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1 INTRODUCTION
The need for undervoltage protection is often overlooked in system design. In many power systems, a sudden and rapid increase in load current (for example, inrush currents to disk drives) results in a power supply line voltage dip. This is due to the rapid increase in cur rent demanded during the transient and the limited response time of the power supply and its connections.
Even when the transient performance of the power supply itself is beyond reproach, the voltage at the load can still dip when the load is remote from the supply, as a result of line resistance and inductance.
When the load variations are relatively small and short-lived, it is often sufficient to pro vide a low-impedance capacitor at the load end of the supply lines to "hold up" the voltage during transient loading. However, for large load variations lasting several milliseconds, extremely large shunt capacitors would be required if the voltage is to be maintained close to its nominal value.
It is possible, by fitting an active "undervoltage suppression circuit," to prevent the undervoltage dip at the load without needing excessively large storage capacitors. The following describes a suitable system.
2 UNDERVOLTAGE SUPPRESSOR PERFORMANCE PARAMETERS
FIG. 1a, b, and c shows the typical current and voltage waveforms that may be expected at the load end of the DC output lines from a power supply when a large transient current is applied by the load.
FIG. 1a shows a large transient load current demand during the period from t1 to t2. FIG. 1b shows the undervoltage transient that might typically be expected at the load during this transient. (Assume that the voltage dip is caused by the resistance and inductance of the supply lines in this example.) FIG. 1c shows the much-reduced undervoltage transient that would be seen at the load when the undervoltage suppressor circuit shown in FIG. 4 is fitted at the load end of the supply lines.
FIG. 1 Characteristics of atypical "undervoltage transient
protection" circuit. (a) Load current transient. (b) Typical undervoltage
transient excursion without protection circuit fitted, (c) Undervoltage
transient excursion with protection circuit fitted.
FIG. 2 shows how an undervoltage suppressor should be connected to the load at the end of the power supply distribution lines. This circuit stores the energy required to eliminate the undervoltage transient in two small capacitors C1 and C2. An active circuit supplies the required current during the transient demand, preventing any large deviations in the supply voltage at the load. C1 and C2 can be quite small because in this circuit up to 75% of the stored energy is available for use. How this is achieved is shown in principle in FIG. 3 a, b, and c.
FIG. 2 Position and method of connection for undervoltage protection
circuit.
3 BASIC PRINCIPLES
FIG. 3a shows the method of energy storage and delivery. When SW1 is open, capacitors C1 and C2 are charged in parallel from the supply lines via resistors R1 and R2.
They will eventually charge to the supply voltage Vs.
If this circuit is now removed from the supply lines and SW1 is closed, C1 and C2 will be connected in series, and a voltage of 2Vs will be provided at the terminals.
In FIG. 3b, this circuit (in its charged state) is shown connected at the input to a linear regulator transistor Q1. Now if, during an undervoltage condition, SW1 is closed, capacitors C1 and C2 will be connected in series and provide a voltage of 2Vs at point A in the circuit.
Since the header voltage at point A (the input of the linear regulator) now exceeds the required output voltage Vs, Q1 can operate as a linear regulator, supplying the required transient current and maintaining the output voltage across the load nearly constant. This can continue until C1 and C2 have discharged to half their initial voltage.
In the active state, C1, C2, SW1, and Q1 form a series circuit. The position of individual components in a series chain has no effect on the overall function of the network; further, SW1 and Q1 can both act as switches, and one of them is redundant. In this example SW1 is to be made redundant.
FIG. 3c shows a practical development of the circuit; SW1 has been removed, and Q1 has been moved to the original position of SW1. Q1 now carries out both the original functions of switch SW1 and linear regulator Q1. Although it is perhaps not obvious, examination will show that this circuit has the same properties as the circuit shown in FIG. 3b.
As previously stated, voltage regulation can continue as long as the charge in C1 and C2 can maintain the required header voltage. Clearly this depends on the load current and the size of C1 and C2. The voltage at point A will not fall below a value where Q1 goes out of regulation until the voltage across each capacitor is approximately half its original value. Since the energy stored in the capacitors is proportional to V2 , and since 1/4 of this energy will be dissipated in the linear regulator, half of the stored energy is available for use.
