Modern µPs place severe demands on the transient response of the power source. For example, Intel's P6 specification stipulates an output-current capability of at least 30A/µsec while maintaining output-voltage regulation within specified limits. Building a power supply that meets this requirement is a challenge, and testing a supply to assure compliance with this specification is also difficult. Commercially available active loads can't provide load-current steps with a 30A/µsec di/dt rate; the good ones do only about 5A/µsec.
Fortunately, you can easily build your own 30A/µsec active load using inexpensive and readily available components (Figure 1). The basic approach to building a transient-response tester is to connect a load (resistor) to the power-supply outputs through a switch (FET) that connects and disconnects the load at a specific frequency and duty cycle. Because this tester is dedicated to Pentium applications, the power supply under test has an output voltage of 2 to 3.5V and requires maximum load currents of approximately 12A.
In 
Figure 1, the load consists of four sections, each comprising five 5.1 ohm/3W resistors in parallel. Each load section makes up an equivalent load of about 1 ohm at 15W, which means that each load section draws a current of 1A/V from the power supply with the FET switch closed.
IC2, a one-shot with adjustable on-time, turns FET switch Q1 on and off. When the signal through C1 triggers IC2, the output at pin 3 goes high, which turns on Q1 for a time interval set by R2. For the component values in Figure 1, you can vary the pulse width from 20 to 400 µsec. IC1, a free-running oscillator, sets the frequency of the pulses. Using R1, you can vary the frequency from 120 Hz to 1.6 kHz. The snubber comprising R3 and C2 connects directly to Q1's drain and source leads to minimize ringing at turn-off.
A 50-m(ohm)/5W sense resistor in series with the negative output helps to easily view the current pulse on an oscilloscope. This sense resistor consists of 20 parallel-connected 1 ohm/0.25W resistors. You can measure the rise and fall times of the current pulse by connecting an oscilloscope to IS(+) and IS(-).
Parasitic inductance throughout the load is the primary factor that limits the rise time of the current pulse. The law of inductance states that
V/L=di/dt,
where V is the applied voltage, L is the inductance, and di/dt is the rate of change of the current in amps per second. In this case, assuming a power supply set to 3.3V, a di/dt of 30A/µsec requires that the total inductance of the load not exceed 0.1 µH. This low inductance requires careful component selection and construction methods.
The load resistors should be metal-oxide, carbon-film, or carbon-composition types. Do not use wirewound resistors. Connecting multiple resistors in parallel minimizes the inductance, because the inductance of a single resistor, which is about 50 mH for metal oxide, divides down by the number of parallel resistors. The value of 5.1 ohm/3W allows the circuit to handle 3.3V indefinitely without exceeding the power rating. You can also place a jumper across Q1 for continuous load testing.
Constructing the 50-m(ohm) sense resistor from 20 parallel 1-ohm metal-film resistors minimizes the added inductance, which slows the current rise time. Any additional inductance also causes ringing, making the waveform more difficult to view on an oscilloscope.
By far the most significant contributors to parasitic inductance are the connecting leads and traces. The best rule to remember is that you minimize the inductance by making a conductor as wide and short as possible. You should minimize the lead lengths of the resistors by bending them straight down and mounting the component flush to the pc board. You should make those connections between the output of the power supply under test and the load using copper foil as wide and as short as possible.
Tests were performed using a lab power supply set to 3.3V with three load sections, which provides a load of about 8A. The 0 to 90% rise time (0 to 7.2A) takes about 220 nsec, an average di/dt rate of 33A/µsec. The fall time is 120 nsec, which corresponds to a rate of 60A/µsec. (DI#1855)