WAC-1020
`
`WAC-1020
`
`

`
`To my wife Annie
`
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`62'l.881'044——~dc21
`
`97-311368C11’
`
`McGra,w-Hill
`A Division ofTl1eMcGraw-Hill Companies
`
`Copyright © 1998 by The McGraw-Hill Companies, Inc. All rights
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`
`567890 DOCIDOC
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`0210
`
`ISBN 0~07—052236-7
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`in ullm: nmlnnmuiml u-man-«on
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`lemw ul am ugu-hum lulu pr iusmlumxl ulmnki ln~ eeml;,gl\l..
`
`

`
`output inductors.
`It can be used from DC input voltages as low as 5 V up to the usual
`rectified 160 V from a 115~V—AC power line by proper choice of the
`secondary to primary turns ratios. By careful design of these turns ra-
`tios, it can also be used off the rectified DC of 160 V from a 115-V—AC
`power line or from the rectified 320 V DC from a 220—V—AC power line
`without resorting to the voltage doubling—-full-wave rectifying scheme
`(switch S1) of Fig. 3.1."
`The latter scheme, although very widely used, has the objectionable
`Feature that to do the switching from 115 to 220 V AC, both ends of
`the switch in Fig. 3.1 have to be accessible on the outside of the supply
`which is a safety hazard. Or the supply must be opened to change the
`switch position. Both these alternatives have drawbacks. An alterna-
`tive scheme not requiring switching will be discussed in Sec. 4.3.5.
`
`4.3 Discontinuous-Mode Flybacks—
`Basic Operation
`
`l\’.¢:('crring to Fig. 4.1, the topology works as follows. Figure 4.1 shows
`:1 master and one slave output. As in all other topologies shown pre-
`viously, a negativefeedback loop will be closed around the master
`V,,,,,. A fraction of Van, will be compared to a reference, and the error
`::i;r;uz1'l will control the Q1 on time pulse width so as to make the sam-
`pl:-(I output equal to the reference voltage for line and load changes.
`’i‘|w slaves will be well regulated against line changes and somewhat
`it ~: ;.~; against load changes. But slave changes with line and load will be
`|l4'l.l.(.‘l‘ than in the previous forward-type topologies.
`In Fig. 4.1, flyback operation can be immediately recognized from
`tho dots on the transformer primary and secondary. VVhen Q1 is on,
`cl: >1. uncle of all windings are negative with respect to their no-dot ends.
`( mtpu t rectifier diodes D1 and D2 are reverse-biased and all the out-
`put loud currents are supplied from storage filter capacitors C1 and
`1
`'15.. ‘l‘hcso will be chosen as described below to deliver the load cur-
`H-ntn with the maximum specified ripple or droop in output voltage.
`imriugg‘ the Q1 on time, there is a fixed voltage across Np and cur~
`wot in it ramps up linearly (Fig. 4.117) at a rate 0fdI/dt = (Vdc —— 1)/LP,
`wl u-H» I,,, is the pr.im:»u'y magnetizing inductance. At the end of the on
`(inn-, the p1',im:.n'_y u11rrom;|l’1as ramped up to II, = (Vac - 1)Tm,/L,,. This
`mm-nl. n*uprv.a.~u.mi.s :1 slmrcd energy of
`
`DC voltage-comro|led
`voriable — width
`pulse generator
`
`<— vac + (Nm/Nplvdc
`
`*‘w Vdc
`
`(d)
`
`gure 4.1 Discontinuous-mode flyback converter. VVhen Q1 is on, all rectifier di~
`les are reverse-biased and all output capacitors supply load currents. Np acts like
`pure inductor and load current builds up linearly in it to a peak 1],. When Q1
`one off, the primary stored energy 1/2LIp2 is delivered to the secondaries to supply
`ad current and replenish the charge on output capacitors which they had lost
`hen Q1 was on. The circuit is discontinuous if the secondary current has decayed
`zero before the start of the next turnon.
`
`ants are not excessive. The feature which makes it Valuable for high
`itput voltages is that it requires no output inductor. In forward con-
`erters, discussed above, output inductors become a troublesome prob—
`:m at high output voltages because of the large voltages they have to
`istain. Not requiring a high voltage freewheeling diode is also a
`[us for the flyback in high-voltage supplies.
`It is also a frequent choice for a supply with many output voltages
`; 10 is not uncommon) in the region of 50 to 150 W. It is attractive For
`multioutput supply ‘because the output voltages truck one another
`ith line and load chan far better thim they do in Llic l'orwzu*-cl
`
`