Because of the efficient use of the stored energy, very much smaller capacitors can be used (compared with what would be required if normal shunt capacitors were used on their own). Further, the load voltage can be maintained within a few millivolts of normal throughout the undervoltage stress period, even though the capacitor voltages are falling. Hence much better performance can be provided with active transient suppression.
It should be noticed that resistors R1 and R2 form undesirable loads on C1 and C2 when the circuit is active (SW1 or Q1 closed); hence their resistance is a compromise selection. A high value of resistance presents minimum loading but requires a longer charging time.
FIG. 3 Undervoltage circuit development steps.
4 PRACTICAL CIRCUIT DESCRIPTION
FIG. 4 shows a practical implementation of this technique. In this circuit, switch SW1 or Q1 is replaced by Darlington-connected transistors Q3 and Q4. These transistors operate as a switch and linear regulator.
FIG. 4 Example of an undervoltage protection circuit.
Although Q3 and Q4 are now shown positioned between the two capacitors C1 and C2, it has been demonstrated above that since they still form a series network, their position in the series chain does not change the function of the circuit.
Q1 and Q2 are part of the drive and linear regulator control circuit. The control circuit is not easily identified as a linear regulator, because it appears to lack the normal reference voltage. However, a relative reference voltage that is proportional to the mean (normal) sup ply voltage Vs is set up on C3. An absolute reference voltage is not what is required here; setting a relative reference voltage on C3 makes the unit self-voltage-tracking. Hence the circuit responds to any transient deviation that is below normal; it does not require preset ting to a particular voltage.
5 OPERATING PRINCIPLES (PRACTICAL CIRCUIT)
Initial Conditions
A bias voltage is set up on the base of Q1 by the current in R1, D1, and D2. Q1 conducts to develop a second bias voltage across R2 of approximately one diode drop (0.6 V). The current flow in R3 is similar to that in R2, and a third bias voltage is set up across R3 that is slightly less than that across R2, since the resistance of R3 is lower than that of R2.
Hence, under quiescent conditions, transistor Q2 is close to conducting. At the same time, capacitor C3 will charge through R4, R2, D3, D1, and D2 so that the voltage on its negative terminal will end up the same as the emitter voltage of Q1. Also, C1 and C2 will charge up to the input voltage Vs via the 10-7 resistors.
6 TRANSIENT BEHAVIOR
When a transient current demand occurs, it will reduce the voltage across the load and input terminals 1 to 6. The negative end of C3 will track this change, taking the emitter of Q1 negative. After a few millivolts change, Q1 will start to turn on, bringing Q2 into conduction. Q2 will drive the Darlington-connected linear regulator transistors Q3 and Q4 into conduction.
This action progressively connects C1 and C2 in series, driving current into the output terminals 1 to 6 to prevent any further reduction in terminal voltage. Hence the circuit can be considered to be "propping up" the voltage by using the charge on C1 and C2.
It should be noted that the circuit is self-tracking-during normal operation the voltage across C3 adjusts to respond to any transient deviation below the normal working voltage.
Because the control circuit is always active and close to conduction, the response time is very fast. The small shunt-connected capacitor C4 can maintain the output voltage during the very short turn-on delay of Q3 and Q4.
Undervoltage clamping occurs as soon as the output voltage has dropped from its nominal value by a defined margin (typically 30 mV). This self-tracking arrangement removes the need to set the operating voltage of the undervoltage protection circuit to suit the power supply output voltage.
This protection circuit can be extremely useful where load current transients are a problem. It is best positioned close to the transient demand, to eliminate the effects of voltage drop in the supply lines. In some applications, extra capacitors may be required to extend the holdup time; these can be connected to terminals 2, 3, 4, and 5 across C1 and C2.
A further possible advantage to be gained from this technique is that the peak current demand on the power supply can be reduced. This may permit a lower current rating (more cost-effective) power supply to be used.
The decision to use such systems becomes part of the total power system design philosophy. Since this is not part of the power supply, it is the system designer who should consider such needs.
FIG. 1b and c shows the performance that may be expected at the load with and without the undervoltage protector. It is clear that even if the power supply has a very fast transient response, the improvement in performance at the load with the UVP unit can be very significant.
7 QUIZ
1. Even if the power supply has an ideal transient response, it is still possible for undervoltage transients to occur at the load. Under what conditions would this occur?
2. What advantages does an active undervoltage protection circuit have over a decoupling capacitor?
Also see: Our other Switching Power Supply Guide