John Guy, Maxim Integrated Products
EDN
Advances in semiconductor technology have allowed ICs to replace many mechanical relays, but relays still dominate in high-current circuits that must stand off high voltages of arbitrary polarity. Contact bounce in those relays, however, can prove troublesome to downstream circuitry.
One approach to contact bounce combines a relay with a hot-swap controller. Such controllers are increasingly popular as the means for switching system components without shutting down the system power. In Figure 1, a relay contact replaces the pin of a mechanical connector.
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| Figure 1. | A hot-swap controller IC and external MOSFET remove contact bounce from relay K1. |
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The drive circuit drives the relay closed, and that relay closure connects the input of the hot-swap circuitry to the power supply: 28 V, in this case. The hot-swap controller, IC1, keeps the p-channel MOSFET, Q1, off for a minimum of 150 msec after the input supply reaches a valid level. That delay allows ample time for contact bounce in the relay to subside. After the 150-msec delay, IC1 drives the MOSFET gate such that the output voltage slews at 9 V/msec. This controlled ramp rate minimizes the inrush current, thereby reducing stress on the power supply, the relay, and capacitors downstream from the hot-swap controller.
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| Figure 2. | The mechanical relay K1 by itself exhibits contact bounce on closure. |
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An example of relay contact bounce shows three bounces with an inrush-current peak of almost 30 A (Figure 2). The top trace is output voltage at 10 V/division, the lower trace is input current at 5 A/division, and the output load is 54 Ω in parallel with 100 µF. Use of the Figure 1 circuit under these conditions yields a better picture (Figure 3). The delayed rise in output voltage is clearly visible, with no hiccups arising from contact bounce. The input current shows much less variation, peaking under 1.5 A before settling to a steady-state value of 500 mA.
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| Figure 3. | The Figure 1 circuit removes relay-contact bounce and reduces inrush current. |
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