Electronic Design

Inside The VTM

A VTM employs a zero-voltage switching and zero-current switching (ZVS/ZCS) topology called a sine amplitude converter (SAC) (see the figure). The power train is a low-Q, high-frequency, controlled oscillator, with high spectral purity and common-mode symmetry, so it generates very little noise.

The VTM control architecture locks the operating frequency to the power-train resonant frequency. The reactive components of loss are cancelled out. Remaining losses can be treated as a lumped resistance RO. So, the SAC-based VTM is a linear voltage/current converter with a flat output impedance up to about 1 MHz. Characterization data shows RO can be as low as 0.8 mW. If that's not low enough, or if more power is needed, VTMs can be paralleled with accurate current sharing.

A SAC VTM's secondary current is a pure sinusoid. The very low, non-inductive RO allows an almost instantaneous response to a 100% step change in load current. There is no internal regulation circuitry in a VTM and none of the attendant loop delays and stability issues, so no internal control action is required to respond to the change in load. The internal ASIC controller continues controlling and synchronizing the operation of the switches to maintain operation at resonance.

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