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SCR Gate Snubber And Drive Indicator

IP.com Disclosure Number: IPCOM000101172D
Original Publication Date: 1990-Jul-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 109K

Publishing Venue

IBM

Related People

Woodworth, GK: AUTHOR

Abstract

The expanding applications of silicon-controlled rectifiers (SCRs) in modern electronic equipment require upgraded methods to control electrical noise. Improved SCRs have higher gain gates that require less drive energy, making them sensitive to disturbance by electrostatic and electromagnetic pickup in their control leads. This unwanted noise can cause uncontrolled turn-on of the SCR, with potentially dangerous and undesirable consequences.

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SCR Gate Snubber And Drive Indicator

       The expanding applications of silicon-controlled
rectifiers (SCRs) in modern electronic equipment require upgraded
methods to control electrical noise.  Improved SCRs have higher gain
gates that require less drive energy, making them sensitive to
disturbance by electrostatic and electromagnetic pickup in their
control leads.  This unwanted noise can cause uncontrolled turn-on of
the SCR, with potentially dangerous and undesirable consequences.

      A snubber is often used to quiet the gate of an SCR and reduce
the effects of stray noise voltage pickup on the gate leads.  A
snubber is usually composed of a parallel resistor and capacitor
connected between the gate and cathode of the SCR, as shown in Fig.
1.  Adding parasitic snubber circuits to reduce the sensitivity of
the gate of an SCR reduces the benefit of using a high gain device.
The capacitor is used to absorb the transient noise voltages present
on the gate, while the resistor is used to bleed off the resulting
charges.  This dissipative circuit is intended to reduce the
impedance of the gate circuit and to shunt some of the gate noise
current back to the cathode.

      The improved snubber design described here utilizes a light-
emitting (LED) diode to increase the noise rejection at the gate of
an SCR, as shown in Fig. 2.  This approach does not require the
snubber components to be as large electrically or physically to
achieve greater noise attenuation.  The higher resistance value also
reduces the gate current required from the drive circuits.  The
reduction in gate capacity reduces the turn-off delay on the SCR and
allows its use in higher speed designs.

      The attached schematic shows the gate lead of an SCR and its
snubber connected to the gate and anode terminals. The turn-on
voltage of the SCR is typically about two volts. Desirable gate
signals or noise impulses of this level can initiate conduction of
the SCR.  The regenerative nature of the SCR causes it to remain
conductive until the source of energy reverses polarity or is removed
briefly.  After the current in the device is stopped, the SCR will
regain the ability to block the passage of current and act like an
open switch.

      If an LED is added into the gate lead, as in Fig. 2, the
noise rejection action of the snubber is improved.  The LED onl...