The power supply regulator (Example: a buck regulator or a LDO) helps to produce the regulated power supply output and the output is always less than the input voltage. There shall be a series pass element and reference section inside the regulator. In our testing experience we might have seen the regulator failing and input voltage appearing across the output directly without any regulation. There can also be chance of some voltage transients across the output causing the voltage to increase well beyond a voltage for a short period of time. While the effect of transients on the load can be less as it exists for a very short period of time (in ns or us) , the effect of input to output short can be catastrophic. If the load is very sensitive to voltage like for example take the case of devices where the voltage to them has very low tolerance, then the load device shall be damaged.
To eliminate this scenario, a SCR based circuit called the crowbar circuit is used at the output side which helps to short the output to circuit in case there is a high voltage on the output. The components required for this circuit are:
- Silicon Controlled Rectifier (SCR)
- Reference circuit
The reference circuit measures the output voltage and drives the gate of SCR when over voltage occurs. The circuit shown below is the implementation of the crowbar circuit.
The above circuit is implementation of the crowbar circuit, where the voltage to be applied to the load is +8V. The zener diode forms the reference circuit. The voltage across R1 is 0V and the gate is not driven. Hence, Q1 is OFF under normal conditions. Now, let us assume a scenario where the input voltage exceeds or becomes 9.1V. In this case, the zener turns ON and voltage is developed across Resistor R1. This fires the Q1 (voltage is called firing voltage) to ON state and the output is shorted. Due to short condition, the current bypassed from the load and a large current flows through the diode Q1. The fuse F1 is designed such that it blows up if the current exceeds the normal load current. When Q1 is ON which happens when over voltage occurs, causes the fuse to blow and hence the load is protected from over voltage. The response time of SCR is very less and hence is a good option in this circuit. Also, selecting the Zener diode with appropriate voltage is important dsign criteria. One of the important parameter in this circuit is that if there are any transients for a shorter time, this may not cause short circuit condition and is always a desired condition as transients should not cause 1 to get triggered.
While Zener diode based simple discrete based approach, there are several chips available in the market which helps to detect the output voltage and drive the SCR. One example is MC3423, which is a over voltage crowbar sensing circuit. The output voltage is fed to this MC3423 whose output drives the SCR. Below is the block diagram of the MC3423 implementation in the circuit.
The implementation details of MC3423 are as follows:
The output is divided using a resistive divider which is fed as sense input to the MC3423. under normal operation, SCR is not fired. When the output voltage exceeds the usual voltage, the resistive divider input also increases causing the MC3423 to fire the SCR. The internal implementation of the MC3423 is as follows:
For more details on MC3423, visit the below link.
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