Under Voltage Protection

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Under Voltage Protection

• Chapter 12
• Switch mode Power Supply Handbook
• Keith Billings

2
What is Under Voltage?

• Under Voltage condition occurs when a load is
  suddenly connected to a power supply.
• The load will start to draw current, this causes
  the voltage to temporarily drop.

3
Graphical Example of Under Voltage

• Graph A shows the sudden rise in current going to
  the load
• Graph B shows the sudden voltage drop without an
  adequate under voltage protection circuit.
• Graph C is the same circuit with a under voltage
  protection circuit.

4
Dangers of Under Voltage

• Premature Shutdown of circuit
• Lose of Data
• Very important in navigation and automotive
  application were data is constantly being
  processed and updated.

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Example of Under Voltage Protection Circuit
6
What makes a good Under Voltage Protection
Circuit?

• Fast response time
• Able to supply a large voltage in order to keep
  the voltage drop small.
• Able to handle voltage drops for extended periods
  of time.

7
Example of a Under Voltage Protection Circuit.
   
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What does this circuit do?

• This circuit prevents the output from going under
  5 mV.
• When it goes under 5mV, S1 will turn on providing
  additional voltage to supplement the load.

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How does this circuit work?

• E2 and R6 form the Thevenin Equivalent of the DC
  power source.
• R1,C1,R2,C2 form the circuit to lift up the
  voltage if needed.
• R3 and R4 form a voltage division.

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How does this circuit work?
How does this circuit work?

• E1 is the voltage reference, if the voltage
  across the load drops below E1. The output of the
  comparator is high.
• If the output of the comparator is high. S1 turns
  on thus provide the output with the voltage of C1
  and C2 in series.

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Results
12
Results
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Advantage and Disadvantages

• Advantages
• Cheap easy to implement.
• Circuit protection level can be adjusted by
  changing the value of R3,R4 and the value of the
  reference voltage.

• Disadvantages
• Can only be used in the mV range.
• Can only be used in high frequency circuit.
  (Above 10 kHz)