Circuit Ensures Smooth "Soft Start" for Isolated Converter, Limits In-Rush Current

Mangjing Xie

Electronic Design

Isolated dc-dc converters are prone to excessive in-rush current, but a small soft-start circuit can solve the problem and limit that current.

Most dc-dc converters require a soft-start circuit to limit the in-rush current at startup. Although a smooth soft start is required for systems with power-on reset (POR), this is difficult to implement in an isolated converter with a controller on the primary side and a limited duty cycle or current.

Figure 1 shows the soft start of a forward converter with a duty-cycle soft start from the primary side. The steady-state output of the converter is 12 V. A 50% load current is applied at 10 V, which is the POR threshold of the system. As soon as a load is applied, the output drops and triggers system shutdown, causing the system power to cycle several times. At the end of soft start, the output overshoots 10%, which is not desirable.

Figure 1.	The output of a forward converter during startup mode, with a load applied at 10 V, shows the system power-cycling and the output overshooting by about 10%.

A simple circuit can implement a smooth soft start for an isolated converter in a system using the LM5025 active-clamp, voltage-mode PWM controller. Figure 2 shows the concept of this secondary-side soft-start approach.

Figure 2.	The basic secondary-side soft-start approach adds an auxiliary circuit to the secondary-side compensation node.

When first applying the input, the converter output (V_OUT) starts to rise, capacitor C_SS is charging up, and the C_SS charging current (I_SS) flows through resistor R_SS. When I_SS is higher than V_BE(ON)/R_SS, Q_SS then turns on and starts to pull current from the secondary-side comp node (SEC COMP), thus reducing the duty cycle. During soft start, the error amplifier saturates and the soft-start circuit dominates the feedback loop. The converter, C_SS, R_SS, Q_SS, and optocoupler form a closed loop. When the output rises to regulation, the error amplifier starts to regulate and I_SS goes down, thus turning Q_SS off.

Equation 1 shows the transfer function from V_OUT to the optocoupler current:

(1)

While effective, this simple circuit might not be stable because the Q_SS forward gain β is high and varies dramatically from part to part. To stabilize this circuit, you need to insert a gain-reducing resistor R_E between the emitter of Q_SS and ground (Fig. 3). Increasing R_E can reduce the feedback-loop gain during startup.

Figure 3.	Adding gain-stabilizing resistor RE stabilizes the soft-start circuit.

Equation 2 shows the soft-start circuit transfer function with R_E:

(2)

At high frequencies, Equation 3 is an approximation of Equation 2:

(3)

For testing, the soft-start circuit to the converter was set up with C_SS = 0.1 µF, R_SS = 100 kΩ, and R_E = 1.18 kΩ. Figure 4 shows the soft-start waveform with these circuit parameters. When the system starts pulling current, the soft-start circuit stops drawing current from the COMP and the duty cycle increases quickly. The converter continues to soft start after a minor dip caused by the load transient.

Figure 4.	The soft-start waveform shows the circuit stops drawing current from the COMP pin; as a result, the duty cycle increases and the converter continues the soft-start process.

The figure also shows that after the load is applied, the converter switching node (VSW) has an additional voltage spike. Figure 5 shows the zoomed-in waveform, which reveals that the system oscillates at 9.5 kHz.

Figure 5.	The zoomed-in soft-start waveform shows system oscillation at 9.5 kHz, which isn’t acceptable.

The controller in this design is a voltage-mode controller. The power stage has 180° of phase drop because of the double poles. It’s necessary to add a zero to improve stability, done by adding capacitor C_E in parallel with R_E(Fig. 6). In order to add 45° to the phase margin, a zero was placed at 9.5 kHz (the measured oscillation frequency). With R_E = 1.18 kΩ, this required a 15-nF capacitor.

Figure 6.	Adding a zero improves stability, here done by placing capacitor CE in parallel with RE to add 45° to the phase margin.

Equation 4 shows the soft-start circuit transfer function with R_E and C_E in parallel:

(4)

Figure 7 shows the startup waveform with C_E = 15 nF. Note that the oscillation is eliminated, while the total soft-start time is 50 ms.

Figure 7.	Adding the zero via the small additional capacitance eliminates the oscillation;the final circuit has an overall soft-start time of 50 ms.

During soft start, the typical optocoupler diode current (I_{OPTO_D}) is between 1.2 and 0.8 mA, which is determined by the LM5025 and the optocoupler forward gain. With R_E = 1.18 kΩ, the voltage across R_SS is

Since V_BE(ON) = 0.7 V, you can calculate I_SS as

I_SS/C_SS sets the output V_OUT, dv/dt. To ensure the effectiveness of the secondary-side soft start, the primary-side soft start should be set to be much faster than the secondary-side soft start.

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