How Crowbar Circuits Work
There are often two types of components that are employed for overvoltage protection. These are TVS diodes and varistors, which can provide protection to fast ESD pulses and reasonable protection against slower processes producing overvoltage. These components are designed to divert energy away from a protected component. But there is another type of circuit that eliminates provision of power to a load, which is called a crowbar circuit.
A crowbar circuit is a simple circuit involving a few discrete components, and its function is to shutdown power output from a supply in the event the supply begins to exhibit overvoltage. Rather than divert the overvoltage away from the load, the system can divert power away from a load and thereby shut down the supply. In this article, we’ll show how a crowbar circuit can be designed and integrated with a larger control strategy that provides momentary or periodic protection in a more complex power system.
Controllable Circuit Protection With Crowbar Circuits
Crowbar circuits operate on a basic principle; they diver current away from the intended output of a supply when an overvoltage event occurs. Crowbar circuits can be added to the output stage of a power supply as shown in the example below.
Basic Crowbar Circuit
A basic example of a crowbar circuit is shown below. In this example circuit, the DC_IN source provides the main DC power that is being generated and regulated by a DC/DC converter, or it could be the power provided by an unregulated source or current-mode regulator. In the event of overvoltage, the circuit diverts the output current away from LOAD and across D3.
Initially, DC_IN is brought up to its operating level slowly, so DC_IN generally requires a significant amount of capacitance, possibly with some slight series resistance to ensure slow rise time up to the value of DC_IN. This will ensure that the SCR (D3) is initially insulating and that the component stays in its insulating state. As long as there is no excessive overshoot on DC_IN, the system can enter normal operation without accidentally tripping the crowbar circuit.
SCR For Overvoltage Protection
The overvoltage protection threshold is set by selecting the appropriate reverse breakdown voltage for D1. For very fine-level protection, this value for D1 should be slightly above DC_IN. When DC_IN is greater than the reverse breakdown voltage of D1, this will apply a voltage to D3, and this applied voltage will cause D3 to begin conducting. As soon as D3 conducts, the supplied current will be diverted from the load, and it will see a voltage drop of 0 V.
More Advanced Control and Reset
Because the above system could use a thyristor, the main rail in this would need to power down in order to completely reset the thyristor to its insulating state. Only once the SCR (thyristor) is reset can the main power be brought back into the system, and then power delivery can commence to the load. However, the system could be reset automatically by disabling the voltage on the main rail by disabling the supply at the input
There is one way to implement this: by bringing the output rail into a feedback loop involving the DC/DC supply at the input of this system. This would involve the use of the following topology:
The topology uses a measurement of the output rail current; once the current is fully diverted through the thyristor, this can toggle an enable feature on the supply regulator rail and shut down the system. In some cases, where the DC/DC converter that supplies the crowbar circuit is isolated, the diverted could be used to control an optocoupler, which then transfers a shutdown signal to regulator control system (either an ENABLE pin, RESET pin, or a microcontroller that controls these).
Current Limiting
Finally, it’s possible to add some additional features to the above circuit to ensure safety and reliability of the regulator providing DC_IN. For example, we could add:
- A resistor in series with D3 to limit current once D3 conducts
- A MOSFET could be used instead of D3 with the gate connected to D1
If D3 has a series resistor (R), the maximum current would be limited to (DC_IN)/R. The value of R should be selected so that the current provided by DC_IN does not exceed the regulator’s maximum current. If D3 were replaced with a MOSFET, make sure the ON-state transition is slow enough that the output does not oscillate.
Whenever you need to design and simulate circuits for overvoltage protection in your PCB design, make sure to design your system with the complete set of simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.
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