Exploring a four-quadrant DC/DC switching regulator
There are many traditional methods to provide a bipolar voltage/current to a load. H-bridge designs are frequently used, but require that neither of the loads terminals is tied directly to ground. Each of the loads two terminals has to swing between the positive supply rail and ground and usually an inductor is placed in series with the load to filter out this chopped waveform. This lack of a direct ground connection to the load can complicate the mechanical and electrical design of the overall system. The H-bridge method also requires four switching elements and a more complex control scheme. There are loads that have a negative terminal that cannot be biased high relative to ground, such as an FPGA back-biasing application.
Another traditional method is to build two power rails, a positive one and a negative one. Various circuits are used to swap in the positive or negative rail with regulation to achieve bipolar voltage operation that can go below ground. This results in a very complex system, generally with poor efficiency, and a non-linear response right as the output voltage crosses the ground potential.
A new DC/DC switching architecture is presented that has the ability to generate true four-quadrant operation, meaning the output voltage can be positive or negative and the current flow can be in either direction as well. Additionally, this new architecture can generate an output voltage that transitions from one polarity to another, through the ground potential, smoothly and without any non-linearity from mode transitions.
Four-quadrant DC/DC converter
Figure 1 shows the basic connections and elements of the four-quadrant converter. NFET, MN and PFET, MP are operated out of phase from one another and at a constant switching frequency. Current mode control is used (not shown) to modulate the duty cycle of MN as needed.
Figure 1: Four-quadrant DC/DC converter topology.
If we assume fixed frequency operation, the duty cycle for the ON time of MN can be calculated as
From this equation, it is clear that with a positive VIN voltage, the output voltage VOUT can be positive (up to VIN) or negative (limited only by practical DC considerations) and can go to 0V as well. In fact, there is nothing special about the 0V output level since the DC of the converter is 50% at that operating point.
The output of this converter can sink or source current regardless of the polarity of the output voltage, making this a true four-quadrant operating topology. The maximum drain to source voltage stress on MN and MP are both 2VIN VOUT. For example, if VIN is +12V and VOUT is -12V, then the BVDSS ratings for both FETs must be greater then 36V.
LT8710 in four-quadrant topology
The recently released controller from Linear Technology, the LT8710, can be used in the four-quadrant topology. Figure 2 shows a complete and fully tested circuit configured in this topology. The input voltage range for this circuit is typically 12V, but allows a range from 11V to 13V. The output can be adjusted from +5V to -5V with an output current capability of 3A. An analogue control signal, VCNTL, is used to adjust the output voltage. The LT8710 is an 80V capable controller so it can be used to build many other versions of the four-quadrant converter with higher or lower voltage and current capabilities.
Figure 2: Four-quadrant converter using the LT8710.
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