Buck DCDC Converters<\/h5>\n
This is bad:<\/p>\n
This is good:<\/p>\n Switcher IC’s with internal mosfets tend to aim towards higher efficiency and therefore lower mosfet on resistance. \u00a0However low on resistance means faster switching times, which contributes to noise and ground bounce. \u00a0Should you consider a less efficient IC with a higher on resistance? \u00a0You can’t easily add resistance externally as the resistance your aiming for is still very low (e.g. 100 or more milli Ohms).<\/p>\n For a boost design get the output capacitor cathode right next to the switchers mosfet ground pin. \u00a0A\u00a0few extra mm of tracking can cause issues on some designs. \u00a0Keep the inductor to diode and diode to capacitor tracking very short.<\/p>\n Can you\u00a0shut-down\u00a0the DCDC converter while doing sensitive measurements? \u00a0If your doing periodic fast AtoD readings then whilst not an ideal solution, including the means to shut-down the DCDC converter whilst the AtoD is sampling can be a compromise solution in suitable applications.<\/p>\n If changing layers for any of the tracking, including the power supply to the switcher, vias can create significant resistance so use multiple vias to remove this issue.<\/p>\n Boost designs where the load can draw high currents can be very problematic.\u00a0 Often the source is batteries where the voltage will drop as the load appears, or for low input voltages the power supply may need to be supplying high currents.\u00a0 Once you build your prototype you may find that the say 3.0V to 5V @ 3A switch mode design which the switcher IC data sheet says will work just fine actually doesn’t.\u00a0 So when designing these types of boost switch mode circuits you may want to consider:<\/p>\n Can you use multiple switch mode circuits to supply different loads, so that the power each has to output is reduced?\u00a0 Often a problem switcher design will work fine if the load current is reduced.<\/p>\n Can you use a switchmode IC with an adjustable switching frequency?\u00a0 If possible use one as it gives you an important parameter to adjust when trying to solve a problem.<\/p>\n Can you use 2oz copper on the PCB to reduce the resistance of power supply tracks?<\/p>\n Reduce the length of power input tracks.\u00a0 If there is a risk that when you come to test your design it fails until you directly wire the power source to the inductor and mosfet consider this now and reduce the the track lengths the power input takes to get there and increase their copper width.\u00a0 This is a classic problem!<\/p>\n Is there a chance you’ll need to increase input or output capacitance for the switcher – if you can add the footprints at the design stage just in case.\u00a0 Remember that low ESR is often more important than capacitor size for switchmode supplies.<\/p>\n Could the tracking be improved in any way?\u00a0 If so do it now instead of finding out the hard way!\u00a0 If the switcher IC data sheet includes a recommended PCB layout does your layout match?\u00a0\u00a0 Could it?\u00a0 Their layout works so if you can match it your chances of success are increased.<\/p>\n Read the switcher data sheet in it’s entirety.\u00a0 Often there are hidden away mentions of additions to the recommended circuit useful in specific applications which may apply to your circuit.<\/p>\n Does the IC have an evaluation board?\u00a0 If so read it’s data sheet to check if there is anything helpful in it you haven’t considered.<\/p>\n The primary cause of ground bounce is poor PCB design, or more typically non optimal PCB design. \u00a0These notes refer to boost switcher, but apply for a buck switcher also by adjusting for the different\u00a0wiring\u00a0arrangement.<\/p>\n A classic method to solving it is to start adding or changing capacitors, but this usually only has a very limited effect.<\/p>\n A good starting point to identifying the problem and curing it is to start by removing as many imperfections as possible:<\/p>\n Power the PCB with 0Vin connected directly to\u00a0the switchers GND point, and with +Vin connected near to the inductor.<\/p>\n If the main capacitor is not connected ideally as above lift it and make it perfect using short wire links if necessary.<\/p>\n These two steps will give you a good starting point. \u00a0Has the ground bounce reduced or gone away?<\/p>\n Scope from the switchers GND point (mosfet ground) to the +V terminal of the main capacitor. \u00a0Is there noise? \u00a0There may be only a very short link but if you have noise between these two points then maybe you have the cause of your problem? \u00a0It might seem impossible for there to be noise from an inch of copper or wire, but it isn’t and if it’s there you need to eradicate it.