Is Lt1170 Substitute For Lt1070

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** Temperature Ranges: C = 0°C to +70°C, E = 40°C to +85°C, M = -558C to +125°C

t Prices provided are for design guidance and are POB USA. International prices will differ due to local duties, taxes, and exchange rates, tt Future product—contact factory for pricing and availability. Specifications are preliminary.

Figure 17.16 (Continued)

manufacturers' devices. This discrepancy is due only to the author's earlier familiarity with LTC products.

17.4 Distributed Power Systems with IC Building Blocks7

Figure 17.17a shows a conventional off-line, multi-output power supply. The alternating current is rectified (with or without power factor correction), and some topology—half, full bridge, forward converter, or flyback—is used to generate a precisely controlled master output voltage on output ground. This master is generally the highest current output—usually +5 V—and is very well regulated against line and load current changes. It is very well regulated because of the feedback loop around it which controls the on time of the power switching devices on input ground.

Additional outputs (referred to as slaves) are obtained by adding more secondaries whose turns are selected to yield the desired secondary DC voltages. Since the time the slave secondary outputs are high is the same as that of the master secondary, the DC voltages of the slaves after their LC filters are also very well regulated against line input voltage changes.

But slaves are not well regulated against load current changes in either the master or the slave outputs (Sees. 2.2.1 to 2.2.3). Slave regulation against line and load changes is generally only as good as ±5 to 8 percent and may change as much as 50 percent if either the master or slave inductors are permitted to go into discontinuous mode (Sec. 2.2.4). Further, the slave absolute DC voltages cannot be set precisely, as the smallest amount by which they can be changed is that corresponding to adding or removing a single turn. Since volts per turn is proportional to the switching frequency from Faraday's law [EIN = Ae (dBitJiW 8) = Ae(dB X 0.4f)(10'8)] at high frequencies, E/N can amount to 2 to 3 V per turn depending on the flux change and core iron area.

These DC slave voltages are, of course, generated physically close to the main power transformer and are piped around to their point of use.

But generally the poor regulation of and the inability to set accurately the absolute DC voltage of the slaves are not too much of a problem. Slaves are used to power operational amplifiers or motors for various computer peripherals, and these can tolerate large output voltage changes.

But in cases where slaves must be accurate and well regulated, they must be controlled by a separate feedback loop around them. This often is done by following the poorly regulated slave with either a linear regulator for low current, a buck regulator for higher current, or a magnetic amplifier (Sec. 10.3).

V0(Slave 1)

V0(Slave 2)

V0(Slave 3)

V0( Master)

Figure 17.17 (a) Conventional scheme for building a multioutput power supply. A feedback loop around a master (highest current output) regulates it against line and load changes. Secondaries on the power transformer yield slaves which are well regulated against line changes but only ±8 percent for load changes.

V0(Slave 1)

V0(Slave 2)

V0(Slave 3)

V0( Master)

Figure 17.17 (a) Conventional scheme for building a multioutput power supply. A feedback loop around a master (highest current output) regulates it against line and load changes. Secondaries on the power transformer yield slaves which are well regulated against line changes but only ±8 percent for load changes.

Distributed power is an alternative which corrects this problem of inaccurate slaves, and it has additional significant advantages, as shown in one of its versions in Fig. 17.176.

The essence of distributed power is that it generates a common DC voltage (not necessarily well regulated) at a central point and buses it around to the points of use. There, standardized well-regulated DC/DC converters—buck, boost, or polarity inverters—convert it to the desired voltage. The availability of the above-described LTC and Maxim standard regulators makes them immediate candidates for such a distributed power scheme.

