EE 136 Power Electronics Class ProjectToday’s TopicsDangers of Over VoltageExample of Over Voltage Protection in IndustryDesired Characteristics of a Good Over Voltage Protection CircuitA common Over Voltage Protection Circuit is the SCR crowbarWhat does this circuit do?How does the SCR crowbar circuit work?ResultsAdvantages and DisadvantagesA more precise SCR circuit using a voltage comparator.Slide 12How does the SCR voltage comparator works?Slide 14Second type of over voltage protectionSimulation results of Voltage limiting circuit.Advantages and Disadvantages to voltage limiting circuits.EE 136 Power ElectronicsClass ProjectPower Supply Handbook Chapter 11and 12 Over voltage and Under voltage ProtectionByGordon WongToday’s Topics•Over Voltage Protection•Under Voltage Protection•Why are these types of protection needed?•Circuit Analysis• Simulations in Simplorer•Advantages and DisadvantagesDangers of Over Voltage•Overheating •Increase Thermal Stress on Components•MTBF will decrease•Waste of energy•Safety Hazards increase risk of fire and electric shockExample of Over Voltage Protection in Industry•The following circuit is from Maxim.•This is a over voltage protection circuit in automotive systems.•Input Voltage 16V maximum•Output Voltage 5 V•How does this work?Desired Characteristics of a Good Over Voltage Protection Circuit•Fast Response•Able to handle large input voltages, from voltage spikes and surges.•Doesn’t intrude with the normal operation of the circuit•Doesn’t overheatA common Over Voltage Protection Circuit is the SCR crowbarR1R2RLoadC1+ VVMLoadE1+ VVM1S2S3R3+ VVM4R5R41000k.0011k.00110k1k1uWhat does this circuit do?•To prevent the output from going over 5 volts the circuit is designed to short the output to ground if Rload goes over 5 volts.R1R2RLoadC1+ VVMLoadE1+ VVM1S2S3R3+ VVM4R5R41000k.0011k.00110k1k1uHow does the SCR crowbar circuit work?•E1 and R6 are the Thevenin equivalent for the DC power supply.•R1 and C2 introduce a time delay in the circuit.•S2 is zener diode that closes if E1 goes over a certain value•S3 is the SCR that closes if the C1 voltage goes over a certain value.R1R2RLoadC1+ VVMLoadE1+ VVM1S2S3R3+ VVM4R5R41000k.0011k.00110k1k1uResults C1.V [V] VM1.V [V] t [s] 10-124680 50m5m 10m 15m 20m 25m 30m 35m 40m 45mVMLoad.V [V] VM1.V [V] t [s] 10-1024680 50m5m 10m 15m 20m 25m 30m 35m 40m 45mAdvantages and Disadvantages•Advantages•Easy to create •The value of C1 can be adjusted to accommodate a wide range of time delays and let thru voltages.•Disadvantages•Premature shutdown of circuit might occur is the C1 value is too low.•The SCR require a manual reset if it goes off.A more precise SCR circuit using a voltage comparator.R2R3R4C1+-OPV51S2E1E2R6+ VVM1+ VVM2R1R510k1k1k1u2.51k1k.001What does this circuit do?•This circuit also prevents the output from receiving more then 5 volts. •This is done by shorting the output to ground if it goes over 5 volts.•However this circuit has a much faster response time then the SCR crowbar circuit.R2R3R4C1+-OPV51S2E1E2R6+ VVM1+ VVM2R1R510k1k1k1u2.51k1k.001How does the SCR voltage comparator works?•E2 and R6 are the Thevenin equivalent of the DC power supply.•R2, R3 form a voltage divider.•E1 is the reference voltage.•R4, C1 introduce a time delay.•S2 closes if the voltage across C1 gets too high.R2R3R4C1+-OPV51S2E1E2R6+ VVM1+ VVM2R1R510k1k1k1u2.51k1k.001Results C1.V [V] VM1.V [V] t [s] 12-100-7.5-5-2.52.557.5100 20m2m 4m 6m 8m 10m 12m 14m 16m 18mVM2.V [V] t [s] 5-0.512340 20m2m 4m 6m 8m 10m 12m 14m 16m 18mSecond type of over voltage protection•This is a voltage clamping circuit.•The zener diode automatically limits the voltages across the load.D_Z_ST_2_4V1R1E1+ VVM1+ VVM2Simulation results of Voltage limiting circuit.VM1.V [V] t [s] 6-10123450 40m5m 10m 15m 20m 25m 30m 35mVM2.V [V] t [s] 2.5-0.250.511.520 40m5m 10m 15m 20m 25m 30m 35mAdvantages and Disadvantages to voltage limiting circuits.•Advantages•Simple easy to use.•Cost effective•Disadvantages•Zener diode characteristic depend on temperature •You have find a Zener diode with exactly the breakdown voltage you