How to use a simple circuit to achieve a smooth soft-start for an isolated converter

    Most DC/DC converters require a soft-start circuit to limit the inrush current at startup. While systems with a power-on reset (POR) require a smooth soft-start, this is difficult for isolated converters with primary-side controllers and limited duty cycle or current.

    Figure 1 shows a soft start for a forward converter with duty cycle soft start on the primary side. The steady state output of the converter is 12V. A 50% load current is applied at 10V (the system’s POR threshold). As soon as the load is applied, the output drops and triggers system shutdown, causing the system to cycle multiple times. At the end of the soft start, the output is over 10%, which is undesirable.

    Figure 1: Output of a forward converter during startup with a load applied at 10V

    In this article, I will use a simple circuit to implement a smooth soft start of an isolated converter. The circuit is applied to an active clamp forward converter with LM5025 as the controller. Figure 2 shows the concept of secondary side soft start.

    Figure 2: Secondary-side soft-start circuit for an isolated converter

    When the input is first applied, the converter output (V OUT) begins to rise. The capacitor (C SS) is charging. The C SS charging current (I SS) flows through the resistor (R SS). When I SS is high, then it is V BE(on) / R SS. Q SS turns on and begins to pull current from the secondary side composite node (SEC COMP), thus reducing the duty cycle. During the soft start period, 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 into regulation, the error amplifier begins to regulate and I SS decreases. Q SS turns off.

    Equation 1 shows the transfer function from V OUT to the optocoupler current:

    Although effective, this simple circuit can be unstable because the QSS forward gain (β) is high and varies widely between different parts. To stabilize this circuit, insert a gain-reducing resistor (RE) between the emitter of QSS and ground, as shown in Figure 3. Increasing RE reduces the feedback loop gain during startup.

    Figure 3: Adding RE to stabilize the soft-start circuit

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

    At high frequencies, use Equation 3 as an approximation of Equation 2:

    I added a soft-start circuit to the converter using the following parameters:

    C SS = 0.1 μF.

    RSS = 100kΩ.

    RE =1.18kΩ.

    Figure 4 shows the soft-start waveform with these circuit parameters. When the system starts sourcing current, the soft-start circuit stops drawing current from COMP and the duty cycle increases rapidly. After a slight dip caused by the load transient, the converter continues to soft-start.

    Figure 4: Soft start waveform of the soft start circuit, as shown in Figure 3

    Figure 4 also shows that the converter switch node (VSW) has an additional voltage spike after the load is applied. Figure 5 shows the zoomed-in waveform. It is clear that the system oscillates at 9.5kHz.

    Figure 5: Zoomed soft-start waveform with soft-start circuit

    The controller in this design is a voltage mode controller. The power stage has a 180 degree phase drop due to the bipolar. It is necessary to add a zero to improve stability; you can do this by adding a capacitor (CE) in parallel with RE. To add 45 degrees to the phase margin, I put the measured oscillation frequency at 9.5kHz. When RE = 1.18kΩ, I added a 15nF capacitor.

    Figure 6: Soft-start circuit with improved stability

    Figure 7 shows the start-up waveforms with CE = 15nF. The oscillation is eliminated. The total soft-start time is 50ms.

    Figure 7: Soft-start waveform, CE = 15nF

    During soft start, the typical optocoupler diode current (I opto_D) is 1.2mA to 0.8mA. This is determined by the LM5025 and the optocoupler forward gain. When RE = 1.18kΩ, the voltage across R SS is V BE(ON) + RE × 0.8mA = 1.644V. VBE(on) = 0.7V. Therefore, you can calculate I SS as I SS = (V BE(ON) + RE × I opto_D) / R SS. 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 much faster than the secondary side soft start.

    The test results show that this simple soft-start circuit effectively realizes the smooth soft-start of the isolated converter.