AN-31
Efficiency Gain Over Diode
Rectification
Output Voltage
L2
5 V
+3%
+6%
+8%
R15
3.3 V
2.5 V
Q1
D3
Q2
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L2
Table 3. Efficiency Gain vs. Output Voltage for Synchronous
Rectification.
(a)
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R16
The forward rectifier MOSFET Q2 turns on when the
DPA-Switch turns on to apply the DC input voltage across the
primary winding. The direction of current in Q2 is from source
to drain. When the DPA-Switch turns off, the reset voltage on
the transformer forces a negative gate-to-source voltage on Q2
andapositivegate-to-sourcevoltageonQ1. SchottkydiodeD3
conductsuntilthegate-to-sourcevoltageonQ1risessufficiently
to exceed the threshold voltage.
R15
Q1
D3
D4
Q2
PI-3475-062204
(b)
Figure 12. Synchronous Rectification (a) Winding Driven DC
Coupled. (b) Winding Driven AC Coupled.
Suitable MOSFETs for this applicationhave threshold voltages
typicallybetween4Vand5V. Thepermissiblemaximumgate-
to-source voltage is usually 15 V to 20 V. These restrictions
limit the range of input voltage for converter operation.
output. For a 5 V output, an efficiency of 85% with Schottky
rectifierswouldtypicallygoto90%orhigherwithsynchronous
rectifiers. Synchronousrectificationgivesthebenefitofgreater
efficiency at lower output voltages as shown in Table 3.
TheintegratedlineovervoltagefeatureofDPA-Switchsimplifies
the design of winding driven synchronous rectifiers. In most
cases it eliminates the need for Zener diodes to protect the gates
of the MOSFETs from excessive voltage. Excess voltage will
not appear on the secondary of the transformer because the
DPA-Switch will not operate when the input voltage is too
high.
DPA-Switch has features that can simplify the design of
synchronous rectifier circuits that are in common use. Circuits
for synchronous rectification with DPA-Switch fall into three
categories of increasing complexity.
a) Winding Driven DC Coupled
b) Winding Driven AC Coupled
c) Actively Driven
DC coupling of the gates in this configuration permits a mode
of operation that may not be desirable in some applications.
Duringshutdown, thevoltageacrosstheoutputinductorwillgo
to zero after its current decays to zero. The remaining output
voltage will then appear across Q1 and D3.
The first two are shown in Figures 11 and 12. MOSFETs Q1
andQ2conductatappropriatetimestoreducethevoltagedrops
associated with the output rectifiers of a forward converter. Q2
performsthefunctionoftheforwardrectifier. Q1operatesasthe
catch rectifier with a parallel Schottky diode. The voltage drop
of each synchronous rectifier is dominated by the on-resistance
of the MOSFETs multiplied by the RMS load current, rather
than by the average current times the minimum voltage of a
Schottky barrier.
If the output voltage is high enough (above the gate threshold
of Q2) it will turn on Q2, allowing reverse current to flow
through L2 and the transformer secondary. The voltage on the
secondarywindingwillsaturatethetransformer,abruptlyturning
off Q2 and generating a voltage spike on the gate of Q1. This
voltage spike may exceed the rated gate voltage for Q1. This
behaviorcanoccurinanydesignusingthisformofsynchronous
rectification with an undervoltage lockout. It is not specific to
DPA-Switch. A solution to this issue is offered below.
Winding Driven DC Coupled Synchronous Rectifier
The simplest way to drive synchronous rectifiers with
DPA-Switch is shown in Figure 12 (a). The gate-to-source
voltage that turns on the MOSFETs is essentially the voltage
at the secondary winding of the transformer. The channel of the
MOSFET will conduct as long as the gate-to-source voltage
exceeds the threshold voltage.
Winding Driven AC Coupled Synchronous Rectifier
The AC coupled circuit of Figure 12 (b) eliminates the high
voltagespikebylimitingtheon-timeofQ2suchthatsignificant
reverse current cannot flow through L2 and the secondary
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