Analysis of LED drivers for topologies such as Boost and Buck-Boost

LED light source manufacturers and designers often refer to solid-state lighting applications, the most obvious advantage is like "the fruit hanging low on the tree." For example, garden path lighting or MR16 cup lights often only need some or even just one LED. For low voltage applications, the most common voltages are 12VDC, 24VDC and 12VAC. These applications often use a Bulk regulator. Although Bulk is the first choice, as mentioned above, in LED lighting applications, as the number of LEDs increases, Boost regulators are also being used more and more. Designers are no longer satisfied with flashlights or individual cup light applications, but are turning their attention to large-scale general lighting and lighting systems that reach thousands of lumens. Such as street lights, apartment and commercial lighting, stadium lighting and interior and exterior decorative lighting.

Still need constant current

As with linear and Buck-derived LED drivers , the main technical challenge in Boost LED driver design is to provide a controlled forward current IF for each LED in the array. Ideally, each LED has a single set of chains installed to ensure the same current through each device. The Boost regulator is the easiest option when boosting the input DC voltage to a high DC output voltage because it allows more LEDs to be connected in series at a given voltage.

Figure 1: Bulk and BoostLED drivers with Vo calculation: buck: VO = nxVF, VO < VIN; boost: VO = nxVF, Vo > VIN.

Figure 1: Bulk and BoostLED drivers with Vo calculation: buck: VO=nxVF, VO VIN.

General lighting system designers often need to design the line voltage to 110VAC or 220VAC. If power factor correction (PFC), isolation, and line harmonic filtering are not required, single-stage non-isolated converters (buck, boost, or various buck-boost topologies) can be directly driven using the corrected output of the AC voltage. Long strings of LEDs in series.

However, in many cases, we need to use an intermediate DC bus voltage that is generated by an AC/DC regulator that uses a universal AC input and PFC, isolation, and filtering. Including legal requirements, a low intermediate voltage bus reduces dielectric breakdown and arcing problems, making maintenance personnel safer.

The EU has proposed the most stringent legal regulations in the world: any source above 25 watts must have a PFC. In the absence of a few years, the same rules have been made in North America and Asia. Safety standards such as UL and CE electrical regulations limit the AC/DC supply output voltage supplied to the boostLED drive. Usually the voltage is specified as 12 and 24V, sometimes 48V. These intermediate voltage buses rarely exceed 60V, which means that ULClass2 is the highest value of the DC voltage.

Boost regulator

Whether we want to control the output voltage or output current, Boost regulators are more difficult to design than Buck regulators. The average induced current in the continuous conduction state (CCM) Boost converter is equal to the load current (LED current) multiplied by 1/(1-D), where D is the duty cycle. Boost voltage regulators require designers to consider the input voltage limits to ensure proper design of the inductor, especially the rated peak current.

The Boost LED driver adds a variable output voltage that affects the duty cycle and therefore the inductance and current rating of the main inductor. To avoid inductor saturation, the maximum average and current peak must be derived simultaneously from VIN-MIN and VO-MAX. For example, calendar processing, drive current, and mold temperature, the VF of a standard white InGaN LED can vary from 3V to 4V. The more LEDs are connected in series, the greater the spacing between VO-MIN and VO-MAX.

Unlike Bulk regulators with output inductors, Boost converters have a non-continuous output current. Therefore, the output capacitor requires the output voltage to continue (as does the output current). Here, the output capacitor in the voltage regulator is designed to have a filter and maintain the output voltage during load transients. In current regulation, it only acts like an AC current filter. The capacitance should be as low as possible and consistent with the desired LED ripple current. The smaller the output capacitance (and the lower the cost and size), the faster the converter responds to the output current, so the dimming response of the LED is better.

Another serious challenge for Boost converters is the control loop. The buck regulator allows voltage mode PWM control, peak current mode PWM control, constant/controlledon-time, and other hysteresis control. Note that the right half plane zero of the CCM's Boost regulator (except low power, portable devices) and the power supply to the output when the control switch is off are almost limited to peak current mode PWM control. To design a BoostLED driver that controls the output current, the control loop must treat the LED as a load for analysis, which is very different from the typical load of a Boost voltage regulator.

