AN-31
PI-2878-032603
60
270
240
210
1-Gain
56 dB Loop Gain
Z1
50
40
Z2
Z3
Z4
0 Degrees Phase
180 Degrees Phase Margin
180
150
120
90
30
20
1-Phase
Gain Margin
20 dB
10
0
0 dB Gain
60
-10
-20
Phase Margin
60 Degrees
30
0
180 Degrees Phase
0 Degrees Phase Margin
-30
-40
-30
P3
P4
P5
P1
P2
P6
-60
-90
-50
-60
0.1
1
10
100
1 k
10 k
100 k
Frequency (Hz)
Figure 10. Gain and Phase of a Typical Feedback Loop for DC-DC Forward Converter with DPA-Switch. Markers Show Locations of Major
Poles and Zeros.
TheoutputcapacitorESRcontributesazerothatcompensatesfor
one of the poles from the filter. However, for low ESR tantalum
or organic electrolyte capacitors, this zero usually occurs too
high in frequency to substantially offset the effects of the filter
withinthedesiredloopbandwidth.Intheprototypeexample,the
output filter capacitors are 100 µF, with a maximum specified
ESR of 100 milliohms. The ESR zero is thus at approximately
16 kHz, well beyond the 4 kHz LC filter resonant frequency.
Actual ESR is approximately 80 milliohms, placing the zero
typically at 20 kHz. In situations where standard low ESR
electrolytic capacitors can be used, the higher ESR may place
the ESR zero at a sufficiently low frequency to add significant
additional phase margin.
required for tight DC voltage regulation. Not shown in the
circuit diagram is the ESR of the output capacitors. The ESR
is also an important element in the frequency compensation of
the feedback loop.
Output LC Filter
Thefilterformedbytheoutputinductorandtheoutputcapacitors
contributestwopolestotheloopresponseatthefilterʼsresonant
frequency. Since the filter is a resonant circuit with relatively
low loss, the gain and phase change rather abruptly near the
resonant frequency. Consequently, the poles and zeros for
shaping the loop response should either avoid this region or
compensate for the resonance.
DPA-Switch Compensation
Proper placement of the resonant frequency of the output
filter will avoid complications in the design of the feedback
loop. The position of the resonant frequency should allow the
designer to shape the desired response with a limited number of
compensationcomponentsofreasonablesize.Therecommended
resonant frequency for an output filter that uses low ESR
tantalum capacitors in a forward converter with DPA-Switch
and optocoupler feedback is between 4 kHz and 6 kHz. This
value is consistent with the inductor and capacitor values for
desirable ripple current and ripple voltage.
ThenetworkofC6andR4attheCONTROLpinofDPA-Switch
provides compensation for the feedback loop in addition to
other functions. The capacitance of C6 with R4 and its own
ESR plus the impedance of the CONTROL pin impedance
provide a pole in the loop gain, followed by a zero from R4
and the ESR of C6.
Suggested values of C6 are between 47 µF and 100 µF. This
range of values will generally be sufficient to provide desirable
C
12 7/04
POWERINT [ Power Integrations ]
