Understanding Peak Inverse Voltage and Surge Current.

Peak Inverse Voltage (PIV): Maximum voltage across a non-conducting diode
PIV Requirement: Must be less than diode’s breakdown voltage
Worst Case: Capacitor-input filter
Symbols for Maximum Reverse Voltage: PIV, PRV, \(V_B\), \(V_{BR}\), \(V_{R}\), \(V_{RRM}\), \(V_{R(max)}\)

Half-Wave Rectifier with Capacitor-Input Filter
- Critical part: Determines reverse voltage
- Worst case: Peak secondary voltage negative, capacitor fully charged
- Peak Inverse Voltage (PIV):
- Example: Peak secondary voltage = 15 V
- Diode must have breakdown voltage greater than PIV

\[\mathrm{PIV} = 2 \times 15\,\text{V} = 30\,\text{V}\]

\[\mathrm{PIV} = 2V_p\]

The Peak Inverse Voltage and Surge Current in Diodes

Full-Wave Rectifier with Capacitor-Input Filter
- Critical part: Determines peak inverse voltage
- Secondary voltage at negative peak
- Lower diode: short (closed switch), upper diode: open
- Peak Inverse Voltage (PIV):

\[\mathrm{PIV} = V_p\]

Bridge Rectifier with Capacitor-Input Filter
- Critical part: Determines peak inverse voltage
- Upper diode: short (closed switch), lower diode: open
- Peak Inverse Voltage (PIV):
- Advantage: Lowest PIV for a given load voltage
- Compared to full-wave rectifier: Requires half the secondary voltage for same load voltage

\[\mathrm{PIV} = V_p\]

SECTION 01

Surge Resistor

Filter capacitor initially uncharged
At power-on, capacitor acts as a short
Initial charging current may be very large
Surge current: Initial rush of current
Charging path: Resistance of transformer windings and diode bulk resistance
Designer’s choice: Diode with sufficient current rating or use of a surge resistor
Surge resistor: Reduces surge current to safe level
\(R_{surge}\) inserted between bridge rectifier and capacitor-input filter
Surge resistors are infrequent in use

SECTION 02

Problem on Peak inverse voltage

What is the peak inverse voltage if the turns ratio is \(8: 1\) ? The diode has a breakdown voltage of 50 V . Is it safe to use the diode in this circuit?

The rms secondary voltage is:

\[V_2=\frac{120 \mathrm{~V}}{8}=15 \mathrm{~V}\]
The peak secondary voltage is:

\[V_p=\frac{15 \mathrm{~V}}{0.707}=21.2 \mathrm{~V}\]
The peak inverse voltage is:

\[\text { PIV }=21.2 \mathrm{~V}\]
The diode is more than adequate, since the peak inverse voltage is much less than the breakdown voltage of 50 V .