The FRS300 firing board is specifically designed for three-phase semiconverter applications. It includes our ASIC-based firing circuit, a current and voltage regulator, and a snubber circuit integrated on a single board. This provides a complete, compact and economical package for battery chargers, welding equipment, and DC bus generation where power quality is not an issue.


  • Economical one-board solution
  • Built-in voltage and current regulation
  • Integral snubber circuit
  • Configured for three-phase SCR/diode semiconverters


The FRS300  includes the following features:

  • Analog Delay Angle Command Signal (SIG HI): Users may choose a variety of DC control signal ranges including 0-5 V, 0-10 V, 4-20 mA, or custom ranges.
  • Soft-Start and Soft-Stop: Upon soft-start, SCR firing is enabled and the delay angle command ramps from the maximum value to the setpoint value determined by the SIG HI command signal. Upon soft-stop, the delay angle ramps from the setpoint value to the maximum value after which SCR firing is inhibited.
  • Phase Loss Inhibit: A phase loss circuit instantly inhibits SCR firing if a loss of one or more phases or gross phase imbalance is sensed on the AC line. Firing will soft-start when such a fault is cleared.
  • Instant Enable and Inhibit: A contact closure (relay, switch, transistor) instantly enables or inhibits SCR firing at the delay angle commanded by the delay angle command signal (SIG HI).
  • Regulator Circuit: The board typically operates in voltage or current regulation mode, depending upon which error signal is larger. This allows a seamless transition from voltage to current regulation or vice-versa.
  • Snubber Circuit: The snubber section consists of three RC circuits connected to the SCR anodes and cathodes.
  • High Current Picket Fence Gate Drive: The transformer-isolated gate drive circuits provide a hard firing initial 13 V open circuit/1.3 A short circuit firing pulse followed by sustaining "back porch" pulses at 6 V open circuit/0.5 A short circuit. The gate pulse burst frequency is 384 times the mains voltage frequency.
  • Analog Delay Determinator Circuit: Enerpro's gate delay determinator circuit is based on the Ainsworth three-phase PLL circuit and implemented with a proprietary ASIC. This circuit adjusts the gate delay firing angle in negative proportion to the SIG HI command. Gate drive phase balance is typically less than ±1°.
  • Control Power Options: The FRS300 is powered from an external 30 VDC or 24 Vac source.
  • Board Construction: All circuit boards are assembled at the Enerpro plant in Goleta, California and are manufactured by a UL-approved fabricator from 2.4 mm thick FR4 fire resistant fiberglass epoxy laminate. All boards are conformal coated (MIL-1-46058, Type UR)



  • DC Bus Capacitor Chargers
  • Battery Chargers
  • Corrosion-Protecting Semi-converters

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