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1 INTRODUCTION
In "direct-off-line" switchmode supplies, where minimum size and cost are a major consideration, it is common practice to use direct-off-line semiconductor bridge rectification with capacitive input filters to produce the high-voltage DC supply for the converter section.
If the line input is switched directly to this type of rectifier capacitor arrangement, very large inrush currents will flow in the supply lines, input components, switches, rectifiers, and capacitors. This is not only very stressful on these components, it may also cause interference with other equipment sharing a common supply line impedance.
Various methods of "inrush current control" are used to reduce this stress. Normally these methods include some form of series limiting resistive device in one or more of the supply lines between the input point and the reservoir capacitors.
These limiting devices usually take one of the following three forms: series resistors, thermistor inrush limiting, and active limiting circuits.
2 SERIES RESISTORS
For low-power applications, simple series resistors may be used, as shown in FIG. 7.1.
However, a compromise must be made, as a high value of resistance, which will give a low inrush current, will also be very dissipative under normal operating conditions.
Consequently, a compromise selection must be made between acceptable inrush current and acceptable operating losses.
The series resistors must be selected to withstand the initial high voltage and high current stress (which occurs when the supply is first switched on). Special high-current surge-rated resistors are best suited for this application. Adequately rated wirewound types are often used, however. If high humidity is to be expected, the wirewound types should be avoided. With such resistors, the transient thermal stress and wire expansion tend to degrade the integrity of the protective coating, allowing the ingress of moisture and leading to early failure.
FIG. 7.1 shows the normal positions for the limiting resistors. Where dual input voltage operation is required, two resistors should be used in positions R1 and R2. This has the advantage of effective parallel operation for low-voltage link positions and series operation for high-voltage link positions. This limits the inrush current at similar values for the two conditions.
Where single-range input voltages are used, then a single inrush-limiting device may be fitted at position R3 at the input of the rectifiers.
FIG. 7.1 Resistive inrush limiting circuit. (Suitable for bridge and
voltage doubler operation, maintaining the inrush current at the same
value.)
3 THERMISTOR INRUSH LIMITING
Negative temperature coefficient thermistors (NTC) are often used in the position of R1, R2, or R3 in low-power applications. The resistance of the NTCs is high when the supply is first switched on, giving them an advantage over normal resistors. They may be selected to give a low inrush current on initial switch-on, and yet, since the resistance will fall when the thermistor self-heats under normal operating conditions, excessive dissipation is avoided.
However, a disadvantage also exists with thermistor limiting. When first switched on, the thermistor resistance takes some time to fall to its working value. If the line input is near its minimum at this time, full regulation may not be established for the warmup period. Further, when the supply is switched off, then rapidly turned back on again, the thermistor will not have cooled completely and some proportion of the inrush protection will be lost.
Nevertheless, this type of inrush limiting is often used for small units, and this is why it is bad practice to switch SMPSs off and back on rapidly unless the supply has been designed for this mode of operation.
4 ACTIVE LIMITING CIRCUITS (TRIAC START CIRCUIT)
For high-power converters, the limiting device is better shorted out to reduce losses when the unit is fully operating.
Position R1 will normally be selected for the start resistor so that a single triac or relay may be used. R1 can be shunted by a triac or relay after start-up, as shown in FIG. 7.2.
Since the start resistance can have a much higher value in this type of start-up circuit, it is not normally necessary to change the start resistor for dual input voltage operation.
Although FIG. 7.2 shows an active limiting arrangement in which a resistor is shunted by a triac, other combinations using thyristors or relays are possible.
On initial switch-on, the inrush current is limited by the resistor. When the input capacitors are fully charged, the active shunt device is operated to short out the resistor, and hence the losses under normal running conditions will be low.
In the case of the triac start circuit, the triac may be conveniently energized by a winding on the main converter transformer. The normal converter turn-on delay and soft start will provide a delay to the turn-on of the triac. This will allow the input capacitors to fully charge through the start resistor before converter action starts. This delay is important, because if the converter starts before the capacitors are fully charged, the load current will prevent full charging of the input capacitors, and when the triac is energized there will be a further inrush current.
FIG. 7.2 Resistive inrush-limiting circuit with triac bypass for improved
efficiency. (Note: Higher inrush current for bridge operation.)
For high-power or low-voltage DC-to-DC converter applications (where the power loss in the triac is unacceptable), a relay may be used. However, under these conditions, it is very important that the input capacitors be fully charged before the relay is operated.
Consequently, converter action must not commence until after relay contact closure, and suitable timing circuits must be used.
5 QUIZ
1. What are three typical methods of inrush control used in switchmode supplies?
2. Describe the major advantages and limitations of each method.
Also see: Our other Switching Power Supply Guide