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Transient load enables power systems workout

10 Feb 2015  | Mike Rose

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One of the more convenient features of this implementation is the two-wire connection to the DUT. For 3.3V and 5V systems, a 12V boost converter is included to power the MOSFET driver and gate. No other connections or power sources are needed. The output of the boost converter can source approximately 350mA with a 3.3V input which may limit the amount of current that can be provided to charge the MOSFET's gate. A low ESR aluminium capacitor, C5, supplies some of the initial gate charging current for faster current edge rates. For 12V operation, a direct connect version can be assembled by replacing C7 with a 0Ω resistor, disabling the boost converter. There will be some voltage drop across L1 and D2, but this will not affect proper operation of the circuit. D3 protects the boost converter circuits from reverse polarity (note that the body diode of Q1 will conduct during reverse polarity, likely leading to excessive dissipation in the MOSFET).

The entire circuit fits comfortably on a 3" × 5" two-layer PCB, including the heat sink and a small 12V fan. With only two wires to connect, operation is very straightforward. The tester's leads must be short and have low inductance to prevent ringing from the lead reactance. DUT connection should be made near the point of load or the remote sense location. The tester's common and the voltage probe return leads should be connected at a single location. This location should also be selected to have a low impedance path back to the power source.

Pressing the momentary pushbutton PB1 starts the astable circuit and the dynamic load begins switching. If desired, quiescent PSU loading can be provided externally. R5 and J2 provide a convenient high bandwidth means for measuring the pulsing current. A straight 50Ω coax can be connected directly to an oscilloscope input for monitoring current with a scale of 1mV/A. The voltage measurement should also be made near the point of load or remote sense point and should be AC-coupled to a second scope input. The voltage probing must be done with care. Probe inductance from a distant ground/return lead will cause misleading measurements. A small series resistor (several ohms) at the probe tip can be added to dampen out the high frequency ringing from the probe's ESL. Also, avoid probing right at the pads of a very low ESR decoupling capacitor which may filter or dampen the voltage response unrealistically.


References
1. Application note 1716.0, Intersil, Paul Traynham and Dan Swank, January 26, 2012

2. Application note AN1733, Texas Instruments, SNOA507, November 2007


About the author
Mike rose is a design and development engineering since 1975. He specialies in power, analogue, signal integrity, and embedded computing.


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