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Choosing PWM controller for step-down conversion

Posted: 29 Jul 2015     Print Version  Bookmark and Share

Keywords:PWM controller  inductor  capacitor  buck converter  power supplies 

The choice of a PWM controller has become less straight forward as leading edge DSPs, FPGAs and CPUs run on increasingly lower supply voltages and consume higher current. Sub 1V voltages are becoming ubiquitous while intermediate bus voltages have stayed the same or increased, depending on the application. System frequencies also have steadily increased to support smaller inductor and capacitor (L&C) filtering. Last year's 500kHz is today's 1MHz.

In high voltage applications where a lower output voltage is required, power supply designers have traditionally relied on modules that increase system cost, or two stage DC/DC solutions that increase solution footprint and complexity. This article highlights the trends influencing narrow on-time point-of-load (POL) conversion and compares the typically used current mode control architectures. A hybrid valley current mode (VCM) architecture with adaptive slope compensation is examined, including its use in a new 60V synchronous buck controller able to deliver stable operation over a wide range of Vin and Vout combinations, and low duty cycle that allows a direct step-down conversion from 48V to a 1V point-of-load.

The need for narrow on-time POL conversion
A buck converter is the most widely used power supply topology, and recent trends indicate that next generation switching controllers must be able to provide stable and efficient operation at very small duty cycle. While the current mode control approach offers many advantages compared to voltage mode control, it has its own limitations depending on the application requirement, particularly in terms of duty cycle limits.

Generally, power delivery systems in telecommunication and industrial applications are based on multi-stage conversion. There has been a continuous power delivery system shift with POL input voltage increasing over time from 3.3V to 5V to 12V. With the increase in power requirements, the use of 12V rails is now common, while 3.3V rails are rarer. This trend towards higher input voltages is due in part to I2R (current to resistance) power losses and associated issues in low voltage traces due to the higher current.

More recently, the trend is moving towards much higher voltages, such as 24V~42V for industrial applications, and 48V for telecom. Consistent technology improvements have made it possible to control narrow pulses. At the same time, new studies show that a higher input voltage enables higher overall efficiency, lowers system cost and contributes to system reliability by reducing distribution path temperature.

Another factor driving the requirement of a narrow PWM pulse is the need for higher switching frequency, which in turn results in higher power density. Operating power supplies at a switching frequency of 1MHz has become common practice. In fact, the switching frequency needs to be above 1.8MHz in automotive infotainment applications to avoid the AM frequency band. A 12V to 1V power conversion at 1MHz would still need to generate a 83ns pulse.

Limitations of low duty cycle operation
An ideal buck convertor can generate any voltage lower than Vin down to zero, However, in practice, there are many limitations such as the reference voltage, internal or external circuit losses, and most importantly, the type of modulator used to generate the control signal. Given a particular input voltage, the reference voltage is the most obvious limitation preventing the controller from covering the entire range 0% to 100%. Most obvious is the reference voltage:

Vout = Vref * (1+R1/R2)

This indicates that the output cannot be regulated below the Vref voltage. The second major limiting factor for the minimum Vout is the minimum on-time of the controller. For a given input voltage (Vin), minimum Vout can be expressed as:

Vout = Ton min* Vin *Fs

For switching frequency (Fs), the on-time of the upper MOSFET will be:

Ton= D*(1/Fs)

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