Contemporary switching regulators not only occupy significant PCB area, but are also plagued by layout-dependent noise and stability issues. New breakthroughs in semiconductor processes and magnetic materials enable integration of the regulator, including ancillary components, into a single package. The result is enhanced control, allowing converters to operate over a wide range of input/output conditions without layout dependant issues. Integration of the inductor makes it possible to dramatically reduce noise associated with the large pulsating currents. Input capacitor loop current is also confined to a geometrically small area. Integration reduces noise, parts count and footprint, speeding design time.
Troublesome Switchers
The two fundamental choices in voltage regulators are linear and switch-mode. Linear regulators offer simplicity, low part count, transient performance, and poor efficiency. Switch-mode regulators offer higher efficiency, but also high part count, layout sensitivity, noise, and a larger footprint.
In the simplest terms, the power MOSFETs act as switches that ¡§chop¡¨ the DC signal up into a pulsed AC waveform. The pulsed wave is then ¡§filtered¡¨ to create a new DC signal. The ratio of the switch on and off times, the duty cycle, determines the new voltage level. A feedback network controls the duty cycle to regulate this output voltage level.
When the switches chop the input they create large pulsed currents that pass through the switch and PCB trace to the output filter. Pulsating current is a source of noise. The filter itself is not perfect, so a significant ripple current voltage remains. The ripple voltage rides on the DC output voltage, creating a noisy supply rail. The ripple current circulates through the switches, the inductor, and the input and output capacitors, acting as loop antennas radiating electrical noise. A further issue is that the switching noise can couple onto the feedback lines and affect regulation or make the regulator unstable altogether. The net effect is that switch-mode converters have the potential to create noise on the circuit board, radiate significant electrical noise, and have poor regulation or even be completely unstable. All of these effects are critically dependent on the layout of the converter components, especially the magnetics.
Making Them Smaller
Integration of Switches, Control, Compensation
The first and most obvious way to reduce the converter footprint is to integrate all the components. Lateral Diffused MOSFETs (LDMOS) transistors on a small geometry CMOS process offer this capability. We can integrate control, compensation, gate drive and the power switches. However, this is not enough to address the noise and layout issues, nor does it result in a footprint reduction that really makes a difference.
Now What? Ah, the Magnetics!
The magnetics, that is, the inductor, is the source of most of the troubles. The layout and placement of this component is the most important element of the switch-mode DC-TO-DC converter design.
. When the power switches open and close, large pulsed currents flow through the switch and into the inductor. This path between the switch and the inductor must have very low impedance or the pulsed current will result in a large voltage ¡§spike¡¨. In a non-integrated converter, the path consists of the wire-bond, the package lead, the lead solder-joint, the board trace, the inductor solder-joint and finally the inductor lead. The total impedance is given by:
The trace and solder joint impedances are the main culprits, having potentially large resistances along with significant reactive components. This leads to noise and ringing from the pulse currents and AC ripple current.
The solution? Bring the inductor inside the package and wire-bond directly from the MOSFETs to the inductor electrode. This eliminates the trace impedance and the solder joints.
The higher level of integration yields a foot print reduction of 50 ¡V 80 percent and solves the troublesome noise and layout issues
But My Inductor and Output Cap are Too Large
Let¡¦s say we are designing a DC-TO-DC converter to step down a Lithium Ion battery voltage, nominally 3.6V down to 1.8V at a load current of 500 mA. We want low current ripple, Assume a ripple current, ƒ´IOUT of 25% of IMAX.
The required inductor would be given by:
This inductor is too large for integration, especially if shielding is required.
Increasing the switching frequency by a factor of five, to 5 MHz, would reduce the needed inductor to a value of 1.4 ƒÝH. This value is much more realistic in a small footprint with low profile.
The choice of output capacitor is also inversely proportional to the switching frequency and is given by:
Where ƒ´Vout is the maximum allowable ripple voltage. Operating at higher switching frequency allows the use of smaller capacitors, yielding further footprint reduction.
Switching Loss
For any semiconductor process, there is a switching loss that increases linearly with frequency. The equation below describes the relationship:
A common parameter used in the power semiconductor field is known as the ¡§figure of merit¡¨(FOM). This is the product of the gate charge and the MOSFET drain-to-source ¡§on¡¨ resistance or RDS(ON), in units of milli-Ohm*nano-Coulombs.
For any given semiconductor process, this FOM is a constant. You can decrease the RDS(ON) to reduce the conduction losses in the switch, but the penalty is that switching-loss will increase proportionately. Likewise, you can reduce switching loss, but the conduction loss will then increase
Semiconductor Process Developments: How to go Faster
To get to the very low FOM target, we must move to a deep sub-micron CMOS process; one that can tolerate higher voltages. Enpirion engineers tackled this problem with amazing results. While the typical FOM for a power semiconductor device, including LDMOS, ranges from 80-400 mĂ*nC, Enpirion LDMOS exhibits a FOM that is below 10 mĂ*nC. Since the gate capacitance is reduced by as much as a factor of ten, the device can be operated at ten times the speed for the same efficiency, with an inductor one-tenth the size.
High Frequency Magnetics
Magnetic materials also have a loss component that is frequency-dependent. Operating at high frequencies translates to higher core losses. It is not enough to move to higher frequencies to get to a smaller inductor value; it is critical to address this AC loss component as well.
The main culprit in AC core loss is eddy current. The common way to deal with eddy current is to restrict the path through which it flows. In transformer design, slicing the core into thin laminations does this. In ferrite the conduction path is confined to an area the size of a grain of the material. Enpirion¡¦s approach has been to take a magnetic alloy, a metal, and disrupt the crystal lattice structure to form an amorphous crystalline solid. Now the eddy current path is restricted to a minute area, providing excellent high frequency performance.
Practical Applications
Enpirion has introduced a family of DC-TO-DC converter modules in a 4mm x 5mm x 1.3mm QFN package. MLC filter capacitors yield a footprint of only 28 mm2. This is about half to one-fourth the footprint normally required and does not compromise on performance or efficiency. Providing expert field technical support is the final piece in the puzzle. External component selection and timely layout advice makes a hitherto complex task a complete turnkey process.