Thermal Management
Judge and jury
DC/DC regulator efficiency can be deceiving so best let package thermal performance be the judge and jury, as Linear Technology’s Afshin Odabaee, Alan Chern and Jason Sekanina explain
A
DC/DC switching regulator’s efficiency of voltage conversion is considered by many to be a final
measure of performance. The efficiency number is a measured value, listed in a data sheet, graphed for a range of voltages and currents and compared when a system designer attempts to select a solution from among several vendors. To achieve high efficiency (a relative term but let’s assume a number above 85%), analogue silicon designers and application engineers painstakingly try every trick in the book such as adjusting the switching frequency and sizing the gate drive for power switches. Surprisingly, the silicon or IC alone can’t promise the highest efficiency for a circuit. Selection of external components does have a profound effect on the performance of an otherwise exceptional IC. The choice of inductors, capacitors and PCB layout, as well as dexterity of the system designer, are crucial factors in designing a high efficiency DC/DC switching point-of-load regulator. However, the story of thermal management does not end with a “high efficiency” number.
Judging thermal performance of a point-of-load DC/DC regulator by its conversion efficiency alone is similar to speculating on the speed of a vehicle by its engine size. For example, a 90% efficient DC/DC step-down regulator with 3.5W heat dissipation housed inside a tiny package with 22ºC/W j-a thermal impedance creates thermal management challenges that make it almost impractical and often too expensive to use.
Wishing and hoping The battle to control heat dissipation and distribute power more efficiently has been intensifying. The optimum operation and reliability of digital equipment and infrastructure depend highly on the performance of DC/DC converters that are used as distributed DC power for FPGAs, ASICs, transceivers and memory modules as well as RF amplifiers and sensors. Aside from the electrical performance such as the accuracy of regulation or transient
10 March 2012
response, thermal performance has become a more crucial factor in selecting a DC/DC regulator. In Figure 1 this 72W solution relies on the accurate current sharing of four µModule regulators and low thermal impedance values to prevent
Figure 1. Four DC/DC µModule regulator systems current share to regulate 1.5V at 48A with only 2.8mm profile &15mm x 15mm of board area for each device.
the board shown in Figure 1 with readings of the temperatures at specific locations and air flow direction and speed. Cursors 1 to 4 show an estimation of the surface temperature on each module. Cursors 5 to 7 indicate the surface temperature of the PCB. Notice the difference in temperature between the inner two regulators, cursors 1 and 2, and the outside regulators, cursors 3 and 4. µModule regulators placed on the outside have large planes to the left and right promoting heat sinking to cool the part down a few degrees. The inner two only have small top and bottom planes to draw heat away, thus becoming slightly warmer than the outside two.
Airflow has a substantial
Figure 2. Thermograph of 48A, 1.5V circuit of Figure 1 shows balanced power sharing among each DC/DC µModule & low temperature rise even without airflow (VIN=20V to 1.5VOUT at 40A).
hot-spots by dissipating the heat evenly over a compact surface area. Each DC/DC µModule regulator is a complete power supply with inductor, MOSFETs and DC/DC controller circuit fitted in an IC form factor package. Each can deliver 12A (or more if paralleled) from a wide input range of 4.5V to 20V, making it a versatile and scalable solution. Parallel system design involves little more than copying and pasting the layout of each device. It occupies only 15mm x 15mm of board area and has a short profile (height) of only 2.8mm. In addition to good efficiency, the package exhibits only 15ºC/W j-a thermal impedance. This short profile allows air to flow smoothly over the entire circuit, removing heat from the circuit (Figures 2- 5). This solution casts almost no thermal shadow on its surrounding components further assisting in optimum thermal performance of the entire system.
Looking beyond efficiency Figures 2, 3, 4 and 5 are thermal images of
Components in Electronics
effect on the thermal balance of the system. Note the difference in temperature between Figure 2 and Figure 3. In Figure 3, a 200LFM airflow travels evenly from the bottom to the top of the demo board, causing a 20°C drop across the board compared to the no air flow case in Figure 2. Direction of airflow is also important. In Figure 4 the airflow travels from right to left, pushing the heat from one µModule regulator to the next, creating a stacking
effect. The µModule device on the right, the closest to the airflow source, is the coolest. The leftmost µModule regulator has a slightly higher temperature because of spillover heat from the other LTM4601 µModule regulators. Figure 5 shows an extreme case of heat stacking from one µModule device to the next. Each of the four µModule regulators is fitted with a BGA heat sink and the entire board is operated in a chamber with an ambient temperature of 75°C. Here is another example
for 3.3Vin systems requiring high load current, up to 15A. The LTM4611 is able to offer efficiency in a small land pattern (only 15mm ◊ 15mm) and low physical volume (at only 4.32mm tall it occupies only one cubic centimeter), in a
thermally enhanced LGA (land grid array) package. Figure 6 shows the device’s efficiency for various combinations of input and output voltage conditions. Besides high efficiency, the power dissipation envelope of the LTM4611 is relatively flat for a given input voltage condition, which makes the thermal design and its re-use in follow-on products easy. For an increasing number of applications, reducing power loss at light
Figure 3. Thermograph of four parallel LTM4601 with 200LFM bottom-to-top airflow (20VIN to 1.5VOUT at 40A)
loads is as important as reducing power loss at heavy loads. Digital devices are being deliberately designed to operate in lower-power states for as long as possible and whenever practical (for energy conservation), and draw peak power (full load) only intermittently.
Thermally enhanced packaging The device’s LGA packaging allows heat-
Figure 4. Thermograph of four parallel LTM4601s with 400LFM right-to-left airflow in a 50ºC ambient chamber (12VIN to 1.0VOUT at 40A)
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