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Posted on 23 July 2019

Smarter Rectification

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Driving up SMPS Efficiency and Power Density

Performance, size and efficiency advantages have made the LLC series-resonant converter the preferred power supply topology in applications such as high-end consumer products. Going forward, improvements to the secondary side architecture are necessary to further enhance efficiency and space savings.

By Helen Ding, International Rectifier

 

The demands for high operating efficiency and small size in consumer electronic products such as home theatre systems, game consoles and LCD televisions is moving power supply design towards resonant topologies capable of operating at high switching frequencies. At the same time as allowing high-frequency operation permitting the use of smaller magnetic components, the resonant converter’s soft commutation enables the SMPS to operate efficiently and with low EMI.

Refining the Resonant Converter

Among the various resonant converter topologies, the LLC converter has become the switching scheme of choice. Although as easy to build as a basic series-resonant LC converter, it overcomes drawbacks such as difficulty in maintaining regulation at light load. It also effectively improves efficiency when the input voltage is high such that switching loss is more dominant than conduction loss. The LLC resonant topology utilises an additional shunt inductor across the primary winding of the transformer, as shown in Figure 1. This is usually realised using the magnetising inductance of the transformer, which is controlled by adjusting the transformer air gap. This topology produces a complex resonant tank with buck-boost transfer characteristics in the soft-switching region.

Half-Bridge LLC series-resonant converter

Referring to figure 1, in normal operation the primary-side MOSFETs operate at 50% duty cycle and the output voltage is regulated by varying the switching frequency of the converter. The converter has two resonant frequencies; a lower resonant frequency (given by Lm, Lr, Cr and the load), and a fixed higher series resonant frequency Fr1 (given by Lr and Cr only). The secondary side half bridge can be soft-switched for the entire load range by operating the converter either above or below Fr1.

The secondary side half-bridge is conventionally implemented using a pair of diodes. However, this arrangement is relatively inefficient, as diode losses contribute significantly to the overall power loss of the SMPS. With increasing current draw in future generations of featurerich consumer products, these losses will continue to increase, since the diode rectifier conduction loss is proportional to the product of its forward conduction current as well as the forward voltage drop. Increasing dissipation also demands the use of larger diodes, leading to progressively more bulky power supplies.

Hence there are two powerful factors forcing designers to demand a more satisfactory secondary side topology for LLC resonant converters. For power supplies in the region of 50W and higher, the demand for higher power density to minimise case dimensions is the dominant concern. In the region of 200-400W, boosting efficiency to satisfy initiatives such as Energy Star and CEC 80+ is a major reason for power supply designers to seek ways to eliminate the losses incurred in the secondary side diodes.

Synchronous Secondary Side Rectification

Using synchronous rectification in the secondary side holds out the promise of reducing the large losses incurred in the half-bridge diodes. Since MOSFET conduction losses depend on I2 x RDS(ON), splitting current between two synchronous MOSFETs reduces the dissipation in each device by four, thereby halving the total dissipation.

However, the most familiar control techniques for synchronous rectifiers are not workable in LLC resonant converters. For example in a primary-controlled synchronous rectifier, where the MOSFET control signals are derived from primary side signals, the LLC resonant converter has a phase lag between the input voltage of the resonant tank and the rectified secondary side current. This prevents the primary side gates from being used to drive the secondary side rectifiers. The alternative self-controlled rectifier technique, where the control signals are derived from the secondary voltage across the power transformer, suffers from timing mismatches between the rectified secondary side current and the voltage across the main power transformer, which is a 50% duty cycle square wave. These prevent satisfactory operation below the resonant tank frequency of the converter.

Using a current transformer, on the other hand, is a workable control technique for resonant converters. The drawbacks of this technique include a high component count, which increases footprint and impairs reliability, and the need for a relatively expensive fast comparator.

