"Seminar 700 Topic 2

Bob Mammano Isolation Requirements face of the board. Clearance denotes the short- A fact of life for all off-line power supply est distance between two conductive parts as systems is the requirement for galvanic isola- measured through the air, for example, the tion from input to output. This isolation is closest spacing of two bare leads as they run primarily in the interest of safety to insure that from the PC board to the point where they there will be no shock hazard in using the become insulated. Finally, the Isolation Barrier equipment, and the requirements have been represents the shortest distance between two quantized over the years by many agencies conductive parts separated by a dielectric which throughout the world, most notably VDE and meets the voltage and resistance specifications. IEC in Europe, and UL in the United States. With an optocoupler, this is the minimum Exanlples of some of the more stringent of spacing of conductors within a plastic molded these specifications are listed in Table 1. Note package. Transformer windings have the addi- that isolation involves mechanical as well as tional requirement for three separate layers of electrical specifications, and as new technolo- insulation, any two of which are capable of gies shrink component sizes, these physical withstanding the required voltage. spacings can often become limiting factors. For those unfamiliar with the terminology, the All AC mains connecu~d power supplies following definitions are offered: must provide this isolation between the input Creepage is defined as the shortest path be- and output sections of ti le supply and, of tween two conductive parts on opposite sides of course, this is normally accomplished with a the isolation as measured along the surface of power transformer. At 60 Hiertz, this represents any intervening insulation. The best example of a big, heavy, and costly solultion, albeit a simple Creepage is the separation between two PC one. With switch-mode POWIer systems, the high board solder eyes as measured along the sur- frequencies make the powelr transformer much Table I --Examples of Safety Standards and Specifications Isolating the Control Loop 2-1 more manageable but a new problem is intro- 3. Isolating the digital signal path. This duced: the fact that the power has to be means after the analog-to-digital conver- switched on the input side, but under control sion which takes place in the pulse width from the output side in order to provide good modulator. Although it is somewhat more regulation. This implies a second crossing of complex in implementation, the advantage the isolation boundary in order to feed back is accuracy and stability although if errors control information, and although this path do occur, they could result in complete involves only information, rather than power, it loss of control. ; must still meet the same isolation requirements. While the isolation within the power 4. Isolating the digital power path. While transformer is not a trivial matter, it is the also offering high accuracy, placing all the purpose of this discussion to address only the control -including the power switch drive problems associated with isolating the control -on the secondary side makes the isola- path. tion task more difficult due to the power levels and stringent waveform require- Alternatives for Isolating Control ments. It also carries along the added Figure 1 shows the block diagram of a basic burden of a separate isolated starting off-line power converter indicating some of the circuit, or possibly a complete auxiliary places where a designer might choose to insert power supply, and eith~r of these means isolation in the control feedback path. Working a third crossing of the isolation boundary. from output back to input, these options are: 1. Isolating the measurement of the output Before embarking on a more detailed discus- voltage. While this can be accomplished sion of the above alternatives, mention should fairly easily, high accuracy is required in be made of an additional choice, and certainly coupling this large control sigital across the simplest one from an isolation standpoint, the isolation boundary to achieve good which is to not have a feedback path at all. output regulation. Conversion Without an Overall Con- 2. Isolating the analog error signal. Certainly trol Loop the most popular approach and several Clearly, the problems of isolating a control techniques will be discussed in detail. The feedback path are sidestepped neatly by elimi- requirements for absolute accuracyare nating the feedback. The approach which offers substantially reduced as it is only the the highest level of performance using this error difference between the output and technique is shown in Figure 2 where a combi- the reference which crosses the isolation. nation of voltage feed-forward or current-mode POWER PATH control, and secondary regulators can give excellent results. This circuit controls the power stage from the primary side with direct communication. This has the added benefit of reliable fault protection since input voltage and current levels can readily be monitored and reacted to with minimum delay. The control circuit must be either powered directly from the line -which would require very low current -or started from the line and then supplied by a primary- referenced, low-voltage auxiliary winding Fig. 1 --Alternative Isolation Points from the power transformer . 