Application Note AN-1150 - International Rectifier

• Option 2: At the expense of current distortion due to increased 2*f AC ripple in
V OUT , increase only the R gm resistor while retaining the same soft-start time.
This has the effect of placing the zero closer to the pole in the power stage.
For example if H1(s).H2(s) attenuation at 2xfAC can be set at 1.5% instead of
0.5%. Then retaining C z =0.56uF, we can recalculate R gm =10kohm and
recalculate Cp=1.4nF. The cross-over frequency and phase margin are 5Hz &
35° at 85VAC and 20Hz and 42° at 264VAC as seen in Fig.14. While this may
cause some distortion (increased 3 rd harmonic current), the EN61000-3-2
harmonic standards will still easily be met.
90
60
30
0
Gain 85VAC Uncompensated
Gain 85VAC Compensated
EA Gain
Phase 85VAC Uncompensated
Phase 85VAC Compensated
EA Phase
90
60
30
0
Gain (dB)
-30
-60
-90
-120
-150
-30
-60
-90
-120
-150
Phase (deg)
-180
-180
0.01 0.1 1 10 100 1000 10000 100000
f (Hz)
90
60
30
0
Gain 264VAC Uncompensated
Gain 264VAC Compensated
EA Gain
EA Phase
Phase 264VAC Uncompensated
Phase 264VAC Compensated
90
60
30
0
Gain (dB)
-30
-60
-90
-120
-150
-30
-60
-90
-120
-150
Phase (deg)
-180
-180
0.01 0.1 1 10 100 1000 10000 100000
f (Hz)
Fig.14: Overall Loop Gain at 85/264VAC, 350W (fast loop + high phase margin,
but possibly increased current distortion)
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