Application Note AN-1150 - International Rectifier

Phase Margin Discussion:
The zero in the error amplifier compensation can provide phase boost to
compensate the phase lag due to the power stage pole. If the zero is more than a
decade away from the power stage pole there is minimal phase boost. The
location of the zero can be brought closer to the power stage pole by increasing
the value of C z or R gm or both. However, the trade-offs are as follows:
• Increasing C z reduces the DC gain of the transfer function and slows
down the loop response (more sluggish response to a load step)
• Increasing R gm increases the low frequency gain of the error amplifier
transfer function and hence the attenuation at 2xf AC may be insufficient to
meet the 0.5% requirement described earlier.
To illustrate the trade-offs, the following examples are presented with the goal of
improving the phase margin:
• Option 1: Increase the soft-start time. For example, if the soft-start time is
increased to 180ms (3x), then Cz=1.69uF, Rgm=3.49kohm, Cp=4.2nF. The
cross-over frequency and phase margin are 2.6Hz & 47° at 85VAC and 9.5Hz
and 33° at 264VAC as seen in Fig.13. Due to the higher C z capacitor, the
transient response behavior is likely to be more sluggish for this
compensation arrangement.
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
90
0.1 1 10 100 1000 10000 100000
90
f (Hz) Gain 264VAC Uncompensated
60
Gain 264VAC Compensated
60
EA Gain
30
EA Phase
Phase 264VAC Uncompensated
30
Phase 264VAC Compensated
0
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.13: Overall Loop Gain at 85/264VAC, 350W (slow loop + high phase margin)
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