2-2 UNITRODE CORPORATION A simpler approach to a "no ~I '~MUM~'r I DCO;:;-TPUT 1 overall feedback" converter is shown in Figure 3. This circuit uses the same low-voltage, primary-refer- AC INPUT r 0--' 1 POWER I SWITCHING1-=--- enced auxiliary winding which was 0-- ~ STAGE IREGULATORI DCO~PUT2 mentioned above as a way of effi- ciently supplying power to the con- r I LINEARI v" trol circuit, but in this case:,a non- FEED FORWARD~ IREGULATOR~PUT 3 isolated feedback loop is used to PWM AND +v SUPPLY 0-- r force the PWM controller to regu- CONTROL I late its own supply voltage. The ISOLATION BOUNDARY theory is that if the diode voltage drops are matched, and the trans- Fig. 2 -FeedfolWard, No Overall Control Loop former windings well coupled, the Either a current-mode topology or a voltage isolated output voltage will track this regulated feed-forward circuit which linearly adjusts the primary-referenced auxiliary voltage. While this width of the PWM output pulse in response to design may provide acceptable performance in changes in input voltage will provide a constant low power applications, the problem is that volt-second product to the primary of the both the above assumptions are weak -particu- power transformer. For circuit topologies other larly the goal in achieving close coupling be- than discontinuous flyback, this will achieve a tween windings which may need 3750 V AC first-order line regulation. Higher accuracy, as isolation. well as regulation for load changes, is accom- Isolating Feedback Control at the plished by the secondary regulators which, as Output shown in Figure 2, can be implemented in An interesting and relatively simple method several ways. In general, a linear regulator is for isolating the measurement of the output the simplest approach while a mag amp will be voltage is shown in Figure 4. This circuit uses the most efficient. a secondary-driven amplitude modulator to As stated above, a high degree of perfor- transmit the DC output voltage value across an mance can be achieved with this configuration. isolating pulse transformer to the primary side. Its main drawback is the potential cost and re- Assuming the transformer is fully reset between duced efficiency of the additional secondary each pulse, when QI is turned on, Cc will regulators -a penalty which accelerates rapidly charge to a value closely approximating the with increasing load current levels. supply's output voltage, and then hold that value -with only a small decay through Rc - v BULK0- < while QI is off. While the easiest approach to ;' I T1 ~ driving QI would be to use the supply's switching frequency from a secondary winding of the N2 . power transformer, there are at least two Vcc IR11 no . 11!~lllcr limitations: First, QI will follow the duty cycle ~.- of the power transformer and it can be seen lEAl i ISOLATION BOUNDARY from the waveforms of Figure 4 that the aver- PWM1C age value of the peak-detected control voltage, r;--- V c , will vary with duty cycle. Secondly, the ~ bandwidth of this system will not be usable much above one-tenth of the switching frequen- Fig. 3 --Primary Control Isolating the Control Loop ~ + formers, between the PWM output drivers and YO O c --I::'i-- the main power switches. This approach, as rJc illustrated in Figure 5, puts all the control on the secondary side and with close DC coupling <RC to the outputs, high accuracy and excellent Cc protection for the load can readily be provided. 0---0 The problem with this method is twofold: First, the transformers which couple the PWM ~ 11-' commands to the power switches handle more VGS ~ than just information -they must also provide nsu drive power to the switches, conditioned to 0--- insure reliable operation under all operating environments. The second complication is that secondary-side control will normally require an isolated power source, adding a third crossing of the isolation boundary and the added com- ponents of this auxiliary bias supply. It is for 1---~ 1---~~--~ Vc these reasons that this approach -while popu- ---i I ~ , ~--AVG lar in the past -seems more recently to be relegated to very high power systems which can , more readily absorb the added overhead cost, ! I. ID I TIME or applications requiring extensive "hand shak- Fig. 4 --Pu/se Transfonner / Peak Detector ing" between the control circuit and output load-grounded logic.

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