Soft-Start Circuits: An In-Depth Look

In this video we discuss soft-start circuits which are used to limit inrush current during power-up of a circuit. We go into detail on three different soft-start circuit architectures and test example versions of them in the lab. Concepts discussed: - What soft-start circuits are and why they're important - NTC Thermistor architecture overview and lab testing - Resistor and switch architecture overview and lab testing - Ramp rate controller architecture overview and lab testing - Overview on selecting the best architecture for your application Chapter Descriptions - Intro o Overview of what a soft-start circuit is - Circuit Input Capacitance o Using an example of an AC Adapter to a voltage regulator, we discuss why large input capacitances are often desirable and how they can lead to high currents during turn-on and how this large current must be reduced with a soft-start circuit - NTC Thermistor Overview o We explain the general operation of the NTC Thermistor-based soft-start circuit - NTC Thermistor Lab Results o I show some oscilloscope and digital multimeter (DMM) measurements I made in the lab with an NTC thermisor and "example" inrush circuit, as well as some static measurements of a bunch of different thermistor circuits - Table of NTC Thermistor Test values o I show a table containing several thermistors I tested to help get a rough feel for the resistances you can expect how various thermistors - NTCs in Series and Parallel o I discuss some things to keep in mind when putting NTCs in series and parallel (generally don't put them in parallel) - NTC Circuit Pros and Cons o I discuss the advantages and disadvantages of the NTC circuit - Resistor and Switch Overview o I explain the general operation of the resistor and switch-based soft-start circuit - Resistor and Switch Lab Results o I go over the test circuit I designed for the lab demo, show the results, and discuss them - Resistor and Switch Pros and Cons o I discuss the advantages and disadvantages of the resistor and switch circuit - Ramp Rate Controller Overview o I explain the general operation of the ramp-rate-controller -based soft-start circuit using a MOSFET as the transistor element - Mechanics of Power MOSFET Turn-on o I go over the mechanics of power MOSFET turn-on and how gate-source and gate-drain capacitance affects the mechanics of that turn-on. An understanding of the turn-on mechanics is required to understand how the ramp rate controller circuit operates and how to tweak the design to serve your application o I show that the results from our examination of the general power MOSFET turn-on pretty straightforwardly apply to the ramp rate controller circuit - Ramp Rate Controller Design Flow and Analysis o I go through an example design flow for a ramp rate controller to show how you come up with the various parameters to set everything up for the ramp rate controller o I also perform all the necessary calculations to get predicted results from our lab test - Ramp Rate Controller Lab Results o I go over the test circuit I designed for the lab demo, show the results, and discuss them - A Note about MOSFET Safe Operating Area o I go over what a MOSFET's "Safe Operating Area" is and how to incorporate this into your design - The Importance of Gate-Source Capacitance o If the gate-source capacitance is too low relative to the gate-drain capacitance, it will close the MOSFET right at turn-on and cause surge currents o I derive the relevant equations and use some example calculations to indicate what happens with a couple different capacitor values to illustrate the point - Ramp Rate Controller Pros and Cons o I discuss the advantages and disadvantages of the ramp rate controller circuit - Ideal Applications For Each Circuit o I briefly go over the ideal applications for each circuit to help decide which one is best to use for your specific application 00:00:00 Intro 00:02:02 Circuit Input Capacitance 00:06:05 NTC Thermistor Overview 00:09:07 NTC Thermistor Lab Results 00:12:02 Table of NTC Thermistor Test Values 00:13:28 NTCs in Series and Parallel 00:14:43 NTC Circuit Pros and Cons 00:19:43 Resistor and Switch Overview 00:20:55 Resistor and Switch Lab Results 00:27:36 Resistor and Switch Pros and Cons 00:30:06 Ramp Rate Controller Overview 00:32:29 Mechanics of Power MOSFET Turn-on 00:43:00 Ramp Rate Controller Design Flow and Analysis 00:47:13 Ramp Rate Controller Lab Results 00:51:34 A Note about MOSFET Safe Operating Area 00:55:45 The Importance of Gate-Source Capacitance 01:00:20 Ramp Rate Controller Pros and Cons 01:01:53 Ideal Applications For Each Circuit 01:02:40 Outro