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Surge Current Limiting for Power Supply

IP.com Disclosure Number: IPCOM000059617D
Original Publication Date: 1986-Jan-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 2 page(s) / 46K

Publishing Venue

IBM

Related People

Morrish, AJ: AUTHOR

Abstract

In a power supply for a display or TV, a surge current limiting resistor is included in series with the neutral input which is short- circuited by a triac during normal operation. Gate current for the triac is provided via an additional resistor. To permit a wide range of input supply voltages, this additional resistor is replaced by a positive temperature coefficient (PTC) thermistor. An existing thermistor used for degaussing may be utilized to heat the PTC thermistor to provide a greater change of resistance. Some method is required to limit the large current pulse in the mains supply when power is connected to a power supply. This is due to the charging current in the primary smoothing capacitors.

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Surge Current Limiting for Power Supply

In a power supply for a display or TV, a surge current limiting resistor is included in series with the neutral input which is short- circuited by a triac during normal operation. Gate current for the triac is provided via an additional resistor. To permit a wide range of input supply voltages, this additional resistor is replaced by a positive temperature coefficient (PTC) thermistor. An existing thermistor used for degaussing may be utilized to heat the PTC thermistor to provide a greater change of resistance. Some method is required to limit the large current pulse in the mains supply when power is connected to a power supply. This is due to the charging current in the primary smoothing capacitors. The conventional method of achieving this is to use a low-value high-power surge limiting resistor (say, 10 ohms, 10 w) in series with the supply to the primary supply rectifiers. This has the disadvantage of dissipating considerable power during normal operation. To overcome this, a triac is used to short-circuit this resistor during normal operation. This triac is turned on by a floating power rail derived from a transformer winding. This article describes an alternative method of providing a triac short-circuit function that does not rely upon a transformer winding. Fig. 1 shows a voltage doubler input stage and smoothing capacitors, although the input stage may equally well be a full-wave rectifier. In series with the neutral supply N is surge limit resistor R1. Across resistor R1 is triac T. Connected to its gate is resistor R2 and zener diode D2 which are connected to capacitor C1. Before switch- on, this capacitor is discharged, and as there is no gate current, the triac is in its off state. Upon power on, capacitors C2 and C3 are charged through R1. Current flowing through resistor R3 and diode D1 charges C...