In this circuit, a single relay limits the in-rush current and turns off the startup circuitry for a switching power supply. Typically, switching power supplies initially use a rectified ac line to power the control circuitry. Once the power supply is running, an auxiliary supply winding provides power. Startup circuits either use a wasteful high-value multiwatt resistor to charge a capacitor until the controller turns on, or a more efficient but costly active circuit that turns off once the controller is running.
High-power switching supplies often use resistors to limit the initial in-rush current caused by charging the filter capacitors (see figure, a). A single-pole, single-throw (SPST) relay shorts across the resistor once the power supply is operating properly. Changing this relay to a single-pole, double-throw (SPDT) relay creates a circuit that's simple and inexpensive, yet wastes little power once the supply is running (see figure, b).
When the supply is initially turned on, the common terminal of relay K1 is connected to resistor R2 to charge up capacitor C1 and supply power to the controller IC. R1 is left in-circuit to limit the in-rush current. Once the power supply starts operating, K1 closes, shorting across in-rush current-limiting resistor R1 and turning off the current through startup resistor R2.
Nice idea - until you start looking at the relay ratings. The DC rating for relay contacts is usually much much lower in current and/or voltage. This means, that a relay, which is supposed to switch 2A@115VAC has a rating (if lucky) of 2A@28VDC and a very low one (if at all) @115VDC which means a much bigger relay than for the case where the limiting resistor is in a AC path. Further, R1 (the limiting resistor) has still to be rated for max. power (High line voltage, full current), in order to avoid catastrophic failure (most certaily fire) if the relay fails to pull in or if the contact is degraded. I've used such inrush current limiters myself often, but i never feel good with them, except when using redundancy and/or emergency cutouts! One good measure is to actuate a cutout or deactivate a (preceding) mains relay if the limiter relay fails to pull in, which of course necessitates a 2PDT relay and some additional circuitry. A simpler, but still ok solution is to use a thermal switch (->Protistor) closely coupled to R1, which is in the mains power activation chain (control for the primary power relay). There are also mains switches having a cutout coil, which could be used for such purposes (->Schurter, P&B..).
Anonymous -May 31, 2006
Excellent source of new innovative ideas....
Jeffg -January 12, 2006 (Article Rating: )
Excellent source of new innovative ideas....
Jeffg -January 12, 2006 (Article Rating: )
Your Comments:
Enter the text from the image below
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
)
