NCP1219PRINTGEVB
http://onsemi.com
9
Figure 13. Optocoupler Frequency Analysis Test
Circuit
From Figure 14, the crossover frequency of the
SFH615A
−
3 is measured at 4.7 kHz. From the dc bias of the
optocoupler, the current transfer ratio, CTR is measured as
41%. These values are used in the optocoupler page of the
compensation tool.
Figure 14. Frequency Response of Vishay’s
SFH615A3 Optocoupler
MAG (dB)
PHASE
(
°
)
This data is entered into the tool and the capacitance
contribution of the optocoupler is calculated.
The pole and zero placement of the type 2 compensation
configuration is provided by the design tool based on the
desired crossover frequency and phase margin entered by
the user. If the desired crossover frequency causes the pole
frequency of the compensation network to exceed the pole
frequency of the optocoupler, then the crossover frequency
is automatically reduced.
The total loop response is provided by the design tool
based on the power stage response, optocoupler pole
location, and the type 2 compensation design. The user can
check the frequency response at various input voltages and
load conditions to verify system stability over all conditions,
as shown in Figure 15.
Figure 15. Screenshot of the Total Frequency
Response Given By the Design Tool FLYBACK AUTO
A bill of materials for the compensation network is
provided by the tool based on the calculations of the
compensation network, as shown in Figure 16.
Figure 16. Screenshot of the Final Feedback Network
Bill of Materials
The design tool provides a good starting point; a solution
that allows the user to quickly set up a stable feedback
network. It does not, however, release the designer from
measuring the frequency response of the system and
optimizing the loop stability and transient response
tradeoffs. Using an AP Instruments AP200 frequency
response analyzer, the frequency response of the power
stage is confirmed, as shown in Figure 17. The measured
gain boost required for a crossover frequency of 1 kHz is
17 dB, slightly higher than estimated by the compensation
tool.
