High-efficiency DC/DC constant current power supply LED driver innovative design solution

1 Introduction

As a new type of light source in the 21st century, semiconductor lighting has the advantages of energy saving, environmental protection, long life and easy maintenance. It is a general trend to replace traditional lighting sources such as incandescent lamps and fluorescent lamps with high-power and high-brightness light-emitting diodes (LEDs). Due to the characteristics of LEDs themselves, constant current sources must be used to power them. Therefore, the design of high-efficiency constant current drive power supplies has become an important research object in LED applications. LLC half-bridge resonant converter has become a popular topology with its high efficiency and high power density. However, it is generally used in constant voltage output applications. Traditional LLC is considered unsuitable for wide-range constant current output. Here, a new design method for half-bridge LLC is proposed to keep high efficiency in wide-range constant current output applications.

Therefore, LLC can be a good topology choice for LED driving.

2 Design method of constant current LLC resonant converter

2.1 Overview of Half-Bridge LLC Conversion Circuit

The circuit principle of the half-bridge LLC resonant converter is shown in Figure 1.

Two complementary switching tubes VS1 and VS2 with a duty cycle of 0.5 form a half-bridge structure. The resonant inductor Lr, the resonant capacitor Cr and the transformer's excitation inductor Lm form an LLC resonant network. The transformer secondary is composed of rectifier diodes VD1~VD4 to form a full-bridge rectifier circuit.

Figure 1. Half-bridge LLC resonant converter circuit topology

半桥LLC 变流器有两个谐振频率。当变压器初级电压被输出电压箝位时，Lm 不参加谐振，Lr和Cr 产生的串联谐振频率为f1;当变压器不向次级传递能量时，Lm 电压不被箝位，Lm，Lr，Cr 共同参与谐振，构成谐振频率f2 为：

2.2 DC gain curve and operating range

Using the fundamental wave approximation method, the DC voltage gain expression of the LLC resonant converter can be derived as:

In the formula: m=Lm/Lr;fn =fs/f1, fs is the switching frequency; Ro is the equivalent output resistance.

Figure 2 shows the DC gain curve of the half-bridge LLC converter under different load conditions. When LLC operates at f1 (i.e. point (1, 1) in the figure), the resonant circuit impedance is minimum and the loss is lowest. Therefore, in ordinary design, the full load operating point is generally designed at this point.

Figure 2 DC gain curve of half-bridge LLC

In the 3rd interval shown in Figure 2, the switch tube works in the capacitive area, and the switching loss is large, so the circuit should be avoided from working in this area in any design. In the 2nd interval, the LLC works in the resonant current discontinuous mode, which can realize the ZVS opening of the primary switch tube and the ZCS closing of the secondary rectifier tube at the same time to avoid reverse recovery. Therefore, in the design of constant voltage output, the working points under all load conditions are generally designed in this interval. However, in the design of constant current wide voltage range output, the load changes greatly, and the corresponding DC gain changes in a large range. It is difficult to ensure that all working points in the full load range are in the ZVS area. And the circuit works on the curve between the maximum gain point and the (1, 1) point. The smaller the gain of this curve, the closer it is to the resonance point. Therefore, the full load working point can only be designed in the interval with high DC gain, that is, fs < f1. The output voltage is small, that is, the light load working point is designed at the resonance point. The full load efficiency cannot be optimized and the efficiency will be very low.

In the range 1 shown in Figure 2, fs>f1, LLC works in the resonant current continuous mode, the primary switch tube can achieve ZVS turn-on, the secondary rectifier tube cannot achieve ZCS turn-off, there will be a reverse recovery process, but it has little effect when the output current is small. The gain curve slope in this range is large, and the DC gain can be adjusted in a wide range, which can meet the requirements of constant current wide voltage range output design.
2.3 Constant current wide voltage range output design

The DC gain of the half-bridge LLC is:

Where: n is the actual transformer winding turns ratio; Uin, Uo are the input and output voltages respectively.