Consider the following advantages of one version of distributed power shown in Fig. 17.176. In that figure, the highest current output, usually +5 V, is generated directly by the main power transformer which has only the one secondary shown. It is controlled by a feedback loop from a sample on the output ground around to control the on time of the power transistor on the input ground. All other outputs are derived from boost or polarity inverters, each having their own feedback

Figure 17.17 (Continued) (b) Distributed power scheme. Here the transformer has only one secondary which is regulated very well against line and load changes. That output is bussed around and generates the slave voltages at the point of use through standardized DC/DC converters operating either as boost, buck, or polarity inverters. The master may not even require regulation by PWM control of the primary side power transistor. That transistor may operate at a fixed on time, and the master regulation may be derived from its own DC/DC converter for sufficiently low output currents.

Figure 17.17 (Continued) (b) Distributed power scheme. Here the transformer has only one secondary which is regulated very well against line and load changes. That output is bussed around and generates the slave voltages at the point of use through standardized DC/DC converters operating either as boost, buck, or polarity inverters. The master may not even require regulation by PWM control of the primary side power transistor. That transistor may operate at a fixed on time, and the master regulation may be derived from its own DC/DC converter for sufficiently low output currents.

loop. In the figure, all the slaves are generated by LTC1174 boost regulators. The advantages are as follows:

1. Simpler and less expensive main power transformer. That transformer is generally the largest and most expensive element in a switching power supply. With fewer secondary windings, it is easier to meet VDE safety specifications.

2. Ease of changing electrical parameters of the transformer. In the conventional scheme of Fig. 17.17a, the initial transformer design often requires repeated versions. Some windings may require adding or removing turns. Leakage or magnetizing inductance may be improper in an initial prototype. Winding sequence may have to be changed to improve coupling between windings or reduce proximity-effect losses.

In the scheme of Fig. 17.176, each output has its own feedback loop, and the exact input voltage is unimportant as the output voltages are constant generally over a 3:1 input voltage range.

3. Ease of changing output voltages or currents without changing the main transformer design.

4. Ease of adding one or a number of new output voltages. Often in a large system design, it is found late in the design that some new voltages must be added.

5. Possibility of totally eliminating feedback from one master on output ground around to controlling the pulse width on input ground. This avoids all the problems of sensing an output voltage on output ground and controlling a pulse width on input ground.

It thus makes unnecessary optocouplers with their troublesome gain variation with temperature, low-power housekeeping supplies on output ground, and a small power transformer to couple a pulse on output ground around to the power transistor on input ground.

All this becomes possible by generating the usual +5-V high current output also entirely from a secondary standard buck converter. Thus, instead of generating +5 V directly from a secondary winding, generate an unregulated voltage of about +20 to +24 V and bus this around to the point of use. There the lower-current slaves could be bucked down to their desired value with LT1074-type buck regulators. The higher-current 5 or 3.3 V could be bucked down from +24 V by high-efficiency LT1270A or LTC1159 regulators.

This scheme using only secondary side regulation would not need pulse width modulation of the power transistor. It would just require setting the controller pulse width to about a constant 85 percent of a half period and using peak rectification with only a capacitor filter in the single secondary. Variation of the peak rectified DC voltage due to line and load changes and ripple at the filter capacitor would be taken care of by the secondary regulators.

In general, in any such distributed power scheme, it is best to choose a relatively high bus voltage such as +20 to +25 V and buck it down to the desired values, rather than a low bus of +5 V and boost it up to the desired voltages. The only time a low bus of +5 V makes sense is when the +5-V current is high—in the vicinity of, say, 10 to 100 A—as such high currents are not usually from DC/DC converters.

Although a distributed power scheme may be more expensive and may dissipate somewhat more power (since the power is handled twice), the above advantages and the quick turnaround time in developing a new multi-output power supply may outweigh its disadvantages.

References

1. Linear Technology LT1070 and LT1074 data sheets.

2. LT1170 data sheet.

3. Carl Nelson and Jim Wilson, LTC Application Note 19, Linear Technology.

4. LT1170 data sheet.

5. Linear Technology Power Solutions, 1997.

6. Carl Nelson, LTC Application Note 44, Linear Technology.

7. R. Mammano, "Distributed Power Systems," Unitrode Corp. publication.

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  • lete
    Is lt1170 substitute for lt1070?
    2 years ago

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