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1 INTRODUCTION
When a power supply is first switched on, either from the line input switch or by electronic means (say from a TTL logic "high" signal), there will be a delay while the power and control circuits establish their correct working conditions. During this period, it is possible for the output voltage to exceed its correct working value before full regulation is established, giving a "turn-on voltage overshoot."
2 TYPICAL CAUSES OF TURN-ON VOLTAGE OVERSHOOT IN SWITCHMODE SUPPLIES
In most switchmode power supplies, a controlled start-up sequence is initiated at switch-on.
Should the turn-on be from a line input switch, the first action will be "inrush limiting," where a resistive element in series with the line input reduces the peak inrush currents for a few cycles while the input capacitors are charged up.
Following this inrush limiting, there will be a soft-start action. For soft start, the converter action to the power switching devices is progressively increased to establish the correct working conditions for transformers, inductors, and capacitors. The voltage on the output capacitors is progressively increased with the intention of smoothly establishing the required output voltage. However, even under this controlled turn-on condition, it is possible for the output voltage to overshoot as a result of race conditions in the control circuit as follows.
FIG. 1 shows the output filter and control amplifier of a typical duty-cycle controlled switchmode power supply. The control amplifier has a simple pole-zero compensation network to stabilize the loop.
When the input voltage is first applied to this supply, and throughout the start-up phase, the control amplifier A1 will recognize the output voltage as being low, and will demand maximum output and hence maximum pulse width from the ramp comparator A2. The high-gain-control amplifier A1 will be operating in a saturated "high" state, with its output near +5 V. Meanwhile, as the output voltage increases toward its regulated output level of 5 V, the inverting input of A1 approaches 2.5 V. Hence, at the end of the start-up phase, the compensation capacitor C1 will be charged to +2.5 V.
During this start-up phase, the pulse width and hence the output voltage will be under the control of the soft-start circuit and amplifier A3. Therefore the control amplifier will remain in its saturated "high" state until the output voltage is within 1 or 2 mV of the required value. At this point, the output capacitors have been charged and a considerable current has been established in the output inductor L1.
As the output voltage passes through the required value, the control amplifier A1 will start to respond. However, a considerable delay will now ensue while the compensation network R1, C1 establishes its correct DC bias. Since the output voltage of amplifier A1 starts near +5 V (far away from the correct mean working point of 2.5 V), and the slew rate of the amplifier is defined by the time constant of R1 and C1, the correct amplifier working conditions are not established for a considerable period. (In this example, the delay will be approximately 500 µs.) During the delay period, the pulse width will not be significantly reduced, as the output of amplifier A1 must be close to 2.5 V before it comes within the control range of the pulse-width modulator A2. This delay, together with the excess current now flowing in the output inductor L1, will cause a considerable overshoot. (The output voltage will go to 7.5 V in this example, as shown in FIG. 2.)
3 OVERSHOOT PREVENTION
The overshoot can be considerably reduced by making the soft-start action very slow, allowing the amplifier to take over before the overshoot is too large. This has the disadvantage that the turn-on delay can be unacceptably long.
FIG. 1 Typical duty ratio control loop, showing voltage control
amplifier with compensation components R1 and C1.
FIG. 2 Output voltage characteristic of the circuit in FIG. 1
during the "turn-on" transient, showing output voltage overshoot.
FIG. 3 Modified control circuit, showing "turn-on" overshoot
prevention components R1, D1, D2, and C1.
A much better arrangement is the linear power control circuit shown in FIG. 3. In this circuit the 2.5-V reference voltage for the control amplifier will be near zero at the noninverting input to the amplifier when first switched on, as C1 will be discharged prior to initial switch-on. The voltage on C1 will progressively increase as it charges via R1 and R4. Thus the reference voltage is arranged to increase at a rate somewhat slower than the soft-start action. As a result, the control amplifier will establish its normal working conditions at a much lower output voltage so that the latter part of the turn-on action is under full control of the voltage control amplifier A1.
The output voltage now increases progressively, as shown in FIG. 4, in response to the increasing reference voltage, under the full command of the control amplifier. Since the correct bias conditions for C2 and amplifier A1 were established at a much lower voltage, there will not be an overshoot when the correct voltage has been established. For optimum turn-on characteristic via selection of R1, R4, and C1, the change in the reference and hence the output voltage is nearly asymptotic to the required 5-V value. Typical turn-on characteristics of this type of circuit are shown in FIG. 4. Small values of C1 will give underdamped and large values of C1 overdamped performance. The same principle can be applied to any switchmode or linear control circuit.
FIG. 4 "Turn-on" characteristics of modified circuit,
showing underdamped, overdamped, and optimum response.
4 QUIZ
1. Give a typical cause of "turn-on" output voltage overshoot in switchmode supplies.
2. Give two methods of reducing "turn-on" output voltage overshoot.
Also see: Our other Switching Power Supply Guide