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Controllable switching power supply soft start circuit application design
In switching power supplies, soft-start and surge suppression serve different purposes, although they both aim to reduce inrush current during the initial power-on phase. Surge suppression directly limits the current flowing into the input capacitor, while soft-start gradually increases the load by adjusting the converter’s control circuit, typically through increasing the pulse width. This gradual start-up not only reduces stress on the output capacitor and internal components but also helps prevent the "double fluxing" issue in push-pull or bridge configurations.
Typically, an AC power supply is connected to a rectifier, with a large filter capacitor linked via a low-impedance noise filter. To avoid large inrush currents when the system is first powered on, a surge control circuit is usually implemented. In high-power systems, this may involve a series resistor that is later bypassed using a triac, SRC, or relay once the input capacitor is fully charged.
To ensure the input capacitor is fully charged before the power converter starts, a delay is often introduced in the startup sequence. If the capacitor isn’t fully charged, a sudden current surge can occur when the surge suppression resistor is bypassed. Moreover, if the converter begins at maximum pulse width, it could lead to excessive current surges into the output inductor and capacitor, potentially causing voltage overshoot due to inductor current and transformer saturation.
To address these issues, the control circuit usually includes a startup delay and a soft-start procedure. This allows the input capacitor to charge fully before the converter starts, and then slowly ramps up the output voltage from zero. This approach ensures the transformer and inductor operate correctly, avoiding the "double fluxing" effect in push-pull circuits. The slower voltage rise also reduces the risk of secondary-side inductor current surges and minimizes output voltage overshoot.
The soft-start circuit is illustrated in Figure 1.9.1. When the power supply is first turned on, C1 discharges. As the voltage on the 10V line rises, the inverting input of amplifier A1 becomes positive, preventing the pulse width modulator from outputting. Transistor Q1 conducts through R2, keeping C1 in discharge mode until the DC line voltage reaches over 200V.
At this point, ZD1 turns on, turning off Q1. C1 then charges through R3, bringing the inverting input of A1 back to zero. The pulse width modulator then starts providing pulses with increasing width to the drive circuit, gradually building up the output voltage. Once the correct output voltage is achieved, A2 controls the inverting input of A1. C1 continues to charge through R3, reverse biasing D2 and isolating it from the modulator. When the power is turned off, C1 quickly discharges through D3, resetting it for the next startup. When the input voltage is high, D1 prevents Q1 from being reverse-biased beyond the forward diode voltage drop.
Figure 1.9.1 shows a controllable duty cycle soft-start circuit for the switching power supply. It provides an on-delay and a soft-start function, ensuring the transformer doesn’t start before the power supply is fully established. This basic principle can vary in design. For example, Figure 1.9.2 illustrates a soft-start system applied to the transistor startup circuit in Figure 1.8.2. Here, ZD2 goes high once the auxiliary capacitor C3 is charged, allowing Q1 to turn off and initiating the soft-start process. In this configuration, both the main and auxiliary power supply voltages must be stable before the soft-start begins, ensuring proper control conditions during startup.