<\/p>\n Check there isn’t an area elsewhere in the circuit causing or contributing to the noise by powering\u00a0the PCB after the switcher (i.e. at its output so it’s no longer used). \u00a0Is the supply to the rest of the circuit now perfectly clean?<\/p>\n With the switcher powered again use a scope with its GND connected to the switchers GND point (DCDC converter IC power ground or external mosfet ground)\u00a0and probe different areas of the PCB. \u00a0Does the noise level change depending on where you probe? \u00a0Probe to other GND points also. \u00a0If so this is likely to indicate areas of the circuit contributing to the noise. \u00a0What is the GND connection to that area like? \u00a0Does the noise reduce if you provide it with a direct dedicated GND connection to the switchers GND point?<\/p>\n Multilayer PCB 0V ground planes are great at reducing signal noise around a circuit, but are not necessarily great at reducing switcher ground bounce issues (as with audio they can often be a cause of new problems). \u00a0Should your power circuitry use a\u00a0separate\u00a0GND to the GND power plane or should the ground plane include some cuts to direct the switcher current? Don’t assume because there is a great big ground plane connecting to an area of the PCB that it’s therefore got a perfect GND connection. \u00a0Remember that true ground will be at one point of the PCB (i.e. at the 0V power connection in or at the GND connection of the switcher). \u00a0All other points returning current are not grounds but just return lines to ground.<\/p>\n Are there big capacitors elsewhere on the PCB that are affecting the switchers main capacitor being able to provide the perfect switch mode supply arrangement? \u00a0Are they part of the current path during switching? \u00a0Ensure the main capacitor is a low ESR (low impedance) type.<\/p>\n Does changing the switchers diode to a different part help?\u00a0 It often shouldn’t but we’ve occasionally found that using a totally different model can sudenly cure much of a noise problem so it’s worth trying in case.\u00a0 Remember that diode switching times are often only specified as a maximum value, not a typical.\u00a0 Slower diodes can sometimes help.<\/p>\n Have a beer, then come back and read the switcher IC datasheet for a 10th time. \u00a0Hidden in the horrible technical detail may be something you’ve overlooked or that sparks a new process of elimination. \u00a0Get a piece of paper and draw out how you think the current is flowing in PCB traces. \u00a0Does that match the actual tracking and physical layout of the PCB?<\/p>\n Remember, the theory is sound but any PCB layout isn’t perfect. \u00a0The problem is being caused by something that isn’t perfect enough – you just need to work through every detail and find it. \u00a0If your new to switch mode design don’t worry, even experienced experts still get bitten by this stuff once in a while and the best way to learn the\u00a0pitfalls\u00a0is to find your way out of them – you only have to learn a mistake once.<\/p>\n More extreme solutions:<\/p>\n Should\u00a0you\u00a0completely\u00a0isolate a sensitive area or the non high power area of the circuit? \u00a0Can you use a\u00a0separate mains\u00a0transformer\u00a0winding or an isolated DCDC converter to power the other section of the circuit? \u00a0If you can this is often be a great solution. \u00a0You can then choose the one perfect point to connect the isolated GND rails. \u00a0Alternatively for more\u00a0extreme\u00a0issues should\u00a0they remain isolated with opto isolated signals between them?<\/p>\n You can never get your hands on enough ideas about solving ground bounce when faced with it!<\/p>\n http:\/\/www.analog.com\/library\/analogdialogue\/archives\/41-06\/ground_bounce.html<\/a><\/p>\n http:\/\/homepages.which.net\/~paul.hills\/Emc\/BecBody.html<\/a><\/p>\n![\"\"](\"https:\/\/ibex.tech\/resources\/wp-content\/uploads\/sites\/3\/switchmode_buck1.png\" "\\"switchmode_buck1\\"")
<\/a><\/p>\n![\"\"](\"https:\/\/ibex.tech\/resources\/wp-content\/uploads\/sites\/3\/switchmode_buck2.png\" "\\"switchmode_buck2\\"")
<\/a><\/p>\nBoost DCDC Converters<\/h5>\n
![\"\"](\"https:\/\/ibex.tech\/resources\/wp-content\/uploads\/sites\/3\/switchmode_boost1.png\" "\\"switchmode_boost1\\"")
<\/a><\/p>\nGround Bounce Design Tips<\/h4>\n
High Current Design Tips<\/h4>\n
Solving Ground Bounce Problems<\/h4>\n
Ground Bounce Resources<\/h4>\n