SR03 datasheet(4/18 Pages) SUTEX

4 / 18 page

SR036/SR037

B092005

Applications Information

Functional Block Diagram

REF

Reg

Gate

Source

GND

OUT

Operating Principle

The SR03x operates by controlling the conduction angle of the

external MOSFET or IGBT as shown in Figure 1. When the

rectified AC voltage is below the VTH threshold, the pass transistor

is turned on. The pass transistor is turned off when the rectified

AC is above HVIN(off). Output voltage (Vunreg) decays during the

periods when the switch is off and when the rectified AC is below

the output voltage. The amount of decay is determined by the

load and the value of C1. Since the switch only conducts with low

voltages across it, power dissipation is minimized.

Switch ON

VTH

HVIN

VREG

VUNREG

not to scale

Figure 1: Typical Waveforms

Power Dissipation

Power dissipation in the SR03 is from 2 sources. The first is due to the bias current (or overhead) required to operate the device. This may

be calculated from PBIAS = VIN2 / 185ký where VIN is the input voltage in VRMS. The second source of power dissipation is the 3.3/5V linear

regulator and may be calculated from PREG = (16V - VOUT) * IREG, where VOUT is 3.3V or 5V, and IREG is the load current on the 3.3/5V output.

The total power dissipated by the SR03x is the sum of these two: P

BIAS + PREG. (These equations are conservative – actual dissipation may

be less.)

To adequately dissipate the power, the underside plate of the SR03xSG should be soldered to at least 2cm2 of exposed copper area on

the PCB.

Power is also dissipated by the pass transistor. Power dissipated by the transistor will be (16V * I

TOTAL) * (1/Eff -1) where ITOTAL is the sum

of the load currents on the regulated and unregulated outputs and Eff is the converter efficiency (see Efficiency Graph next page). The

transistor should be soldered to at least 5cm2 of exposed copper area on the PCB for heatsinking.

Transformers

SR03 Datasheet(PDF) 4 Page - Supertex, Inc