MC3423 ,OverVoltage Crowbar Sensing Circuit3AN004E/DThe first comparator is designed to initiate a stable time The minimum value of the gate c ..
MC3423P1 ,OverVoltage Crowbar Sensing CircuitAPPLICATION NOTEABSTRACT due to high efficiency and light weight. However, theEMI/RFI generated by ..
MC34261DR2 ,Power Factor ControllerOrder this document by MC34261/D* **The MC34261/MC33261 are active power factor controllers specif ..
MC34261P ,POWER FACTOR CONTROLLERSOrder this document by MC34261/D* **The MC34261/MC33261 are active power factor controllers specif ..
MC34262 ,POWER FACTOR CONTROLLERSELECTRICAL CHARACTERISTICS (V = 12 V (Note 2), for typical values T = 25°C, for min/max values T i ..
MC34262D ,POWER FACTOR CONTROLLERSThermal CharacteristicsP Suffix, Plastic Package, Case 626Maximum Power Dissipation @ T = 70°C P 80 ..
MC74VHC1G07DTT1 ,Noninverting Buffer with Open Drain Outputbuffer and an open drain output which provides the capability to set theoutput switching level. Thi ..
MC74VHC1G07DTT1G , Single Non-Inverting Buffer with Open Drain Output
MC74VHC1G08 ,Single 2 Input AND Gate3MC74VHC1G08VCCOUTPUTINPUTInput A or B50% VCC50%CGND L*t tPLH PHLVOHOutput Y *Includes all probe an ..
MC74VHC1G08 ,Single 2 Input AND GateELECTRICAL CHARACTERISTICST = 25C T 85C 55C to 125CA AV VCC CCMin Typ Max Min Max Min MaxSym ..
MC74VHC1G08DFT2 ,2-Input AND Gateup to 7.0 V are applied, regardless of the supply voltage. This allows theMC74VHC1G08 to be used to ..
MC74VHC1G08DFT2G ,Single 2 Input AND GateMAXIMUM RATINGSSymbol Parameter Value UnitV DC Supply Voltage 0.5 to 7.0 VCCV DC Input Voltage −0 ..
2N6508-2N6509-MC3423
OverVoltage Crowbar Sensing Circuit
AN004E/D
Semiconductor Consideration
for DC Power Supply
Voltage Protector Circuits
ABSTRACTThis paper addresses the requirements for the
semiconductor sensing circuitry and SCR crowbar devices
used in DC power supply over/under voltage protection
schemes.
INTRODUCTIONIt is uncommon now to find several hundred dollars
worth of microprocessors and memory chips powered from
a single low DC supply.
If this supply on the board doesn’t have overvoltage or
undervoltage protection, potentially large sums of money
can literally go up in smoke due to component failure, or,
for instance, a tool may be accidentally dropped across the
supply buses of different voltages during testing or repair
of the system.
Since a couple of years, computer and industrial
manufacturers agree to put additional small investment in
Over Voltage Protection (OVP) and Over/Under Voltage
Protection (OUVP) circuitry to prevent disasters.
ON Semiconductor chose the “crowbar” sensing circuit
technology. This system senses the overvoltage condition,
and quickly “crowbars” or short circuits the supply, forcing
the supply into current limiting or opening the fuse or
circuit breaker. Before detailing this technology, three
questions should be considered: Why OVP? To save money and increase the
reliability of the system. Where OVP?
–Everywhere over/under voltage is a problem.
–Everywhere a power supply system is used.
–Everywhere a switchmode system is designed. How OVP? There are several types of sense
circuits presently being used in OVP applications.
They can be classified into three types:
a) Zener
b) Discrete
c) MC1723 (voltage regulator in OVP
configuration)
This document may contain references to devices which are no
The Zener Sense CircuitThe simplest way to protect against overvoltage is to use
a Zener diode to sense the output voltage (Figure 1). When
the Zener goes into avalanche, it triggers the SCR.
There are problems with this kind of protection: No threshold adjustment, except by selecting
different Zener diodes. Inability to ignore momentary transients. Poor SCR reliability caused by inadequate
trigger–current rise time when slowly varying
voltage is sensed.
Common
Figure 1.
The Discrete Sense CircuitA technique which can provide adequate gate drive and
an adjustable, low temperature coefficient trip point is
shown in Figure 2.
This circuit includes the Zener reference voltage (Z1),
the comparator section (Q1, Q2), band gap circuit (Q3,
Q4), potentiometer (R1), trip point, and output section (Q5,
R2, R3, D1).
While overcoming the problems of the Zener sense
circuit, this technique also brings many disadvantages: This technique requires many components (12
here). Cost is very high. This method is not particularly noise immune and
often suffers from nuisance tripping.