Placing a controlling IC in the secondary side to manage the switching of the MOSFETs potentially offers a simpler and more cost-effec- tive alternative. By eliminating the current transformer and fast comparator, an ICbased solution saves size and component count. However, to implement all the required functions in a single component requires certain competencies, such as the ability to co-integrate the control functions with high-voltage sensing capabilities in the same device, as well as managing high switching frequency and high current-driving capabilities. International Rectifier’s IR1168, for example, uses high-voltage IC (HVIC) technology and patented techniques to deliver a secondary side rectifier driver IC designed to drive two N-Channel power MOSFETs used as synchronous rectifiers in resonant converter applications. In addition to providing two gate drivers, the device also provides adaptive shoot-through protection to prevent both channels from simultaneously turning on. It is also capable of operating in normal and burst-mode conditions. In addition, clamped gate-driver operation significantly reduces power dissipation.

Single-Chip Control

Figure 2 illustrates a typical application schematic for the IR1168. In normal operating mode, the IC senses the voltage drop across each MOSFET at pins VS1/VD1 and VS2/VD2, and turns each MOSFET on and off via pins GATE1 and GATE2.

Typical secondary-side application circuit for IR1168

At the core of this device are two high-voltage (200V) high-speed comparators. These differentially sense the drain to source voltage of the MOSFET, using the RDS(ON) of the device as a shunt resistance, and hence determine the polarity and level of the device currents. Dedicated internal logic then manages the turning on and off of each device in close proximity to the zero-current transition. The device uses the SmartRectifier™ control technique, which compares the sensed voltage across the MOSFET with two negative thresholds to determine the turn-on and turnoff transitions for the device. The most negative of these two thresholds, VTH2, detects current through the body diode and hence, controls the turn on transition for the power device. Similarly, a second negative threshold, VTH1, determines the level of the current at which the device turns off. A third threshold voltage, VTH3, acts as a reset threshold governing the resetting of an internal oneshot when the cycle is completed and the VDS voltage turns positive and starts to increase again. This way the system is ready for next conduction cycle.

By governing the drive level of the secondary side MOSFETs according to these three thresholds, the IR1168 ensures accurate performance without the need of a PLL or external timing sources. In fact, the high accuracy of turn-off transitions is a key benefit of this technique, since it prevents reverse current across the MOSFETs and also minimises the body diode conduction time. Additionally, internal blanking logic is used to prevent spurious gate transitions and guarantee operation in fixed- and variable-frequency operation modes.

The waveforms of Figure 3 show the IR1168 in normal operating mode. When the conduction phase of the MOSFET is initiated the current begins to flow through the MOSFET body diode, generating a negative VDS voltage. Since the body diode has a much higher voltage drop than that caused by the MOSFET RDS(on), this negative VDS triggers the turnon threshold, VTH2. At this point, the IR1168 will turn on the gate of the MOSFET. This, in turn, causes the VDS voltage to drop down to a value defined by the MOSFET drain current (ID) multiplied by RDS(on).

Operation of SmartRectifier™ synchronous rectification control with IR1168

Since this fall in voltage is usually accompanied by some amount of ringing that could trigger the input comparator to turn-off, a fixed Minimum On Time (MOT) blanking period is used that will maintain the power MOSFET on for a minimum time duration.

Once the MOSFET has been turned on, it remains on until the rectifier current decays to the level where the voltage will cross the fixed turn-off threshold VTH1. Once the threshold is crossed, the current will start flowing again through the body diode, causing the VDS voltage to jump negative again.

Depending on the amount of residual current, VDS may once again trigger the turn-on threshold. Hence, VTH2 is blanked for a time duration called TBLANK after VTH1 is triggered. The period TBLANK is shown in the diagram, and is terminated when the device VDS crosses the positive reset threshold VTH3. The IC is then ready for the next conduction cycle.

Conclusion: Performance Advantages in Next-Generation Products

Figure 4 compares a 240W multi-rail power supply for an LCD TV application built using the IR1168 synchronous-rectifier controller against a conventional resonant converter design with secondary side Schottky diodes. The IC is housed in a low-cost SO-8 package and delivers a single-chip solution capable of generating the control signals for both MOSFETs. The four Schottky diodes were replaced by IR1168 with four SO-8 MOSFETs daughter card. The two large heatsinks cooling the Schottky diodes for the 12V and 24V rails are also eliminated.

IR1168 retrofit in a 240W LCD TV

In practice, this smaller SMPS featuring secondary side synchronous rectification has shown a 1.5% increase in efficiency. A 25°C reduction in operating temperature for the SMPS has also been recorded, leading to a significant increase in overall system reliability.

 

 

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