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Posted on 17 July 2019

Maximise Resonant Converter Efficiency in Flat-Panel Displays with Self-Timed Synchronous Rectification

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Switching losses are negligible as the synchronous MOSFETs switch on and off very close to the zero current point

Flat-panel LCD displays have rendered CRT-based TVs and computer monitors obsolete, with today’s consumers vying for the largest screen area, best picture quality, and slimmest profile that manufacturers can offer.

By Yong Ang, Senior Applications Engineer, Diodes Incorporated

 

A key sub-system in the race for the display slimness that’s clearly so attractive is the power supply, which invariably uses a switch-mode topology. Of the topologies that suit offline conversion power levels of typically 100 - 600 Watts, resonant switch-mode converters are attracting increasing attention from product designers. Able to exploit high switching frequencies, resonant converters shrink the size of the supply’s magnetics and filter components while achieving high conversion efficiencies with minimal EMC issues—at build costs that compete with other topologies.

Yet timing issues make it difficult to substitute efficient synchronous rectification for conventional and relatively lossy secondaryside diode rectifiers within a typical resonant converter. A new self-timing approach overcomes this issue, and helps flat-panel display manufacturers to meet minimum efficiency standards that the US leads with its Energy Star program and that other authorities are following.

Zero-voltage switching is key

A universal-input offline supply for a flatscreen LCD typically comprises an input rectification stage followed by an active powerfactor correction circuit that attenuates power-supply harmonics to meet IEC61000- 3-2 requirements for electromagnetic compatibility. Most often, a boost regulator generates a high-voltage DC bus level of around 400V. It’s then necessary to downconvert this voltage as efficiently as possible to the low voltages that suit the display’s subsystems. Efficiency becomes ever more significant with increasing screen size, with a typical LCD TV application requiring three main output levels that can be derived from multiple secondaries wound around a common transformer core.

Of the available resonant converter topologies, the most attractive option for downconverting the high-voltage DC bus uses the LLC arrangement that figure 1 outlines. Here, the controller regulates the supply’s output by modulating the drive signals to the primary-side MOSFETs Q1 and Q2. These switches drive the tank circuit that results from the transformer’s magnetising inductance Lm and its leakage inductance Llk reacting with resonant-mode capacitor Cres.

Resonant LLC converter with centre-tap secondary winding

A key advantage of this topology is its use of zero-voltage “soft” switching for Q1 and Q2, which greatly reduces the peak currents and edge losses that hamper traditional hardswitching topologies. As a result, this softswitching approach improves efficiency and reduces heat generation and stress within the switching elements. It also minimises EMC generation that the sinusoidal nature of tank-circuit currents and voltages further constrains. This attribute is especially welcome in display applications, where noise can severely compromise picture quality because the power-factor correction circuit provides a pre-regulated DC supply, it’s possible to optimise the resonant converter to operate close to the series resonant tank frequency at the nominal DC input voltage. It’s then possible to regulate the output voltage over a wide load variation with narrow changes in switching frequency. As the load level approaches zero, the converter’s frequency reaches its maximum before entering a pulse-skipping mode that minimises power consumption.

Increasing efficiency with synchronous rectification

Yet in a typical LLC converter, the secondary- side rectifiers are almost invariably Schottky diodes that have relatively large forward voltages. It’s therefore desirable to replace these lossy elements with MOSFETs configured as synchronous rectifiers (S1 and S2 in figure 1).

Precise gate-drive control is essential for synchronous rectification, with two control schemes being possible. The first synchronises the gate-drive signals for the secondary- side MOSFET rectifiers with the primaryside MOSFET’s gate drive using pulse transformers. But for LLC resonant converters that operate over a wide load range, this method has difficulty in producing secondary gate-drive signals that are usable over the full operating envelope. Because the converter operates above its resonant frequency under light load conditions, the output rectifier currents become discontinuous and phase differences exist between the resonant tank voltages and currents. The resulting timing mismatch causes reactive power flow between the output and the power transformer due to the output capacitor discharging during the discontinuous rectification interval, causing inefficiencies that limit this scheme to resonant converters that operate over a relatively narrow load range.

An alternative scheme employs signals derived from the secondary side to drive the synchronous MOSFETs. The basic idea relies on sensing the current through the MOSFET rectifiers to derive these drive signals, conventionally using a current-sense transformer and analogue comparator to switch each MOSFET in response to the current flowing through each device. Issues include high component count and the timing delay that the comparator introduces. Figure 2 shows a lossless drain-voltage sensing technique that takes advantage of IC design techniques to overcome these problems, improving conversion efficiency while reducing system cost by dispensing with the current-sense transformer and discrete comparator.

