VI BRICK VTM (Voltage Transformation Module):
Sine Amplitude Converter

The VI BRICK VTM (Fig. 1) — one of the building blocks of FPA — is a wide voltage range input, high efficiency voltage transformation unit using a proprietary Zero-Current Switching / Zero-Voltage Switching (ZCS / ZVS) Sine Amplitude Converter™ (SAC™).

Figure 1 - VI BRICK VTM Click image for larger view

A simplified schematic of a Sine Amplitude Converter is shown in Fig.— 2. The power train is a low-charge (Q), high-frequency controlled oscillator, with high spectral purity and common-mode symmetry, resulting in essentially noise-free operation. The control architecture locks the operating frequency to the power train resonant frequency, optimizing efficiency and minimizing output impedance by effectively canceling reactive components. R_out can be as low as 0.8 milliohm from a single VTM. If that is not low enough, or if more power is required, VTMs can be paralleled with accurate current sharing. Quiet and powerful, the SAC-based VTM can be considered as a linear voltage / current converter with a flat output impedance up to about 1 MHz.

Figure 2 - Simplified schematic of the Sine Amplitude Converter Click image for larger view

The secondary current in a SAC VTM is basically a pure sinusoid. Selected SAC VTM operating waveforms (Fig. 3) show the purity, low output impedance and fast response of a typical VTM. Note also that the time scale in Fig. 4a is only 200 nanoseconds per division and that the waveform in Fig. 4a is with no external output capacitance across the load. The very low, non-inductive output impedance of the VTM allows an almost instantaneous response to the 100% step change in load current of Fig. 4b. Because there is no internal regulation circuitry in a VTM, and none of the attendant loop delays or stability issues, no internal control action is required to respond to the change in load. The internal ASIC controller simply continues its function of controlling and synchronizing the operation of the switches to maintain operation at resonance.

Figure 3 - Selected SAC VTM waveform Click image for larger view

Figure 4 - VTM dynamic response Click image for larger view

VI BRICK VTM – Transformation and Isolation

The VTM offers speed, density and efficiency levels designed to meet the demands of DSP, FPGA, ASIC, processor cores and microprocessor applications at the point of load while providing isolation from input to output. Its response time is less than 1 µs, and it delivers up to 100 A with very high efficiency.

The VI BRICK VTM can be considered a fixed-ratio DC-DC transformer with the following capabilities:

• input range compatible with 48 V and 24 V PRMs
• power up to 300 W or 100 A
• power density up to 390 W/in³
• efficiency up to 97%
• isolation to 2,250 Vdc in 2.08 in² package
• low power dissipation at point of load
• low output impedance enabling fast transient response.

For DC-DC power conversion, the VTM is designed to operate with the PRM (see next section), which provides soft start, regulation, and the initial Vcc pulse at start up. This pulse is received through the VTM control (VC) pin. Standalone VTM operation is possible if a Vcc is available see Application Note AN:007 "Using VTMs as 26 — 55 V Input Bus Converters".

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