It can be seen that in order to obtain the optimal design point (i.e. the resonance point), it is only necessary to take the desired transformer winding turns ratio Nnor=Uin/(2Uo).

As shown in Figure 2, the smaller the curve gain, the larger the slope. If the full-load operating point is designed at the resonance point, the operating frequency will reach more than 2 times the resonance frequency when the output voltage drops to half (that is, Gdc drops to 0.5), and the operating frequency range is very wide. In order to narrow the operating frequency range, a section with a large slope of the gain curve can be selected, that is, Gdc < 1. From formula (3) and the Nnor calculation formula, it can be seen that if n < Nnor, then Gdc < 1. Figure 3 shows the gain curve and operating point when n = 0.88Nnor.

Figure 3. Operating point of constant current LLC

In Figure 3, the dotted line is the Gdc corresponding to Uo changing from 200 to 100 V, and the solid line is the gain curve of the equivalent load when Uo is 200 to 100 V. The intersection of the solid line and the dotted line when Uo is the same is the actual operating point of the circuit. In this design, when Uo changes from 200 to 100 V, the operating frequency range is 1.22f1 to 2.11f1.

3 Parameter analysis and optimization

3.1 f1 selection

Considering the design of magnetic components, it is ideal to design the operating frequency of the circuit at full load at around 100 kHz. To ensure the half-load working efficiency, the half-load frequency cannot be too high. Therefore, the section with a larger slope in the gain curve should be selected, that is, Gdc < 1. The actual operating frequency of the circuit is always greater than f1, so f1 < 100 kHz should be selected, and it is more reasonable to design at 60~70 kHz.

3.2 Resonance parameters Cr, Lr

当f1 一定时，Cr 越小，Lr 越大，Q 越大，增益曲线的斜率越大，故减小Cr 可使半载的工作频率显著降低。从提高半载效率的角度考虑，Cr 越小越好，但Cr 越小，其两端的电压峰值则越大，要降低Cr 的电压应力，Cr 应取越大越好。设计中应该折中考虑。Cr 确定后，根据f1 可计算出Lr 为：

3.3 Selection of n and Lm

In order to narrow the range of switching frequency, the actual transformer winding turns ratio should be smaller than the expected transformer winding turns ratio. n < Nnor.n decreases, the operating frequency at half load decreases, but at the same time the operating frequency at full load increases, the operating point deviates far from the resonance point, and the circuit operates in a more continuous state. When the resonant current is still large, the MOSFET is forcibly turned off; when the diode is turned off, the rectifier current flowing through it is also large. In this way, the switching loss of the MOSFET and the rectifier tube will increase, which is more obvious in high current situations. Therefore, n should not be too small.

From the perspective of reducing the conduction loss of the switch tube, the larger the value of the transformer Lm, the smaller the effective value of the primary current, and the smaller the conduction loss of the switch tube, so it is hoped that the larger the Lm, the better. However, when Lr is constant, the larger the Lm, the larger the m, and the smaller the slope of the gain curve. In order to ensure the required Uo, the operating frequency range of the converter is widened, which will affect the efficiency when Uo is reduced to half. Therefore, under the premise of ensuring a certain switching frequency range, the larger the Lm, the better.

The design of all the above parameters requires comprehensive consideration of multiple factors, making reasonable trade-offs based on the design goals, and finding the best design parameters for specific applications.

Portable power applications are broad and diverse. Products range from wireless sensor nodes that consume uW average power to medical or data acquisition systems that can be carted around and consume hundreds of watt-hours of battery power. However, despite the wide variety of applications, several trends have emerged, with designers increasingly needing to provide more power to their products to support an increasing number of features while also considering how to charge the battery from any available power source. To meet the first need, the battery capacity must be increased. Unfortunately, most users are impatient, and after the capacity is increased, it must be fully charged in a reasonable time, which leads to an increase in charging current. To meet the second need, the battery charging solution requires great flexibility. This article will discuss these issues in more detail.