In peak current mode control, the load impedance has a large effect on the DC gain and the low frequency pole of the control to output transfer function. For voltage regulators, the load impedance is determined by the ratio of the output voltage to the output current. The LED is a diode with dynamic resistance. This dynamic resistance can only be determined by making a VF (IF) curve and then using a tangent to find the slope of the desired forward current. As shown in Figure 1, the current regulator uses the load itself as a feedback divider to close the loop. This reduces the DC gain by (RSNS/(RSNS+rD)) times.

We tend to compensate for BoostLED drivers with a simple integrator at the expense of stable bandwidth. In fact, most or many LED driver applications require dimming. Whether dimming is done by linear adjustment of the IF (analog dimming) or by high frequency switching on or off (digital or PWM dimming), the system requires high bandwidth and fast transients like voltage regulators. Respond.

Buck-boost regulator

The development of LEDs for lighting is much faster than the development of solid state light source standards. A large number of different types of LEDs have many different supply voltages. The number and type of LEDs in series and their different processing and mold temperatures produce different output voltages. For example, high-end cars are transitioning to using LEDs as daytime running lights. Three 3-watt white LEDs make up a 12V1A load. Automotive voltage systems typically need to operate continuously from 9 to 16V and can be extended to 6 to 42V, making the system non-destructive, but performance may be discounted. In general, the Buck regulator is the best LED driver , followed by Boost, but in this application, they have no advantages or disadvantages. If you must use a Buck-boost regulator, the hardest decision is which topology to use.

The most basic difference between any topology Buck-boost regulator and a Buck regulator or Boost regulator is that Buck-boost never directly connects the input supply to the output. In a portion of the conversion loop, Buck and Boost regulators connect VIN to VO (via inductors and switches/diodes), which makes them more efficient.

All Buck-boost stores all the energy to be transferred to the load or magnetic field (inductor or transformer) or electric field (capacitor), which results in high peak current or higher voltage in the power conversion. The special point is to consider the corners of the input voltage and the output voltage because the peak switching current occurs at VIN-MIN and VO-MAX, but the peak transition voltage occurs at VIN-MAX, VIN-MAX, and VO-MAX. In general, this means that a Buck-boost regulator with such an output power is larger and less efficient than a Buck or Boost regulator of the same output power.

The single-inductor Buck-boost can be built like a Buck or Boost regulator, making it attractive in terms of system cost. One disadvantage of this topology is that Vo is inverted (Figure 2a) or referenced to VIN (Figure 2b). Some converters must be used for the leveling movement or reverse bias circuit. Like boost converters, they have a discontinuous output current and require an output capacitor to maintain a continuous LED current. The power MOSFET is subjected to a current with a peak of IIN plus IF and a voltage with a peak of VIN plus VO.

Figure 2: High-end buck-boost(a); low-end buck-boost(b)

Figure 2: (a) high-end buck-boost (b) low-end buck-boost

Other topology

The SEPIC converter has the advantage of continuous input current, which is generated by the input inductor and the positive output voltage. Like boost and single inductor buck-boost, they require an output capacitor to maintain a smooth LED current. Another advantage of a SEPIC converter is that almost any low-side regulator or controller can be configured as a SEPIC that does not require a reverse bias or leveling mobile circuit.

Figure 3: SEPICLED driver

Figure 3: SEPICLED driver

The Cuk converter, which is rarely used as a voltage regulator, has emerged as an LED driver. Both the input and output currents are continuous. The polarity of the output voltage is reversed like the high-end buck-boost, but the output capacitor is eliminated like a buck converter. In addition to Buck-boost and boost, Cuk is the only practical non-isolated regulator with this capability.

Figure 4: Cuk regulator

Figure 4: Cuk regulator

Due to the high complexity of Boost and Buck-boost regulators and their inadequate selection of peripheral circuits, inefficiencies (especially Buck-boost) and control topologies, they are not the first choice for converting LED drivers. But they are all essential for more and more lighting applications in LEDs. Some system architectures can be replaced with buck or even linear regulator-based LED drivers. For example, a large light source similar to a street light requires one hundred or more 1W+ LEDs. In general, LEDs for general illumination range from low power consumption to high power consumption, and in the middle stage, such as automotive headlights and small optical components, boost and buck-boost regulators represent the best choice for constant current drive. (Edit: Technology)

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