Noise has always been a component of switched mode power supplies and as with most designs there are trade-offs you have to consider for you SMPS design. Paul Greenland, Marketing Director of National Semiconductor gives you a shopping list of items to look out for when designing with switchers, so you can minimize your trade off gotchas.
In the late 1980s Hewlett-Packard published a booklet entitled "What's all this noise about switchers?" HP, now Agilent, chose to issue this booklet at the same time as the introduction of a 1 kW laboratory switched mode power supply to a notoriously conservative test and measurement customer base.
Switching regulators are inherently noisy as they regulate by varying the duty cycle or frequency of a power switch. One of the key trade-offs in switching regulator design is that of efficiency versus noise. Switching loss is generated when there is voltage across, and current through, the power switch simultaneously, each time it switches on or off. This power loss increases with switching frequency. In order to maximize efficiency, the slew rate at switch on or off must be high to minimize any overlap. However, high slew rate, particularly at small duty cycles generates high-frequency harmonics which are more difficult to filter out. If the switch is a MOSFET, switching speed can be adjusted by varying the gate resistance. Load-line shaping or snubbing modifies the slew rate of bipolar transistors.
Two parasitic elements, stray inductance and capacitance exacerbate the situation. Stray inductance results in voltage overshoot and ringing at switch off and stray capacitance results in over-current at switch on. Both phenomena cause electromagnetic interference. The stray capacitance can be found in the switch itself, in the freewheeling diode and in the filter inductor. The stray inductance is in the bond wires of the switch, diode and the printed circuit board layout.
The turn-to-turn and turn-to-core capacitance in the filter inductor constitutes a low impedance path for high-frequency noise. Certain types of regulator, specifically quasi-resonant and multi-resonant regulators incorporate parasitic elements into resonant circuits and recycle the energy. These regulators are inherently quiet; however, there is a trade-off with increased conduction loss. Particular care should be taken in selection and location of the filter inductor.
If the inductor core is gapped, magnetic fringing flux radiates from the gap, and will induce eddy currents and interference into conducting material placed less than three gap widths from the gapped face of the core. This is a serious issue in multiphase regulators where high input impedance current-sense traces monitor phase currents in sense resistors or the DC resistance of the filter inductor.
Diode reverse recovery characteristics also contribute to electromagnetic interference. Fast recovery diodes can have a "snappy" or "soft" recovery characteristic. Snappy recovery diodes should be avoided as they excite ringing in the parasitic elements mentioned above.
Synchronous buck or step-down regulators suffer from shoot-through current which occurs when the highside, or control, switch turns on before the low side, or synchronous, switch turns off. Shoot-through current not only degrades efficiency but also generates electrical noise.
There are many techniques to mitigate shoot-through current, most involve modifications to the gate drive; such as increased dead time and break-before-make logic that ensures both devices cannot be on simultaneously. More sophisticated approaches such as predictive gate drive allow a regulator to operate without should-through currents in the power stage, regardless of the MOSFETs selected.
Design for low EMI should start right at the beginning of the development process when scaling and selecting power components. Pay close attention to PCB layout, making noisy runs broad, short and direct. An example of a trace carrying switching current is the PCB track between the input filter capacitor and the drain/collector of the buck power switch. Star returns and ground planes are invaluable for local decoupling and reduction of ground bounce or common-resistance effects.
Remember, filter components are non-ideal; inductors have shunt capacitance and series resistance, capacitors have equivalent series inductance and resistance. All these non-idealities degrade the filter's performance reducing high frequency rejection. A seasoned engineer once told me that applying EMI countermeasures to a badly designed switching regulator was like "wearing a rubber glove because your fountain pen leaks". Close attention to the items herein may save considerable heartache at the system-integration phase of the project.
Paul Greenland, Marketing Director, Power Management Products, Tel: 408-721-3210, Cell: 408-718-3149
Paul is an industry veteran with over twenty years of experience in power management design, applications and marketing. He is the author of numerous articles, training seminars and white papers, and a speaker at conferences including Power Systems World, PCIM-Europe, APEC, Intelec, ISPSD, Power-UK and the IEE colloquium on power electronics. His interests include quasi-resonant and multi-resonant power conversion techniques.