`
`Now when Q1 turns off, current in the magnetizing inductance
`:‘orces reversal of polarities on all windings. Assume for the moment
`zhat there are no slave windings and only the master secondary N,,,.
`Since the current in an inductor cannot change instantaneously, at
`:he instant of turnoff, the primary current transfers to the secondary
`it an amplitude Is = Ip(Np/Nm).
`After a number of cycles, the secondary DC voltage has built up to a
`nagnitude (calculated below) of Vom. Now with Q1 off, the dot end of
`V,,, is positive with respect to its no-dot end and current flows out of it,
`Jut ramps down linearly (Fig. 4.1c) at a rate dls/dt = Von,/Ls, where Ls
`s the secondary inductance. If the secondary current has ramped
`iown to zero before the start of the next Q1 on time, all the energy
`ltored in the primary when Q1 was on has been delivered to the load
`and the circuit is said to be operating in the discontinuous mode. Since
`in amount of energy E in joules delivered in a time T in seconds rep-
`resents input power in watts, at the end of one period, power drawn
`:‘rom Vdc is
`
`1/2L,,(I,,)2
`P - ——T7—W
`
`3ut Ip = (Vac — 1)T(,,,/LP. Then
`
`P _ l:(Vdc "' 1)Tcn]2 ~ (x/:icTon)2
`P
`P
`”‘
`ZTL
`"
`2TL
`
`W
`
`(4.221)
`
`(4.2b)
`
`as can be seen from Eq. 4.2b, the feedback loop maintains constant
`Jutput voltage by keeping the product Vd,,T,,,, constant.
`
`l.3.1 Relation between output voltage
`rersus input voltage, on time, output load
`
`kssume an efficiency of 80 percent.
`
`Input power = 1.25 (output power)
`
`1.25(V,,)2 _ 1/2(L,,I,,2)
`R,
`‘
`T
`
`3ut Ip = Vd,,T,—,,:/L,, since maximum on time T0,, occurs at minimum
`iupply volgge Vac, as can be seen from Eq 4.2b.
`
`‘Jo "7:
`
`10¢:
`
`>'~
`
`J1)
`
`’l‘lius the fccclback loop will regulate the output by decreasing T0,, as
`i
`\/’,_, or It” goes up, increasing T0,, as Vac or R,, goes down.
`
`4.3.2 Design relations and sequential
`decision requirements
`
`4.3.2.1 Establishing primary/secondary turns ratio. There are a number
`of" decisions which should be made in the proper sequence. The first is
`to choose the primary/master secondary turns ratio Np/Nsm; this de-
`l.¢u'111ln(-ZS the maximum off-voltage stress V,,,,, on the power transistor
`in the absence of a leakage inductance spike. Neglecting the leakage
`::])ik0, the maximum transistor voltage stress at maximum DC input
`V4,. and for a 1-V rectifier drop is
`
`NP
`V,,,,, = Vac + ——-— (V, + 1)
`Nsm
`
`(4.4)
`
`where Vms is chosen sufficiently low so that a leakage inductance
`spike of 0.3Vdc on top of that still leaves a safety margin of about 30
`percent below the maximum pertinent transistor rating (Vceo, Veer, or
`V......v)~
`
`4.3.2.2 Ensuring care does not saturate, circuit remains discontinuous.
`Recall that to ensure that the core does not drift up or down its hys-
`teresis loop, the on vo1t—second product (A1 in Fig. 4.1d) must equal
`the reset volt—second product (A2 in Fig. 4.1d). Assume that the on
`drop of Q1 and the forward drop of the rectifier D2 are both 1 V:
`
`N
`__
`(YE — 1)T,,, = (V. + 1)N" T,
`
`‘
`(4.5)
`
`where T, is the reset time shown in Fig. 4.10 and is the time required
`for the secondary current to return to zero.
`To ensure the circuit operates in the discontinuous mode, a dead
`time (Td, in Fig. 4.10) is established so that the maximum on time
`Ton, which occurs when Vdc is a minimum plus the reset time T, is
`only 80 percent of a full period. This leaves 0.2T margin against un-
`expected decreases in R0, which according to Eq. 4.3 would force the
`feedback loop to increase T0,, in order to keep V0 constant.
`As for the boost regulator (which is also a flyback type), Secs. 1.4.2