Feedback generator avoids optocoupler use in SMPS design

Article By : Gheorghe Plasoianu

Using an optocoupler in a switched mode power supply has some drawbacks, which this approach avoids.

Optocouplers are frequently used in isolated switch-mode power supplies (SMPS) for galvanic separation between the primary and secondary sides as well as from the feedback generator. There are several disadvantages to use an optocoupler, however, including performance and durability issues. Here is an alternative to the optocoupler that uses a digital isolator, instead.

SMPS power converter designs depend upon feedback about their output voltage to maintain regulation. This feedback signal typically passes through an optocoupler to maintain galvanic isolation between the primary and secondary sides. One of the key concerns when using an optocoupler, however, is that it introduces an extra pole in the control loop. This pole reduces the feedback path’s bandwidth. In addition, an optocoupler has large unit-to-unit variation in, and temperature and lifetime degradation of, its current transfer ratio. This variability affects the control loop’s calibration and long-term drift.

The design below shows an alternative to the optocoupler in an SMPS design using the Si8642 digital isolator from Silicon Labs to form the barrier between the converter’s primary and secondary sides. It requires generating a feedback signal based on the converter’s output instead of using that output signal directly.

Figure 1 Schematic of an SMPS voltage converter using a digital isolator-based feedback scheme

The feedback generator starts with a 10 MHz clock signal sent over the isolation barrier to the secondary side, where the circuit R2C2 converts the clock signal into a triangular waveform. This waveform drives high-speed comparator U3’s inverting input. The non-inverting input receives a scaled value of the converter’s output voltage. Whenever the triangular signal is smaller than the scaled output voltage, the comparator’s output goes high. The comparator output signal’s duty cycle will thus be proportional to the converter’s output voltage, as shown in Figure 2.

Figure 2 Comparing a triangular clock signal (green) with a scaled version of the SMPS output voltage yields a signal (yellow) with duty cycle proportional to the SMPS output.

The comparator’s output passes through the isolation barrier back to the primary side, where circuit R1C1 low-pass filters the signal and applies that result to the switching controller’s feedback pin.

To evaluate the feedback generator’s linearity, I applied a ramp signal to the comparator’s non-inverting input and watched the output signal (at C1). The results are shown in Figure 3.

Figure 3 The feedback generator’s linearity is tested using a triangular wave at the non-inverting input.

In order to tune the converter’s control loop, we need to know the position of the additional pole this feedback scheme has introduced. To determine this, I checked the generator’s AC behavior with a Bode-100 phase-gain analyzer, and the results are shown in Figure 4:

Figure 4 The gain magnitude and gain phase of the SMPS feedback generator

The pole occurs at 85 kHz, mainly due to the R1C1 filter. By choosing smaller values for these components it is possible to push the pole even higher.

Gheorghe Plasoianu has a Masters degree in electrical engineering from the Polytechnics Institute of Bucharest.

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