Here is how to reduce Miller Effect to enhance an SMPS' performance and reduce component count.
Power-supply designs requiring high-performance isolated feedback often use an error amplifier similar to the one in figure 1, which relies on a second amplifier, IC1B, to provide the necessary inversion to keep the optocoupler, IC2, referenced to ground. To prevent bias-supply noise from entering the feedback path and causing oscillations, the amplifier relies on its ground reference and power-supply-rejection characteristics. The power supply's output drives a voltage divider comprising R1 and R2 that maintains the amplifier's inverting input at the same voltage as the reference voltage that IC3 provides. C2, R3, and C3 comprise frequency-compensation components for the power supply's stable operation. This component-intensive error-amplifier configuration requires two operational amplifiers, one precision shunt-voltage reference, four capacitors and often a fifth in parallel with R6, and seven resistors.
Figure 1: A conventional isolated-feedback circuit requires an extra operational amplifier and adds several passive components to a representative pulse-width-modulated power-supply design.
Figure 2 shows an alternative single-amplifier design in which IC3, an LM4040 precision-voltage reference, drives optocoupler IC2 with a "stiff" positive-voltage source over a wide current range. The voltage reference suppresses any noise present on the bias-supply rail. Variations in the reference and power-supply voltages appear in common mode at the amplifier's inputs and thus provide additional noise immunity. A resistive-voltage divider comprising R2 and R3 reduces the reference voltage to equal the power supply's regulated output voltage, which drives IC1's inverting input through R1. Given its single voltage divider, the error-amplifier circuit of figure 2 provides the same output voltage as the circuit of figure 1 and requires a single operational amplifier and precision shunt reference, four capacitors, and six resistors.
Figure 2: Clamping an optoisolator's voltage excursion improves the PWM-regulator loop's transient response.
Miller-effect coupling of collector-emitter-voltage transitions into a typical phototransistor-based optocoupler's high-impedance, optically sensitive base region introduces a bandwidth-limiting pole, which dramatically slows the device's response time. Holding the phototransistor's collector-emitter voltage constant and allowing only its collector-emitter current to change provide an order-of-magnitude switching-speed improvement.
National Semiconductor's LM5026 active-clamp current-mode PWM controller, IC4, provides a convenient method of reducing an optocoupler's Miller-effect-induced slowdown. Figure 2 shows the LM5026's internal current mirror driving what would normally serve as a frequency-compensation pin. Optocoupler IC2 connects directly between two constant-voltage sources comprising the current mirror and a voltage reference. The resultant decrease in response time relocates the bandwidth-limiting pole and improves the circuit's transient response.
The values of C2, C3, R3, and R1 apply only to this design and may require modification for other applications. Select R1 to provide equal impedances at both of the op amp's inputs. C2 forms a high-frequency noise filter. After you measure the converter's overall gain, calculate values for C3 and R3 that will provide proper gain and phase response. Several methods of calculation are available, most of which will provide adequate results.
This article is a Design Idea selected for re-publication by the editors. It was first published on March 1, 2007 in EDN.com.