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1 INTRODUCTION
Soft-start action is quite different from the inrush limiting discussed in Section 7, although the two functions are complementary. Both actions reduce the inrush current to the sup ply during the initial switch-on period. However, whereas inrush limiting directly limits the current into the input capacitors, soft start acts upon the converter control circuit to give a progressively increasing load, typically by increasing the pulse width. This progressive start not only reduces the inrush current stress on the output capacitors and converter components, it also reduces the problems of transformer "flux doubling" in push-pull and bridge topologies. It is normal practice with switchmode supplies to take the line input directly to the rectifier and a large storage and/or filter capacitors via a low-impedance noise filter. To prevent large inrush currents on initial switch-on, inrush-control circuitry is normally provided. In large power systems, the inrush limiting often consists of a series resistor that is shorted out by a triac, SCR, or relay when the input capacitors are fully charged. (Part 1, Section 7 shows typical inrush-control circuits.) To allow the input capacitors to fully charge during start-up, it is necessary to delay the start-up of the power converter so that it does not draw current from the input capacitors until these are fully charged. If the capacitors have not been fully charged, there will be a current surge when the inrush-control SCR or triac operates to bypass the inrush-limiting series resistor. Furthermore, if the converter was allowed to start up with maximum pulse width, there would be a large current surge into the output capacitors and inductors, resulting in overshoot of the output voltage because of the large current in the output inductor, and possibly saturation effects in the main transformer.
To deal with these start-up problems, a start-up delay and soft-start procedure are usually provided by the control circuit. This will delay the initial switch-on of the converter and allow the input capacitors to fully charge. After the delay, the soft start control circuit must start the converter from zero and slowly increase the output voltage. This will allow the transformer and output inductor working conditions to be correctly established. This will prevent "flux doubling" in push-pull circuits. As the output voltages are slowly established, secondary inductor current surge and the tendency for output voltage overshoot are reduced.
2 SOFT-START CIRCUIT
A typical soft-start circuit is shown in FIG. 9.1. This operates as follows:
FIG. 9.1 Soft-start circuit for duty-cycle-controlled SMPS.
When the supply is first switched on, C1 will be discharged. The increasing voltage on the 10-V supply line will take the inverting input of amplifier A1 positive, inhibiting the output of the pulse-width modulator. Transistor Q1 will be turned on via R2, keeping C1 discharged until the 300-V DC line to the converter circuit has been established to a voltage exceeding 200 V.
At this point ZD1 will start to conduct and Q1 will be turned off. C1 will now charge via R3, taking the voltage on the inverting input of A1 toward zero and allowing the output of the pulse-width modulator to provide progressively wider pulses to the drive circuit until the required output voltage has been developed.
When the correct output voltage has been established, amplifier A2 takes over control of the voltage at the inverting input of amplifier A1. C1 will continue charging via R3, reverse-biasing diode D2 and removing the influence of C1 from the modulator action.
When the supply is turned off, C1 will quickly discharge through D3, resetting C1 for the next start action. D1 prevents Q1 being reversed-biased by more than a forward diode drop when the input voltage is high.
This circuit not only provides turn-on delay and soft start, but also gives a low-voltage inhibit action, preventing the converter from starting until the supply voltage is fully established.
Many variations of this basic principle are possible. FIG. 9.2 shows a soft-start system applied to the transistor start circuit of FIG. 8.2. In this example, the input to ZD2 will not go high and initiate soft start until the auxiliary capacitor C3 has charged and Q1 turned off. Hence, in this circuit, the input and auxiliary supply voltages must be correctly established before the soft-start action can be initiated. This will ensure that the converter starts under correctly controlled conditions.
3 LOW-VOLTAGE INHIBIT
In many switchmode designs it is necessary to prevent power converter action when the input supply voltage is too low to ensure proper performance.
The converter control, drive, and power switching circuits all require the correct sup ply voltage to ensure a well-defined switching action. In many cases, attempts to operate below the minimum input voltage will result in failure of the power switches because of ill-defined drive conditions and nonsaturated power switching.
Normally, the same voltage inhibit signal that prevents the initial start-up action until the supply voltage is high enough to ensure correct operation will also be used to shut the converter down in a well-defined way should the voltage fall below a second minimum voltage.
The low-voltage inhibit circuitry is often linked to the soft-start system, so that the unit will not turn on by normal soft-start action until the correct operating voltage has been established. This also provides the delay required on the soft-start action and prevents start-up race conditions.
A typical soft-start circuit with a low-voltage inhibit is shown in FIG. 9.2. In this circuit, sufficient hysteresis action is provided by the auxiliary winding to prevent squegging at the turn-on threshold. (In this context, "squegging" refers to the rapid "on-off " switching action that would otherwise occur at the low-voltage threshold as a result of load-induced input voltage changes.)
4 QUIZ
1. Under what conditions may an impulse-type start circuit be considered a suitable start technique?
2. Under what conditions would impulse start circuits not be considered suitable?
3. What is the function of a soft-start circuit as opposed to inrush limiting?
4. What is the function of input low-voltage inhibit in switchmode applications?
FIG. 9.2 Combined low-dissipation transistor auxiliary start circuit,
with duty ratio control (pulse-width modulator) and soft-start characteristic.
Also see: Our other Switching Power Supply Guide