1. EE 136 Power Electronics Class Project Power Supply Handbook Chapter 11and 12 Over voltage and Under voltage Protection By Gordon Wong

2. Today’s Topics Over Voltage Protection Under Voltage Protection Why are these types of protection needed? Circuit Analysis Simulations in Simplorer Advantages and Disadvantages

3. Dangers of Over Voltage Overheating Increase Thermal Stress on Components MTBF will decrease Waste of energy Safety Hazards increase risk of fire and electric shock

4. Example 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?

5. 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 overheat

6. A common Over Voltage Protection Circuit is the SCR crowbar

7. What 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.

8. How 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.

9. Results

10. Advantages 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.

11. A more precise SCR circuit using a voltage comparator.

12. What 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.

13. How 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.

14. Results

15. Second type of over voltage protection This is a voltage clamping circuit. The zener diode automatically limits the voltages across the load.

16. Simulation results of Voltage limiting circuit.

17. Advantages 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 want.