How to Protect a Voltage Regulator against Reverse Polarity, ESD, Over-Voltage, and Noise

Voltage regulator protection circuit for reverse polarity, ESD, over-voltage, and noise

What it does

This circuit protects a voltage regulator against reverse polarity, electrostatic discharge (ESD), over-voltage conditions, and noise.

Why it matters

Circuit designers must consider both nominal and fault conditions when designing circuitry to ensure robust and reliable performance. Fault conditions for voltage regulator circuits include:

• Reverse polarity on VIN

• Positive ESD events on VIN

• Negative ESD events on VIN

• Over-voltage on VIN

• Noise on VIN

How it works

Nominal operation. Under normal operating conditions with an input voltage of 24 VDC, current will flow through R8 (anti-surge thick film resistor) and D4 (Schottky diode) to U13 (voltage regulator), producing the desired 5-Volt output. There are four critical parameters in this situation:

• The voltage drop across R8 must not reduce the voltage at U13 below its drop-out voltage

• The power rating of R8 must not be exceeded

• The maximum DC forward current (IF) of D4 must not be exceeded

• The breakdown voltage (VBR) of D2 (unidirectional TVS diode) must be greater than the nominal 24 VDC input voltage to avoid conducting

Reverse polarity operation. If the 24 VDC input is accidentally connected to the circuit with reverse polarity, current will try to flow backwards through the circuit and cause damage. D4 prevents this by acting like an open (current can only flow one way in a diode), protecting the circuit from reverse polarity. There are two critical parameters in this situation:

• The breakdown voltage (VBR) of D4 must be greater than the nominal 24 VDC input voltage

• The breakdown voltage (VBR) of D3 (bidirectional TVS diode) must be greater than the nominal 24 VDC input voltage to avoid conducting

Positive ESD event. If a positive ESD event occurs on the 24 VDC input line, current from the high-voltage surge will flow through R8 and D4 before being shunted to ground through D2. There is one critical parameter in this situation:

• The clamping voltage (VC) of D2 must be less than U13's maximum input voltage (VIN_MAX)

Negative ESD event. If a negative ESD event occurs on the 24 VDC input line, current from the high-voltage surge will flow from the bottom of the circuit to the top through D2, D3, and R8 (D4 will block the reverse current). There are two critical parameters in this situation:

• The negative of D2's forward voltage (VF) must be less than U13's minimum input voltage (VIN_MIN)

• The clamping voltage (VC) of D3 must be less than the breakdown voltage (VBR) of D4

Over-voltage. An over-voltage on the 24 VDC input line will behave similar to the positive ESD event situation described above, with the current flowing through R8 and D4 before being shunted to ground through D2. There are four critical parameters in this situation:

• The power rating of R8 must not be exceeded

• The maximum DC forward current (IF) of D4 must not be exceeded

• The steady-state power dissipation (PMAV) of D2 must not be exceeded

• The clamping voltage (VC) of D2 must be less than U13's maximum input voltage (VIN_MAX)

Noise. Electrical noise on the 24 VDC input line will be filtered by the low-pass RC network formed by R8 and C21 with a cutoff frequency of 1/(2*pi*R*C). There is one critical parameter in this situation:

• The RC network and U13 Power Supply Ripple Rejection (PSRR) should be analyzed to ensure U13 output noise is within acceptable levels