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Power Tips #125: How an opto-emulator improves reliability and transient response for isolated DC/DC converters



In high-voltage power-supply designs, safety concerns require isolating the high-voltage input from the low-voltage output. Designers typically use magnetic isolation in a transformer for power transfer, while an optocoupler provides optical isolation for signal feedback.

One of the main drawbacks of optocouplers in isolated power supplies is their reliability. The use of an LED in traditional optocouplers to transmit signals across the isolation barrier leads to wide part-to-part variation in the current transfer ratio over temperature, forward current, and operating time. Optocouplers are also lacking in terms of isolation performance, since they often use weak insulation materials such as epoxies or sometimes just an air gap.

A purely silicon-based device that emulates the behavior of an optocoupler such as the Texas Instruments (TI) ISOM8110 remedies these issues since it removes the LED component, uses a resilient isolation material such as silicon dioxide, and is certified and tested under a much more stringent standard [International Electrotechnical Commission (IEC) 60747-17] compared to the IEC 60747-5-5 optocoupler standard (see this application note for more details).

An optocoupler’s lack of reliability over time and temperature has meant that many sectors, such as automotive and space, have had to rely on primary-side regulation or other means to regulate the output. An opto-emulator contributes to improved reliability and also provides substantial improvements in transient and loop response without increasing the output filter.

Typically, the limiting factor in the bandwidth of an isolated power supply is the bandwidth of the optocoupler. This bandwidth is limited by the optocoupler pole, formed from its intrinsic parasitic capacitance and the output bias resistor. Using an opto-emulator eliminates this pole, which leads to higher bandwidth for the entire system without any changes to the output filter. Figure 1 and Figure 2 show the frequency response of an isolated flyback design tested with an optocoupler and opto-emulator, respectively.

Figure 1 Total bandwidth of an isolated power supply using the TCMT1107 optocoupler. Source: Texas Instruments

Figure 2 Total bandwidth of an isolated power supply using the ISOM8110 opto-emulator. Source: Texas Instruments

The target for both designs was to increase the overall bandwidth while still maintaining 60 degrees of phase margin and 10dB of gain margin. Table 1 lists the side-by-side results.

 

Optocoupler

Opto-emulator

Bandwidth (kHz)

8.6

38.2

Phase margin (degrees)

60.2

67.4

Gain margin (dB)

18.7

11.62

Table 1 Optocoupler versus opto-emulator frequency response results.

The increased bandwidth of the opto-emulator helps achieve nearly a quadruple increase in the overall bandwidth of the design while maintaining phase and gain margins. Figure 3 highlights the changes made to the compensation network of the opto-emulator board versus the optocoupler board. As you can see, these changes are minimal and only require changing a total of three passive components. Another benefit of the opto-emulator is that it is pin-for-pin compatible with most optocouplers, so it doesn’t require a new layout for existing designs.

Figure 3 Schematic changes made to the compensation network of the opto-emulator board versus the optocoupler board. Source: Texas Instruments

Only the compensation components around the TL431 shunt voltage regulator were modified from one design to the other. Other than C19, C22 and R20, the rest of the design was identical, including the power-stage components, which include the output capacitance.

Because of the quadruple increase in the bandwidth, we were able to improve transient response significantly as well, without adding any more capacitance to the output. Figure 4 and Figure 5 show the transient response of the optocoupler and opto-emulator designs, respectively.

Figure 4 The transient response for the optocoupler design. Source: Texas Instruments

Figure 5 The transient response for the opto-emulator design showing a greater than 50% reduction in overall transient response. Source: Texas Instruments

The load step and the slew rate were the same in both tests. The load-step response went from –1.04 V in the optocoupler to –360 mV in the opto-emulator, and the load-dump response decreased from 840 mV to 260 mV. This is a > 50% reduction in the overall transient response, without adding more output capacitors.

Opto-emulator benefits

Because of the significant bandwidth improvement that an opto-emulator provides over an optocoupler, designers can reduce the size of their output capacitor without sacrificing transient performance in isolated designs that are cost- and size-sensitive.

An opto-emulator also provides more reliability than an optocoupler by enabling secondary-side regulation in applications that could not use optocouplers before, such as automotive and space. With the increase in bandwidth, an opto-emulator can provide higher bandwidth for the overall loop of the power supply, leading to significantly better transient response without increasing the output capacitance. For existing designs, an opto-emulator’s pin-for-pin compatibility with most optocouplers allows for drop-in replacements, with only minor tweaks to the compensation network.

Sarmad Abedin has been a systems engineer at Texas Instruments since 2011. He works for the Power Design Services team in Dallas, TX. He has been designing custom power supplies for over 10 years specializing in low power AC/DC applications. He graduated from RIT in 2010 with a BS in Electrical Engineering.

 

 

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The post Power Tips #125: How an opto-emulator improves reliability and transient response for isolated DC/DC converters appeared first on EDN.

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