How safe is your power supply? Introduction Overvoltage (OVP) is an important security feature in power supplies. By using this feature, the device under test (DUT) can be protected from a higher voltage than the DUT was designed to handle. It also lets users address a variety of application in electronics manufacturing and educational market. Overvoltage protection is also available if required during constant current operation. The constant voltage / constant current (CV/CC) and constant voltage / current limit (CV/CL) automatic crossover circuitry is ideal for these purposes since it allows the user to select the maximum safe current or voltage for the particular load device. Overvoltage Crowbar The new Agilent U8000 series power supply is short-circuit proof and can operate under any current overload condition indefinitely without risks of internal damage. It uses an overvoltage crowbar in which the crowbar circuit is connected across the output terminals, and provides protection against any output overvoltage condition which might occur because of operator error or failure within the power supply or load. This application note is going to show you the architecture of OVP and its applications and what you need to pay attention when using this feature. An operator error or a component failure within the regulating feedback loop can drive a power supply’s output voltage to many times its preset value. The function of the crowbar circuit is to protect the load against this possibility. The circuit insures that the power supply voltage across the load will never exceed a preset limit. This protection is valuable because of the extreme voltage sensitivity of present-day semiconductor devices. The basic elements used in most crowbars are: some method of sensing the output voltage, a silicon controlled rectifier (SCR) that will short the output, and a circuit that will reliably trigger the SCR within a time period that is short enough to avoid damage to the load. The sense circuit can be a simple bridge or voltage divider network that compares the output voltage to some internal crowbar reference voltage. The best trigger circuit is the one that turns the SCR on the fastest. Fastest SCR turn-on is accomplished by a fast rise-time pulse circuit such as a blocking oscillator or Schmitt trigger. Figure 1: General Crowbar Overvoltage Protection Circuit Figure 1 shows the general crowbar overvoltage protection circuit. The circuit compares the output voltage with a reference voltage +V. The overvoltage potentiometer adjusts the reference voltage on the comparison amplifier and sets the voltage level at which the crowbar will be activate. Normally the overvoltage control is located on the front panel and can be adjusted from approximately 20% to 120% of the maximum rated output voltage of the power supply. When the output voltage exceeds the reference, the comparison amplifier triggers the blocking oscillator which then sends firing pulses to the SCR. When the SCR fires, it places a very low impedance across the output, reducing the voltage to near-zero. • An auxiliary winding is included on the blocking oscillator transformer for connection to an There are several beneficial features for crowbar circuits: • An overvoltage indicator appear on front panel screen when the SCR fires; the indicator conducts a holding current to prevent the SCR from oscillating on and off. additional crowbar. Tandem crowbar operation is then available for coincident firing of all crowbars in a system. Attention when using OVP -Connecting power supply in series One OVP application is connecting power supplies in series for higher voltage • The crowbar circuit output. Set the OVP creates an extra current shutdown voltage in each path during normal unit so that its shuts down operation of the supply, thus changing the current at a voltage higher than its output voltage during autothat flows through the tracking operation. When a current monitoring master unit shuts down, it resistor. A diode keeps programs any slave units to this extra current at a zero output. When a slave fixed level of which compensation can then be unit shuts down, it shuts down only itself, and the made in the constant current comparator circuit. master continues to supply output voltage. • In pre-regulate supplies, the crowbar turns off the pre-regulator circuit when the SCR fires, reducing the voltage drop across the series regulator and the current flow through the SCR. -Battery charging Some power supplies contain an overvoltage protection circuit, which will short the power supply’s output if an overvoltage condition occurs. The OVP circuit is typically a crowbar SCR that operates independently of the power supply’s regulation circuit. When you use a power supply to charge a battery, it is possible to change the battery to a voltage greater than the OVP limit. Once the OVP limit is exceeded, the SCR will short the output and continuously sink a large current from the source, possibly damaging the power supply. To avoid damaging the power supply, connect a diode in series with the power supply to insure that current only flows out of the power supply. crowbar the output, protecting output loads from the hazardous ESD current. - False shut downs Conclusion The voltage at the output terminal can be increased inadvertently a variety of ways: from the front panel, or by a change in the load. False OVP shutdowns can occur if the OVP shutdown voltage has been set too close to the power supply’s operating voltage. Set the OVP shutdown voltage 4% higher or at least +2.0V above the desired output voltage to avoid false shutdowns from loadinduced transients. Overvoltage is an important feature for power supplies. It does not only protect you and your device under test, it also protects your investment by preventing down time of your test system due to testing fault. A common cause of false OVP shutdowns is an OVP limit set too close to the output voltage when you are using remote sense. Remote sense will increase the voltage at the output terminals to compensate for a voltage drop in the wire between the power supply and the unit under test. Long wire runs and small gauge wires can create a significant voltage drop. Another possible cause of cause OVP false shutdowns is strong electrostatic discharge to the power supply, which can trip the OVP limit and eventually Agilent U8000 series power supply come with a thoughtful design that provide you with value pack features at affordable price.
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