Datasheet

Start-Up

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5 Start-Up

Power supplies have a soft-start circuit(s) to control their output voltage when input power is applied. The

soft-start sequence limits the peak inrush current as the output capacitors are charged, and prevents the

output voltage from over-shooting. In most power supplies there are primary side and secondary side soft-

start circuits.

The primary side soft-start circuit is generally in a primary side controller and the soft-start time is set with

an external capacitor. The function of the primary side soft-start circuit is to slowly increasing the duty

cycle of the controller from zero to the maximum duty cycle. The maximum duty cycle varies based on the

controller and the circuit topology.

The secondary side soft-start circuit connects a resistor/capacitor from the secondary side voltage

reference to the positive input of the error amplifier. The soft-start time is set by the resistor/capacitor time

constant and works by ramping up the voltage reference on the secondary side error amplifier. The output

of the error amplifier is fed across the isolation boundary to the primary side controller compensation input

which is connected to the controller PWM input. The voltage at the compensation input increases to a

value required for regulation as determined by the voltage feedback loop. The secondary side soft-start

along with the primary side soft-start work together to control the duty cycle on start-up to controlling the

power supplies output start-up time, and limit the stress on the power components.

Figure 3 shows the primary and secondary side soft-start sequence using the LM5027 into an electronic

load. After Vin is applied the primary side soft-start ramps up. When the voltage on the LM5027 SS pin

reached 1.0 V the output drives start and power is delivered to the secondary of the transformer. The

power supply output rises and the secondary side soft-start circuit begins to ramp-up. The output of the

DC-DC converter monotonically increased with no overshoot to 3.3 V out.

Figure 3. Soft-Start

6 Pre-Bias Load Start-Up

Figure 4 shows a typical Forward Converter topology with an active clamp using self-driven synchronous

rectification. It’s simple and very efficient; however there are some disadvantages when starting this

topology into a pre-biased load. The first occurs because the synchronous rectification is on the secondary

side of the transformer and without adding intelligence the output current will flow into the converter via the

output choke and the free wheeling MOSFET when the converter starts-up or shut down with pre-bias

voltage.

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

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