CreatingaFastLoadTransient

Creating a Fast Load Transient

Abstract: A simple circuit generates fast load transients for thorough testing of power supplies.

Many of Maxim's devices show a typical operating characteristic (TOC) graph with a very fast load transient. Our customers sometimes ask us how we create such fast changes in load. Although commercially available dynamic load boxes are capable of supplying transients, they do not provide the fast rise times and fall times necessary for an attractive-looking TOC.

To obtain fast load transients, we combine a power MOSFET and a couple of resistors right src="/data/attachment/portal/201007/ET29203201007211251151.gif">
Figure 1. Component connections.

Determining the value of R1 and selecting M1 depend src="/data/attachment/portal/201007/ET29203201007211251152.gif">
Figure 2. The MAX8654 load transient response.


Figure 3. Load transient response setup.

To achieve the best performance, keep the signal generator's duty cycle to 10% to reduce self-heating. Also, some converters are capable of supplying more than 25W, which is sufficient to cause minor injury or destroy the components if operated above 10% duty cycle.

The physical location of the components is critical for best performance. Usually they have to be cobbled onto an existing PC board. Keep both the MOSFET and R1 as close to the DUT's output capacitor as possible. Make the conductors as wide and short as you can.

Measurement of the DUT output should be done directly across the output capacitor. If the device is insensitive to 1000pF loading, use a 1X scope probe. The 1X probe provides the highest signal-to-noise ratio to the oscilloscope.

Whatever probe is used, though, make sure to have the ground lead as close to zero length as possible. Ideally, a scope-probe jack connected directly to ground and the output works very well. Any high-speed edge to be displayed on the scope is subject to this criterion, not just transient response.