Drain voltage sensing synchronous controller with analogue gate drive for resonant converter.

Drain voltage sensing synchronous control IC

The ZXGD3101 integrates the voltage-detection circuitry, comparator, and MOSFET gate-driver stages to implement a drain-voltage-sensing synchronous controller. As figure 2 shows, the voltage between the chip’s GND and DRAIN pins is proportional to the current flowing through the resistance that the MOSFET’s source-drain channel presents. Each MOSFET structure also intrinsically includes a forward- biased body diode in parallel with the source-drain terminals. This diode starts to conduct when current in the rectifier starts to flow, generating a negative voltage at the drain pin. Upon detecting this voltage, the controller turns the MOSFET on.

At the end of the conduction cycle, efficiency considerations make it imperative to switch the MOSFET off as closely as possible to the zero-current point, and without allowing any reverse current flow. By comparing the drain voltage with a negative threshold voltage and pulling the MOSFET’s gate-drive voltage down when the drain voltage is more positive than this threshold value, the controller guarantees to switch the MOSFET off at the optimum time. An interesting feature of the controller’s implementation is its use o

f an analogue signal to control the MOSFET’s gate, rather than the digital high voltage that traditional gate drivers use to enhance the MOSFET throughout its conduction period. The device’s proportional gate-drive approach reflects the fact that in a resonant converter, the load current is sinusoidal and losses reach their maximum at the peak drain current point.

Figure 3A shows the controller quickly ramping up the gate voltage as the drain current level rises to around 25% of nominal load capacity, fully enhancing the MOSFET to minimise its on-resistance while it carries significant current. The controller then gradually ramps down the gate-drive voltage in response to falling load current. This adaptive reduction in gate-drive voltage lowers the level of gate charge in the MOSFET, minimising switching power losses while speeding the device’s turn-off to prevent reverse current flow. Figure 3B shows a cycle at full output loading, when the gate-drive voltage takes on a more rectangular shape to minimise the conduction losses that dominate due to the MOSFET’s on resistance.

ynchronous rectifier control waveforms in resonant converter (a) 25 percent load

Synchronous rectifier control waveforms in resonant converter (b) full load

Lowering thermal stresses in backlight supplies—example

A representative LCD TV power supply might provide +24V for the screen’s backlight, +12V for the audio amplifier, and +5V for analogue and digital circuitry. The +12V and +5V supplies are low current rails that can use Schottky-diode rectifiers without significant compromises. But at full brightness, a 32” screen needs around 144W for the backlight, rising to about 264W for a 42” display. These levels respectively equate to about 6A and 11A that challenge the Schottky rectification approach, with each diode dissipating around 1.275W at 6A and 3.12W at 11A. Heatsinks then become essential to manage heat dispersal, which the sealed nature of TV supplies further complicates.

Compared with the relatively static power dissipation of Schottky diode rectifiers, the drain-voltage-sensing synchronous rectification technique is more complex to analyse. Assuming that the resonant tank circuit filters the harmonics of the input voltage under full load conditions, the RMS drain current of each MOSFET is around 3.33A for a 6A output. Because the controller only turns on the rectifier when it detects conduction within the MOSFET’s body diode, there is a theoretical efficiency loss due to the delay in gate activation time that becomes more significant with rises in switching frequency.

In practice, the switching losses are negligible as the synchronous MOSFETs switch on and off very close to the zero current point. As a result—and taking into account body-diode and MOSFET channel losses for a converter switching at 80kHz—each power device’s dissipation is about 192mW for a representative 100V part with 9mÙ onresistance. This means that each MOSFET can safely operate with a junction temperature of just 92ºC within an ambient temperature of 80ºC. The same MOSFET dissipates approximately 935mW at the 11A level that a 42” display requires. Assuming a surface-mount D2PAK footprint that uses the PCB’s copper for heat dispersal, the device still operates reliably with a 29.5ºC junction temperature rise.

As well as slimming the assembly and its bill-of-materials, power-supply build costs fall yet further because dispensing with through-hole heatsinks allows manufacture to become a one-stage assembly process.

 

Reference:

1. Ang, Yong: “Synchronous rectifier reduces conduction loss in LLC resonant power supplies”, application note AN69, March 2009, Diodes Incorporated.

 

 

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