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Design Article

# Use scope's math channel to analyze hot-swap circuits

## 3/16/2012 4:28 PM EDT

Most engineering labs have digital oscilloscopes, but many engineers don't fully explore their features. Among the more interesting features of a digital oscilloscope is its "math" channel, which can help you analyze circuits such as hot-swap and load-switching circuits.

Math functions can yield detailed information about hot-swap circuit parameters that can aid you in design and troubleshooting. For example, you can use an oscilloscope's math functions to calculate load capacitance, which can reveal a MOSFET's transient power dissipation during startup or shutdown.

Oscilloscope setup

To give you an idea of how to use math functions, I've chosen an integrated-MOSFET hot-swap device (MAX5976). It combines an internal MOSFET switching element with current-sensing and driver circuitry that form a complete power-switching circuit. The test method also applies to hot-swap control circuits built from discrete components.

Connecting oscilloscope probes to the hot-swap circuit shown in Figure 1 gives the oscilloscope access to the signals you need for calculations. Voltage probes connected to the circuit's input and output provide the voltage drop across the MOSFET. A current probe offers the easiest way to sense load current through the device.

Figure 1. Connect voltage probes across a MOSFET to measure VDS and a current probe to measure ID.

The same basic connections apply for a non-integrated hot-swap circuit. Connect the input and output voltage probes before and after the MOSFET (internal to a MAX5976 but external to a MAX5978), and place the current probe in series with the circuit's current-sense resistor. To get an accurate measure of current flowing through the switch element itself, you should place the current probe after the input bypass capacitor and before the output capacitor. The probe must measure the current that passes through the controller. Capacitors COUT and CIN can't be between the controller and the current probe.

MOSFET power dissipation
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