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PFC Controllers Deliver Compact Designs
The NCP1653 and NCP1601 power factor correction (PFC) controllers are designed for use in power supplies with input levels ranging from 75 W to several kilowatts. These controllers enable low-cost and compact PFC designs using few external components. A high level of noise immunity in these devices lessens their sensitivity to pc board layout.
The NCP1653 is a fixed-frequency current-mode PFC controller designed to efficiently drive continuous-conduction mode (CCM) boost converters with mid- to high- power (100-W to 3-kW) needs. In addition to the usual fixed-output voltage operation, it also can operate with the output voltage tracking the input voltage, called follower boost. Despite its small number of I/O, the 8-pin packaged chip offers many advanced features, such as average current-mode or voltage-mode control, soft start and VCC undervoltage lockout with hysteresis. The controller also guards against undervoltage, overvoltage and overpower conditions and provides thermal shutdown with hysteresis.
The NCP1601 is a controller designed for PFC boost circuits in power ranges up to 300 W. It operates in either fixed-frequency discontinuous conduction mode (DCM) and/or variable frequency critical conduction mode (CRM). In DCM, the controller can be synchronized to an external frequency source or set at a fixed frequency.
As the load and line conditions cause it to approach the continuous conduction mode, the output drive is delayed to cause it to operate in CRM, in which case operating frequency becomes variable. In both modes, the usual challenges of the recovery (soft turn off) of the PFC diode are avoided and component stresses are reduced. Filtering requirements are simplified and light load efficiency is improved by limiting the maximum frequency.
Several factors contribute to the devices' layout insensitivity and noise immunity. One is the small number of external components, which reduces the number of paths susceptible to noise pick up. In addition, most pins receive filtered signals, minimizing the chances for noise injection into the IC. Finally, the current-sense processing is performed using a current signal that is mirrored inside the IC. Traditionally, the current-sense pin is one of the most noise-sensitive pins in PWM or PFC ICs. In lots of 10,000, unit pricing is $0.66 for the NCP1653 and $0.36 for the NCP1601.
High-voltage MOSFET Technology
The fourth generation of CoolMOS technology from Infineon Technologies lowers the RDS(ON) of high-voltage MOSFETs into the double-digit milliohm range, while improving system performance for ac-dc power supplies by enabling lower cost, higher efficiency and new design options. The first devices fabricated in the new CoolMOS CS technology include a 600-V MOSFET with 99 mΩ in a TO-220 package; a 45-mΩ device in a TO-247; and a 380-mΩ device in a DPAK. These MOSFETs achieve switching speeds of 150 V/ns. For perspective, 600-V MOSFETs produced in Infineon's third-generation process provide 160 mΩ in a TO-220 and 70 mΩ in a TO-247.
To demonstrate the benefit of CoolMOS CS, the company incorporated a 99-mΩ CoolMOS CS transistor in a 1000-W server power supply. In this application, the MOSFET increased efficiency by 1.5% compared to a design using two 250-mΩ MOSFETs in parallel. The added efficiency lowered the system cost per watt by 10%.
CoolMOS CS overcomes a performance barrier known as the silicon limit, which dictates that doubling the blocking voltage typically increases on-resistance by a factor of five. Dr. Gerald Deboy, an Infineon engineer, describes how the company's superjunction technology addresses this problem. “To come as close as possible to zero resistance, we add more and more charge in the device for current conduction,” said Deboy. “This charge is then counterbalanced by exactly the same amount of charge of the opposite type. The two charges are separated locally in the device by a very refined technology. In the end, we get a pattern with very fine pitch. The finer the pitch can be made, the lower the on-state resistance will be. With every CoolMOS generation, we increase the fineness of the pitch, moving ever closer to the zero resistance point without losing voltage blocking capability.”
To reduce RDS(ON) while lowering capacitance (for faster switching), the size of the MOSFET dies are made smaller. These changes produce a device capable of switching in the megahertz range, though most applications fall into the 100-kHz to 300-kHz range.
The improved performance of CoolMOS CS devices provides designers with the option of switching to a simpler power architecture. Up until now, ac-dc power supplies above 1000 W were typically implemented using a phase shift zero voltage switching bridge converter. This resonant topology achieves high efficiency and low output ripple but has complex timing requirements.
Because of the CoolMOS CS performance, power supplies at 1000 W may now be designed using the interleaved two-transistor forward converter topology. This hard switching topology requires fewer components and can improve efficiency across the range of load conditions. Unit pricing for the 99-mΩ MOSFETs in the TO-220 is approximately $5 in quantities of 10,000.
Power Module Boosts Current Density
A multichip module (MCM), the PIP212-12M from Philips Semiconductors combines two power MOSFET switches and a driver IC in an 8-mm × 8-mm QFN, enabling synchronous buck converters to achieve current densities as high as 130 A/in3. That represents a doubling of current density when compared with existing discrete designs, according to the company. By using the latest TrenchMOS technology for the MOSFETs and by minimizing parasitic layout inductance, the module enables buck converters to achieve efficiencies as high as 94% at a 500-kHz switching frequency.
Capable of operating over a wide input voltage range (3.3 V to 16 V), a single module can deliver as much as 35 A of output. When multiple modules are employed in a multiphase synchronous buck converter, higher currents are possible. The company has applied the PIP212-12M in applications up to four phases and demonstrated output levels beyond 30 A per phase when converting 12 V to 1.2 V at a switching frequency of 500 kHz.
Such designs can achieve high efficiencies even when switching at 1 MHz per phase. For example, when converting 12 V to 1.5 V, a 120-A design achieves efficiencies above 90% at a 500-kHz per phase switching frequency and efficiencies up to 87% when switching at 1 MHz.
The PIP212-12M simplifies buck converter layout by integrating a 6.5-mΩ control MOSFET, a 2-mΩ synchronous rectifier MOSFET and parallel Schottky diode, a MOSFET driver, and a bootstrap diode. The 56-pin module also adds features, such as power sequencing, lost-phase detection, overtemperature detection and a 5-V output to power a PWM in a 12-V system. The PIP212-12M eliminates deadtime losses using a deadtime control technique that relies on the accurate sensing of the voltage directly across the sync FET silicon — a technique that would be impractical in a discrete design. The device costs $2.25 each in quantities of 10,000.