	*What's the definition of efficiency, and how can it be optimized?*
	Efficiency is defined as the ratio of the output power to the input power, the difference between the two being the losses. The power losses of the transformer are comprised of core losses and copper losses. The core should be selected for the range of operating frequency and the level of induction desired. In general, for a given VA rating, the larger the transformer size, the more efficient it will be.

	*How does Underwriters Laboratories classify insulation systems?*
	UL specifies various temperature levels based on a property called "thermal life" - the highest temperature the material can withstand without encountering degradation of its essential properties. The UL temperature classifications used most commonly with transformers are class A (105°); class B (130°); class F (155°); class H (180°).

	*In general, how does one realize a low-profile design at high frequency?*
	For a given frequency and power level, the designer has a multitude of ferrite core geometries from which to choose. Each core configuration accentuates a certain characteristic, i.e., low radiated noise, low profile, surface mount, etc. Various core geometries that lend themselves to a low-profile design are: EFD, EPC, LP, Planar, U-U, and U-I. These geometries exhibit low build height and high throughput power density.

	*How does one reduce common noise in SMPS?*
	Common mode noise reduction in switchmode power supplies is accomplished by suppressing it at the source. Radiated noise is generally reduced by following layout and wiring techniques that minimize leakage inductance, i.e., short high-frequency current loops and twisted pair when possible. In addition to good layout techniques and common mode filtering, conducted common mode noise is further reduced when incorporating Faraday shields into the switching devices of the circuit, i.e., power transformer and power transistor.

	*What design parameters are used by Signal's engineers, i.e., how is cm/A and core size determined?*
	The level of circular mils per ampere used in a design will directly affect the transformer's temperature rise. Typically, our designs average 750 circular mils per ampere. The typical product of window area and core area (WaAc)...1 (4) per 100 VA at 60 Hz.

	*What core materials/thickness does Signal employ in a typical transformer design, and why?*
	The eddy current component of core loss will vary as the square of the thickness of the core material; for the 50 Hz to 500 Hz frequency range we generally use either 0.014 in. or 0.018 in. thick laminations. These laminations are silicon steel (less than 1%Si) and can be either grain oriented or non-grain oriented.

	*Does Signal offer a standard line for offline switching applications? How does one specify a switchmode transformer?*
	Signal offers a standard line of planar transformers/inductors for offline applications, in the 200 W, 100 kHz to 500 kHz range. Switching transformers can be specified by defining the following general parameters: DC output power - voltage and current High-line and low-line DC input voltages Switching frequency and maximum duty cycle Magnetizing inductance (if necessary) Maximum leakage inductance allowed Maximum transient volt-microseconds Maximum ambient temperature Maximum operating temperature complete application circuit

	*How does one specify a current-sensing transformer?*
	To specify a current-sensing transformer, one must first specify the current transformation ratio and accuracy needed. Operating frequency and sensing voltage and/or terminating resistance must also be specified.

Signal Transformer Inc.

500 Bayview Avenue, Inwood, NY 11096
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