[Engineering Essentials]
Regenerative Current Transformation Delivers Sub-Volt Regulated Output
Rengenerative current transformation coupled with dc-dc conversion provides a 0.1- to 3-V adjustable supply for wide range and high overall efficiency.
Regenerative topologies and systems have been explored extensively in regards to providing regenerative loads for burn-in systems,1 enabling efficient burn-in of power supplies. Regenerative burnin systems are desirable, as the power supply ideally functions as both source and load. This means a substantial energy savings (up to 90%) when attempting to provide a burn-in function.
For example, in a non-regenerative burn-in system, a 100-A, 100-W dc-dc supply will need a 100-A, 100-W load for burn-in (a 100-W bank of resistors). Let’s also assume in this example that the supply is 90% efficient. The total amount of energy used in the burn-in process is 110 W (100- W load plus 10-W power-supply losses). The regenerative version of this system, rather than powering a load, converts the load current back to the initial voltage and returns it to the input. In this case, the power used in the system is equal to the power dissipation of the device-under-test (DUT) supply plus the dissipation of the regenerative dc-dc converter.
Thus in our example, the supply still provides 100 A and dissipates 10 W, but a second supply back to the input converts the 100 A. Let’s say the regenerative supply was 80% efficient. Now the total losses would be 30 W (10-W supply + 20 W in the regenerative supply). This reduces energy by 72% compared to the first system.
A second example of energy recovery and regenerative circuitry comes from topologies that utilize subcircuits to recover primary (or secondary) circulating currents and return some of that energy back to the input instead of dissipating it in magnetics, snubbers, or switching losses.2 Energy is saved here as well, along with the additional benefit of increased power-converter efficiency.
This circuit was designed to enable a 48-V to sub-volt conversion, where duty cycle and outputrange limitations would normally not allow for low voltage with regulation. It’s intended to provide a wide-range capability for testing microprocessors as part of an automated-test-equipment (ATE) system. An alternative approach would be to provide 48 to 0.8 V regulated,3 with a linear post-regulation stage to 0.5 V. However, at high currents (up to 300 A), the additional losses would come close to 90 W total. The proposed solution dissipates an additional 45 W at 300 A.
The design uses three V•I chip modules:
• The bus converter (BCM-U4) is a compact and efficient 48-V input step-down converter. • Downstream, a power regulator (PRM-U2) is an isolated zero-voltage switching (ZVS) buck-boost regulator that can operate with input voltages from 1.5 to 400 V and can step up or step down over a 5:1 range with a conversion efficiency up to 98%. • The isolated dc transformer (VTM-U3) is essentially a non-isolated current transformer, used at the point of load. For input conditioning, U1 is a multi-amplifier chip that provides: • Differential-sense at the point of load with 80-dB common-mode rejection capability • Error-amplifier functionality with closed-loop regulation bandwidth of up to 100 kHz • A buffer stage for analog reference input from the system
It should also be noted that the Load terminal is tied directly to the –IN terminal of U2 and designated as ground. No other minus terminals are connected to ground.
Feedback from the isolated dc transformer (VTM) to the upstream regulator (PRM) is used to perform load regulation. As a current transformer, the VTM multiplies the current (and divides the voltage) by a “K” factor. This takes place with essentially a 100% transformation duty cycle; therefore, there’s no loss of efficiency at high values of K. Consequently, the bus voltage provided by the bus converter module (BCM) can be greater than 12 V. In fact, it’s limited only by safety concerns.
Bulk capacitance at the VTM input reflects itself at the point of load with a gain equal to the square of the VTM current gain, K. Only very small amounts of ceramic bypass capacitance, effective over a short time scale of less than a microsecond, are needed at the load. This approach also allows precise control of the load voltage through the isolation barrier without long, noise-sensitive feedback lines or optocouplers.
CONFIGURATION
The circuit is designed to provide a regulated 0.5 V (or lower) output from a 48-V, ±10% input (see the figure and table). U2 and U3 provide a regulated 48- to 2.5-V output. U2 provides a regulated 40-V rail from the 48-V input.4 U3 is an isolated dc transformer that provides a 2.5-V output proportional to the input by a factor of 1/16.5 This factor is henceforth referred to as K1.
As can be observed from the circuit, the output of U4 is connected in series opposing the output of U3.6 U4 is a second isolated dc transformer that provides an output proportional to its input by a factor of 1/32. This factor is henceforth referred to as K2. U4 is essentially the device that provides the regenerative function in the circuit.
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