More power

Consider a modern handheld device, both consumer and industrial, that may include a cellular modem, a Wi-Fi module, a Bluetooth module, a large backlit display, and so on. The power architecture of many handheld devices is very similar to that of a cellular phone. Typically, a 3.7V Li-ion battery is used as the primary power source because Li-ion batteries have high energy densities by weight and volume (Wh/kg and Wh/m3 respectively). In the past, many high-power devices used 7.4V Li-ion batteries to reduce current requirements, but the availability of low-cost 5V power management ICs has driven more and more handheld devices to lower voltage architectures. Tablets illustrate this point well: a typical tablet has an extremely high number of features and a very large display (for a portable device). When powered by a 3.7V battery, its capacity must be measured in thousands of milliamp-hours. To fully charge such a battery in a few hours, thousands of mA of charging current are required.

However, even this high charge current does not prevent consumers from using USB ports to charge high-power devices without a high-current AC adapter. To meet this demand, the battery charger must be able to charge at high current (>2A) when an AC adapter is available, and when no AC adapter is available, the battery charger must still efficiently utilize the 2.5W to 4.5W power provided by the USB port. In addition, the device must protect sensitive downstream low-voltage components from damage caused by possible overvoltage conditions, while seamlessly delivering high current from the USB input, AC adapter, or battery to the load and minimizing power consumption. In addition, the IC must safely manage the battery charging algorithm and monitor key system parameters.

Overcoming Power Challenges in Single-Cell Battery-Powered Portable Products

Although it may seem impossible to find a single IC that meets these requirements, consider the LTC4155, a high power, I2C controlled, high efficiency PowerPath™ manager, ideal diode controller and Li-Ion battery charger. Designed to efficiently deliver up to 3A from a variety of 5V supplies, the IC can generate more than 3.5A of available current for battery charging and system use (see Figure 1). With efficiencies ranging from 88% to 94%, the LTC4155 can still ease thermal budget constraints even at these high current levels (see Figure 2). The LTC4155’s switching PowerPath topology seamlessly manages power distribution from two input sources (such as a wall adapter and a USB port) to the device’s rechargeable Li-Ion battery, while giving priority to powering the system load when input power is limited.

Figure 1: Typical application circuit for LTC4155

Figure 2: Typical efficiency of the LTC4155

Compared to a typical linear mode charger, the switching regulator in the LTC4155 acts like a transformer, allowing the load current on VOUT to exceed the current drawn by the input supply and greatly increasing the available power for battery charging. The previous example shows how the LTC4155 can charge efficiently at currents up to 3.5A for faster charging times. Unlike ordinary switching battery chargers, the LTC4155 can turn on instantly to ensure that system power is immediately available as soon as the power plug is plugged in, even if the battery is dead or deeply discharged.

Even when charging the battery at a high rate, it is important to monitor the battery for safety. The LTC4155 automatically stops charging when the battery temperature drops below 0°C or rises above 40°C (measured by an external negative temperature coefficient (NTC) thermistor). In addition to this autonomous function, the LTC4155 also provides a 7-bit extended-scale analog-to-digital converter (ADC) to monitor battery temperature with a resolution of approximately 1°C (see Figure 3). Combined with the four available float voltage settings and 15 battery charge current settings, the ADC can be used to establish a customized charging algorithm based on battery temperature.

Figure 3: 7-bit thermistor ADC showing preset temperature trip points for the LTC4155

The results of the NTC ADC are available through a simple two-wire I2C port, which can adjust the set points of the charging current and voltage. The I2C port also provides USB compatibility by controlling the set points of 16 input currents (including USB 2.0 and 3.0 compatible settings). The communication bus allows the LTC4155 to indicate additional status information, such as input power status, charger status, and fault status. Because USB OTG is supported, 5V power can be provided to the USB port in turn without any additional components.

