Switching power supplies & inductive (relay) loads - Page 1
Forum section: electronics / projects

author: JDW
author_rank: Frequent Contributor
timestamp: August 20, 2019, 12:25:02 am
content: I am pondering use of a slight variant of the following 6V-75V to 3.3V switching (buck) power supply:https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=085D149D69F04A23For those of you who cannot open the above link for some reason, here is the schematic:The application circuit will be used in 12V and 24V vehicles, hence the broad Vin range. My actual design will slightly deviate from the above design in that I will add a TVS diode on Vin to clamp spikes to 70.1V (Vc) and also have an inline Schottky diode (B2100-13-F) for reverse polarity protection (and I will use that same Schottky p/n for D1 in the above circuit as well). I will also use two 330uF 50V capacitors instead of the suggested 33uF Cin to better maintain Vin during cranking, and I will use a single 15.4uF (derated capacitance at 3.3V, originally 22uF) 10V X7R ceramic output capacitor (for greater stability over temperature) instead of the two X5R caps suggested in the above design. And I will of course have a 0.1uF ceramic cap near Vcc of my MCU and near other IC's in the circuit.But my question centers on use of an inductive load-- specifically, a 3V relay (50mA@3.3V, with 1N4004 diode across the coil to suppress kick back). I'm not an expert on switching power supplies, hence this post. I do know that we need to keep output impedance in check with proper PCB layout techniques, and choice of the output capacitor also needs to keep impedance low as well. As such,how negatively would my 3V relay's coil impact my 3.3V power rail?I am also thinking I should probably add a low costaluminum electrolytic capacitor (47uF or 100uF) near the relay coilto avoid voltage dips when the relay switches, especially because my MCU and a fingerprint sensor (with its own MCU) will also be powered on the same 3.3V output voltage rail, so any thoughts on that would also be appreciated as well.Thank you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 20, 2019, 03:22:02 am
content: A relay coil won't affect the source, as long as the source itself is stable, has nominal transient response, and is adequately rated for the load.You can test transient response by measuring the output voltage with an oscilloscope, and pulsing a load. Typically an "idle" load will be present, drawing 10 to 50% of rated current, and then the remainder (the remaining 90-50%) is turned on and off say with a switch or relay or MOSFET. (It doesn't have to be fast, a few microseconds is fast enough. Mechanical contacts actually switch in fractional nanoseconds, making some very fast edges that can make it harder to trigger on the 100s-microseconds scale transient you're actually looking for. Something to keep in mind!)Tim

author: JDW
author_rank: Frequent Contributor
timestamp: August 20, 2019, 04:11:07 am
content: Quote from: T3sl4co1l on August 20, 2019, 03:22:02 amThank you for sharing your thoughts, Tim. The transient response has not yet been bench-tested because I've not built the circuit yet, but at the URL mentioned in my opening post, there is a simulator that can test the transient response and it seems reasonable to me. The low current state of my circuit will draw 2mA or less (not including the Iq of the power supply), and draw 90mA when the fingerprint sensor is engaged, and another 50mA when the relay is switched. So the max current draw would be on order of 150mA or less, and I designed the supply to handle 250mA over the full range of Vin. It's just that I've never used a low voltage signal relay before, and certainly not on the same voltage rail as an MCU, so I was curious if there might be any ill effects from the relay in that 3.3V voltage rail, even though I intend to use a diode across the relay coil. Since the diode should suppress the relay coil voltage spikes, the main consideration would be capacitance on Vout so that voltage rail doesn't dip too low when the relay switches. The relay will be switched on for some hours at a time, as long as the vehicle's engine is running.