include an opto-isolator if the LED output current is to be sensed and fed back to the input to control the pulse-width modulation (PWM) duty cycle. Since the opto-isolator adds costs and reduces reliability, some designs eliminate it by sensing the primary current to regulate the power.
This method works on the assumption that the input voltage and load don’t change significantly and therefore regulating the input current will provide a constant output power to a fixed voltage load such as a string of LEDs.
This method is generally adequate for LED drivers designed to drive a specific load from a fixed supply voltage such as 120 V ac. Designs intended to work over a wide input voltage range or with different loads need a closed loop with an opto-isolator.
Over-voltage protection is easily implemented if there is already an opto-isolator providing feedback isolation in the circuit. The output current and voltage information can be OR’d together and fed back to the PWM control circuitry at the primary. However, where there’s no opto-isolator, the voltage needs to be sensed indirectly at the primary.
The primary control IC and circuitry normally requires a third winding of the inductor to provide its VCC supply, which is also useful for providing voltage feedback information since it is proportional to the load voltage. It also can provide zero crossing information to the control IC, enabling it to detect when stored energy has been transferred and initiate the next switching cycle.
Power Factor Correction
The flyback topology is an extension of the basic boost topology widely used in power-factor-correcting front-end stages in many power supplies and electronic ballasts. It is therefore possible to obtain a high power factor without adding additional switching stages and inductors simply by operating the circuit from a full-wave rectified voltage without dc bus smoothing and using a similar control approach.
This can be done in flyback configuration in the same way as in a boost. A relatively large capacitance is necessary at the output to remove low-frequency ripple though. A small voltage ripple translates to a large current ripple in an LED load due to its steep voltage to current gradient.
High-brightness LEDs operate from low voltage and high current, which means that the output capacitor needs to be on the order of 1000 µF to supply a typical 350-mA LED load with acceptable low-frequency ripple.
This necessitates the use of an electrolytic capacitor selected for longest possible life and small dimensions. A 105°C rated capacitor with an 8000-hour operating life rating is a good option.
As a general rule of thumb, the life doubles for every 10°C lower operating temperature. The LED driver operating life, then, doesn’t have to be restricted by the electrolytic capacitor if the maximum operating temperature is kept at least 20°C below the 105°C limit and voltage and ripple current ratings are higher than the maximum output voltage and ripple current of the LED driver.
The regulating control loop speed must be relatively slow, requiring several ac line cycles to adjust the PWM on time so that during a single line half cycle the on time varies by only a very small amount. The ac line input current can maintain a reasonably sinusoidal shape to provide a power factor above 0.9.
Although the on time is effectively constant during the ac line cycle, the off time varies because more energy is stored at higher instantaneous ac line voltages. The stored energy per cycle is given by E = ½.L.Ipk2 where Ipk = V.Ton/Lpri and Lpri and Ton are constant so the energy stored per cycle is proportional to the square of the instantaneous line voltage. The change in off time during the cycle does not distort the sinusoidal input current as a variation in on time would.
Efficiency And Power Limits
The basic flyback converter can provide efficiencies of above 80% at power levels up to about 50 W. The fact that the boost/flyback circuit uses indirect energy transfer, meaning that energy is stored in the on phase and transferred during the off phase, explains why the flyback topology is not efficient at higher power levels as the inductor would need to be large with inherent losses due to imperfect coupling and parasitic elements.
Other switched-mode power-supply (SMPS) topologies using direct energy transfer offer higher efficiency and smaller size at higher power levels. A dimmable one-stage high-power-factor flyback LED driver may have an efficiency of 80% at 120-V ac input with 33-V dc output at 350 mA, which is 11.5 W.
LED drivers requiring a high output current often use a two-stage topology comprising a boost front end followed by a flyback output stage. Since the first stage produces a regulated high-voltage dc bus, the design of the flyback second stage can be greatly simplified and optimized for efficiency with only a small output capacitor to remove high-frequency ripple.
Almost all dimmers are based on a simple triac-based circuit, and most of them use a very basic input and output connection with no connection back to the neutral line. A simple timing circuit produces a firing pulse to the triac gate at some point during the ac line half cycle, which can be adjusted by a potentiometer, allowing the triac to be fired at almost any point.
The triac switches on and conducts only after firing and continues to conduct until the current flowing through it drops behold a fixed threshold known as the holding current, which normally occurs at next zero crossing. This works with purely resistive light bulbs because of the relatively low resistance path back to neutral for the load current, which allows the gate trigger circuit to be referenced to the output.
Simply having a high power factor does not enable an LED driver to be dimmable. Connected to capacitive loads such as electronic ballasts and power supplies, dimmers are susceptible to erratic switching on and off caused by the high-voltage ringing caused by filter inductors and capacitors. This can cause the triac current to drop below the holding current and switch off and then back on again several times during a cycle, resulting in severe flickering.
Even a high-power-factor power-supply input circuit represents a capacitive load, but it does have the advantage of drawing current from the ac line for the complete cycle or from the triac firing point until the next zero crossing. For smooth dimming, the challenge is to avoid false triac switching caused by the ringing produced by the large dV/dt that occurs when the triac is initially fired.
The flyback-based LED driver makes sense at power levels below about 50 W where isolation is a requirement. But since it is less efficient than a buck regulator, it makes little sense for non-isolated designs. With the techniques discussed here, a simple and cost-effective design can be produced with high power factor and dimming capability. The flyback is less suited for drivers required to produce more than 350-mA output since large-output capacitors are needed, and it is relatively bulky and inefficient above around 50 W.