Minimizing Transients with RC Circuits

Mar 1, 2002 12:00 PM
By Keith Billings, DKB Power Inc., Guelph, Ontario, Canada

Difficulties are associated with meeting the regulation needs of modern microprocessors during the very fast and large transient current changes they demand. This is particularly troublesome for low-voltage applications in the range 2V to 3.5V.

The figure shows the circuit I patented in the 1960s to aid transient suppression (the patent has lapsed). It operates as follows: Terminals + and - are connected directly across the supply leads between the power supply output and the load, (best connected at the load where the transient occurs). Under steady-state conditions, capacitors C3 and C4 charge through resistors R6 and R7 to the supply voltage. During this charging stage, capacitors C3 and C4 are effectively in parallel.

If Q1 were to be fully turned on, capacitors C3 and C4 would now be in series, and would be able to drive the terminals + and - (and hence the load voltage), to twice the supply voltage. In practice however, Q1 is only turned partly “on” — just sufficient to maintain the terminal voltage constant during the transient. This can continue until C3 and C4 discharge to half the original starting voltage. If the transient continues beyond that point, the circuit will not maintain the voltage.

The energy stored in the capacitors is proportional to Vc², hence three quarters of the energy stored in C3 and C4 can be effectively used to maintain the output voltage constant during a current transient. Typically in a microprocessors application, the maximum permitted voltage deviation may be only -100 mV or even less. Transient current requirements of 20A for 50μs are not unusual. To meet this need with a single output capacitor would require 10,000 μF or more — assuming no help from the supply. Typically, this may be reduced by a factor of 20 or so with the proposed circuit.

Normally, the power supply has small electrolytic or tantalum capacitors connected across the power supply output, to help meet the transient requirements. However, since the permitted voltage drop is so small, very little of the energy stored in these capacitors can be usefully exploited. So, relatively large capacitors are necessary. By using the active under voltage transient suppresser shown in the figure, you can use much smaller capacitors for an even better result.

Looking at the control circuit in the figure, R1, R2, R3, and R4 are voltage dividers across the supply lines. R3 and R4 develop a voltage reference across C1 a short time after initial switch on. The size of C1 is chosen such that its voltage doesn't significantly drop during the transient. R1 and R2 monitor the applied voltage and apply this to the inverting input of amplifier U1. Although the values of R1, R2, R3, and R4 can be the same, a small transient voltage drop is permitted by making R4 less than R3, to provide a slightly lower hold-up threshold. This ensures the power supply control circuits will be activated to provide more current.

During a transient, the voltage on the inverting terminal of U1 will fall as the output voltage decreases. U1 will be turned on to activate Q1 and the circuit will provide the difference in the transient load current and the existing power supply current, through the network C3, Q1, C4. It will supply sufficient current to maintain the terminal voltage constant at the elected hold-up voltage. The system maintains the voltage constant until C3 and C4 have discharged to about half the supply voltage. However, during this period, the power supply should have increased its output current to meet the new needs. The line voltage will remain constant.

Because the circuit is self-contained, it may be positioned close to the load, reducing the voltage drop on the supply leads and providing better transient performance.

Keith Billings is President of DKB Power Inc., (dkbp@rogers.com), and author of the Switchmode Power Supply Handbook published by McGraw-Hill. He will present Abe Pressman's “Modern Switching Power Design Course” this fall. For more information visit Web www.apressman.com.

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