Power Electronics

Buck/Boost Regulator

Duryea’s brushless motor/generators employ an electronic commutator, used in the same manner as a brush DC machine. Unlike a brushless amplifier, the integrated electronics don’t provide speed control or current limiting. As a result, an external device is necessary for serving those functions and ensuring the desired output for applications such as battery charging. These items are essentially DC/DC converters. Buck regulators step down voltage, with a DC motor controller being the typical example. A boost regulator, employed in applications such as regenerative braking, step voltage up. A simple way of considering this is that buck regulators trade volts for amps, while boost regulators do the opposite.

Duryea’s two-stage power conversion topology doesn’t apply high-frequency PWM to the motor, thereby allowing the regulator, which has built-in filtering, to operate at frequencies far exceeding the norm for motor control. Further, state-of-the-art power supply design includes the use of interleaved phases working in parallel. These two features significantly reduce current ripple, which, in turn, reduces the amount of bus capacitance required, minimizing size.

Advanced power supplies also incorporate aluminum-clad circuit boards for MOSFETs and IGBTs to provide the lowest possible thermal impedance. However, these PCBs are single layer and limited to surface mount parts, which means control components and capacitors must be located on mating circuit boards.

The latest product has ratings of 36VDC, nominal, and 300A continuous. Dimensions are 6.25” x 3.25” x 4”, weight is 2.5 lbs. Buck, boost and throttle (with regen) modes can be selected, as can voltage and current limits. A version for boosting to 48VDC is also available.

One of the uses for the boost regulator function is to charge a higher voltage battery from a lower voltage source. A classic example of this is using a typical 12-volt alternator to charge a 24 or 36-volt battery pack. The advantages to this include the ability to operate a system with lower current or higher efficiency, with higher peak performance also possible. In effect, lower average power derived from the alternator is being used to provide much higher power for brief periods. For example, 300A @12VDC = 3.6kW, or 5hp, whereas 300A @24VDC = 7.2kW (9.6hp) and 300A @36VDC = 10.8kW, or 14.4hp.

A buck/boost regulator is also a key component of an Integrated Starter/Alternator (ISA) system. In this application, both motor (buck) and generator (boost) modes must be accommodated with voltage and current limits, just as is the case in a propulsion system with regenerative braking.

Mechanical factors also play into ISA systems. In particular, torque requirement, as well as maximum speed and voltage must be carefully considered. While it may seem attractive to generate a significant voltage at low speed, the voltage will surely be excessive at high speed. Further, because boost mode trades amps for volts, a substantial amount of current is needed at low speed/voltage to achieve a modest amount at a higher voltage. On a related note, this is why SolidSlot motor/generators, with their very high current capacity, are useful.

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