Two electronic circuits that generate controlled power-turn-on and power-turn-off transients have been developed. These circuits are parts of a suite of test equipment used to measure the responses, to power-bus transients, of power supplies and power-consuming electronic circuitry aboard the space shuttle. These circuits can also be used in testing responses to power-bus transients in other closed electronic systems that include power sources and power-consuming equipment.
The figure is a simplified block diagram of the relationships among these circuits, a power source, and a piece of equipment under test (EUT). The power-distribution wiring between the power source and the EUT is modeled by a line impedance stabilization network (LISN), which is a circuit that contains lumped resistance(s), capacitance(s), and/or inductance(s). One of the circuits, called a “precision phase-controlled power switch” (“emission switch” for short), is designed specifically for turning on and/or turning off the power supplied to the EUT. The other circuit, called a “switching transient simulator” (“STS” for short), injects transients from a switched resistor-and-capacitor load into the connection between the LISN and the EUT to simulate the switching of power to circuits other than the EUT.
Prior to the development of the emission switch and the STS, turn-on and turn-off transients for testing were generated by use of mechanical switches. One disadvantage of mechanical switches is that they are susceptible to contact bounce and arcing, which give rise to nonreapeatability of switching waveforms. Another disadvantage of mechanical switches is that their opening and closing times cannot be controlled precisely; this is especially important in the case of an ac power supply, inasmuch as the open-switch voltage an infinitesimal time before turn-on or the closed-switch current an infinitesimal time before turn-off depends on the phase (that is, the time from the beginning of the power-supply cycle).
In the emission switch and the STS, the primary switching elements are field-effect transistors (FETs), which, unlike mechanical switches, are not susceptible to bounce or arcing. Moreover, FETs can be turned on or off starting at precise instants of time and with repeatable rise or fall times, by use of control signals with suitably timed and shaped waveforms. Both the emission switch and the STS can be synchronized with an ac power waveform so that they generate the required “on” or “off” transients at specified phases of the power cycle.
An additional notable feature of the emission switch is the inclusion of a timed mechanical relay switch. This switch is turned on 10 ms after the FET has been turned on and is turned off 10 ms before the FET is turned off. Thus, except for 10 ms intervals at the beginning and end of the “on” time, the mechanical switch bypasses the FET. This feature reduces the time-averaged power dissipated in the FET, thereby reducing the FET heat-sinking requirement.
This work was done by Ken Javor of EMC Compliance for Marshall Space Flight Center.