For when low power has to be efficiently sourced from a high voltage line.

The VIPer26K is a member of the VIPer Plus family of devices from STMicroelectronics that was released in January of this year. The units are designed to implement low-power, switched-mode AC to DC power supplies up to a maximum power of 15 watts. This is typically a situation that can come up when there is plenty of high voltage power available, and there is a device that needs only a very small amount of power, at relatively low voltage, and perhaps only intermittently.

 

The VIPer26K. Image from STMicroelectronics

 

One of the most prominent applications this component could be used for is smart meters.

 

Smart Metering

The archetypical use situation is a power meter. It is read only infrequently, so there is no need to feed it power continuously. It is typically a node on an IoT network—you can learn more about how such nodes can be activated (however frequently or infrequently) in my article on u-blox's BLE modules released in February.

The VIPER26K readily works with three-phase voltage supplies, and at 230 VAC input, it draws less 30 mW during no-load periods. This makes it well suited for powering LED display drivers or microcontrollers, as well as for smart meter applications.

The device includes both a MOSFET and a controller. Having both vital components in one tight package will greatly simplify the design of physically small power supplies where power efficiency and reliability are prime concerns.

The standout feature of the VIPER26K is that the MOSFET has a breakdown voltage of 1050 volts. This eliminates the need to stack two MOSFETs with lower breakdown voltages, enabling simpler, smaller and more reliable designs with smaller BOMs. This also serves to allow the device to operate over a wide range of input voltages, as well as enabling a reduction in the size of the DRAIN snubber circuit.

The device includes an error amplifier and a current-mode PWM controller and can support such switched-mode power supply topologies as:

  • Isolated flyback with secondary-side or primary-side regulation
  • Non-isolated flyback with resistive feedback
  • Buck converters
  • Boost converters

The device's switching frequency is internally fixed at 60 kHz, ± 4kHz. Combined with control of the MOSFET gate during turn on and turn off, switching-noise emissions are minimized. 

 

Operational Notes

Block Diagram

Block diagram VIPER26K. Image from STMicroelectronics

 

Pin Assignments

  • VDD: Supply voltage of the control section (providing the charging current of the external capacitor)
  • FB: Inverting input of the error amplifier
  • COMP: Output of the error amplifier
  • DRAIN: High voltage drain pin.

If the VIPer26 is deployed in a non-isolated topology, the feedback signal from the output voltage is applied directly to the FB pin. The output of the chip’s internal error amplifier sources and sinks the current, ICOMP, respectively to and from the compensation network connected on the COMP pin.

If the device is the basis of an isolated power supply, the internal error amplifier is disabled by shorting the FB pin to GND. Then, an internal resistor is connected between an internal reference voltage and the COMP pin.

 

The MOSFET

The power section of the VIPer26K is implemented with an N-channel power MOSFET with a maximum RDS(ON) of 7 Ω. Its sense-FET structure to supports virtually lossless current and thermal sensing.

The MOSFET was designed to supply a controlled gate current during both turn-ON and turn-OFF. This serves to minimize common mode EMI.
 

The Oscillator

Internally fixed at 60 kHz, the switching frequency is modulated by approximately ±4 kHz at a 230 Hz (typical) rate. The result is a spread spectrum

that distributes the energy of each harmonic of the switching frequency over a number of sideband harmonics. The energy as a whole is the same but each sideband has a smaller amplitude.

 

Thermal Shutdown

When the device’s temperature reaches a critical point, it shuts down to avoid heat damage. The device does stay alive in order to be able to resume operation once the heat dissipates.

 

Packaging

The unit is available in an SO16N package. Different grades of STM’s ECOPACK packages are available as needed.

 

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