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Power-supply optocoupler basics

Posted: 05 Feb 2008 ?? ?Print Version ?Bookmark and Share

Keywords:optocoupler-isolated power supply? optocoupler basics? photodetector isolators?

By Yeo Siok Been
Avago Technologies

An optocoupler-isolated power supply is often the safest and most practical way to go when it comes to performance and protection. Here's the basics on today's LED/photodetector isolators and what you need to know to apply them to your system.

Beginners' guide
The junior system designer often places the system's power requirements at the end of the list, and thus overlooks the importance of an isolated, vs. non-isolated AC/AC, AC/DC, DC/AC, or DC/DC converter. True isolation (transformer at the input, optoisolator in the supply's feedback control loops) virtually removes any direct conductive path between the power supply's input stage and its output terminals/load. That's especially important in the high-power-density applications that are becoming more the rule than the exception, and for more demanding system requirements that often place power supplies in explosive or otherwise hazardous environments.

The use of an optocoupler also acts to break ground loops, and this functionality is valuable in eliminating common-mode noise, especially for systems working at the higher operating voltages. When different power supplies in a system are tied together, ground loop currents tend to be induced due to slight differences in ground potential.

In addition, power supplies tend to see transient noise in equipment that switches between various power states (today's optocouplers are able to withstand up to 40kV/?s transient common-mode voltage).

Typical optocouplers for performing this so-called galvanic isolation functionin essence to connect intrinsically safe circuitry to circuits that pose a safety riskcomprise an LED, a photodetector and appropriate connecting circuitry in the supply's output-to-input feedback loop (Figure 1).

In general circuit operation, the optocoupler, driven by the supply's pulse-width modulation (PWM), serves as the link to maintain the supply's desired output voltage. When the output voltage deviates either due to line and/or load changes, the supply's error amplifier attempts to compensate. It compares its input with a reference voltage, and the error signal thus controls the output of the PWM. In turn, the PWM directs the primary-side power MOSFETs via the optocoupler.

(Click to view image.)

The standards
Regulatory agencies such as UL in the United States, CENELEC in Europe, CSA in Canada, and TIIS in Japan, set the power level needed to make circuitry intrinsically safe. In essence, the standards set the requirements for the galvanic isolation barrier between the "safe" circuitry and the outside world.

For best results, choose optocouplers with additional reinforced insulation as suggested by IEC EN-60747-5-2. Reinforced insulation ensures protection from electric shock as well as provide a fail-safe mode. Fail-safe techniques terminate system operation and leaves system processes and components in a secure state when a failure occurs.

The input-voltage level usually defines the insulation voltage rating, which typically ranges from 500V for some telecom applications to 3,500V for universal line-voltage capability. The regulations you need to know about, and the specs you should study, include IEC60950, EN55022, and IEC 61000. IEC 61000 in particular covers electromagnetic compatibility (EMC), and part 4 of that document (IEC61000-4-4) covers fast transient/burst immunity testing.

Electrical Fast Transient (EFT) testing discussed in part 4.4 addresses interference simulated in inductively loaded switches. In this standard, the modules will be subjected to the following test levels, depending on the designed environment: Level 1 (Well protected); Level 2 (Protected); Level 3 (Typical Industrial Environment); and Level 4 (Severe Industrial Environment), where test voltage peaks at the power supply ports are 0.5kV (5kHz repetition rate), 1kV (5kHz), 2kV (5kHz) and 4kV (2.5kHz), respectively.

(Click to view image.)

There are alternatives to the optocoupler. These include magnetic and capacitive couplers, and they may be suited to your application. But be aware of the tradeoffs. Optocouplers have the advantage of operating down to DC whereas magnetic isolators like transformers are usually specified at some AC bandwidth. Magnetic isolators require a built-in data refresh function to sense the steady-state signal. In addition, short bursts of high-frequency energy from switching-supply transients can be a source of EMI and RFI may impact the functionality of the magnetic isolator. In that context, optocouplers are generally more robust because electromagnetic signals will not interfere with optical signals, as opposed to the magnetic signal that transfers across the magnetic isolation barrier in the magnetic coupler. The optocoupler also comes with a reinforced insulation certification.

[1] J. Seah, A. Jaus, P. Sullivan and T. B. Chua, Building a Safe and Robust Industrial System with Agilent's Optocouplers, publication number: 5989-1774EN, 17 Nov 2005.
[2] IEC 61000-4-4, Electromagnetic compatibility (EMC) " Part 4-4: Testing and Measurement Techniques- Electrical Fast Transient/Bus Immunity Test, 2001.

About the author
Yeo Siok Been
is the product manager for high-speed digital optocouplers at Avago Technologies. She earned a masters degree in electrical engineering at the University of Singapore.

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