The LTC4155's dual input, priority multiplexer autonomously selects the most appropriate input (i.e., wall adapter or USB) based on a user-defined priority (the default priority is the adapter input). An overvoltage protection (OVP) circuit protects both inputs from damage due to inadvertent high voltage or reverse voltage. The LTC4155's ideal diode controller ensures that sufficient power is always available to VOUT, even when input power is insufficient or absent.

For many portable applications such as tablets, industrial barcode scanners, etc., being able to manage two inputs (such as USB and AC adapter) is sufficient. However, designers of portable devices have been looking for ways to charge the battery from any available power source.

Multiple input sources

There are several reasons why users want to charge batteries from multiple input sources. Some applications may need to be independent of the grid and powered by a solar panel. Other applications may require the convenience of charging from an AC adapter, car battery, or high-voltage industrial and telecom power supplies. Regardless of the reason, this requirement places a significant burden on the battery charging system. Most battery chargers use a step-down (switching or linear) architecture to charge the battery from a voltage source that is higher than the maximum battery voltage. Previous charger products have generally been limited to input voltages of approximately 30V. This limitation prevents designers from using telecom power supplies or solar panels with a 42V open circuit voltage as viable input sources. In some cases, it is desirable to use input supply voltages that cover a range above and below the battery voltage. Designing a solution to this challenge usually requires a mix of high-precision current sense amplifiers, ADCs, a microprocessor to control charging, a high-performance DC/DC converter, and an ideal diode or multiplexing circuit.

Powerful charging solutions provide unmatched flexibility

The LTC4000 converts any externally compensated DC/DC power supply into a full-featured battery charger with PowerPath™ control. Typical DC/DC converter topologies that can be driven by the LTC4000 include, but are not limited to, buck, boost, buck-boost, SEPIC, and flyback topologies. The device provides precise input and charge current regulation and operates over a wide input and output voltage range of 3V to 60V, enabling compatibility with a variety of input voltage sources and battery packs of different sizes and chemistries. Due to its versatile configuration, the device's typical applications are extensive, including high power battery charger systems, high performance portable instruments, battery backup systems, equipment with industrial batteries, and notebook/subnotebook computers.

In addition to being able to be combined with many different DC/DC topologies, the LTC4000's high voltage capability allows the device to form a powerful battery charging solution that can use almost any input power source (see Figures 4 and 5). To ensure that power from these inputs is delivered to the appropriate load, the LTC4000 uses an intelligent power path topology that prioritizes powering the system load when input power is limited. The LTC4000 controls the external PFET to provide low-loss reverse current protection, low-loss charging and discharging of the battery, and instant-on operation, which ensures that system power is immediately available when plugged in, even when the battery is dead or deeply discharged. External sense resistors provide input current and battery charge current information, allowing the LTC4000 to be used with converters covering the mW to kW power range.

Figure 4: LTC4000 and LTC3789: 6V to 36VIN, 4-cell 5A Li-Ion battery charger

Figure 5: Efficiency vs. VIN for the circuit in Figure 4 when the system load is 4A

The LTC4000's full-featured battery charge controller charges a variety of battery chemistries, including Li-Ion/Polymer/Phosphate, Sealed Lead Acid (SLA) and Nickel chemistries. The battery charger also provides accurate current sensing, allowing lower sense voltages in high current applications.

in conclusion

Designers of modern portable products have a challenging job, especially when it comes to power. Customers continue to demand more features, which results in more power, resulting in larger batteries. At the same time, customers want the convenience of charging these batteries from almost any available power source. While these trends in portable power create design challenges, the LTC4155 and LTC4000 make the design job much easier. In low voltage systems, the LTC4155 efficiently delivers up to 3.5A of charging current and offers many high performance features. The LTC4000 forms a powerful charging solution and can use almost any input, providing unparalleled performance and flexibility.