Design Article
Design a wide-voltage-band buck regulator
Michael Harney
1/27/2012 4:09 PM EST
This article describes the design of a wide-band buck regulator, which is a standard buck regulator, but with modifications that allow the output voltage to vary across a wider range. This allows it to accommodate electronic devices that also operate over a wide range.
A modified buck regulator which generates an output voltage in the wider voltage range of today’s ICs will use less power. It does this by keeping its FET switch off more often (by keeping the feedback-voltage constant) while operating in this wide range, thus increasing its efficiency.
A standard buck regulator maintains regulation by measuring the output voltage and comparing it with an internal reference. When the divided-down output voltage drops below the internal reference of the regulator, a FET switch turns on more often (due to increased duty cycle of the drive to the gate) to increase current to the output capacitor, raising the output voltage.
Similarly, when the divided-down output voltage rises higher than the internal reference, the FET turn-on time decreases (due to decreased duty cycle of the gate drive) and the capacitor voltage is allowed to drop, based on current draw from the load.
By increasing the allowable range of the regulator output, the voltage output is allowed to drift based on load requirements, which reduces the duty cycle of the FET gate drive, much like a virtual zero power (VZP) controller (Reference 1). This reduces the I2R losses of the FET and increases the efficiency.
The approach to designing such a regulator starts with identifying the range of the output voltage that is less than the limits of the operating range of the load. We refer to these limits as VH (for Voltage High) and VL (for Voltage Low).
These limits should be chosen to be slightly less than the actual limits of the load to account for some ripple at these extremes. Once the limits are chosen, the regulator feedback voltage is modified through a pathway that uses comparators to determine if the output voltage is greater than VH or less than VL (Figure 1).
Figure 1: Schematic of a wide-band regulator (2.48 V – 4.13 V).
[Click here to see enlarged image.]
The figure shows a wide-band regulator with a range of operation between 2.48 V and 4.13 V, set by the -¼ gain of summing circuit (the “summer”) around IC1, with a mid-range output of 3.3 V. The outputs of the comparators (IC2 and IC3, which are assumed to be rail-to-rail outputs) are summed relative to the internal reference voltage; in example of Figure 1, this voltage is 1.25V).
The summer (IC1) generates a new feedback voltage for the regulator that is either equal to the reference voltage Vref (if operation is between VH and VL), 25% above the reference voltage (if operating above VH) or 25% below the reference voltage (if operating below VL). This will keep the regulator within the range of VH and VL (excluding some ripple), which keeps the duty cycle of the gate drive constant.
In a standard buck regulator, where a single feedback voltage is targeted, the duty cycle varies whenever the divided-down output voltage changes slightly from the reference voltage. This causes the FET to turn on more often than if the system had two feedback voltages between which the output could vary (Figure 2). The requirement to turn on the FET at a lower output voltage reduces I2R losses, increasing efficiency.
Figure 2: Range between two feedback voltages results
in fixed duty cycle.
[Click here to see enlarged image.]
A few additional requirements for the circuit in Figure 1 are necessary: the need to generate voltage references (implemented with Zener diodes from the input supply to the regulator) for VH and VL; and an op-amp supply that is exactly twice the reference voltage.
This supply voltage of 2×Vref is necessary for operation between VH and VL, where the output of IC3 generates 0 V as an input to the summer and the output of IC2 generates a 2×Vref input to the summer. The net result is that there is no current coming into or out of the summer, since the summer’s non-inverting terminal is fixed at Vref, which becomes the virtual reference for the inverting terminal as well.
In this situation, with operation in the range between VH and VL, the output of the summer is exactly equal to the reference voltage, and Vref and the duty cycle of the regulator do not change.
The output of the summer (IC1) has a gain of -¼, which generates a higher feedback voltage (1.25×Vref) or lower feedback voltage (0.75×Vref) for operation outside of the VH to VL range, respectively. The gain of IC1 also sets the range of the output voltage to 1.25×3.3 = 4.13 V to 0.75×3.3 = 2.48 V with 3.3V as the mid-range output. IC4 inverts the output of the IC1 for positive drive into the regulator, preserving negative feedback of the entire voltage control loop.
There is usually a need to account for loop dynamics in the design, including the op amps in the feedback loop, which should be chosen to have low phase shift so as to preserve overall phase shift of 180⁰ and reduce the chances of instability. The phase margin of the loop can be adjusted by placing appropriately chosen capacitors in parallel with R6 and R8.
In conclusion, the wide-band buck regulator can offer efficiency improvement by allowing the output voltage to drift with load requirements over the operating voltage range of the load. This approach can also be applied to synchronous and multiphase buck architectures.
References
- http://areeweb.polito.it/eventi/6thISMST/pdf/ISMST6.105_2.vzp.pdf
About the author
Michael Harney is an Electrical Engineer working in industrial electronics. He is the holder of five patents and has a Bachelor of Science in Electrical Engineering from Utah State University.
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alager12345
2/3/2012 11:39 AM EST
For all the extra components, what is the expected efficiency gain? Also don't forget to account for the power consumption of the additional op-amps and 3 voltage references.
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PowerCycle
2/3/2012 4:44 PM EST
The op-amps drive high-impedance inputs and can have 10 uA quiescent current and the zener diode references have low bias current (they also source low current to the op-amps). For a buck regulator with a Vfeedback input of 100K ohms (which the last op-amp stage drives), the op-amps and zeners+bias could be under 100 uA at 3V max (assuming Vref = 1.25 V). If the inductor and FET see much more current than this (say in the high mA), the added ineffciency of the additional components is very small compared to the power of the overall system. These days most bucks are at 90% efficiency or better and so a few extra percent is all that may be realized, but even 1% improvement is worth a lot if the system is high power (like a cell phone).
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