The use of SCR crowbar overvoltage protection (OVP) circuits is a popular method for providing protection from accidental overvoltage stress for a power supply load. The sensing function for this type of OVP circuit can be provided by a single IC, the MC3423, as shown in Figure 6-70.
THE CROWBAR TECHNIQUE
One of the simplest and most effective methods of obtaining overvoltage protection is to use a crowbar SCR placed across the equipment’s de power supply bus. As the name implies, the SCR is used much like a crowbar would be, to short the input of the de supply when an overvoltage condition is detected. A typical circuit configuration is shown in Figure 6-71.
The MC3423 operates from a VCC minimum of 4.5 V to a maximum of 40 V. The input error amplifier has a 2.6 V reference between the non-inverting input and VEE. The inverting input is Vsensel (Pin 2) and is the point to which the output sense voltage is applied. This is usually done through a resistor voltage divider which sets the trip point (Vref) at 2.6 V. The output of the device. Pin 8, then triggers the gate drive terminal of the SCR. A basic OVP circuit is shown in Figure 6-72.
When VCC rises above the trip point set by Rl and R2, an internal current source (Pin 4) begins charging capacitor Cl which is also connected to Pin 3. When triggered, Pin 8 supplies gate drive through the current limit resistor (RG) to the gate of the SCR. The minimum value of RG is given in Figure 6-73.
The value of capacitor C determines the minimum duration of the overvoltage condition necessary to trip the OVP. The value of C can be determined from Figure 6-74. If the overvoltage condition disappears before C is charged. C discharges at a rate which is 10 times faster than the charging rate, and resets the timing feature until the next overvoltage condition occurs.
ACTIVATION INDICATION OUTPUT
An additional output for use as an OV indicator is provided on the MC3423. This is an open-collector transistor which saturates when the OVP circuit is activated. It will remain in a saturated state until the SCR crowbar pulls the supply voltage. VCC, below 4.5V.
This output may also be used to clock an edge-triggered flip-flop whose output inhibits or shuts down the power supply when the OVP trips. This method of protection reduces or eliminates the heat sinking requirements for the crowbar SCR.
REMOTE ACTIVATION INPUT
Another feature of the MC3423 is its remote activation input. Pin 5, which has an internal pull-up current source. This input is CMOS/TTL compatible and, when held below 0.8V, the MC3423 operates normally. However, if it is raised above 2V, the OVP is activated regardless of whether an overvoltage condition is present. This feature may be used to accomplish an orderly and sequenced shutdown of system power supplies during a system fault condition.