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Secondary Side Soft-Start Reset

9 Secondary Side Soft-Start Reset

When input power is supplied to the LM5027 Evaluation Board the LM5027’s internal VCC Regulator

turns-on providing power to the VCC pin, the primary side soft-start voltage increases, and the output

drives are enabled. When the drive outputs are enables the voltage on the transformer secondary

increases, the Secondary Bias rises supplying voltage to the reference and error amplifier, refer to

Figure 10. During this time FET Q1 is turned-on holding the reference voltage at the positive input to the

error amplifier low (zero volts). When the voltage on the secondary bias capacitor (C

BIAS

) rises above the

Zener diode> 3.6 V, the Secondary Bias Power Good (the collector of Q2) goes high. This turns-off FET

Q1 allowing the secondary soft-start capacitor to charge up. This solution of reseting the soft-start

capacitor to zero (0 V) on start-up works for pre-bias loads as well as loads that do not need to start into a

pre-biased condition. This allows for a monotonic start-up under both operating modes.

Figure 9. VREF with Pre-Bias Load

SNVA400B–August 2009–Revised May 2013 AN-1976 LM5027 Evaluation Board

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