Tame Switching Supply Noise While Maintaining Efficiency

A number of situations could benefit from the high efficiency of a switched power supply, save for the supply’s intrinsically high noise level. For example, an optical communications application using a large number of laser diodes could employ a switched power supply to avoid the need for heat-removal techniques.

However, the laser diodes require a noiseless environment. A hasty decision to use a switcher would not be rewarding unless much attention is paid to noise reduction. Furthermore, RFI and EMI could cause problems during compliance testing.

The technique illustrated in the figure uses an ultra-low-noise regulator, a type of switching IC designed to reduce conducted and radiated electromagnetic interference.¹ It also employs a relatively new type of adjustable resistor called a Rejustor, which can complement the ultra-low-noise switcher.

The regulator, in this case an LT1533, reduces noise and EMI by allowing users to control the output voltage and current slew rates and to program the device to optimize the switcher’s harmonic content versus efficiency. The result is as much as a 40-dB reduction in high-frequency harmonic power with only minor losses in efficiency. The Rejustor is nonvolatile, requiring no power to hold its adjustment, and can be readjusted many times bidirectionally to very high precision using only electrical signals.²

The adjustments, which can be made after assembly, consider not only natural component variations (i.e., inductance and capacitance) but also board specifics like layout and type of load. For simplicity, the components and pins of the adjustment circuitry (dashed box) that aren’t essential for this discussion are omitted.

R2 and R4 define the output voltage of the switch regulator U1. The output voltage is fixed. When the load is connected, the current through R1 being converted into voltage by instrumentation amplifier G1 is the measure of the regulator’s efficiency. R1 may be an integral part (a sense resistor) of an electronic circuit breaker, hot-swap controller, or any current-monitoring circuitry.

G1’s output, which represents the controller’s efficiency, is conveyed to the Microbridge Rejustor Calibration Tool (RCT).³ U2, an LTC1968 rms-to-dc converter, measures the regulator’s noise value. The converter acquires the regulator noise at In1 and passes the noise value to the RCT through buffer A1, which acts as an interface between the converter and the RCT.

Properly selecting the differential gain (G1), as well as R5 and R6, ensures that the appropriately weighted coefficients can be applied to measure either regulator efficiency or noise. These coefficients help to optimize the RCT performance. A PC-driven calibration procedure sets the dual Rejustor’s resistor values, which adjust the feedback to the regulator. (Linear Technology advises that the resistor values be equal for most cases.) The two control loops are combined internally to provide a smooth transition from current slew control to voltage slew control. The user must choose the correct balance between efficiency and noise.

See associated figure